r
f'T EXPLORAnLE ELECTROTACTILE
DISPLAY
by
Robert Michael Strong
B.E.E., Villanova University
(1965)
S.M., Massachusetts Institute of Technology
(1966)
SUBMITTED IN PARTIAL FULFILLMENT OF THE REC4UIREMENTS
FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
February, 1970
Signature of Author
Department of Electrical Engineering, October 1, 1969
Certified by
Thesis Supervisor
Accepted by
Chairman,
~~1.........~)
Departmtal Commi tee on Graduate Students
MAR 20 197 0
--IBRARIES
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-2AT EXPLORABLE ELECTROTACTILE
DISPLAY
by
Robert Michael Strong
Submitted to the Department of Electrical Engineering on
October 1, 1969 in partial fulfillment of the recqiirements
for the Degrree of Doctor of Philosophy
ABSTRACT
The feasability of an explorable tactile display for
the presentation of two dimensional graphical information
to the blind is demonstrated. The display is based on the
use of an electrical stimulus and an array of small active
electrodes to which the user applies his finger tips. The
sensations which can be elicited in this manner have been
explored and experiments are presented which result in an
initial measurement of the characteristics of such a de-
vice relating to the line drawings or shaded area pictures.
Two distinctly different sensations were elicited, resulting from different mechanisms. The first of these sensations is apparently the result of a current driven mechanism, and is similar to previously reported electrotactile
sensations. Its continued use is unpleasant to most display users, and the experimental evidence shows that it is
not as useful for display purposes as the second sensation.
The second sensation is one of texture, that is, that
the surface of the display has acc-uired a texture in those
areas where the electrodes are excited. This sensation
appears to be caused by a voltage related mechanism rather
than a current related one. A model for its production
is proposed, and it is shown that the force expected from
the proposed mechanism is of the same order of magnitude as
that measured in the analogous mechanical stimulus experiment. 71 of the experimental evidence relating to the
texture sensation is consistent with the model proposed.
The texture sensation is the preferred sensation for
most users.
It allows distinctions to be made between
textures on the basis of both stimulus amplitude and stimulus frequency (pulse repetition rate).
The shape of the
stimulus oulse anpears to be non-criticAl, indeed having
little effect on the sensation if the peak voltage applied
remains constant.
-3The use of these sensations in displays for line drawings is explored by way of the distinguishability of the
elements of such a presentation. Their use in a shaded
(or textured ) area display is explored by way of the distinguishability of the resulting textures and the localizability of the interstimulus boundaries. In both cases,
useful resolution is shown.
THESIS SUPERVISOR: Donald E. Troxel
TITLE: Associate Professor of Electrical Engineering
-4-
ACKNOWLEDGEMENT
The author wishes to acknowledge the financial
assistance of the Fannie and John Hertz Foundation
whose fellowship aid paid for three and one half years
of his studies at the Massachusetts Institute of
Technology.
The intellectual assistance of the members of the
Cognitive Information Processing Group of the Research
Laboratory of Electronics, particularly of my thesis
committee and Dr. Kenneth Ingham, whose assistance
in helping me to understand the needs of the blind
has been invaluable, will always be appreciated.
TArLE OF CONTENTS
Section Number
Abstract
Pa e
.
.a..................
Acknowledgement. .
Chanter
I
.
.
*
2
.
4
.
Introduction. .
.
.
.
.
.
.
.
.
8
.
I-a
The Underl,7ina Problems . . . . . . . . .
I-b
The Problem of Tactile Information
Transfer
.
.
.
.
.....
..
8
11
....
Requirements to be Met by the Display
.
.
17
A Review of the Available Stimuli and
General Display Design . . . . . . .
.
28
II-a
Kinds of Stimuli. .
.
28
II->
Kinds of Stimulus Sources .
II-c
Reasons for the Use of an Electrical
.
........
Stimulus...
I-c
Chapter
II
.
.
.
.
.
.
.
.
.
.
30
.......
.
34
A Brief Summary of the Results.
.
.
.
.
.
35
The Experimental Apparatus. .
.
.
.
.
.
39
III-a
The General Structure of the System . . .
39
III-b
A Description of the Electrodes
III-c
A Description of the Stimulus and Its
II-d
Chapter III
.
.
.
.
.
48
.
.
.
.
.
.
.
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.
53
On the Use of Subjects. .
.
.
.
.
.
.
.
.
58
The Sensations Elicited by Electricity
in a Freely Explored Environment . . .
63
IV-a
A Brief History ..
63
IV-b
The Qualities of the Sensations
Variables.
III-d
Chapter
.
IV
.
.
.
.
..
.
.........
.
.
.
.
.
77
-6-
P ac.re
Section Number
IV-c
Chanter
V
V-a
The Mechanisms.
.
.
.
.
.
.
.
90
Some Factors Affecting the Use of an
Electrical Stimulus in an Explorable
Display. .
.
. .
..........
.
117
Experiments Relating to the Use of the
Sensation in General . . . . . . . .
.
118
119
Experiment
a-I
.
.
.
.
.
.
A Scale for Amplitude
.
.
Experiment a-II
A Scale for Frecquency
.
. 128
Experiment a-III
Threshold Variations.
.
.
142
Experiment a-IV
The Effect of Electrode
Area on Threshold.
.
.
149
Experiment a-V
Pin Electrode Size.
.
.
159
Experiment a-VI
Large Area Thresholds
.
.
161
Experiment a-VII
The Effect of Pulse Sha pe
on the Sensation
.
. . 161
Experiment a-VIII
The Effect of Pulse
Repetition Rate. . .
ExDeriment a-IX
V-b
.
.
167
The Amplitude Dynamic Range 168
Experiments Relating to the Use of the
Stimuli in an Explorable "Textured
Area" Display. .o
.
........
o..
170
Experiment b-I
The Just-NoticeableDifference for Amplitude 173
Experiment b-II
The Just-NoticeableDifference for Frequency 178
Experiment b-III
Area Boundary Localizations 182
Experiment b-IV
Two Area TransitionsAmplitude Differences. . 184
Experiment b-V
Two Area TransitionsFrequency Differences. . 186
-7Section Number
V-c
Experiments Relating to Point-and-Line
Displays . . . . . . . . . . . . . .
Form Separation
Measurements . .
Experiment c-I
.
.
a Grounded Field of
Electrodes . . . . .
Experiment c-III
Small Pattern
Discriminability
Chapter VI
A Summary and Some Conclusions.
Appendix
The Stimulus Sources Used .
.
A-!
An Amplifier Based System
.
.
A-2
A Switch Based
.
.
.
.
. .. ...
Biographical Note .
.
.
. 191-
Localization of Points in
Experiment c-II
Bibliography .
.
. 188
.
System .
.
.
. . . . . . ..
..
.
.
.
.
.
.
.
.
. .
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. 192
.
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.19
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9
212
. . . . 222
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222
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226
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. 230
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236
-8Introduction
The work which culminated in this thesis grew out
of specific recuirements posed by two distinct problems.
As the nature of the results is determined by these
considerations, a review of those problems will be
undertaken first.
The Cognitive Information Processing Group in the
M.I.T. Research Laboratory of Electronics has in progress,
at the time of this writing, a project to develop a
"Reading Machine".
This is a computer based system
which scans the pages of an ink print book, processes
the information found there and translates it into a
form usable by the blind, finally presenting the results
to the user.
All -of this must be done at a rate approacha
ing the user's ability to absorb the information presented to him.
It is clear now that the system can be made to
handle all of the linguistic information by extension
of present technirues.
However, there yet remains the
problem presented by thenonlinguistic information,
in particular, the pictures, line drawings, graphs,
etc., often found in technical works and reference
material.
Here, as opposed perhaps to pictures found in
recreational reading, the pictures form an important part
-9of the information content of the work.
In some
circumstances, such as electrical circuits and mechanical structures, this is the only reasonable way to
convey the required information.
It is desired to convey this two dimensional
information to the user in a form as close to the
original as possible.
This avoids the necessity to do
large amounts of preprocessing before the presentation,
and simultaneously minimizes the need for user training.
It is also pertinent to point out here that there
is considerably more information contained in most
pictures than any one user will want to remember.
It
is therefore desirable to permit the user to seek out
that information which is useful to him, and to ignore
the remainder, much as a sighted person would do in
examining the original.
This consideration brings us to a second problem;
we simply do not understand the process by which people
"feel" objects.
We understand fairly well the proper-
ties of the perception of the world about us given by
the visual apparatus, but this information is distinctlv lacking when it comes to the perceptions formed
by blind persons on the basis of tactile information.
It is clear from what the blind tell us about
their perception of the world in which they live that
there are strikina differences between their perceptions
and those of the sighted.
The most obvious of these
differences is the absolute lack of any sense of projective geometry.
As a result of the physical pro-
perties of light and the nature of the visual apparatus
we use projection extensively in formin
our pictures,
whether they are generated by camera or by an artist.
Lacking any reason to presume that projection is the
normal state of affairs, the blind, apparently tend to
perceive the objects around them in the basically three
dimensional form in which they receive the information.
They have essentially no contact with representational
descriptions, such as two dimensional representation
of a three dimensional object.
This seems also to be
true of those recently blinded as well as the congenitally blind, and it becomes worse with time as tactile
experience replaces visual experience.
It is therefore desired to develop a display which
can be used to study the way in which a tactilely
oriented person perceives a two dimensional presentation.
Similarly, it would appear advantageous for the blind
individual living in a world organized basically by and
for sighted individuals to learn the representation
schemes commonly used by the sighted.
A device and
technicrue is needed which will allow us to teach this
as we now teach Braille.
I-b.
The Problem of Tactile Information Transfer
The study of the sense of touch and the associated
perceptions is not a new idea; it has been going on for
at least a century.
The early emphasis was placed on
the isolation of specific aspects of the nerve structure
which serves the tactile sense; i.e., the set of nerves
whose stimulation results in sensations of touch.
More recently, people have become interested in
transmitting information through the skin.
This work
has forced us into an interesting distinction, that
of the difference between the classical "tactile sense"
and what has been called the "haptic system".
The
distinction is basically the difference between a single
one way information pathway and a multipath feedback
system in which several "senses" of the classical type
as well as the voluntary motor system of muscles and
nerves participate.
Another view would be that it is
the difference between being touched (tactile) and
actively touching (haptic).
The primary reason for making this distinction
comes as a result of two observations.
The first is that
when presented with a new environment, whether it be a
dark room, or a visual scene which is new, people go
through an exploratory procedure, and that this procedure is usually goal directed.
The second is that
the nerve structures which serve as primary sensors for
-,
2-
all of our perceptual systems are structured to operate
most efficiently on changing presentations.
The first phenomenon is perhaps best demonstrated
by measuring the eye movements of a subject viewing a
picture.
It can be easily demonstrated that the eye
movements and the fixation points can be caused to vary
from presentation to presentation by changing the
subject's goals, that is, by asking him cuestions about
what he sees before each presentation.
The second
observation is true for both spatial and temporal changes,
though the primary concern here is for temporal changes,
that is, the case in which the spatial pattern changes
with time.
An experiment performed some years ago by J. J.
Gibson (1962) shows fairly clearly that it is necessary
to consider both of these phenomena in the design of
any display.
The experiment consisted of the presentation
for identification of one of a set of six cooky cutters.
The presentations were made by placing the cooky cutter
in the palm of the subject's hand.
The subject knew
what shapes were included in the set.
The presentations were made a) by holding the
object stationary in the subject's palm, b) by having
the experimenter move it about while the subject remained passive, and finally c) by permitting the subject
to control the movement of the object about his skin.
-13-
In all cases, the area of skin used was the same.
significant result is that in case (a),
The
the subjects
were able to identify approximately 49% of the presentations correctly.
In case (b) the correct answers
were up to 72%, and in the last case the identifications
were 95% correct.
The rather dramatic results of this experiment had
been anticipated, and it has often been suggested that
experiments be performed to distinguish the effects of
various kinds of active and passive motion, to isolate
the perceptual phenomena involved, and perhaps most
importantly, to rank the strengths of these effects.
Returning to the work which has been done on
specifically tactile displays, we find that almost all
of the recent work has been on what will be termed here
reading-rate experiments.
The most common form of this
situation is an experiment in which the subject is
presented with letters selected from a more or less
arbitrary alphabet.
The letters are always presented
very briefly, and the subject is usually constrained,
either by the brevity of the presentation or by the
nature of the display, to take in all of the information
he can at once, with no second looks or exploratory
activities.
In addition, he is often required to
accept subseruent letters on the same sensor area.
-
.
WN-
The measure of performance is usually the rate
at which letters, letter combinations, or words can be
recognized.
These experiments have been performed with
a wide variety of displays (see Geldard, 1957; Bliss
et al, 1965; Troxel, 1967).
The most startling characteristic of all of these
experiments is not which displays or senses give the
best results, but that as Troxel (1967) has noted,
it does not seem to make much difference to the readingrate limit which sense, or which display system is used.
The implication is that the fundamental limit is not a
sensory one at all, but is more probably related to the
brain's method of processing such information.
There
is some evidence that the key lies in the size of the
pieces of information which are remembered (Miller, 1956).
It is clear however, that the limit, found to be
generally less than 40 words per minute for letter-ata-time presentations, does not apply to ordinary visual
reading.
Either the brevity of the presentation pre-
vents a necessary feedback or reinforcement operation
in preventing a second glance, or the "letter" is too
small a piece of information.
In either case, it might
be conjectured that the subject is being prevented from
utilizing the redundancy and excluding the extraneous
information found in normal text.
Readers wishing to explore further the characteristics and limitations of the tactile sense are referred
to the original work of von Frey (1915), and to the
more recent work of Geldard (1960) and von Bdkesy, (1959).
The latter work also contains a fair analysis of the
relationships which exist between the tactile sense
and visual and auditory senses.
The best discussion
of the relationship of the tactile sense to its associated
haptic information gathering system is given by J. J.
Gibson (1966).
Unfortunately, there seems to be no good
collection of what is known about the performance
limitations of the haptic system as a whole.
Our knowledge of the information transfer capabilities of the tactile nervous system, and to some
extent the haptic system, is best divided according to
the kind of display stimulus used, and the references
given are representative of the work which has been
done.
The usefulness of vibratory mechanical stimuli
has been explored by Geldard (1957), who as the
originator of Vibratese was the first to make a major
attempt to fit the alphabet of a reading-rate experiment
to the capabilities of the sensory system being used.
More recently, C. E. Sherrick (1966) and R. Verrillo
(1965) have explored in detail the phenomena involved
in the vibratory sensations which Geldard used.
J. C.
Bliss (1965) and his associates at Stanford Research
Institute have done considerable work using several
different types of stimulators, including air jets,
mechanical vibrators, and a mechanical transducer
with some unusual properties called a Bimorph.
Stimulation of the skin senses using electric
currents has also been of interest.
The most recent,
and perhaps the most comprehensive work has been done
by R. H. Gibson (1967).
Some work has also been done with what are termed
poke probe mechanical stimuli.
The basic physical
distinction between this and the vibratory type is that
the stimulus is either very brief, usually one cycle of
+-ibrator motion, or remains available but without the
motion after application.
Here again, the reader is
referred to von Bekesy's work.
The work described in the references above, with
the exception of that of Bliss, is that primarily of
psychologists whose main interest was in gaining an
understanding of the operation of the tactile system.
Studies relating more directly to the problem of transmitting complex patterns of information, particularly
two dimensional information, should be made in areas
related to the manner in which a person uses his hands
to perceive such complex patterns, and on the form of the
tactile images which he forms from the information he
has gathered.
Unfortunately there has been little
7-
systematic research in these areas.
I-c.
Requirements to be Met by the Display
The requirements which the display must meet are
set by two factors, the nature of the information to
be displayed, and the characteristics of the sensory
system to be used.
The fundamental assumptions are
that an explorable display should be built if possible,
and that we want to avoid preprocessing the pictures.
The basic reasons behind these assumptions are the
experience of J. J. Gibson (1966), the feeling that a
human being can be expected to operate best in the
kind of environment he is used to and that since processing the pictures before presentation requires that
something be thrown away in some sense, we cannot do
this without prior knowledge of the goals of the user.
Since a very wide variety of visual images can be
found in ink print books, all two dimensional, but
ranging from simple line drawings and graphs to color
photographs, the approach will be taken that it is
necessary to first examine the nature of the pictures
we might run into without restricting the set at all,
then to examine the capabilities of the display in the
hands of a potential user, in simulation when possible
of the use situation, and finally to reduce the set of
-1-8
pictures which we hppe to handle according to the
results of the experiments.
The need for and kinds of
preprocessing to be done are also determined on the
basis of the experimental results.
In analyzing the material we wish to display,
it is necessary to do two things, to show that a tactile
analog of a visual scene does exist, and from that
anAlog to determine what characteristics the display
must have.
It is clear that if one wants to specify
a picture in an optimum way he should take into account
the nature of the picture.
However, since we do not
know what pictures we will encounter, let us for the
time being consider all pictures, even line drawings,
as specific cases of a picture composed of colored
areas, modulated in intensity and bordering on areas
of other colors.
If we consider the color of an area in combination
with the fine detail of the intensity modulation to
be the analog of the textures common in the tactile
world, then most visual scenes can be broken into areas
of distinct texture.
This is not possible in cases where
two colors blend smoothly into each other, although there
may he an analog for that too.
A few examples may
help to illustrate what a visual texture might be.
-19We wish to think of texture as the property of an
area which is more or less visually uniform on a large
scale; thus the regular pattern of a window screen
viewed against the sky constitutes a visual texture.
On
a slightly more complicated level there is the leaf
structure of a tree when viewed from a great enough
distance that the viewer is not tempted to look at
individual leaves.
A brick wall has yet another
texture, and in many instances color plays an important
part in it.
It is granted that the areas of relatively constant
texture in a visual scene tend often to be very small,
but this is a matter of the high resolution of the
visual system, and will be treated later.
We do need,
however, a means of specifying the resolution which
we might need to achieve, and this can be done by specifying the knowm performance of the eye when reading,
and the capabilities of the tactile sense in use on the
explorable display to be constructed, after experimental
studies of those capabilities.
It should be clear that specifying line drawings,
electrical circuit schematics, etc. in terms of texture
may be a much too restrictive way of looking at the
situation, and it wouild be wise to explore the tactile
analog of a line drawing as well as in the experiments
.
,
REEF-W-*-
-20with the display.
It would seem then that the tactile
analog of a visual picture does exist.
There is of
course no guarantee at this point that it will be
possible to generate tactile textures, much less with
the variety to be found in the visual world, and there
are other uncertainties such as resolution and dynamic
range limits.
Conceptually, however, it is possible
to get around the spatial resolution problem to some
extent by scale changes.
The question of dynamic range
will have to be explored experimentally.
It
should be noted that while the analog of a
visual scene discussed above is intended to include all
visual scenes, whether of a line drawing, a photograph,
a painting or live objects moving about in the real
world, the intent is to imply that the human eye is
being replaced with a television camera, and the
objective information contained in its output is passed
on to the tactile sense of the user.
No attempt has
been made to be concerned with the relationship between
the perceptions of a sighted person viewing the scene
in the original and a blind user getting information
about the same scene from the display.
It is hoped that
with a display available such questions can be explored.
The first of the constraints placed on the system
by the human user is speed.
There are two distinct
speed factors to be considered in the present case.
The
nature of the presented materiAl and the fact that we
have chosen to avoid the information reduction task
force us to maintain the display for a considerable
length of time, and conversely allow us to spend a bit
more time in establishing the display for any given
presentation.
The fundamental limit on set up time is
the patience of the user in any real situation.
At
the other extreme, the presentation must be maintainable
for an indefinite period of time since we cannot specify
a priori how much time any given user will require with
any given presentation.
On the other hand, since the display must be
applicable to several fundamental perceptual studies,
it must be fast enoucgh to permit direct examination
of Phenomena whose characteristic times are very small.
Phonomena have been noted, by others, with characteristic
times as short as 0.1 milliseconds.
If the known
phenomena are examined it is found that the response
times of the system can be specified.
The result is
a recuirement that any individual point in the display
must be able to change value in less than 046
milli-
seconds, and that an entirely new presentation must
take less than 30 milliseconds to generate.
The first number, 0.1 milliseconds, turns out to
be the required transducer response time; the second
limit, 30 milliseconds, is of no real conse-uence
unless the display becomes very large since electronic
equipment can be made to operate much more rapidly than
this.
It is safe to assume that at the moment the
primary speed condern is the transducer response time,
with the note that when a full sized display is built,
the trade off of speed fam controller complexity may be
the primary consideration.
The question of display resolution probably represents the most difficult case of a user imposed restriction on the system design.
We know that in real world
situations, the human hand is extremely good at its
normal task.
We know for instance that a tailor can
make very fine distinctions in the quality of a piece
of cloth on the basis of its texture and that a
machinist can detect with his fingers burrs which may
be difficult or impossible to see.
Another situation
which is perhaps more meaningful is the examination
of the detail on the surface of a small carving such
as a chess piece or a cameo pin.
While there seems to
be no good way to measure a subject's performance
at tasks of this sort, it is generally true that those
who try these experiments find that people do remarkably
well.
In attempting to teach the blind about the world
around them,
people have sometines resorted to the
generation of "Braille pictures", that is, the embossed
paper analog of a visual picture or line drawing.
They
are generally made in one of two ways, both of which
have the characteristic that they are two-amplitude
presentations, a given point is either raised or not
The first of these operates on a fixed grid principle in that a preselected grid of points is laid over
the original picture and if under a given grid point
the picture is "black", a dot is raised, and if the
picture is "white" it is not.
This system lends itself
well to machine implementation.
Under most circumstances,
the grid is relatively coarse, perhaps one-tenth inch
between dot centers, and the dots are about half that
in di ameter.
The second method requires a human artist.
It is
accomplished by the use of a device known as a star wheel,
a small toothed wheel attached to a handle which when
run across one side of a piece of heavy paper, will
cause a line of raised dots to appear on the other
side.
The dot spacing in this case is usually much
smaller than in the first scheme, as is the dot size.
Here,
the line rather than the dot is
the structural
element of the picture, and by providing a variety of
different star wheels, a variety of lines can be
obtained.
After a few experiences with such pictures, one
comes to three conclusions.
The first is that the
resulting "pictures" are not very good.
They present
real and serious perceptual difficulties when the user
tries to relate what he feels to the known "feels" of
real objects.
The problem is much more serious here
than in the visual case, apparently because the
representation is "wrong" in some sense.
The second
conclusion is that while the subject can feel individual
dots, he will usually not attempt to do so unless the
dot is isolated, rather he prefers to group the dots
into lines and forms when he can.
The third conclusion
is that the star wheel method produces better results
than the dot array method.
is ver y important.
The reason for this fact
As it turns out, the second method
lends itself to the inadvertant generation of texture,
for example in an artist's attempts to draw hair about
a girl's face.
The picture in figure I-1 will perhaps
give the reader a better feeling for why this might
be true.
The basic conclusions with respect to resolution
then are that we should strive for the best possible
spatial resolution in the display, both to present
I
STAR WHEEL
DOT ARRAY METHOD
METHOD
FIGURE 1-1
EXAMPLES OF
TACTILE DRAWINGS
accurately the details of a shape and perhaps to generate
textures in some fashion.
It is expected that texture
will prove important enough to justify extra expense
to achieve good textures even if there were no gain in
the ability to discriminate detail in the presentation.
The final consideration from the point of view
of the user is the overall size of the display.
Ex-
perience with the Braille pictures quickly brings out
the fact that as a sensing organ, the fingers encompass
a very small area.
The situation for a blind person
exploring a ten inch square tactile picture can be
likened to a sighted person trying to get an overall
view of a ten block square section of a city by running
through all its streets.
It takes time, and one is
likely to forget where he has been and what he has seen,
therefore having to go over it again and again.
A careful analysis of a blind person's actions when
exploring such a picture, and the methods he uses to
remember where he has been, lead one to conclude that
no display of this sort should be more than two hand
spans across in any direction, suggesting a ten inch
square display at most, with six or seven inches being
perhaps a better choice.
The use scale changes to over-
come resolution limits presents no real problems if one
allows himself the luxury of multiple presentations.
-27-
The first presentation might provide the general arrangement of the parts of the figure, and subsequent presentations at appropriate magnifications might provide
knowledge of the detailed structure.
The discussion given in the introduction is intended
to outline for the reader the nature of the general
problem area and to present a few of the salient characteristics of the human user which bear on the display
envisioned.
On this basis the research program is laid
out to encompass three areas.
The selection of a stimulus source and a basic
structure for the display are discussed in chapter II.
Part two of the program is a study of the phenomena
which occur when the user is permitted to freely explore
the selected display.
This of course involves not
only questions of the perceptions available, and the
phenomena responsible for them, but also such questions
as the safety of the user,
and the stability of the
perceptionsnover a period of time.
are explored in chapter IV.
These questions
Part three is a program
of tests of the system, and these results appear in
chapter V.
CHAPTER II
A Review of the Available Stimuli
and General Display Design
Kinds of Stimuli
II-a.
Three distinct kinds of stimuli can be generated
by the classical methods of stimulation of the tactile
sense.
It should be emphasized that the discussion
is based on experiments of the distinctly tactile rather
than of the haptic type, that is, the subject is a
passive observer, and the stimulator is fixed in location
on the skin surface.
These stimuli are best distinguished on the basis
of the sensation they produce.
The first sensation,
both historically, and in relation to ordinary tactile
experience, is that produced by poke probe stimulus.
The use of the term poke probe is historical.
Stimuli
of this type are inherently"on" for a considerable
length of time, and they feel as though the skin were
being deformed by the application of continuous pressure
to the skin viaasmall, dull probe or rod.
The important
properties of this stimulus type are the relatively long
drration and the direct relationship to the real world
situation of bringing an object into contact with the
skin.
The long period of stimulation permits accomo-
dation effects to take Dlace which will eventually
-29permit the stimulus to fade away.
An example of this
phenomena is the disappearance of the "feeling" of the
presence of a pair of eyeglasses after wearing them
for a period, or in the visual system, the accomodation
of the eyes to a bright scene.
It
The second type is the pulse-like stimulus.
is characterized by its brevity which requires that
all of the information which is to be transmitted to the
subject be perceived in a very short time span.
The
subject is deprived of the ability to mentally explore
his skin surface, and is forced to rely on short term
memory for any second look which he needs to take.
Unlike the poke probe case, fatigue becomes important,
particularly if the skin is cycled through a relatively
large local deformation repeatedly.
In most of the sit-
uations which have been studied, the probe repeatedly
hits the same spot on the skin, aggravating this
problem.
This stimulus and the vibratory stimulus to be
described next have been used extensively in the letterat-a-time reading-rate experiments mentioned in chapter I.
The last of the stimulus types is the vibratory
one, the most thoroughly studied of the possible stimuli.
It avoids the accomodation problem of the poke probe type,
and some of the effects due to the brevity of the pulselike stimulus.
In some circumstances, it might be a
rapidly repeating pulse stimulus.
Probably the primary
-30-
advantage of this stimulus is that it takes advantage
of the fact that the tactile sense is better able to
deal with changes than with constant stimuli.
The vibratory stimulus has its disadvantages.
most prominent problem seems to be fatigue.
The
The vibratory
motion leads to waves which propagate through the
tissues, fatiguing them, and simultaneously leading to
stimulation of large areas of skin.
The latter effect
would make it difficult to discriminate between very
closely spaced stimulators.
In fact, interference can
and does occur among stimulators which are widely
spaced over the body (Geldard, 1957).
II-b.
Kinds of Stimulus Sources
Using the second method of distinguishing between
systems, we note specifically differeces in the manner
in which energy is transferred to the skin.
Roughly
these can be characterized as either solid mechanical
systems, fluid systems, such as air jets, and electrical
systems.
Energy is transferred by inducing motion of
the skin or by passing electric current through it.
Solid contactor mechanical stimulators have received
the most extensive study both in the vibratory stimulus
form (Geldard, 1957; Verillo, 1963) and in the poke
probe form.
In addition to the difficulties of fatiguing
and indistinguishability mentioned above, this source
type tends to present some difficult engineering problems
when an attempt is made to construct a high density
display.
Most such systems have, for reasons of simplicity
and size, electrical sources driving electro-mechanical
transducers.
solenoid.
The normal form for the transducer is a
Solenoids inherently require a considerable
amount of space.
This consideration and that of the difficultyin
distinguishing between individual stimulators in multiple
stimulator systems have resulted in an emphasis on widely
separated stimulators, often greatly restricting the user
(see Geldard, 1957; Geldard and Sherrick, 1965).
These difficulties are not so severe with the poke
probe modes since the drivers are smaller, and because
the interference effects mentioned in the previous
section can be circumvented to some extent.
Densities
of up to six points on one finger have been attempted
(see Troxal, 1967) and have given results, in terms
of letter-at-a-time reading rates, which are as good as
those obtained with widely separated vibratory sources.
The majority of the work with air jets has been
done by J. C. Bliss et al (1965).
The normal mode of
use involves a nozzle some distance from the finger tip.
The air flow when the stimulator is "on" can be either
continuous or fluctuating.
tory sensation,
The latter produces a vibra-
and seems to be easier to detect.
The construction difficulties noted to occur with
solid mechanical systems do not occur here, and so it
becomes possible to produce displays with densities
limited only by the methods used in machining the nozzles.
Bliss has used densities of 100 per square inch.
It should be noted that in the systems which Bliss
has explored, the subject's hand does not come in
direct contact with the display surface.
This has
allowed him to test several kinds of artificial motion,
including scanning the information across the skin area,
and simulating the nystagmus of the eye (see Bliss et al,
1965).
The results show marked improvements in many
cases, but since the tests were of the reading-rate type,
it is difficult to relate them to the explorable display
case.
A third type of electromechanical transducer is
also in use.
Again, the development has been done by
Bliss et al.
The device, called a "Bimorph", is a lead
zirconate crystal with Iiezo-electric properties.
is used to drive a solid contactor via a long rod.
It
The
device is small, and thus it presents fewer difficillties
in building high density displays.
Bliss has built dis-
plays with densities of 200 per square inch.
The real advantage of this stimulator type lies
in the fact that it can be used to simulate in some
ways the real world situations which were discussed in
the first chapter.
Displays can be made explorable,
and though the bimorph is basically only a two state
device, through the use of vibratory modes, it may be
possible to produce a wide variety of effects.
However,
apparently most of the work to date is of the readingrate type and has not been concerned with long term
display of information in an explorable form.
The last of the stimulus sources is the purely
electrical source.
In this system electrodes are placed
directly on the skin, and currents are passed through
them.
By properly controlling the currents it is possible
to elicit sensations of touch or of vibration without
causing sensations of pain or muscle contractions.
Experiments in this area have been going on for some time,
and in fact they date back at least to the work of von
Frey in 1915.
The most recent, andbhy far the most
complete work is due to R. H. Gibson and his students
(1967, 1963).
The primary characteristics of the electrical
stimulus source, apart from any perceptual considerations,
are that it is inherently reliable, relatively cheap, and
that high density stimulator arrays are easily constructed.
II-c.
Reasons for the Use of an Electrical Stimulus
An examination of the requirements put forth in
chapter I will show that from the point of view of the
construction of a display, the electrical system has
distinct advantages.
It was shown that the system should
be relatively fast, a characteristic easily achieved
with electrical systems.
The best mechanical vibrators have response times
of 1 millisecond or longer, too slow to meet the high
speed requirements set forth in chapter I.
are a bit faster.
Air jets
The fluid part of such a system has
inherent limits considerably better than we ask for,
but such systems invariably contain an electromechanical
valve, and again the inherently slower mechanical
speeds prevail.
The bimorph, particularly in a case
where it.is asked to drive very little mass, can achieve
the desired speeds, but it is limited in usefulness
by the small excursions available from small units.
On the other hand, electrical systems are potentially
free of such problems, and should be easily capable of
ten times the desired speed.
It was argued that the density of stimulators must
be made high in order to more accurately simulate the
sensations available in the normal tactile environment.
Modern methods of construction should nermit electrode
densities much higher than the demonstrated resolution
of the skin senses.
These construction properties, and the possibility of
using integrated circuits, help to alleviate the problem
of the construction of a large display,
With all of
the mechanical systems described above, the transducers
inherently occupy more space than the associated contactors in a display of any reasonable density, say a spacing
of about 0.1 inches, the density which must be met in
order to compete with embossed paper displays in resolution.
Between the elimination of the electromechanical
transducer and the potential for integration, a considerable saving in cost can be expected if the perceptual
properties of the electrical display can be shown to
compete favorably with those of the more usual mechanical
displays.
II-d.
A Brief Summary of the Results
In the paragraphs that follow, a few representative
results are given to give the reader a capsule view of
the discussions to appear in later chapters.
These
results are presented without proof and are by no means
complete.
They are offered in order to help the reader
grasp the implications of the results without the obscuring details.
Some thirteen subjects have used the devTice, seven
of them for over 18 hours each.
The sensations reported
by these people have been consistent, and the results
repeatable over both the subject set and long periods
of time.
Two sensations were elicited, one being apparent-
ly related to the current controlled sensations reported
by others, and the other being a voltage controlled
effect which has evidently not been thoroughly investigated yet.
The current controlled sensations are reported
by the subjects to feel like a tingle or pins-and-needles
sensation deep within the finger.
effect is
electrode.
The voltage controlled
reported as the acquisition of texture by the
The electrode is often reported to have
become"rough" or "brushy".
The texture effect is apparent-
ly the more important one for display use as it is more
pleasant and offers better resolution.
Resolution limits have been measured in three
ways.
The presentation consists of a rectangular
group of electrodes.
The electrodes were one-tenth inch
between centers in the vertical and horizontal directions
and were mounted flush in a plastic table top.
The elec-
trodes were connected either to the source or to the
neutral electrode to form geometric patterns which the
subject was allowed to explore with his fingers.
In all
of the results reported here the subject was using the
texture sensation.
-37-
The first measure of resolution is the distance
between two figures, perhaps two points or two parallel
lines, which is needed to be certain that the elements
will be perceived as separate.
The measured value,
for several different kinds of patterns , is about three
electrode spacings or three tenths of an inch.
*.t
shorter distances the patterns are sometimes felt as
joined.
The second measure is intended to determine how
well the subject is able to localize a boundary between
two electrode groups excited by different signals.
In
order to make this measurement, the "just-noticeabledifference" (JND) for some parameter, for instance the
amplitude of the voltage, is measured.
A boundary local-
ization experiment is then performed with the areas
on either side of the boundary excited by signals which
are two JND's apart along a selected dimension, forinstance amplitude.
The localization errors measured
have a standard deviation of about 0.15 inches.
The third resolution measure is acuired by having
the subjects identify simple geometric figures.
The
conclusion is that with the display used, a minimum
dimension of seven t'enths of an inch should be used.
It
should be noted that the size of the array point spacing
may be a limiting factor in this case since the figures
must be apProximated by points in that grid.
The two important results are that textures can be
generated, and are the rule, not the exception, and that
the system resolution is sifficient to present relatively
small scale figures.
The only serious difficulty encountered was a
sensitivity of the texture sensation to skin resistance.
The mechanism requires that the user's skin have a very
high resistance.
This is achieved by the user in a
process usually described as "warming up the finger".
The most striking stimulus failure occurred when a subject
had difficulties off and on for two weeks, the only apparent common element being that during that period
he suffered from a chronic cold.
Subsequent exoeriments showed that the texture
stimulus could always be reliably produced by interposing
an insulating sheet between the electrodes and the subject's finger.
This procedure, however, also tended to
adversely effect the spatial resolution available.
Aside from that one difficulty the effect is rguite
stable, shows no fatigue or accomodation effects not
present in ordinary usage of the sense of touch, and
shows no signs of loss of sensation or hypersensitivity
either over protracted periods of disuse or prolonged
continuous use.
r
-39CHAPTER III
The Experimental Apparatus
The apparatus used in these experiments was designed
in the form of a prototype display.
It was deliberately
constructed so that it would be possible to rearrange
the structure of the system.
This permits displays
applicable to several different kinds of information
to be simulated without major reconstruction of the
equipment.
Since this system consists of large arrays of identical elements, an effort was made to minimize the cost
of the elements of that array.
The system .1sed has
the advantage too that it permits a wide variety of
pulse shaped to be attempted.
The application of direct.
currents was presumed to be undesirable (cf. Gibson,
1963).
III-a.
The General Structure of the System
The overall structure of the system is shown in
figure III-1.
The system consists of two computer
interfaces, each with its own local controls; a source
array; a stimulus distribution system, controlled via
the interfaces; and the display unit itself, mounted
in a table at which the user sits.
In, addition, for
SWITC H
ARRAY
INTERFACE
I I
/
--
Av
A
489
100
IN,
100-
/90
/
RESPONSE
UNIT
rTABLE
ELECTRODE
/8
ALF5
TO and FROM COMPUTERS
AMPLIFIERS
18
p
0
I
SHAPER
INTERFACE 2
[Response DiSp.
CLOCKS
/4
FIGURE TEL-I
SYSTEM STRUCTURE
experimental purposes, a subject response unit returns
information from the subject back to the experimenter,
via a visual display on the interface ecuipment, and to
the computer.
The first interface accepts, on command, an 18-bit
data word which is interpreted as twelve bits of true
data, four bits of information specifying the routing
for the data, and two other bits of command information.
It contains storage for 48 bits of information, which
in this s'stem are used to control forty--eight reed
switches in the stimulus control section.
This inter-
face was originally built to operate an array of poke
probe stimulators, and is described in a thesis by
David Peterson (1967).
The second interface accepts independently two
9-bit data words.
The first is internreted as an octal
constant defining the multiplication factor of a digital
attenuator.
It was commonly used to control the amplitude
of the stimulus.
The second is interpreted as a command
to the interface itself.
It is used to establish the
meaning of two pulses generated by the computer, and to
determine the disposition of data returned by the subject
response unit.
Four of its bits are supplied to the
stimulus control section for use in the co ntrol of four
pulse generators.
This interface also accdpts the data returned by
the subject, nine answers and four instructions and passes
this information on to the computer in a fashion governed
by the control word.
To facilitate non-computer-controlled
use of the equipment, the subject's response, the current
command word and a 9-bit status word which contains the
instantaneously current state of the machine are presented
visually to the experimenter.
Similarly, all of the
computer input and output data can be generated and/or
displayed locally.
The display itself consists only of a table in
which are flush mounted the active electrodes.
The ground
or neutral electrode is partially encased in a plastic
block, and slides around on the table.
It was designed
to rest und!er the heel of the hand when in use.
The
electrodes are fed by cable from the rack containing
the remainder of the equipment.
More will be said of
the electrodes themselves in section III-b.
The interfaces described above are both designed to
operate through data link facilities which existed between
the laboratory in which the work was done, and the PDP
1/TXO computer complex of the Electrical Engineering
Department.
The construction of two independent inter-
faces allowed the independent use of both computers.
Under ordinary circumstances, however, they are treated
as though they were in parallel, and the computer
-A.
11-
distinguishes them by issuing independent command pulses.
The source system consists of three segments, a set
of pulse generators or clocks, a set of pulse shapers,
and a set of amplifiers.
The two primary source types
used in these experiments are described further in the
Appendix
The pulse source section, the clocks, can be turned
on and off by commands from the computer or the experimenter.
In addition, their period can be controlled over
a very wide range, from 0.1 milliseconds to 15 seconds
in four stages, by the experimenter but not by the
computer.
A small patch facility allows each of the clock
outputs to be connected to any source.
The clocks can
also be made to report back to the computer, enabling
them to control computer function timing when necessary.
The pulse shaping section of the system ordinarily
generated bipolar rectangular pulses, but can accept
externally generated pulses of arbitrary shape.
In the
normal condition, bipolar pulses are generated, and fed
through a second order pre-emPhasis network, and then
applied to the amplifiers.
The pre-emphasis network
corrects for the major deviations from the ideal in the
frecuencv response of the amplifier-cable-electrode
system, and might be used to correct contact and tissue
effects if they were sufficiently well known.
This latter
use was not attempted, however, and it would appear to be
-/A.4-
unnecessary.
The details of the variables available
with this stimulus are discussed further in section
III-c.
The last element of the source section is the
amplifier.
Figure 111-2 shows a simplified representa-
tion of the system used.
The signal source is essen-
tially a voltage source, feeding a very high impedance
amplifier input.
The amplifier load consists of a
resistor and a transformer, the secondary-of which is
connected to the electrodes via the control network.
Under the expected conditions of finger load, apm
proximately 20,000 ohms, the source has an apparent
outout impedalce of 200,000 ohms.
variable bi
resistance.
This value is
changing the transformer ratio or the series
Under all conditions, the impedance seen
by the amplifier was maintained at its nominal output
impedance to optimize the frecuency response of the
system.
During some of the experiments, in which the
subjects exhibited extremely high skin resistances, it
was necessary to load down the tran&)rmer secondary in
order to maintain good control over the nulse shape.
The last section of the system to be discussed is
the control and distribution portion.
The primary
purpose of this portion of the system is to allow the
simulation of many different system structures with a
minimum of elements, thus the fifteen stimulus sources
-1
AMPLIFIER
FROM PULSE
SOURCE
SOURCE
IMPEDANCE
CONTROL
/TO CONTROL
SECTION
4zn
I5
TRANSFORMEF
TO NEUTRAL
ELECTRODE
FIGURE 111-2
STIMULUS SOURCE
4]
available in the maximum configuration, the switches
mentioned earlier, and one half of the total number of
electrodes are made available at any given time on a
patch block. (Figure 111-3)
This section of the system consists of two patch
facilities and a set of 48 reed switches.
The switches
are controlled by the first interface described earlier.
They are single pole single throw switches, and can be
used either in parallel with any electrode or in series
with selected active electrodes.
The electrodes selected
for use with the switch in the "series" condition were
the right most five columns of the array, forming an
array of five by ten elements.
Due to the difference
in these two numbers, two electrodes in the lower left
corner of this array were left unconnected.
The patch facility at the left side of the figure
permits any number of switches to be connected to any
given source.
The patch facility on the right permits any
switch to be connected to any number of electrodes.
Both
of these patch facilities permit the use of standard
resistors, shown in the figure in dotted form, in place
of the usual patch wires.
This facilitated the addition
of external load or current limiting resistances when
needed, and the isolation of multiple switches fed from
a single source.
15 SOURCES
48
DIODES
90 LINES -Io
ELECTRODE S
48
REED SWITCHES
T
0
F
/
F
(
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L
E
C
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T
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0
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E
F
S
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R
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48 LINES to ColuMnS I to 5
-'I
48 CONTROL LINES
frorn INTERFACE I
FIGURE MI-3
CONTROL SECTION
In addition to these elements, this section also
contains an over-voltage protection device in the form
of a selenium diode clipper.
One such device is as-
sociated with each switch, and serves to protect both the
subject and the switch from excessive voltages.
The
device has a practical voltage limit of 180 volts in
either direction.
The primary purpose of this device is
to limit the voltage on any electrode to a value small
enough to prevent sparking when initial contact is made
with an excited electrode.
The danger in even the most
insignificant sparks is that they cause holes through
the surface skin layer which constitutes the majority of
the subject's skin resistance, leading to high local current
densities and potentially to painful sensations.
III-h.
A Description of the Electrodes
The electrodes used in these exneriments are of
two types, large solid areas and arrays of very small
electrodes.
In both cases they were designed to be used
on the fingertips, and in such a manner that the subject
moves his finger around on the electrode or array at
will.
The subject alone has control over the pressure
which he applies to the electrode, and over the motions
he makes with his hand or fingers.
-9
To make the situation as easy on the subject as
possible, and to minimize the current paths, the neutral
or return electrode for all of the active electrodes
was placed under the subject's palm, and again, was not
attached to him in any way except by the pressure he
applied.
The neutral electrode consisted of a stainless
steel block covered with insulator except for a two square
inch area on its upper surface, which was contoured to
fit the palm.
The contouring permitted the subject to
slide it around on the table with very little effort.
A picture of the electrodes in one section of the
table appears as figure 111-4.
The picture is taken from
the point of view of the subject, with the return
electrode appearing in the foreground, four one-half inch
diameter solid electrodes appearing on the right, and
the main array of small electrodes appearing on the left.
All of the electrodes are of similar construt'tion.
A plastic block was drilled so that a rod of appropriate
diameter would fit very tiqhtlly.
The rod was then pushed
into the hole, and the surface of the assembly was ground
off until a smooth, flat surface was obtained.
Noother
attempt was made to treat the surface, either by further
oolishinr or coating it.
There was also no attempt made
to ensure that the co-efficients of friction were the
same for the electrode and plastic areas.
If
Ii~~III~i
I
I
01
FIGURE IBI-4
EXPERIMENTAL DISPLAY
reported that they were unable
Subjects initially
to locate unexcited electrodes in the plastic area, and
this was confirmed by blindfolding them.
After considera-
ble use, the surface developed "differences" which made
localization of the electrodes possible.
This effect
was apparently due to differential wear, and was eliminated
by simply regrinding the surface.
The electrodes of the array of similar ones are
made of brass, and those of the half inch set are made
of stainless steel.
It was found necessary to clean the
electrodes periodically with soap and water, but no
difficulties arose which could be attributed to corrosion
in any of the work with either type of electrode.
The larger electrodes are circuar in form, and are
either one-half or one inch in diameter.
The electrodes
of the main array are 7X10-2 inches in diameter and were
set in a square array with 0.10 inches between the centers
of adjacent electrodes.
A drawing showing the pertinent
dimensions of the array is given in figure 111-5.
The array has ten electrodes in the vertical direction, and eighteen in the horizontal.
is therefore 1.0 inches by 1.8 inches.
The overall area
The area of one
electrode element is approximately 3.85X10-3 inches
scuare so that 38.5% of the area within the array is
electrode, the remainder being plastic insulator.
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-53Only one-half of this array can be independently
handled by the control and distribution section, and only
the right most five columns are available directly in
the series switch mode.
The remainder of the array
was used for experiments in which fixed segments, such
as lincs or other point groups, were used.
These were
created by soldering at the back of the array.
III-c.
A Description of the Stimulus and Its Variables
The normal stimulus in the experiments reported in
later chapters was a bipolar rectangular pulse.
111-6 shows an idealization of this pulse.
Figure
Figure 111-7
shows photographs of two such pulses in practice.
The
one on the left was taken with the system correctly
loaded, and that on the right was taken with no load on
the transformer and is therefore a "worst case" result.
Referring now to figure III-6, the reader will
see that there are essentially three timing variables and
two amplitude variables to be specified; the first time
to be accounted for is the internulse interval, T ,
or its inverse the pulse reoetition rate which is the
equivalent of the stimulus frec'uency.
The clock system
is capable of any interval from 100 microseconds to 15
seconds, but the maximum speed of this system is ordinarily set not by the clock but by the width of the
overall pulse.
The system limits T1 to be greater than
i
VoIta
e
tiMe
T
%a
(.71
0 VOLTC.
79
Vb
C:3
+Tb]
FIGURE I1-6
AN IDEALIZED
STIMULUS PULSE
-55-
17 I
______
______
L
I______
I______
T_
.F ....
2Ov.
T
IDEAL LOAD (20 ka)
TIME
T-----
1l
.-
i
1
IL
50v.
+
T
I
NO
IFI17
LOAD (OPEN CIRCUITED ELECTRODES)
FIGURE I[L-7
SOURCE OUTPUT PULSES
T
-56-
Ta + Tb,
and any attempt to further reduce T
will
result in a reduction of Tb.
The two pulse width parameters, Ta and Tb have
upper limits of four milliseconds each, and lower limits
of approximately ten microseconds.
With exception of the
pulse interference mentioned above, all three time
parameters are independently controllable from the
front panel of the interface system.
In addition,
the pulse repetition rate is computer controllable by
switching clocks.
The amplitude parameters have been specified either
as the two pulse heights, Va and Vb, or in terms
of Va
and a ratio A = Vb /Va.
In most circumstances V is
computer controllable via the digital attenuator, and both
Sand Va are controllable from the interface panel.
Referring back to the amplifier discussion in
section III-a, we can clear up a remaining ambiguity.
The- system was designed around the stimulus parameters
suggested by R. H. Gibson (1963).
He recommends a bi-
polar rectangular current pulse, and recordb skin im-nedances on the order of ten to forty thousand ohms for
the dry stainless steel electrodes which he uses.
Using the value of 20,000 ohms nominal, and
choosing to allow a one percent variation in the current
pulse amplitude, it is possible to derive the design
-57-
parameters given earlier.
However, the fact that the
subject freely moves his hands about the display, and
the apparent nature of the texture effect place special
requirements of the display system.
The most important
is that it must have a voltage limit low enough to
protect the subject, but high enough to permit stimulation.
It is for this reason that the 200 volt clip-
ping diodes are included in the switch network.
The texture effect presents an entirely different
difficulty.
IV,
As will be explained in detail in chapter
it involves a high, very high, skin impedance,
usually a megohm or more.
Furthermore, it appears to
be governed by voltage, not current.
Therefore, the
source should be a VOLTAGE source, or should have a much
lower output impedance than the impedance of the load.
Since the electrodes were not insulated, a current
limit is also recuired, so that if the subject's skin
resistance lowers, the system will revert to the current
source form.
It was found that the amplifier system
would perform this task ruite well in the resistance
range encountered.
The pulse shape is badly effected in texture usage
by cable capacitances, etc. unless the source was
partially loaded artificially.
This results in a source
which is inefficient but extremely inexpensive.
The
variations in pulse shape which occur did not have a
significant effect on the perceptions reported, and it
is conjectured that the variations in sensation attributable to short term variations in the use of the fingers
were-so much larger as to make the pulse shape
variations negligible.
III-d.
On the Use of Subjects
Over a period of experimentation which extended
from June 1, 1968 to April 15, 1969, thirteen subjects
were exposed to the display.
Of these, seven had more
than seventeen hours of experience each with the system.
The remainder had less than one hour of experience on
an informal basis.
All but two of the subjects, both in the short
term group,
were sighted.
The use of sighted subiects
was necessary since a visual reference was needed to
facilitate the rerorting of responses.
There appeared
to be no significant difference between blind and sighted
subjects after the initial learning period, though the
sample is much too small for a good verification of that
as fact.
The number of hours of experience for each
subject in the long term group is
given in table III-1.
-59-
Subject
Total Hours
Group I:
1.
WM
34
2.
CG
18
1.
NN
54
2.
JD
65
3.
LB
79
4.
RK
85
5.
Jc
25 (Oct. 1, 1968 to
Dec. 1, 1968)
Group II:
Table III-1
Total Subject Experience
-60-
The formal experiments were divided into two gxmps,
with two subjects participating in the first section and
five in the second.
The first group of experiments
was primarily exploratory, to determine what perceptions
could be elicited and whether these perceptions were stable and repeatable as well as non-painful and at least
marginally useful.
The second group consisted mainly
of the experiments reported directly in the chapters
to follow, with some preliminary experiments to determine
the similarities of the sensations across the group of
subjects and to provide a common minimal tactile
exnerience, as none of the subjects in the formal experiments had ever particinated in any tactilE; experiments.
The subjects were one high school senior, two
undergraduates and two graduate students, one female and
four males, who were paid on an hourly basis.
None of
the subjects reported having experienced severe electric
shock and none renorted any undue fear of electricity.
None of the subjects showed visible scarring of the
finger tips, and none reported unusual medical conditions
such as chronic illness or continued use of drugs.
All of the experiments of the first group, and the
early experiments of the second group were of the free
association or "tell me what you feel now" type.
This
was done deliberately in an effort to simulate as closely
-6as possible the situation of normal haptic exploration.
The questions did, however, attempt to distinguish the
characteristics of the sensation from the apparent
characteristics of the surf ace.
The second group of experiments was entirely of the
directed activity type.
The subject was told specifically
what task he was expected to perform and often in what
termsbe was expected to respond, although the description
of his sensations was left entirely to his own discretion
with no attempt on the experimenter's partto limit the
set of descriptions.
All of the subjects who participated in these experiments exPerienced both of the sensations which are
described in detail in chapter IV.
In all but one of
the cases both sensations were reported without telling
the subject what he was expected to feel, thourrh in two
cases the learning was sped up by instruction in how to
feel the surface.
The one subject who recuired extensive coaching was
1NTN.
It was necessary to artificially prevent the normal
current mediated sensation by the use of a thin insulating
layer before she was able to detect and make judgements
on the texture effect.
This subject exhibited a
oreference for the classical electrotactile effect even
after training in the ac-uisition of the texture effect,
in distinct contrast to all of the others.
She was
-62subjected to the entire series of experiments in spite
of this preference, and was permitted to use her fingers
as she wished.
Therefore, most of the experiments re-
ported in the following chapters will make a distinction
between this subject and the remainder of the group.
An examination of the table will indicate that one
of the subjects in the second group had considerably less
experience than the rest.
This subject was forced to
discontinue the program early, and so does not appear
in the data for many of the experiments reported in
chapter V.
-63CHAPTER IV
The Sensations Elicited by Electricity
in a Freely Explored Environment
IV-a.
A History of Electrotactile Studies
Scientific interest in electrotactile sensations
appeared as early as 1915 (see Von Frey).
The early
work in the field is reviewed in Gilmer (1935).
It was
not until the early 1950's that investigators began to
get useful results.
Since that time most of the research
has been done with electrodes fixed to the skin surface
in an effort to determine such things as the characteristics of the sensations, the best stimulus and the
mechanism by which the stimulus produces a sensation.
The debate over the mediating processes has centered
around two groups of phenomena.
The first amounts to
direct stimulus of the tactile nervous system by the
passage of currents through or adjacent to the nerves
themselves, and the second group contains a host of ways
in which mechanical deformation of the skin could be
caused by electric currents, such as local muscle
stimulation.
-64-
In 1953 Vernon published the results of an attempt
to determine whether or not the sensations generated by
electrical stimuli in fact were mediated by motion of
the skin.
The experiment consisted of simultaneous
electrical and mechanical stimulation of the skin at the
same locus.
Both sensations were vibratory in nature
when felt alone.
He was unable to find evidence to
support the electrically driven vibration theories
and many people took that to mean that such vibration
could not be generated.
However,
Mallinckrodt et al (1953) also published
a comment which at least in retrospect sheds a different
light on the situation.
Mallinckrodt describes a phe-
nomenon which is probably familiar to every housewife
who uses electric appliances.
Suppose that the case
of a household appliance inadvertantly gets connected
to the hot side, that is ungrounded side, of the 10-volC
power line.
Then particularly if the surface of the
appliance is painted or lacquered, the surface of the
appliance will feel different when the appliance is
on than when it is off.
After several informal experi-
ments, Mallinckrodt proposed a model which involves
mechanical motion of the skin, driven by an electrostatic force.
The model is similar to that which will be
presented for the texture effect later in this chapter.
-65-fhe sensation which Mallinckrodt describes is in fact
apparently identical to the texture effect.
It is important to note that the effect which
Mallinckrodt et al, and perhaps many of the other
investigators who proposed mechanical motion theories,
describe is fundamentally different from that for
which Vernon was looking.
In Vernon's experiment the
electrode was stationary with respect to the skin
surface, but Mallinckrodt indicates that motion of the
skin over the electrode surface is a necessity.
It
was not until 1958 that Hahn spelled out the parameters
of the electrotactile stimulus which is now accepted as
optimum.
Since that time, the principle investigations
of electrotactile perception have been carried out by
R. H. Gibson (1963,1967) and have been concerned with
dry electrodes at fixed locations on the skin surface.
Since Gibson's work represents the state of the art at
the time that this thesis was begun, a capsule summary
of his results as they pertain to this thesis follows.
Professor Gibson's work has been done primarily
using unidirectional current pulses of rectangular shape.
An idealized version is shown in figure IV-l.
The
available stimulus variables in Gibson's experiments
are pulse height, pulse width, pulse repetition rate,
and the number of pulses in any given stimulus.
A
current
t±1ne
T
I
1
1~
0 AMPERES_
0
N PULSES PER STIMULUS
FIGURE IV-1
STIMULUS USED BY
R, H,GIBSO N
-67-
In an effort to simultaneously minimize the threshold
for sensations of touch, and maximize the ratio of
pain threshold to touch threshold, Gibson selects a
pulse width of 0.5 milliseconds.
Shorter pulses cause
undue increases in the touch threshold, and longer pulses
cause a reduction in the pain threshold, with the pain
threshold reaching approximately the level of the touch
threshold at 10 milliseconds.
Using this value, 0.5
milliseconds, for the pulse width, the pulse repetition
rate and the number of pulses in a burst can be varied
to obtain the relationships shown in figures IV-2 and IV-3
between these variables and the pulse height required
to generate threshold sensations of touch and pain.
This data is abstracted from Gibson's work (1963).
The curves shown are for stimulation of the hairless regions, the finger pad, the lip and the palate.
Note that neither pulse repetition rate nor the number
of pulses in a given stimulus burst affects the touch
threshold greatly and that in either case, a limit is
reached at large values of the variable.
The pain
threshold on the other hand decreases rapidly at first,
and then it too reaches an apparent limit.
Since we
expect to be working with high repetition rates and continuous pulse strings, we might expect to find touch
thresholds of 1.2 milliamps or more, if we were using
the same stimulus waveform.
-68-
FINGER
PAD
4
o PAIN
wi
(DULL)
*TOUCH
u.
0
_-0
-o
lo xr
PULSE REPETITION RATE (PPS)
.(4 and 20 PULSES)
frorn
R.H.GIBSON 1963
FIGURE ]T.-2
THRESHOLD vs, REPETITION RATE
-69-
FINGER
6r-
PAD
4
uJ
PAI N(DULL)
eTOUCH
o
0
u
LIi'
a_-
0
12.
4
7
13
NUMBER OF PULSES
frohn R.H.GIBSON,1963
FIGURE =-3
THRESHOLD VS. NUMBER OF PULSES
-70-
The rapid decrease in threshold for short times
shown on the curves is attributed by Gibson to a temporal summation effect, which it
will be noticed
affects the "pain reception" more than touch.
It
is in
part, however, this same sort of effect which is
responsible for the change in threshold with a change
in pulse width, and again it should be noted that the
process goes on longer for pain than for touch.
Gibson
suggests that the touch and pain systems have integrating
properties with time constants of 30 milliseconds and
200 milliseconds respectively.
This kind of phenomenon also apparently occurs with
respect to the spatial dimensions in that for small
electrodes, an increase in electrode size produces a
decrease in the touch threshold value, where now threshold
is to be measured in units of current density rather
than peak current value.
These results perhaps can
best be summarized from experiments performed by
C. M. Leung (see Gibson, 1967).
The apparent critical measurement of an electrode
is the area enclosed by its perimeter, that is the skin
area within the electrode boundaries, not the electrical
area it presents.
The "cleanest" variable for the
measurement of thresholds is current density, not peak
value.
Finally, the relationship between electrode
-71enclosed area, A, and current density at threshold, J,
can be given approximately b-,:
J =
(I /A)
=
K(A)- 0 '8
where K is a constant, varying slightly from subject to
subject or with electrode material.
The relationship between current density and enclosed area can be thought of as Gibson's spatial
summation effect.
The meaning of such a strangely
calculated current densityI is however quite unclear,
and the result should not be given physical significance
as it stands.
Leung has demonstrated this relationship
for electrodes ranging from 25 mm 2 to 800 mm 2 on the
palm of the hand.
Note that what has just been said implies that an
electrode which is in electrically distinct pieces, or
whose center is removed leaving an annulus, is ecuivalent to any other electrode having the same boundary,
at least in terms of the perception generated by a given
current.
Since, however, skin resistance is a function
of the electrical area of the electrode, the solid
electrode will require less voltage to produce the required current than a thin annulus.
These facts become very important in the light of
the fact that one of the main causes of perceived pain
in cases of electrical stimulation is electrical puncture
of the skin surface.
-72When that happens, the locus of
the puncture becomes reddendd, sore, and unusable for
as long as several days.
A second skin characteristic
of interest is that it is sensitive in "spots" to
particular kinds of stimuli.
The spots vary in size,
being smallest on the finger tips, but if the electrodes
are made too small, the spot size and the electrode
size start to-interfere with each other resulting in
unstable sensations, or sensations which cannot be
reliably well localized.
As one might expect, there are interactions between such parameters as electrode size, pulse width,
number of pulses, etc. and the localizability of a
stimulator.
As Gibson (1963) indicates, single pulses
applied to hairless areas like the palm of the hand
"...feel somewhat like a tap from the blunt end of a
fountain pen."
Such sensations can be nicely localized,
and appear to be smaller than the electrode when the
electrode is of moderate size.
For smaller electrodes,
an increase in electrode size may make localization
easier (Gibson, 1963).
The subjective area may be de-
creased by increasing either the pulse repetition rate
or the number of pulses in the string.
For very long
pulse trains the sensation may be one of a tone.
-73Vernon (1950) ardothers have elicited a wide variety
of sensations, ranging over touch, pain, sting, heat,
cold, "tingle", vibrations, etc., sometimes predictably,
and sometimes not.
On the finger tips there is an ad-
ditional problem not usually encountered elsewhere which
leads to stimulation of the deep nerve structures,
particularly at the joints.
It is due to increases in
the current density at such locations.
The finger is
not nearly as uniform electrically as muscular or fatty
tissue.
Because of the uncertainties encountered with small
electrodes, it was suggested by R. H. Gibson (1967;b)
that a considerably better sensation miqht result if
the very small electrodes, which are desirable from the
point of view of increased resolution, of the display
were grouped together to form one electrode with a
diameter of perhaps finger pad diameter.
This would,
it was thought, utilize the spatial summation effects
to achieve a stable and hopefully predictable sensation.
It would, however, obviate the possibility of always
having the information available at all points of the
display, and would require some form of scanned mode
presentation.
This, of course, brings up questions
such as the tactile"persistence" time, and other time
sensitive processes.
ticular
interest.
Two such phenomena are of par-
-74Studies of the kinds of interference which occur
between the stimuli of a tactile presentation have led
investigators to the conclusion that there are at least
Von
two distinct phenomena causing the interference.
Bekesy (1960) working with mechanical stimuli and
R. H. Gibson (1963) workingr with electrical stimuli
have isolated these phenomena in the tactile sense.
They are commonly referred to as phantom phenomena and
are distinguished as a phantom of position and a phantom
of motion.
They are analogous to similar events in the
visual and auditory systems.
Phantom motion occurs when two locations are
stimulated with an onset delay of 100 to 300 milliseconds between stimuli.
The sensation is as though
the stimulus were moving from one source locus to the
other.
The position phantom occurs when two locations
are stimulated nearly simultaneously, i.e., with less
than ten milliseconds delay.
The most common sensation
is that there is only one stimulus, located somewhere
between the locations of the real stimulators.
Its
location and apparent size are governed by the amplitude
and time parameters of the stimuli.
The phenomenon is
not particularly stable and at times one or bothieof the
stimulators may also be felt at ita proper location,
-75and the phantom may be entirely absent.
These phenomena have caused considerable difficulty
with experiments of the reading-rate type.
Little is
known, however, of the effects they might have on an
explorable presentation, or whether in fact they will
appear.
In this respect, the situation which was established
in the first chapter is an extremely complex one.
The
phantom phenomena, should they exist, and should they
be stable, repeatable entities, might well be useful in
such tasks as the presentation of smoothly curved lines.
On the other hand, it is entirely possible that they
would be nothing but a distracting influence on the
subject.
In spite of several attempts to elicit phantoms in
the present work, no instances were identified, and there
is no evidence that they occurred in any of the experiments, of whatever sort, which constitute the core of
this thesis.
Several investiators have reported that
the phantom sensations are not always consistent
(cf. Gibson, 1963; Bliss et al, 1965),
and that they
are more often "observed" than "located",
that is,
is
not looking
more likely to observe them when he is
for them.
one
Furthermore, there are apparently no recorded
instances of such effects in a situation in which the
.
-
-
environment can be activel', searched.
It
is this author's opinion that while such effects
may occur in the case where the subject is a passive
observer, active searching will always permit them to
be distinguished from real stimuli, and in fact that
they will never be consciously noticed in an explorable
presentation.
They can therefore neither help nor hinder
the task we have set out to accomplish.
We now explore the relationships which exist between the stimulus used by Gibson et al, and that which
has been applied in the current experiments.
The first
difference is that continuous pulse trains are to be
used unless it were absolutely necessary to apply a
scanning technique.
If that is so, we would expect the
thresholds mentioned earlier, about 0.4 milliamps for
touch and about 1.2 milliamps for pain.
Should a
scanning mode become necessary, a touch threshold of 0.7
milliamps peak and a pain threshold of 2.0 milliamps
or more would be expected.
Of course, this is predicated
on the use of the same pulse shape used by Gibson.
The effect of the pulse shape difference is cgiven
by Gibson (1963).
He indicated that the use of bi-
directional pulses has the effect of reducing the reddening and discomfort which sometimes accompany lonq
sessions, particularly if painful stimulus is applied.
It is unclear what effects the use of bidirectional
-
|
-77pulses might have on the thresholds, but it would be
axticipated that if
the width of the pulse in each
direction were close to 015 milliseconds, the thresholds
would be the same or perhaps slightly lower.
In any
case no more than a 25% shift would be anticipated.
Gibson indicated (1967;b) that segmentation of
the electrode, equivalent to connecting many of the
points in the array together,
should have no serious
effect if the overall size of the array in contact with
the finger is kept large enough.
Unfortunately, the
concept "large enough" is dependent upon, among other
things, the locus of stimulation.
In this respect the
finger pad is ideal, being one of the most heavily
inervated areas of the body.
Thus even a relatively
small electrode may be "large enough".
The experimentation was begun first
with a single
large electrode under the finger pad, the electrode
being one half inchh in diameter.
With this set-up,
Gibson's findings were substantially confirmed.
IV-b.
The Qualities of the Sensations
Two distinct types of sensations were elicited
during the course of the experiments.
They differ not
only in the qualities of the sensations, but also
apparently in the mediating cause and in the properties
-78-
which make them useful for displays of the sort envisioned.
Their qualities will be discussed in this section, and
their mechanisms in the next.
For simplicity in the following discussion, the two
sensations will be distinguished either in terms of the
sensation itself or in terms of the apparent mechanism.
To facilitate the first distinction, the terms "tingle"
and "texture" will be used.
These terms are derived
from the descriptors used consistently by the subjects.
The terms "electrotactile" and "electrovibratory" will
be used to distinguish between the two sensations on
the basis of the most probable mechanism.
Therefore, the
first sensation noticed, and the first to be discussed
will be referred to as either the tingle sensation
of the electrotactile sensation, and the terms are
equivalent as names for the sensation.
The second
sensation will be referred to as either the texture
sensation or the electrovibratorm, sensation.
The Electrotactile Sensations:
As was indicated, these sensations seem to be
related to the classical effects in both mechanism and
sensation.
WTith single pulses,
widely spaced in time,
the sensations are as Gibson (1963) described, dull taps.
This seems to hold to frequencies of perhaps 20 to 50
pulses per second; at higher fre-'iencies other sensations appear and even at freq-uencies below 50 pulses
per second the sensation is not well localized.
It
often apparently feels as though the entire finger pad
were being stimulated, no matter how small the electrode
group.
As the pulse repetition rate was raised, the
surface sensation of being tapped disappears and other,
less desitable sensations appeared.
These sensations
are the reason for the "tingle" description.
Perhaps
this could best be described in terms of the pinsand-needles sensations one gets when the blood supply
to a limb has been cut off for some time and is then
returned.
The most common sensation at high pulse rates is
of a tingle in large portions of the finger, independent
of the location of the electrode on the finger pad.
The amount of the finger involved tends to grow as
the intensity of the stimulus is increased, and at times
it has involved the entire finger without elicitinr
pain.
The sensation is reported to be interior to the
finger and may first appear in any location but usually
appears near the tin of the finger rather than over the
electrode under the pad.
This sensation may move within
the finger without apparent external cause.
The second sensation, experienced b1
all subjects
sooner or later, is of a highly concentrated spot of
It is usually associated with a fixed
tinale sensation.
spot on the skin, apparently near the surface, and tends
to remain fixed in location, but to come and go from
day to da".
When it occurs, it usually supercedes the
more generalized tingle mentioned above, and can be
guite annoying.
Again, its occurrence bears no apparent
relationship to the locus of stimulation so long as it
is on the finger pad.
The last and most disturbina of these effects is
a sensation which occurs in the joints.
Again, the
sensations come and go from day to day, though in this
case it sometimes also accompanies the first sensation
mentioned at high stimulus amplitudes.
The sensation
is again one of tingle, localized in the joints, and
sometimes accompanied by.' a feeling of joint stiffness.
The feeling of stiffness may last for some minutes
after the stimulus is turned off.
The three sensations described seemed to vary
somewhat from Person to person, particularly. as to the
location of the "hot spots", btit the general outlines
were always the same.
What is more important perhaps
is that these sensations were invariably described as
not very pleasant.
None of the subjects felt that they
could be described as painful at reasonable stimulus
levels, and none objected to being asked to perform
tasks such as pattern analysis using these sensations,
low
but the experimenter got an unmistakable impression
that they would have preferred to get paid for a more
pleasant task.
This, it should be noted, holds true
for the subject who preferred these sensations to the
texture sensations as well as for the other subjects.
The hand and finger motions used by subjects while
exploring the display and using this sensation mode
reveal some of the difficulty in its use.
Invariably
the subjectapplies a fairly large pressure to his finger,
probably as a means of lowering his finger resistance.
(Note that in spite of any precautions which might have
been taken, he has some control over the system in this
manner).
He then uses his fingers in one of two well
definrod ways, each related to a particular task.
In
searching for the general location on an "on" area, he
will wipe his finger slowly across the arravr, often
deliberately involving large portions of his finger pad.
In attempting to identify whether or not a specific pin
or group of pins in the array is "on", he tends to use
a patting motion with his finger tip or pad, always
endeavorinq to limit the area in contact to the pins he
is "interrogating".
The reasons for this kind of behaviorwill be clear
if the reader remembers the fact that none of these
three sensations is related to the location of the
-82-
electrode on the skin.
Thus the only way the subject
has of knowing whether a particular electrode pin is
"on" or "off" is the position of his ginger at the
onset or offset of the sensation.
This of course limits
the resolution to the smallest area which the subject
can touch reliably, a measure which can be aided to a
great extent by sight if the subject is allowed to examine
closely the contact of his finger with the display.
Subjects were not permitted to do this.
The relationship between this situation and the
classical sort of electrotactile experiment should now
be clear.
First of all, the sensation does not seem to
be related directly to a property of the surface, in
fact it is described as though it were a property of the
finger.
The sensation is that "...my finger tingles."
As will he seen, this is in direct opposition to the
situation with the texture sensation.
The connection
between the tingle sensation and the properties of the
display surface is only indirect and the subject thinks
of himself observing his tissues, not the displa,.
Something is happening to him, not to the display.
The above effects are essentially independent
subjectively of the pulse repetition rate at more than
perhaps 100 pulses per second.
apparent intensity'do occur.)
(Minor changes in the
On the other hand, overall
changes in size, either in width or amplitude change
the sensation level, and tii:thappears to be all, provided
it does not become too strong or too wide.
Any distortion which results in an appreciable
leakage current during the interpulse interval, due to
changes remaining in the tissues at the end of a stimulus
pulse, tends to aggravate the problems with joint involvement, and even to make the whole finger tingle at a
low intensity.
The solution. to this annoying situation
is to adjust the pulse width ratio Ta/Tb or the amplitude
ratio X so that the currents in the interpulse interval
are minimized.
Electrovibratorv Effects:
This sensation, unlike that described above, is a
sensation that the surface of the display has changed
properties.
The best apparent description of the sen-
sation is one of a textured surface.
The subjects
variously describe the surface as having become "...like
sandpaper" or "...brushy".
In addition to being described as a property of the
surface, this sensation has other properties which are
texture-like.
The most important is that the sensation
is available only in response to the subject's exploratory
activity.
at all.
The finger must be in motion to feel anything
In this respect it is like normal textures which
cannot be felt by passive contact of the finger with
the surface.
-84The experimenter's introspective impressions
would suggest that the sensation is like that of
rubbing a patch of very short stiff fur or velvet the
wrong way, or to the sensation of rubbing a very
nearly dry painted surface.
The finger or a portion of
it seems to alternately stick and move, as in the
relaxation oscillation of chalk Pushed backwards across
a blackboard.
Subject observations are consistent with
this view.
The hand motions of subjects reporting this sensation are dominated by a light brushing motion With
the finger tip.
The effect is independent of the direc-
tion of finger motion.
The motion tends to be repetitive,
with motions of the whole hand serving to carry the
finger tip over the display.
These motions are not
dependent on the goal of the subject, and he tends to
do the same thing whether searching for a pattern or
analyzing it.
The subjects report with a single electrode turned
on, the sensation seems to be localized on the surface
of the display.
The sensation occupies an area on the
skin at most very slightly larger than the size of the
electrode.
This sensation has none of the unpleasant aspects
of the tingle sensations.
Fatigue from the repetitive
finger motions is the only reported aftereffect.
Subjects
-85-
have performed pattern analysis tasks continuously for
periods of up to 45 minutes, and sessions have lasted
as long as three hours.
The requirements for generating this sensation
are relatively simple.
As was indicated in section IV-a,
a 60 Hz sine wave will do, although a narrow pulse
generated a more distinct or "sharper" texture.
The
pulse width limit for narrow pulses is set by the possibility that the tingle sensation may be felt instead.
Narrow pulses recquire higher stimulator voltages,
thus increasing the possibility of this sort of trouble,
or of the more dangerous skin puncture.
As the pulse
width increases, the apparent sensation level rises
until a saturation point is reached.
The increase in
apparent intensity was essentially complete for Ta
Tb
0.5 milliseconds, and so for the most part the
value of half-pulse width used was 0.6 milliseconds.
Any further increase tended to make the sensation more
diffuse.
Both frequency of stimulation and Pulse amplitude,
which for this stimulus is best specified in terms of
neak voltage, affect the cuality of the sensation.
If
two subjective variables are defined, let us call them
"intensity" and "coarseness",
the physical variables,
it
is foind that both of
call them "amPlitude"
and
-86"frequency", affect both of the subjective variables.
Frequency has by far the greater effect on coarseness
and amplitude has by far the greater effect on intensity.
The available frequency range was limited at the
high end to 1000 Hz by the interference of adjacent
pulses, and at the low to 100 Hz by the fact that infrequent pulses result in difficulties in making localizations, and apparently in some instabilities in the
quality of the sensation.
Limits of amplitude variation are established by
threshold values and by the occurrence of unpleasant
sensations.
The latter were usually reported either as
the onset of a tingle sensation or large scale vibration
of the finger.
In either case, the ratio of annoyance
threshold voltage to touch threshold voltage was regularly greater than five.
If the model given in section
IV-c is correct this is equivalent to a 28 db range
for tangential vibratory mechanical stimulus.
The task of acquiring and maintaining the sensation
is not an easy one, in spite of the fact that all but
one of the subjects achieved it withou4 coaching.
The
normal procedure which evolved required that the subject
be given about five minutes to "warm up his finger" on
a suprathreshold signal.
This was required at the
beginning of every session, and shorter periods were
-87often required in the middle of a session if the subject
was away from the display.
This situation did not im-
prove significantly even after many hours of use.
The difficulty that seems to plague the use of this
sensation is the necessity of drying out the skin surface
And drying off the display surface.
If the display
surface is dirty or damp, or if the subject's finger is
damp, the sensation is reported to disappear, and another
"warm up" period is required to recover it.
On several occasions subjects found the sensation
either difficult or impossible to accuire or found it
unstable after acquisition.
This did not occur often,
and when it did, it was always accompanied by a great
reduction in the subject's skin impedance, as measured
in the experimental situation.
Skin resistance measurements for a representative
two week period appear in table IV-l.
There are two
cases during that period when the subjects reported
difficulty.
These are January 17 for LB and January 24
for RK.
No cases of total failure occurred during the
period.
LB and RK experienced one case of total
failure each during the six month period of the experiments.
JC experienced two such periods during two months.
JD was not bothered by the failure phenomenon, nor was
NN who did not use the texture sensation.
-88-
I'm
Suhiec t:
Date
JD
Resistance
(ohms)
Date
Resistance
(ohms)
Jan.
9
i.IM
150k
Jan. 11
2.3M
Jan. 16
190k
Jan. 15
71.5M
n. 23
Ja7
160k
Jan. 18
4.6M
JaF
n.
190k
Jan. 23
2.3M
n.
JaE
7
20k
J2n. 14
28
Subject:
LB
Date
RK
Resistance
Resistance
Date
(o'hms)
(ohms)
Ja n.
8
2.3M
Jan.
Jan.
10
1.1M
Jan. 10
570k
Jan.
13
2.3M
Jan.
13
570k
Jan.
15
2.3M
Jan. 17
570k
Ja n.
'17
570k
Jan.
350k
Ja n.
20
1. IM
8
24
1.*1M
Note: Res istance values accurate to plus or minus
ten percen ,t.
I
le IV-1
Skin Resistance Values
-89The table shows resistance reductions on the two
dates mentioned.
It also indicates that NN has a much
lower normal skin resistance than the others when
searching for sensations.
It is not known whether this
is the result of some action she takes or of a ohVsiological difference between her-and the other subjects,
both male and female who participated.
Attempts to correlate this effect with everything
the experimenter could think to ask about,
to and
including the food he had eaten and what he had had
to drink resulted in only two common events.
Instability
would occur most often on very humid days, with reported
humidities of over 95 percent.
all
cases of total
Most instabilities and
failure occurred while the suibject
was reporting a mild head cold.
The one long term
failure occurred over a period of three weeks during
which JC had a chronic and severe cold.
.ll of the
subjects experienced this nroblem at least once, but
the lost time was 5% or less for all but JC.
We will
speak of this again in the concluding chapter.
An Unusual Sensation:
In the process of oerforming one of the experiments,
a strong capacitive coupling occurred between the outputs
of two stimulus sources which were at the time operating
at different fre-uencies.
This resulted in interference
-90between the two pulse trains causing pulses to occur at
a low repetition rate which were twice as large as usual,
overlaying the normal texture presentation.
The result-
ing sensation is described as very startling and totally
unlike anything any of the subjects had ever felt before.
This may have been due to the fact that the resulting
"features" could not be attributed to either the display
surface or the skin sensation.
Some informal experiments indicated that this
scheme allowed the presentation of very low freq-uencies
in a controlled manner, and that it might very well be
possible to use this effect to imProve vrpon the range
of textures available,
or to improve the subject's
ability to distinguish among freruencies.
Unfortunately,
the system could not be set unP to do this reliably in
a controlled manner,
and so it
is left for another
investigation.
TV-c.
The Mechanisms
Let us first
look at the tingle sensations.
It
will
be remembered that these sensations were always felt
to be interior to the finger,
and unrelated to the actual
location of the electrode on the finger pad.
It will
also be remembered that the feeling spread as the amplitude was increased, and that it
would sometimes involve
-91the joints before the growing tingle volume had reached
them.
Since the sensation is unpleasant, it would be
worthwhile to examine the causes which lie behind them.
The currents measured at the threshold of tingle are
normally in the 0.6 to 0.7 milliampere range at peak
value, consistent with Gibs.n!3 findings.
It is believed
that the tingle sensations have the same cause in current
through the tissues as the tactile sensations reported
by Gibson.
It is also asserted that the reason for the
less pleasant sensations observed in the current experiments is simply the unfavorable structure of the finger.
The consensus in the literature seems to be that
the electrotactile sensations are generated by currents
passing through or in the vicinity c~tbenerves associated
with the sensation elicited.
If
this is
so then one
would expect that the current density in a region and
the population of susceptible nerves would be the principle determinants of the sensation.
This analysis would suggest that the way to achieve
localized sensations on the finger tips would be to
force the current to flow through a localized area at
the skin surface. IEfforts by other investigators to
achieve this have evidently been in vain.
It may simply
be that the nerve endings common to glabrous skin are
-92insensitive to electric currents in their vicinity,
as many of them are encapsulated in -accessory structures.
The very irregular structure of the finger points
to the reasons for some of the interior sensations of
tingle.
The joint sensations can be related to the
fact that the large amount of area in the joint occupied
by bone results in higher current densities in that
region than might otherwise occur.
The "hot spots"
may be related either to high nerve concentrations or
to locally low resistivity of the tissues.
In any case
it would seem that the structure of the fingers and the
relative nerve sensitivities have combined to make
electrotactile stimulation of the fingers in the manner
in which it has been practiced by Gibson unpleasant, and
that no way has yet been found to avoid the unpleasant
aspects of such stimulation.
There is similarly no good explanation for the
nature of the sensations except perhaps that these
nerves are in the locations of higher current density
or are more susceptable to stimulation than the nerves
related to normal tactile sensations.
However, given
that these current-related sensations are unpleasant and
that it does not seem possible to improve them easily,
it was decided to encourage the subjects to use thetexture effect in performing all of the resolution
experiments.
The one subject who continued to. prefer
-93the tingle sensation was permitted to continue using it
for comparison purposes.
Since the display surface was not insulated, it
was found desirable to acquaint the subjects well with
the tingle sensations, even though they had managed
to bypass them in the early sessions,
as these sensations
were likely to be encountered inadvertantly.
The Electrovibratory Mechanism:
The texture phenomenon requires very little current$
it involves a different mode of finger use and allows a
different class of distinctions to be made than the tingle
phenomenon, as will be shown in chapter V.
In order to
explain the observed characteristics of the sensation,
a model for its production is proposed in which the
skin surface acts as an electromechanical transducer.
We wish to show that the model predicts the observed
phenomena, including the observed failures.
Presume first of all that the skin resistance is
primarily concentrated in a very thin layer at the surface, and that layer can be dried out slightly, raising
the resistance further.
Now if a charge can be built
up across this layer, by the stimulator, an attractive
force will be felt by the charges inside for the charges
outside, that is, on the electrode.
This force acts
to squeeze the skin surface against the electrode.
The
-94laws of physics, in particular, friction, act to transform this local. increase in the force perpendicular to
the electrode surface into a local increase in the tangential force under the influence of the finger motion.
This force in turn causes local stretching of the skin.
It can be demonstrated that there is in fact mechanical motion of the skin involved.
an audible sound is produced.
It was discovered that
If the sensation is
present, the sound is present, and the sound is absent
when the sensation is absent.
is small.
The amplitude of the sound
It was first identified by the subjects who
indicated that it came from the finger itself, a fact
later verified by a microphone survey.
There is no
light emitted by the process, thus eliminating the
possibility that sparking is responsible.
of the sound, is shown in figure IV-4.
The spectrum
It will be
noted that the spectrum consists entirely of harmonics of
the excitation frequency of 200 pulses per second.
One other conceivable cause of the sound is a
vibration due to the irregularities in the coefficient
friction of the surface.
The subjects report no apparent
change in either the sensation or the sound with small
variations in the speed of finger motion about the normal
rate.
The latter fact was confirmed by spectral analysis.
3>
C11
-<
z
c~)
P1
.0
m
~
I
N
0
-f--
C-)
m
c-rl
~
~
AMPLITUDE
~
I
~
~
'U
~A.
~4~I
~*:4~*~*
~
~~"'
~
m
0
C
-ri
N
0
N)
-I
m
0
Ah
AMPLITUDE-
~III~~
c...
r
'~
01
-96-
One reason to believe that capacitive effects are
responsible is the fact that an insulator can be placed
over the surface of the display without destroying the
sensation.
This procedure requires a higher amplitude
stimulus and has the effect of makltngc, the stimulus more
diffuse, as might be expected; the sensation, however,
is still there in all its aspects.
It will now be shown that the force generated by the
mechanisms and model which have been proposed aie sufficient, at least to order of magnitude, to produce the
sensations described if we relate this mechanism to
tangential vibration induced by an electromechanical
vibrator in the more classical kind of experiment in
which the subject is a passive observer.
In the case
at hand, the subject is not a passive observer, but that
situation seems to be the best available analogy.
This analysis is fraught with difficulties raised
by the lack of certain data relating both to the model
itself and to the structure and properties of the skin.
When these come up, the best educated guess has been
made, and it will be indicated as such.
The following
symbols will be used in the discussion:
fv()
The vertical component of force, measured
in pounds, between the skin and the electrode.
AG (t)
The variation of function G(t) with time about
its average value. (Units of G(t))
-97-
F
The force applied by the subject in the
V
vertical direction.
(It is assumed to
be essentially constant.)
(pounds)
The coefficient of dynamic friction for the
skin-display surface pair. (dimensionless)
f
t
=4f
V
The total tanfrential force in steadv, state
finger motion in pounds.
The mechanical stiffness, in pounds per inch,
of the skin in the direction tangential to
the skin surface.*
The position of the local skin surface with
respect to a coordinate frame fixed to the
finger. (inches)
T.
The thickness of insulator laTIer i. (inches)
i
The relative dielectric constant of insulator layer i.
A
T.
i -T./s.
i i
The effective thickness of insulator layer i.
(in inches).
*It has been shown (see Franke (1951), and Alles (1968))
that the mechanical impedence of the skin, for small
-telative&dibplaceihents, is essentially a spring in
nature, with a constant stiffness in the range of
displacements encountered in these experiments.
-98A
The area of skin contact with the electrode.
(square inches)
C
The capacitance of the skin surface in contact
with the electrode. (farads)
v(t)
The instantaneous voltage applied to the
subject. (volts)
Vi
The peak voltage applied to the finger in case i.
Subscript s applies to the subject's skin.
Subscript p applies to the plastic insulator.
9
8.854 x 102 farads
The permittivity of free space.
The pertinent forces are shown in figure IV-5.
Mathematically, this system can be described as follows:
fv = Fv
ft
v
=Lfv
A f t=V
x(t)
= ft/Zm
We wish to show that x(t) is of the same magnitude
as the motion required for mechanical stimulation of the
skin in a tangential direction, at frequencies near the
pulse frequency used to elicit the texture sensation.
The values measured for mechanical thresholds in the
tangential direction are similAr to those for vertical
motion, and about 10 microns (about 4 x 10-4 inches)
peak to peak displacement. (Alles, 1968; Verillo, 1962,
1963).
MOTION
FINGER
FINGER MOTION
TABLE SURFACE
SKIN SURFACE
ELEC TRIODES
INSULATING
FIGURE 1V-5
FORCES ON THE SKIN
t
PLAST IC
ID
-100The value of mechanical impedence in the tangential direction has been measured as about 0.48 lb./in.
(Alles,
1968; Franke,
1961).*
It
is
not practical to
directly measure the coefficient or friction, as it
depends on a great many factors including applied finger
pressure, atmospheric humidity, and individual skin
factord, especially perspiration.
However, subjects
indicate a coefficient 1 , for the finger in contact
with the dry uninsulated electrode, of the order of one,
with a maximum variation in that judgment of less than
a factor of three in either direction.
This value was
the result of simply asking the subjects to estimate the
ratio of horizontal to vertical force in their finger
motions.
We would therefore expect to require a
vertical force variation on the order of 2 x 10-4
lb.,
peak-to-peak.
*This value was not measured on the finger pads.
It
was measured for small excursions (6 mils) and we can
expect that the skin of the fin7er pad is not significant1P stiffer over small excursions than the skin of the
unper arm where the measurement was made. Franke's
measurements unfortunately do not apply directly because
of the large areas of contact and large excursions involved,
though they, indirectly support the figure given.
-101The electrical model used in the form of a pair
of closely spaced parallel plates of the same size,
is shown in figure IV-6.
The electrically induced
force between the two "plates" and the capacitance of
the system are given by:
A
fv
(v(t)) 2
J
A
p
2
C
0 A
T
+ T
In order to extract the parameters of this model,
in particular the skin thickness, an experiment was
performed.
It consisted of measuring the threshold of
sensation for each of three conditions.
The active
electrode consisted of the entire right half of the
display array, much larger than the subject's finger
tip.
This was done three times in scrambled order, and
the results averaged to yield the values given in
table
.V-2.
CONDITION
AVERAGE PEAK VOLTAGE AT
THRESHOLD
No Insulator
17 volts
0.5 mil Insulator
73 volts
1.0 mil Insulator
110 volts
TABLE IV-2
-102-
LOW RESISTANCE
INTERIOR STRUCTURES
SKIN
s\\\\\\\\\\\\\\\\\
f
!I
DRY SURFACE
LAYE R
INSULATOR
I NSULATO R
ELECTRODE INSULATOR
(A)
PLATE FORMED BY SKIN INTERIOR
T
Tp l
'I
I 1
ELECT ROD E
(B)
FIGURE I-6
THE ELECTRICAL
MODEL
SKIN SURFACE
INSULATOR
-103These values were measured on only one subject,
and only one day.
While variations in the specific
voltage values do occur, the same calculated values
were obtained with another subject.*
It should be noted,
that these figures also reflect a large difference in
the coefficient of friction between the insulated and
uninsulated cases, so that the insulated and uninsulated
cases cannot be directly compared.
The insulator used was polyvinylidene chloride
(Dow "Saran")" its pertinent properties are shown in
table IV-3.
PROPERTIES OF THE INSULATOR
Dielectric Constant
3.5 to 5.5
(approx. 4.5 at 20OH2)
Dielectric Strength
350 volts per mil
Resistivity
10
14
cm.
TABLE IV-3
In the experimental situation there is a return
electrode under the palm of the same hand in addition
to the contact at the finger tip.
The results of other
*Skin thicknesses computed using the model change
considerably, but calculated forces do not.
-104-
experiments, not reported here, indicate that this
electrode is usually quite wet from perspiration, and
can thus be neglected when compared to the dry finger
tip contact.
Skin resistances measured in this experi-
mental situation on the order of 2 x 10 6 ohms with no
interposed insulator and dry finger electrodes, and 5
to 10 thousand ohms with both electrodes wet.
No
measureable current flows with the insulator between the
finger and the electrode.
It has been assumed that the tangential force
variation on the skin is the same at all three threshold
measurements shown in table 1V7-2. The voltage nulse train
applied to the user's finger is shown in fig-ure IV-7(A,).
The assumption is made that the time constant (RC nroduct)
of the skin is much smaller than the voltage pulse width,
in order to guarantee the rectangular shape of the force
oulse.
The variation in tanrential force is thus just the
peak value of the electrically produced force
A (V.)
£
or:
2
ti
2 0
S
+ $
p
)2
Giving
a
)
0o
s
2
T
2
A(V
A
(A(V 2
2.. -4 2-a32(T
92
s
3
p
2
+ T
2(T
p2
+T
)
p3
-105-
-5h,,sec.
'
V
Max
0.6 hmsec.
0
0.61msec.
-V
Imac
(a )
-:
.rnsec.
Max
-- 1.2 msec.
0
(b)
FIGURE ]Y-7
VOLTAGE AND FORCE PULSES
-106Manipulation of the force functions for the two
insulated cases, noting that T
A
=
A
2 T
3
2
3
and
p2
can be made to give:
(2V
A
T
S.
=T
P
V
2
3
(V 3
V2
The insulating material used has an effective thickness,
for each of 0.50 x 10-3 inches of. physical thickness,
of:
T= 0.111 x 10
-3
inches = T
p
2
This, qie- an effective-thicknesseof -theddry surface
skin layer to be:
5
0.108 x 10-3 inches
The variation in vertical force generated by the model,
using this value of skin thickness and the threshold
voltage measured for the no-insulator case of the
experiment is
f = 93.3 x 10
-6
lb./electrode area
Because of the excessively high voltages needed to elicit
sensation for small groups of insulated electrodes, an
area larger than the finger area (greater than one-half
inch on a side) was excited.
Since we are unable to
measure the area A of finger in contact with the electrode
in these circumstances, the area of one electrode point
has been used.
-107Note that a larger area is probably
involved, perhaps as many as five such points.
This value of force does not of course take into
account the surface coefficient of friction.
However,
unless the subjects have consistently and repeatedly
overestimated the coefficient of friction, the resultant
tangential force would be greater than the expected force
needed to cause sensation by the proposed mechanism.
Earlier it was assumed that the RC time constant
for the skin was small compared to the pulse width.
To
show that this is true, we calculate the value of this
surface capacitor:
A
TS
C = 8.56 picofarads per point electrode
Thus, the maximum value of capacitance is about 45
pf., and the corresponding RC product, using the skin
resistance for the case of two wet electrodes, becomes
45 x 10-8 seconds.
This value is clearly much smaller
than the pulse width of 10-3 seconds.
Thus we can expect
the full effect of the rectangular force pulse.
Many of the assumptions required in the development
of the model have been *ossed over above.
We will now
examine the reasons for ignoring the errors due to uncertainty in the mechanical impedance value and the
-108-
variations in the subject's activities.
The relationship between the pulse-like force applied
here, and the sinusoidal stimulus with which it is being
compared is not a simple one.
Mechanical impedance
measurements indicate that the skin acts primarily like
a simple spring with little or no damping and negligible
mass for small displacements.
In the given situation,
there may be a considerable amount of static stress on
the skin in addition to the small variation which causes
the stimulation.
Experimental observations indicate that
the users tend to minimize this static force by carefully
adjusting the finger pressure applied.
This would tend
to reduce any effect of a nonlinear mechanical impedance.
The skin mechanical impedance is reported to be linear
to displacements of greater than 6.0 x 10-3 inches,
(Franke, 1951 ; Alles, 1968).
Errors in the mechanical
impedance are not therefore expected to have much effect
on the figures given above.
Since the skin acts primarily like a spring, we
can also assume that the maximum displacement indicated
bv the force calculation is reached on each voltage
pulse if the subjects' activities are adeciuate.
If the
subject does not move his finger fast enough, or if he does
not make contact with the surface he will not feel
effect.
the
The subject is motivated to do as well as he
-109can, and observation indicates that the task is easily
accomplished.
The subject control over the skin displacement rates
seems to be the primary difference between the stimulus
in use here and the fixed electromechanical vibrator
in common use.
The maximum rate of skin displacement
is given by the finger velocity.
That is, the best that
the display can possibly do is to stop the local skin
motion in the display reference frame generated by the
finger motion.
We can, therefore, calculate that the
finger velocity must be greater than one-third of an
inch per second with the given pulse width and the exPected threshold.
observations indicate that the sub-
jects voluntarily take up scanning speeds on the order
on two inches per second.
Further support is given to
this observation by the observation that the pulse
width seems to be a non-critical parameter, and increasing it has little or no effect on the threshold.
Skin Resistance Effects:
With this model and its
characteristics in mind,
we can look again at the observed phenomena, in particular the warm up procedure and the qualities of the
sensation.
Let us examine first the warm up procedure
that the subjects use, and the kinds of failures which
occur.
It was mentioned in section IV-b that the two
-130common characteristics at the times of failure were low
skin resistance and sometimes high humidity.
Let us
first examine the effect of skin resistance on the model
given above.
An electric circuit model is shown in figure IV-8.
It consists of the effective capacitance of the skin
surfacelin parallel with the leakage resistance of that
same layer, and this pair in series with the resistance
of the remainder of the circuit.
For the purposes of this
analysis we will treat the source as a voltage source
with an internal impedance of 200 kilohms.
The variables of interest
are:
Vs
The source open circuit peak voltage
Rs
The source internal impedance
Rt
The impedance of the electrode-tissue system,
excluding the skin surface, assumed purely
resistive.
R
si
The surface layer resistance per point electrode area.
C
The surface layer capacitance per point
electrode area
H
=
slRt
V
(Rt + R s)
C
R
Rt
(s)
= H(s)V 0 (s)
-II-
INTERIOR RESISTANCE
SURFACE
CAPACITANCF
SR
RS
FI
AS
SIs
UE
CT \/
EA
N
C
E
SI
ACTIVE ELECTRODE
FIGURE IV-8
ELECTRICAL MODEL
FOR THE FINGER
NEUTRAL ELECRODE
-112and the maximum peak voltage across the skin surface is:
V sl-
V0 Rsl
Rt
si
The maximum load current is given by:
Rt +Ri
It will be remembered that the condition for an
acceptable sensation is that the threshold for texture
be below the threshold for tingle.
If we require a two
to one ratio between the two thresholds to allow for
difficulty the subject might have in achieving the ideal
finger motion, and use the voltage cited in the model
development for the uninsulated case, we arrive at the
fact that R 8 1 must be reliably greater than 80 kilohms.
Typically when failure occurs the measured value of
R l + Rt, at the driest state, is 40 kilohms, and the
value of Rt, measured by wetting the finger tip is 5
kilohms.
The observed failure phenomenon is therefore
predicted.
There is also another phenomenon which probably
contributed to the occurrence of unreliable sensations.
It will be remembered that this tended to occur on
humid days.
It is conjectured that this made it diffi-
cult to keep the finger at a constant state of dryness,
a problem complicated by the fact that the subject was
-113being asked to perform other tasks, not simply to keep
his finger dry, which he was to regard as a secondary
job.
In section IV-b it was mentioned that the senaation
was texture like, and a description of the sensation
in terms of a relaxation oscillation was given.
The
model proposed is consistent with these descriptions.
In particular, the alternate increase and decrease
in the tangential force as a finger slides across a
surface would be expected to feel similar to the alternate
gripping and sliding of a relaxation oscillation.
Similarly, the fact that the force is small and f&lls
off as the scuare of the distance from the surface would
suggest that developing enough downward force to cause
sensation, and at the same time permitting sufficinet
space to exist between the finger and the display surface
that an appropriate skin motion could occur would require
a very difficult and a very accurate control of the
finger.
It is not surprising, therefore, that this
sensation, like the sensation of texture in the real
world, is not felt unless the finger is in motion.
The currents and voltages which accompany the texture
sensation are illustrated by the typical waveforms of
figure IV-9.
These waveforms were recorded on several
successive pulses by a storage oscilloscope, and a
picture was taken of the result; the voltage measured
-1 14-
-4
__
t4>
o0
0
TIME
VOLTA GE
-i
LI I ~
F
-0
I
25A
T
TIME
CURRENT
FIGURE MW-9
TYPICAL WAVEFORMS
w
0
I0~
- 1 5-
is the voltage across the electrodes, and the current
is the current in the common return electrode.
The
stimulus represented is considerably suprathreshold.
The mechanism which has been explored in part
above has not been thoroughly proven to be the cause of
the texture sensation.
However, it accounts well for
all of the observed phenomena associated with that
sensation.
Its primary value is probably not in what
it tells us about the phenomena themselves, but in what
it tells us about how to improve on the display as it
now stands.
For instance, clearl, one of the first
things which should be done is to find a way to make the
drying-out or warm up process as simple as pose~ble, and
at the same time reliable.
A second area where immediate improvement may be
possible is brought out by the possibility of coating
the surface of the display to permit higher stimulus
levels without eliciting tingle sensations.
This also
may permit the use of smaller electrodes, permitting
higher resolution to be obtained.
The model would indicate that the coating should be
as thin as possible, have a high dielectric strength and
a high dielectric constant; it should be extremely smooth,
but have a high coefficient of friction.
Smoothness
is demanded by the subjects who have often complained
-116that the slightest bit of dust on the display will destroy the sensation.
We will return again to possible
display improvements in the last chapter.
-117CH{APTER V
Some Factors -Affecting th
Use of an Electrical
Stimulus in an Explorable Display
In this chapter are presented the results of
several experiments, primarily designed to determine
the usefulness of the sensations described in chapter IV
as the stimulus in a large explorable fto':eimensional
tactile display.
The discussion is divided into sec-
tions relating to displays in which only points and lines
are presented, and displays in which the fundamental
element is assumed to be the textured area.
These two nearly independerit lines of investigation
are first, related to the two distinct kinds of visual
images which are most often encountered in ink print
books, and second, related to displays of distinctly
different structure.
The line drawing type display is
most applicable to the presentation of information,
often originally in line drawing form, such as graphs,
electrical circuit diagrams and chemical structure
drawings, whereas the textured area display is more
applicable to shaded or colored picture presentation.
As might be anticipated, a display for line drawings
-118only can have a considerably simpler structure than
a display for textured areas, and thus is of interest.
In all of the discussions which follow, "line
drawing presentation" is meant to imply that all of the
excited matrix elements are connected to the same source.
The term "textured area oresentation" is meant to
indicate a situation in which a group of active electrodes filling an area larger than the finger pad was
excited from a single source, and an adjacent similar
group was excited from another source.
While either
area might at any time be "off" or grounded, there was
never any attempt to emphasize the boundary between two
such areas by outlining an area with "off", reduced or
higher amplitude stimuli.
All of the presentations are of relatively long
duration, and unchanging during the presentation so that
exploration was useful, and normally occurred.
The
scanned array type of display was not attempted as it
seemed unnecessary to do so.
V-a.
Experiments Relating to the Parameters of the
Sensation in General
Early in the program several experiments were per-
formed to determine the requirements for stimulation
-119and the latitude along the primary stimulus variables
available for use in a display system.
The subjects
in these experiments were the five subjects in group
two mentioned in chapter III.
The stumulus applied in
all cases was of the bipolar rectangular type with
equal amplitude segments, that is, Va
Vb (see
figure 111-6).
The nrimary variables of interest were amplitude
and frequency, and the secondary variables were the pulse
width Parameters.
This chouce was dictated by the sub-
jects who indicated that there was in general insufficient variation in the sensation to distinguish between
two presentations varying only in pulse shape,
but that
both amplitude and frequency differences were easily
dectable.
Experiment a-I:
A Scale for Amplitude
In this experiment the subjects were asked to associate a numerical magnitude with the effect of varying
values of pulse amplitude.
The applied pulse was
symmetrical with a half width, T,
of 0.25 milliseconds.
The pulse repetition rate was 125 pulses per second, and
a single active electrode of 0.5 inch diameter was used
under the finger tip.
-120The subjects were first asked to specify the
maximum strength which they would rate as "acceptable",
and the next convenient amplitude less than that value
was selected as the maximum stimulus to be applied.
These values are givenin table V-a-1.
The subject was then presented with the maximum
stimulus to be used in the scalin-rexperiment and told
to note everything he could about it.
The stimulus was
turned off, and the subject again asked to examine the
surface of the electrode.
The subject was then told
that the first sensation was to be assigned a value of
100, and the second (unexcited) sensation, a value of
zero.
This procedure was repeated four times, and the
subject was asked to identify the primary difference
between the two presentations.
The subject was then presented with 250 samples,
selected at random from the range shown in table V-a-1.
The presentations were made in a random sequence on the
same electrode.
The length of the presentation was not
predetermined, but was simply made long enough that
the subject could make a decision, and it was terminated
as soon as an answer was given.
between stimuli.
One second was allowed
The training procedure described in the
previous paragraph was repeated between presentations
175 and 176.
-121-
Sub iect
Approx.
Threshold
Approx.
Maximum Stimulus
Range Used
In Experiment
NN
51.lv
109. Ov
JD
10.5v
67.lv
Ov to 44.9v
LB
28.4v
51.8v
Ov to 44.9v
RK
16.8v
79v
Ov to 67.2v
Jc
35.4v
57. Ov
Ov to 44.9v
Ov to 102v
Table V-a-l
Upper Limit Determination for Amplitude Sealing Experiment
-122The data were treated by averaging the responses corresponding to identical presentations and smoothing the
resultant seguences by averaging each vAlue with its
two nearest neighbors.
The results are shown in graphical
form as figures V-a-1 through V-a-5.
Recorded on the
figures are the sensations reported by the subjects
during the experiment.
DATA ANALYSIS:
Two distinctly different kinds of scales
appear in the data.
Subjects NN, JC, and LB all exhibit
a nonlinearscale of approximatelI the same form.
remaining two subjects exhibit a linear scale.
The
There
seems to be no reason for this difference in the recorded
data or the experimental situation.
As noted in the
figures, all subjects except NN reported that they were
experiencing the texture sensation.
NN reported a tingle
sensation.
Functions which closely approximate the two classes
of scales are:
For the linear case,
R
a(t-t )
2
For the nonlinear case,
R = b(t-t
0
)2
where R is the response value, a and be are constants,
and t
is the threshold amplitude for touch sensations.
The source amplitude, t, is given in peakvolts and the
units of a and bcare assumed to make R dimensionless.
The graphs for NN, JC and LB are reploted in the form
sr-uare root of the response versus source peak voltage
lOG I
I
I
I
I
I
9080
0
.
0
0
700
60
0
0
0
50-
uJ
S
40k
S
-J
9
30F-
0
0
xJ
0
0
2 0 I-
Z:20 0
C.,.4
01
0
I
i
I
i
10
20
40
30
AMPL ITUDE (PEAK VOLTAGE)
FIGURE V-a-I
AMPLITUDE SCALE
SUBJECT RK
I
50
60
70
I
100
I
I
I
i
I
90
0
80-
0
0
70 60
0
u
50(D
40-
V)
4
0
u
30200
101
0
0
0
0
II
10
II
Ii
iI
20
30
40
AMPLITUDE (PEAK VOLTA GE)
FIGURE V-a-2
AMPLITUDE SCALE
SUBJECT JD
i
50
60
70
_______-
100
I
.
-
I
-
-1
-,
I
90
0
80.4
70
0
600
w
-j
50-
0
40
Lii
0
(I)
0
3020
0
100L
0
2
0
i
IV0
ado
4v
3u
40
AMPLITUDE(PEAK VOLTAGE)
FIGURE V-83-3
AMPLITUDE SCALE
SUBJECT JC
50
60
70
-
0c -
1
I
i
i
%.------------
I
90
800
700
60Lu
50-
z 40r
0
0)
300
20-
6
0
0
I0I-
0
0'
0
0
0
0
0
0
i
i
10
40
30
20
AMPLITUDE (PEAK VOLTAGE)
FIGURE V-a-4
AMPLITUDE SCALE
SUBJECT LB
I
I
50
I
60
70
-J
100
I
I
I
I
I
90
0
800
70
0
0
60
50-
z
(D
40
30
0
20-
0
0
10
Ol
0
&
0
0
0
I
i
10
20
* i
0
0i
I
Ii
* 1 0
0
*1
30
40
50
60
AMPLITUDE(PEAK VOLTAGE)
FIGURE V-cd-5
AMPLITUDE SCALING
SUBJECT NN
Ii
70
|I
80
|i
90
140
-128as figures V-a-6 through V-a-8.
It should be noted that this experiment was performed after approximately three hours of experience for
each subject taking part.
It should also be noted that
while the square law response function provides a better
fit to the nonlinear data, the data is such that an
exponential form also provides a reasonable fit.
Experiment a-II.
A Scale for Frequency
Immediately following the above amplitude scaling
experiment, a similar frequency scaling experiment was
performed.
In this case the amplitude was fixed at a
comfortable level just above threshold, and chosed by
each subject.
It was not possible to standardize the
amplitudes due to the wide variation in subject preferences at this Point.
The pulse shape and electrode
used were the same as in the previous experiment.
The
pulse repetition rate was varied from 100 to 1000 pulses
per second, and the presentations were made in the same
manner as above.
The high frequency limit was assigned
a scale value of 100, and the low frequency limit was
assigned a value of a zero in the training sessions
which were performed in the same manner as above.
Ninety presentations were made to each subject,
with a single training presentation of both the upper and
100
90-
-
80
70
60~.50
~30
20-100I
0
I
10O
20
I
I
40
30
AMPLrTUDE(PEAK VOLTAGE)
FIGURE V-a-6
SQUARE ROOT SCALE
50
60
70
SUBJECT JC
2
100
I
I
I
i
90
0
80
0
70
0
60-V-Lii
0
50S
z 40-
0
0
0
0
30-
0
20IOF
01
0
I
10
ff
40
30
20
AMPLITUDE (PEAK VOLTAGE)
FIGURE V-a-7
SQUARE ROOT SCALE
SUBJECT LB
D.
50
a
60
70
tS
00
807060<50
SO-
-
z 40 --
S30200
10
0o100o
3
6
0
AMPIUtPA VOLTAGf
FIGURE V-a-8
SQUARE ROOT SCALE
SUBJECT
NN
o
9
-132lower limits following both presentations 30 and 60.
All subjects reported difficulty in making the decisions.
The raw data from these presentations were again averaged
at each presented value of frequency; no smoothing of
the results was done.
Graphs of the results appear in
figures V-a-9 through V-a-13.
DATA ANALYSIS:
appear.
Again two distinct different results
In this case, however, the data do not seem
precise enough to justify more than the observation
that the subject using a tingle sensation seems unable
to detect differences in frequency, while the others
can at least detect the differences, though not at all
accurately in most cases.
suggest a linear scale.
The results of JC and JD
The responses of NN show no
clear trend, and are believed to represent random guessing.
The remaining two subjects have given responses
showina the same trend as those of JC and JD but with
considerably' less judgement consistency.
The indications given of the subjective variable
used by the subjects are not clearly related directly
to frequency, but are consistent with the results
including the existence of a linear scale for texture
usage.
Subjects using texture sensations report variables
such as "brushi-ness", "coarseness" and "number of bumps".
I00
00
-
I
I
I
I
I
90-
800
a
706050
cOJ
(jJ
4 -0
w
D 40
-J
4
0
0
30-
w
z
(9
(J-)
2010
0
0
q')
-
n I
00
0
I
200
i
I
300
i
I
I_
_ _
600
5 00
400
PULSE REPETITION RATE (PPS)
FIGURE V-c3-9
FREQUENCY SCALE
SUBJECT RK
wo 0
_
_
_
_
_
_
(u0
_
_
_
_
_
?j uU
go90-80706050UJ
w
4 0
00
30z
S2 2o0100
*
I
100
I
I
200
300
400
PULSE REPETITION RATE(PPS)
FIGURE V-a-jO
FREQUENCY SCALE
SUBJECT JC
500
600
700
800
-1
100
0
9C
8C
0
7C
6( )
S
S
w 5( )
CM,
u
(D
4( )
0
)
2(
0
0
0
)
I
1oo
200
I
300
-~
400
500
PULSE REPETITION RATE(PPS)
FIGURE V-a-Il
FREQUENCY SCALE
SUBJEC JD
600
ruu
1 uu
11
-A
100
8070-60-*50-60
40--
30-.
L0)
2010-
100
200
300
400
Soo
PUL SE REPETITION RATE(PPS)
FIGURE V---2
FREQUENCY SCALE
SUBJECT L13
600
700
Soo
-I
100
9080700
6050-40-2.-
10-
100
200
300
400
500
PULSE REPETITION RATE(PPS)
FIGURE V-c3-13
FREQUENCY SCALE
SUBJECT NN
600
700
800
-138The last description would suggest a linear scale since
an increase in frequency would produce a linear increase
in the number of pulses felt per scan or per inch if the
subject's finger motion parameters were constant.
Several measurements of the electrotactile threshold
for various electrode configurations and various source
conditions are presented in the remainder of this
section.
A common method of stimulus presentation and
data treatment is used in all of these experiments,
is explained below.
and
This discussion will also apply to
any mention of threshold results in later sections.
All threshold measurements were made by the tracking
method developed by von B6kesy.
This method was pre-
ferred over a "method of adjustment" or a two alternative
forced choice presentation, since the resulting data
are to be used to predict subject performance on presentations over which he has little control but which
have long persistance times.
Subjects were instructed
to be certain that the electrically induced sensation
was either present or absent before so reporting.
This
was done in order to discourage overly critical or low
confidence judgements.
This instruction was repeated
periodically during the experiments.
-139Von Bgkdsy's method of threshold measurement
recuires that the strength of the signal be changed
at a constant rate, first increasing until the subject
reports detecting the signal, then decreasing until
signal loss is reported.
This process is repeated,
recording the points where loss and recovery of the signal are reported until sufficient data are accumulated.
The data from this type of presentation is of the form
shown in figure V-a-14.
Notice that the resulting curve has stable and
unstable regions.
This method of taking data has the
disadvantage that the subject may change his criterion
during the experiment and thus move the results to a new
"threshold".
This is one cause of the behavior of the
curve shown.
Another cause which has been positively
identified is change in the subject's manner of using
his finger.
This seems to occur most often in long term
experiments where fatigue resulting from -the repetitive
finger motion is evident.
Referring again to figure V-a-14, each stable region
is assumed to represent a threshold measurement.
The
data are treated by plotting the data sequences and
visually isolating each such stable region which is five
response pairs or more in length.
The large average
sloPes and data dispersions of the unstable regions
50
40REGION I
THRESHOLD
SAA
030-
AD
RA
RD
>
<
REGION
19.8 v
22,G v
16 .9 v
27v
14
34v
-
020-
UNSTABLE
REGION
<
10 -
0
0
.
_
50
I
L
100
ELAPSED
150
20Q
TIME (SECONDS)
250
.
..
a
300
a
350
FIGURE V-c3-14
THRESHOLD MEASUREMENT
EXAMPLE
_______
-~~.1
-141facilitate the identification of the stable regions.
An
experiment is said to have failed if no such stable
region can be found.
Several such regions are often
found in a single sequence of responses.
Due to the
relative inacurracy of measurements containing a low
number, n, of samples, the value of n will be recorded
with the threshold value.
The two sequences, ascending and descending values,
which result for each stable region are then treated
to obtain the following parameters:
The
The
The
The
range of ascending values
average ascending value
range of descending values
average descendingialue
The root-mean-square average of
AA and AD
RA
AA
RD
AD
T
This last value is treated as the actual threshold.
All
of these measurements will be reported on a linear scale
of peak voltage.
Results for NN will also be reported
in this manner though it should be remembered that the
more correct usage is probably peak current.
The reasons behind this method of data analysis
are based on an analysis of the distributions of the
ascending and descending results and on the model proposed in chapter IV.
The mean, median and root-mean-sruare
values of both the ascending and descending secuences for
several trials with each subject were examined and it
*1
-142was found that the median consistently
coincided with
the mean rather than with the root-mean-scquare value,
indicating a more symmetrical distribution in the measured peak voltage domain than in the peak force
(proportionAl to the square of the voltage) domain.
The
ascending and descending sequences were therefore simply
averaged.
The model suggests that the stimulus force
should in fact be proportional to the square of the voltage, and therefore a root-mean-square average was used
to combine the two resultant sequence averages, that is,
in effect, the force was averaged and the voltage needed
to generate the average force was calculated.
The rate of change of amplitude used in these
experiments was 1 volt (peak) per second.
It was noted
that higher rates of change tended to drive both the
ascending and descending sequence values down.
In the
event that several flat regions appeared in any one set
of responses, each region was treated as a separate
threshold, and so in several cases more than one threshold
is reported as the result of a single sequence.
Such
cases will be identified.
Experiment a-III.
Threshold Variation
The first of the threshold experiments is concerned
with the variation of the thresholds from day to day and
-143from subject to subject. It also serves to indicate the
spread which is encountered in the measured data within
a single secuence of data points.
The thresholds were
measured during each of four consecutive sessions (three
for NN) and in some cases the measurement in question
was made more than once in a given session.
All such
measurements are presented.
The electrode nattern used is shown in figure V-a-15,
as is the pulse applied to the electrodes.
The most
important parameter in these experiments is the pulse
repetition rate.
Of only secondary importance are the
pulse width parameters.
This source pulse train will be
used in most of the experiments reported in this chapter,
and has been used unless otherwise indicated.
Each
measurement consisted of 15 ascending and 15 descending
data points, taken as indicated above.
Normally less
than 15 points are usable in the calculation of the
threshold.
The data are presented in tabular form for each
subject by date in tables V-a-2 through V-a-5.
The
threshold values are rearranged in table V-a-6 in
chronological order by subject for more direct comparison.
As can be seen, a day to day variation of as much
as a factor of two is not uncommon.
This much variation
indeed has occurred for one subject in a single day.
It
-144-
000
0
0
0
0
ol
0.I
000
ELECTRODE
01
PATTERN
5.Orns.
T
Vc,
Vc!
0.6mte
SOURCE PULSE
FIGURE V-a-15
STIMULUS FOR
THRESHOLD VARIATION TEST
-145-
Parameter
Date
Dec.
T
Dec.
1 63.4v
1* 77.lv
1* 68.6v
Dec.
3
Dec.
5
Dec.
5
5* 56.5v
Dec.
Dec.
52.2v
53.7v
53.6v
RA
! ?
AD
RD
N
66.5v
2.85i T 61.2v
3.56v
5
81. Ov
4.64v
73.Ov
9.25v
5
72.5v
54.8v
4.28v
66. lv
5.69v
7
3.41v
49.4v
3.14v
9
56.6v
2.36v
4.20v
6
55.9v
3.41v
50.8v
51.Ov
2.36v
7
59.Ov
4.20v
53.9v
1.83v
6
*This result and the one above are from the same measurement sequence.
T = Threshold Value
AA = Average Ascending Value
RA = Range of Ascending Values
AD = Average Descending Value
RD = Range of Descending Values
N w Number of Samples Used
TkbLe
V-a-2
Threshold ,,Variatioiit,-,& ,Subj4ct NN
-146Parameter
Date
T
2V
RA
AD
RD
N
Nov.
23
24.6v
31.8v
12.2v
14.lv
24.4v
Nov.
23
24.4v
5.5v
17.lv
12.2v
Dec.
19.6v
36.2v
3.7v
6.9v
15. 2v
6
Dec.
4
4
29.8v
26.8v
14
10
47.0Ov
10.9V
20.6v
Dec.
7
26.4v
34. Ov
14
20.6v
27.8v
15. 5v
9.OV
6
9
Dec.
8.5v
12.2v
3.7v
16.4v
8. 5v
12
Table V-a-3
Threshold Variations - Subject JD
Parameter
RA
AD
RD
N
Nov. 25
Dec. 2
Dec. 4
Dec. 4
48.9v
30.5v
62. Ov
8.5v
30.2v
9. 7v
9
38.6v
6. av
19.lv
47.lv
56.6v
19.5v
35.2v
12.8v
15.8v
12
8
39.9v
2.4v
12.2v
9
36.5v
6. lv
2.4v
28.lv
Dec. 16
49. Ov
46.lv
23.2v
Table V-a-4
Threshold Variations - Subject LB
5
r
-147Parameter
Date
T
AA
RA
AD
RD
N
Nov. 24
28.8v
35.8v
5.49v
19.6v
6.7v
9
Nov. 25
36.6v
45.lv
6.lv
25.4v
7.3v
8
Dec.
2
25.7v
32.4v
9.lv
17.3v
4.9v
14
Dec.
6
42.9v
52.5v
17.lv
30.3v
6.7v
6
Dec.
6*
35.6v
42.3v
7.3v
24.9v
10.4v
6
*Same measurement
as above.
Threshold Variations -
Subject RK
-148-
Threshold Value (volts peak)
NN
Date
Nov.
23
Nov.
24
Nov.
25
Dec.
1
Dec.
2
Dec.
3
Dec.
4
LB
JD
RK
24.6/24.4
28.8
48.9
36.6
30.5
25.7
63.4
77.1/68.6*
52.2
19.6/36.2
47.1/39.9
53.7
53.6/56.5*
Dec.
5
Dec.
6
Dec.
7
26.4
Dec.
14
20.6
Dec.
16
42.9/35.6*
36.5
*This value and the value to its left are derived
from the same measurement.
Table V-a-6
Threshold Variations - Comoarison
-149is interesting also to note that the dispersion of the
data remains nearly independent of the magnitude of the
threshold.
Experiment a-IV.
The Effect of Electrode Area on
Threshold
The largest group of threshold measurements, from
which the above measurements were taken, was related to
the effect of the electrode area and the area enclosed
by the electrode on the threshold of touch.
Over the
same period of time as the measurements presented in
experiment a-III, measurements were made of the thresholds
for nine different patterns of excited electrodes.
These
patterns are shown in figure V-a-16.
The patterns consist of five compact sets of
electrodes, three ring-like groups and one partially
filled ring.
Measurements of each threshold were made
on at least two different days, and the results averaged
to arrive at the result presented.
Each measurement
represents again 15 ascending and a like number of
descending data points.
The results are then presented
as points, plotted threshold voltage versus an area
parameter, measured in terms of unit electrode areas.
In figqure V-a-17 are plotted the averaged results
for those subjects using the texture sensation for the
compact set of patterns.
Figure V-a-18 gives the same
-
1.
0
0
0
0
0
2.
000
000
150-
6.
0
0
0
00
00
00
00
00O
000
'3.4.
O000
00000
8.
00
9.
0
00
000
000
000
000
5-
0
0000
000
00000
00000
0
v
0
0 0 0
0
0
000
FIGURE V-d-16
ACTIVE ELECTRODES
FOR PATTERN THRESHOLDS
1
50
45-
oAVERAGE THRESHOLD
(THREE SUBJECTS)
403530
ofr-
000
X>20 0
Lij
510 -
I
I
I
I
2
4
6
8
I
I
I
I
I
10
12
14
16
18
AREA (ELECTRODE AREA UNITS)
FIGURE V-c-17
EFFECT OF AREA ON
TEXTURE THRESHOLD
I
I
I
20
22
24
26
28
70
6560555045-
0
c-fl
40-
0
0
14
35-
:
30-
SUBJECT NN
SOLID PATTERNS
251201
0
I
i
2
4
I
I
I
I
14
10
12
6
8
.16
AREA (ELECTRODE AREAS)
FIGURE V-a-18
EFFECT OF AREA ON
TINGLE THRESHOLD
I
I
I
18
20
22
24
.
26
---
-153Notice
results for NN only, who used a tingle sensation.
that the results of NN show a rising threshold with increasing area, in agreement with R. H. Gibson's results,
while the others show a reduction in threshold, as
predicted by the model proposed in chapter IV.
In
particular, the model predicts that aside from other possible effects the threshold should be proportional to
the inverse of the scruare root of the area.
curve is also shown in figure
Such a
V-a-17 for comparison
purposes.
The results for the remaining patterns allow an
examination of whether the active electrode area or the
area enclosed by an electrode pattern is the more
important variable (figures V-a-19 and 20).
It
would
appear that for the subjects using the texture effect,
the electrical area is the more important.
Insufficient
data exists to draw any such conslusion for NN and the
tingle sensation, though these results do not seem to
agree with those of Gibson (1967).
For purposes of comparison across subjects, the
individual results for the subjects using texture sensation are plotted in figure V-a-21 through 23.
These
results show that the subjects tend to have different
threshold levels, but all show the same trends as the
average.
dw -
,50
II
50
II
I
I
I
-
I
I
I
I
------------ # -
I
I
I---------
I-
I
I
II
--
1
I
45402
0
351-
A
2
E0
3060
0
I
I
A
0 5.
A
5
E6
2520-
(~71
0
15OPEN PATTERN THRESHOLD vs.
ELECTRODE AREA a
10-
ENCLOSED AREA
A
5
AVERAGE FOR THREE SUBJECTS
0
01
2
4
6
18
16
14
1I
10
8
AREA(ELECTRODE AREA UNITS)
FIGURE V-8-19
OPEN PATTERN
TEXTURE THRESHOLDS
20
22
24
26
"1
80
I
I
I
i
.
I
I
I
I
I
I
I
I
75
70-
5
A
6
A6
o
2
60[
0
A
2
55(~l1
50lct
0
45OPEN PATTERN THRESHOLD vs.
ELECTRODE AREA o.
A
ENCLOSED AREA
40_
35-
SUBJECT NN
30'
0
I
2
I
I
it
I
I
I
10
12
14
8
6
4
AREA (ELECTRODE AREA UNITS)
I
I
16
18
FIGURE V-a-2O
OPEN PATTERN
TINGLE THRESHOLD
I
20
i
22
24
I
26
------%,,0-
I-
.........
60
0
SOLID PATTERNS
RK 3
LB o
55150[h
0
JD A
45
8
40A
0
35U,
30-
A
8
0)
A
0
r
250
0
fl)
LUJ
T
I-
20A
A
15 10
I
I
I
2
4
6
I
I
_
I
I
I
8
10
12
14
16
AREA(ELECTRODE AREA UNITS)
FIGURE V-a-21
SUBJECT COMPARISON
THRESHOLD vs. AREA
I
I
I
I
I
18
20
22
24
26
-I
5C
- -I I
I
I
I
45-
I
I
I
I
I
I
I
0
40-
2
35-
01
I
F-
-3
0
25
A
6
cn
0
0
mJ
0
LI
OPEN PATTERNS BY
ENCLOSED AREA
RK o
LB o
JD ,
5H
01
0
16
18
10
12
14
6
8
2
4
ENCLOSED AREA(ELECTRODE AREA UNITS)
FIGURE V-d-22
SUBJECT COMPARISON
OPEN PATTERN THRESHOLDS
20
22
24
26
-
-'I-----
----
5C 'I
45
10
*
40 2
3 5'I
+
30
5 &
6
0
25-
A
200
0
Un
OPEN PATTERNS vs.
ELECTRODE AREA
RK +
Ll o
JD,&
15~
Lo
0i
cJ
I-
I0 5h
0
I
2
I
I
4
6
8
10
14
16
12
18
ELECTRODE AREA(ELECTROD E AREA UNITS)
FIGURE V-8-23
SUBJECT COMPARISON
OPEN PATTERN THRESHOLDS
I
I
I
20
22
24
26
Experiment a-V.
-159Pin Electrode Size
An examination of the results of the previous
experiment point to a potential problem with the texture
sensation.
Since the threshold rises sharply with a
reduction in electrode area, an effort to achieve higher
resolutions by reducing the pin size will lead to higher
differences in sensation level between single points
and filled areas, already reported to be a problem by
the subjects.
It also leads to higher threshold voltages
with correspondingly smaller dynamic ranges available,
since the mechanism limiting the maximum stimulus has
a threshold which is reduced with a reduction in electrode
area.
To acquire some feeling for the limit in pin size
which can be allowed, an experiment was performed in the
same manner as the pattern thresholds above to compare
a single isolated point electrode of standard size with
two smaller circular electrodes.
presented in table V-a-7.
The results are
Based on these results and
subject complaints, it seems clear to this experimenter that
ver- little reduction in electrode diameter can be tolerated without some protection for the sUbject, perhaps
in the form of a limit on the minimum number of electrodes
in any isolated "on" group.
-160Electrode
Standard
0.070 in.
diameter
No. 14 wire
0064 in.
-diameter
Parameter
T
AA
RA
AD
RD
No. 16 wire
0.051 in.
T
159.lv
AA
RA
167.8v
-.
"diameter
T
AA
RA
AD
RD
N
57.9v
72.lv
22.8v
38. 5v
19.4v
5
79.4v
97.4v
2.3v
55.5v
5.3v
78.5v
94.3v
30.4
58.4v
14.4v
5
13113v
143.8v
8. Ov
118.5v
27.4v
5
96.1
115.3v
32.6v
71.6v
21.2v
6
177.Ov
188.5v
13.6v
164.8v
4.5v
5
194.9v
212.2v
17.lv
175.2v
18.2v
5
178. lv
195.3v
202.8v
22. Ov
148.Rv
54.6v
5
210.2v
171.7v
4.8v
160. lv
9. OV
5
N
RK
165.8v
N
AD
RD
LB
JD
NN
15.9v
159.lv
4.8v
5
181.6v
188.9v
38.3v
169.lv
34.6v
7
T = Threshold
AA
Average Ascending Value
RA = Range of Ascending Values
AD = Average Descending Value
RD = Range of Descending Values
N = Number of Sample Points
Table V-a-7
Electrode Size Results
'5
20.5v
179.Ov
8.3v
5
-161Experiment a-VI.
Large Area Thresholds
The thresholds for a large area of pin electrodes
with no interior electrodes unexcited are given in
table V-a-8.
Each measurement represents 30 ascending
and 30 descending data points.
Each of these results
is for a single occurrence of the measurement.
The
source pulse had a repetition rate of 125 pulses per second,
and a half width of 0.25 milliseconds.
The pulse was
symmetrical.
Due to the differences in both pulse width and pulse
repetition rate, these results cannot be directly compared
with those of experiment a-IV.
However, it will be noted
that the results fall into the same range as those
previously encountered.
Results for NN are again
presented for comparison.
Experiment a-VII.
The Effect of Pulse Shape on the
Sensation
This experiment and the remainder of the experiments
offer some slightly less precise results which are
nevertheless felt to be of interest to investigators
who might work with such a display in the future.
results are primarily of a qualitative nature.
The
The
subjective nature of these results, and the fact that the
subjects were often unable to find sufficient distinctions
for their own satisfaction force us to be content with
-162Subject
RK
Parameter
Date
T
AA
RA
AD
RD
N
LB
Date
T
AA
RA
AD
RD
N
JD
Date
T
AA
RA
AD
RD
N
Jc
Date
T
AA
RA
AD
RD
N
TSTN
Date
T
AA
RA
AD
RD
N
Oct. 16
54.3v
62.7v
15.9v
44.3v
1l.4v
9
Oct. 20
38.6v
47. lv
12.lv
27.6v
13.7v
18
Oct. 18." Oct. 18* Oct. 21
92.9v
63.8v
60. lv
106.9v
79.5v
68.5v
2.3v
76.2v
21.3v
48. 8v
45.5v
11.4v
14.4v
14.4v
12.6v
5
9
5
Oct. 21*
73"2v
82.3v
8.2v
62. 5v
17.5v
6
Oct. 24
48.3v
54.6v
8.3v
Oct. 24*
54.8v
65.6v
41. Ov
10.6v
43.Ov
Oct. 15
28. 7v
35.4v
13.7v
19. 7v
11.2v
25
Oct. 24
50.Ov
57.6v
18. 2v
41. lv
14.4v
26
Oct. 21
89.6v
93. 0v
6. lv
85.2v
9. lv
6
Oct. 21* Oct. 23
77.6v93.5v
80.6v
95.7v
Oct.
Oct. 17*
119.7v123.9v
14.4v
116. 4v
14.4v
17
100.8v
104. 9v
5.3v
96. Ov
4.3v
5
6. v
74.5v
6.5v
5
8
9
14.4v
10.6v
14
Oct. 23
95.5v
2.7v
91.-v
4.8v
7
98.2v
1. 7v
92.6v
3.1v
6
Oct. 22
126.2v
129. 5v
18. lv
123. lv
16.5v
9
Oct. 22*
136.0v
138.7v
9. 6v
131-.9v
10.7v
5
T w Threshold (volts)
AA = Average Ascending Value (volts)
RA
Range of Ascending Values (volts)
AD
Average Descending Value (volts)
RD
Range of Descending Values (volts)
N
Number of Sample Points
*This result occurred in the same test as the one to the
left.
Table V-a-8
Arrav Texture Thresholds
-163less hard evidence for the results than might be desired.
In many cases we must be content with subject's verbal
comments only.
It is suggested that in future investi-
gations the questions treated here in brief be more
thoroughly explored.
The first test of the effects of pulse shape was
a two alternative forced choice preference test.
The
subject was presented with pairs of stimuli, simultaneously
available in different portions of the display, and
was asked to state his preference on the basis of texture
cquality and the sharpness of the boundary of the excited
area.
The stimuli differed in pulse shape, but not in
frequency or apparent amplitude.
The apparent amplitude
differences had been carefully balanced out.
Twenty presentations of pairs selected from six
stimuli were made.
The preference responses for each
subject were treated by assigning a value of one to the
stimulus when a stimulus was preferred over another, and
minus one if the other was preferred.
These values were
added, and the result is given as table V-a-9.
The preferred form would appear to be a wide sym=
metrical pulse.
The stimuli used here are not by any
means a complete set.
However, subjects report that the
differences even among this set are.almost too small to
detect, and that no difference at all can be detected
-164Preferred Texture Quality
Shape
(Ta x Tb in millisec.)
Percent
Total
Subject
JD
JC
LB
RK
+5
-3
+6
0
-4
-4
-4
+4
0.60 x 0.60
0.40 x 0.10
+2
+5
+;5
+3
-2
+1
-7
0.65 x 0.15
0.80 x 0.25
-4
-1
-5
-5
+3
-1
-31
0.40 x 0.40
0.25 x 0.25
+2
Shape
+3
-3
+12.5
+15
-9
-55.5
-33.3
-10.7
-3
.211.1
JC
LB
RK
Total
0
-2
+2
0
0
x 0.40
0
0.25 x 0.25
0.60 x 0.60
-3
-4
-2
0
+3
+5
0.40 x 0.10
+2
+1
-1
+3
-3
0.65 x 0.15
0.80 x 0.25
+1
+3
-3
+3
-3
-1
-1
0
-13.0
Percent
Total
Subject
(Ta x Tb in millisec.)
0.40
Total
Table V-a-9
Pulse Shape Preferences
-9
0
-39.2
+11
-1
+40.7
-3.7
-7
+21.4
-26.4
-165between an unsymmetric pulse and its mirror image in
time.
NN was unable to detect any differences at all
except the amplitude difference.
It is on this basis
that the symmetric pulse with a half-width of 0.60 milliseconds is used in the majority of the experiments.
Wider pulses were excluded when subjects reported that
they resulted in an indistinct texture sensation.
In a second attempt to determine what kinds of
effects the shape had, a presentation similar to that
used above wasmade and the subjects were asked to rank
the stimuli according to amplitude, coarseness of the
texture and sharpness of the stimulus boundaries.
The
variable in this case was the location of the nositive
to negative transition in an otherwise fixed length
pulse (see firure V-a-24).
Pulse amplitude and repetition
rate were also fixed.
In this case, however, the experimenter used the
responses to generate and to confirm a characteristic
curve while the subject was responding, thus it was
possible to emphasize in the presentations regions of the
curve which gave trouble to the subject.
This procedure
,i elds only the characteristic shape of the curve in
q-uestion.
The curves characteristic of all three
subjects using the texture sensation are shown in
figure V-a-25.
It should be remembered that the subjects
K Q8s
ES
DH
GA
ER
P
N
E
S
S
FIGURE V-a-2 4
PULSE SHAPE VARIATIONS
0
0.2
0.4
0.6
T
08
a)
a)
C
0
A
R
S
E
N
E
S
S
A
m
p
L
T
U
D
E
0
'
0.2
0.4
06
Q8
0
0.2
FIGURE V-ci-25
PULSE SHAPE EFFECTS
0.4
0.6
a
-167feel that these are very minor effects.
The final tests of pulse shape were made simply by
asking the subjects whether they made any difference.
In
this case, many possible alterations were tried to
determine whether or not the apparent size of a single
excited electrode would vary.
No variation of any
repeatable kind could be elicited from pulse shape
changes.
At the same time, subjects were asked to match the
amplitudes of two stimuli, one of which was of normal,
symmetric shape, while the other was identical to it
except for a very poor rise time.
On many trials, at
several amplitudes, all of the subjects using the texture
sensation matched the amplitudes so that their oeak-topeak voltages were identical.
This effect was not tested
by threshold measurement.
Experiment a-VIII.
The Effect of Pulse Repetition Rate
A two alternative forced choice preference test was
performed to determine the effects of pulse period on
the apparent amplitude, the texture coarseness and the
sharpness of the excited area boundaries.
The presen-
tation was simultaneous and on adjacent electrode areas.
The standard symmetrical nulse was used, at an amplitude
ten percent above the apparent threshold.
-168The results were analyzed by assigning a value of
one to the preferred or higher ranked stimulus, and a
minus one to the lesser ranked stimulus.
are given in percentage form.
The results
Approximately 35 presen-
tations were made to each subject, selected at random
from nine pulse periods ranging from 1 to 9 milliseconds.
The results (table V-a-10) show preferences on the basis
of amplitude and edge sharpness near the mid range, about
200 pulses per second.
The results for texture coarse-
ness are strangely mixed.
Subject NN did not take part
in these tests.
Experiment a-IX.
Amplitude Dynamic Range
The dynamic range available for display use is not
so much determined by the pain threshold of the subject,
as by what might be termed his threshold of annoyance.
Measurements of this value were attempted twice for each
subject, once at the beginning of the study and again
about two thirds of the way through.
In each case, only
one ascending trial was made, and so the result is quite
imprecise.
The reasons given by the subjects for reporting an
annoying sensaton are quite uniform, for both sets of
measurements.
Those subjects using a texture sensation
-169-
Amplitude Preference
Pulse Period
(milliseconds)
1
2
3
4
5
6
7
8
9
(larger amlitude)
Percent
Suject
Total
Jc
JD
LB
RK
+1
-6
+3
+4
+6
-2
+2
+4
-4
-7
-4
-7
42
+2
+8
+12
0
-4
-9
-6
+1
+5
0
+4
-1
+3
-6
-2
-3
Total
-15
0
+8
+4
+4
+2
-4
-9
+20
£ji9
+21
0
-65.3
+7.2
+15"0"
,-62.5
+22.5
+44.7
0
-14
-28
-87.5
-77.8
-13
±6
-56.5
-21.4
+15.0
+25.0
+35.0
Edge Sharpness (sharper)
1
2
3
4
5
6
7
8
9
+1
+5
+4
-2
0
+1
-4
-2
-3
Coarseness
1
2
3
4
5
6
7,
8
9
+3
+1
+6
+2
-2
-5
-4
0
+1
-2
-3
-2
+4
+2
+2
0
0
-1
-6
-5
+4
+2
+8
+6
+2
-4
-7
-6
-3
0
+4
+4
+4
-4
-3
+6
:8
+14
+13
+2
-10
-14
+27.6
+5.0
-62.5
-38.9
(coarser texture)
*6
-5
-2
+2
0
-2
-2
0
+3
+6-1
-5
-8
+2
0
-2
0
+4
+7
-6
-5
+2
-4
+4
+4
+6
+2
-1
+9
-14
-2
-2
+2
-5
0
+6
+8
Table V-a-10
Frequency Preference
+39.1
-50.0
-5.0
-16.
+5.0
-10.6
0
+3715
+22.2
I
-170reported the onset of one or another.tingle phenomenon.
Subject NN reported that the sensation either became
unbearably strong, or came to involve the entire finger,
sometimes accompanied by a feeling of joint and muscle
stiffness.
The results (table V-a-ll) show an increase in the
tolerable range with time, or perhaps with experience.
They also show a dynamic range of a factor of four or
five times the touch threshold for the subjects using
the texture sensation, and about half that for NN.
V-b.
Experiments Relating to the Use of the Stimuli in
an Explorable "textured area" Display.
In the development of the textured area form of the
display, the primary considerations have been the ability
of the subject to distinguish between textures and to
locate the boundar'y between two areas of different textures, in a situation similar to that expected in the
use of an explorable display.
The presentations for the
experiments described below are simultaneous presentation
of two different textures on two adjacent areas of the
display array.
In all cases the size of an area was
larger than the finger pad so that the subject could feel
the textures independently if he wished.
The boundary
between the two areas was always a straight line, and
I --
-171-
Subject
Threshold
nate
Maximum
Range Factor
Oct.
3.
51.lv
109. Ov
2.12
Mar.
9
20.2v
46.9v
2.32
Oct.
10
10.5v
67.lv
6.40
Mar.
8
18.7v
82.5v
4.41
Oct.
4
28.4v
51.8v
1.82
Mar. 5
15. Ov
60. Ov
4.00
Oct.
8
16.8v
79.0v
4.70
Feb.
24
22.4v
118.2v
5.30
Oct.
10
35.4v
57. Ov
1.60
Oct.
14
32.2v
50v
1.55
NN
JD
RK
JC
Table V-a-l
Threshold of Annoyance and Dyrnamic Ran7e Results
7
-1
-172its location was alwas soecified along an axis perpendicular to it.
In preliminary experiments with this presentation
it was found that the onlyv stimulus variables which would
allow the subject to be confident about the texture
differences were amplitude and frequency.
The failure
of NTN to detect frequency differences was .confirmed_
and so she does not appear in the results relating to
the frequency variable.
While the pulse shape parameters
affect somewhat the apparent stimulus amplitude, they
could not produce any distinctdfferences in texture of
use to the subjects.
Similarly, relative delay between
two otherwise identical pulse trains did not produce a
distinguishable boundary when one area was excited with
each.
The remainder of this section is therefore devoted
to experiments measuring first the Just-NoticableDifference (JND) for both amplitude and frequency, and
second, the ability of the subject to localize the
boundary between the two regions under three conditions;
no excitation on the second area, excitations differing
to JND's in amplitude only and excitations differing two
JND's in frequency only7.
Exeriment b-I.
-173The Just-Noticable-Difference
Amplitude
for
This measurement was made by fixing the amplitude of
one area, and varying the amplitude of the other.
each measurement 100 nresentations were made.
For
In each
presentation the lower area was at the fixed center
amplitude, and the upper area was at an amplitude selected
by a uniform random choice from a predetermined range.
The range was determined by having the subject specify
when he was comfortably certain that one area was stronger
than the other.
A range of three times the difference
thus determined was used to ensure that noticable differences on both sides of the center value were included.
The stimulus pulse used was symmetrical with a
half width of 0.60 milliseconds.
rate was 200 pulses per second.
The pulse repetition
The array electrode
shown in figure 111-5 was used and the presentation
pattern is as shown in figure V-b-l.
The resulting
stimuli differed only in amplitude and location.
The
experiment performed *iMe times, once each at 10% above
the ascending threshold, at the "most comfortab&e"
level selected by the subject and at approximately 10%
below the maximum level the subject would accept.
The
midrange value was normally approximately 50% of the
maximum value.
-174-
REGION I
REGION 2
000000000
000000000
000000000
N=5
000000000
000000000
000000000
000000000
000000000
000000000
-000000000
0
0
SOURCE I
SOURCE 2
FIGURE, V-b-I
PRESENTATION FOR
JND EXPERIMENTS
-175-
During the range determination it was made certain
that the difference which the subjects were detecting
was an amplitude difference, and then they were asked
to specify which of the two areas was stronger, in a
two alternative forced choice presentation.
That
constituted the only instruction to the subjects.
One
such response was recorded for each presentation.
The resulting data was treated by determining the
percentage of times the response "bottom is stronger"
was given and graphing the results.
are shown in figure V-b-2.
Two typical curves
A smooth curve was fit by
eye to the plotted results and the locations of the
75% and 25% transitions determined.
is
one half the interquartile
The reported JND
range thus determined,
or where percentages are given, the percentage which
this figure represents of the center value of the same
measurement (table V-b-1).
Analysis of the Results:
Since these results are based on
onl- a very few presentations per data point, their
value is limited to an estimate of the order of magnitude
of the JND.
The curves shown in figure V-b-2 illustrate
the cuality of the data.
The upper plot is of the sort
expected, and this kind of result was obtained in about
half of the measurements.
In the other cases flat regions
or slone reversals would appear in the curve.
These are
-176100
-0
080
0
*
-
-0
O
m6 O
O-
-
-
z
u
-
-
0
-
*
-
0
w2
LU
1ni
I..,
tSAME AM PLI TUDE
AMPLITUDE OF UPPER REGION
icrn
1'
I
I
-
I
I
I
*
0
-
B
-
e
*
z
-
-
04o
0.
-
0
0*
e
,
*
0
0
tSAME AMPLITUDE
AMPLI TUDE OF UPPER REGION
FIGURE V-b-2
AMPLITUDE JND
EXAMPLES
-177-
NN
Level
JD
LB
RI
Near Threshold
JND
Center
Percent
1.45v
1. 20v
1.32v
1. 77v
29.5 v
23.2 v
23.2 v
30.9 v
4.9 %
5.2 %
5.7 %
5.7 *%
1.78v
2.24v
3.!7v
2.35v
37.0 v
39.6 v
43.4 v
38.6 v
4.8 %
5.8 %
7.3 %
6.1 %
1.42v
3.25v
5.85v
6.04v
51.7 v
54.1 v
54.1 v
77.2 v
Midrange
JND
Center
Percent
Near Maximum
JND
Center
Percent
2.7 %
6.0 %
10.8 %
Table V-b-1
Aomplitude Just-Noticeable-Differences
7.8 %
-178believed in some cases to be the result of the small
number of samples.
In other cases, and particularly
with one subject, this appears to be the result of the
subject having allowed himself a third choice, "no
decision",
contrary to the experimenter's instructions.
In any case where such an anomaly in the curve resulted
in more than one distinct crossing of either the 25%
of 75% boundary the crossing resulting in the larger JND
measurement was used.
Subject NN was basing her:answers on the tingle
sensation, while all the others used a texture sensation.
There are two interesting points about the tabulated
results.
First, we find that the JND's are of about the
same size for both sensations.
Second, it is interesting
to note that for NN the JND appears to be of a constant
value, whereas those subjects using the texture sensation
tend to exhibit a constant percentage rather than a
constant value.
This distinction needs further experimen-
tal confirmation.
Experiment b-IT.
The Just-Noticable-Difference for
Freciuency.
In order to determine approximately the JND for
frecqiencyr, the subjects were presented with the same
situation described in the previous experiment, but with
stimuli which differed only in pulse renetition rate.
-179In all, 75 presentations were made for each measurement.
In each presentation the lower area was fixed in fregiencv
at 200 pulses rer second.
only one determination was
made, and the range from which the samples were selected
for the upPer area was the entire useful frecnuency range
of 100 to 1000 pulses Der second.
The same pulse shape and spatial oresentation were
used as in experiment b-I, and again the subject was
permitted as much time as he wanted to make a decision.
Decision times averaged about 10 seconds for all subjects.
The presentation amplitude was set at the subject's "most
comfortable" level, and the amplitudes of the two areas
were identical.
No attempt was made, however, to correct
for the effect of pulse repetition rate on the subjective
strength, and so the stimuli were subjectively the same
strength only when the frequencies were identical.
The subjects were asked to designate which of the
two areas was "coarser", a term which had come into common
use to designate frecruency based texture differences.
The resulting data were treated as in experiment b-I.
The results have the same form and most of the same
characteristics (table V-b-2).
Analysis of the results:
In agreement with the
inability of subject NN to scale frequency, at least
over the range used in these experiments, she was unable
-180-
Subject
JND
Percent
JD
76 pps
38.0%
LB
83 pps
41.5%
RK
77 pps
38.5%
17,
Not Applicable
Center frequency is 200 pulses/second
Table V-b-2
FPrequency Just-NoticableiDifferences
to detect any distinct diEW'5nce between the adjacent
areas on the basis of frecuency differences alone.
The
subject indicated that she could only guess, and the
results appear to be essentially random.
The results for the other subjects are consistent
with their reports in other situations, though the
JND appears to be somewhat larger than might have been
hoped for.
The remaining three experiments in the textured
area group were performed with the same stimulus arrangement.
In each case, two areas of the same form used in
the JND experiments were used, but with the location of
the transition from one stimulus region to the other
variable.
In each case the stimuli remained constant
througwhout the experiment.
50 presentations.
Each measurement represents
Each presentation had a number, N,
of rows of pins in the upper electrode group, connected
to one source, and the remainder of the pins in the
array connected to a second source.
The subject was permitted to explore the display for
an unlimited period of time but was requested to respond
quickly.
He was asked to specify the number of rows in
the upper area.
The resulting data were in each case
analyzed to derive the mean and variance of the localization error in one dimension.
For comparison purposes,
-182the variance for a random guess in these experiments
would be 10.5.
Experiment b-III.
Area Boundary Localization
The boundary between an "on" and an "off" or
grounded area was explored at three amplitudes,
Jis ten percent, twice that value,
"best" amplitude.
threshold
and near the subject's
In all. cases the pulse applied in
the uper area was symmetrical, 0.60 milliseconds in
half-width, and the pulse repetition rate was 200 pulses
per second.
The lower region of the array was grounded.
The results are presented in table V-b-3.
The
similarity between the results of NN and those of the
other subjects is more apparent than real.
The method
she used recuired close visual examination of her finger
locations, and took about four times as long as the
methods of the other subjects.
Decision times for those
subjects using the texture sensation were on the order
of thirty seconds.
Subject RK reported having difficulty maintaining the
sensation on the day these data were taken, and his skin
resistance was abnormally low.
It is presumed that this
is the reason for the large variance which his results
exhibit.
-183-
1)
Near Threshold:
RK
NN
JT)
LB
0.16
0. 7A
,Variance
0.14
1.09
2.02
0.32
Amplitude
22.9N7
24. 3v
15.4v
15.4v
ean
-1.86
1.04
Error
2)
Twice Threshold:
JD
LB
RK
0.19
0.21
-1.58
0.18
0.38
0.25
2.48
0.17
45.7v
48.5v
30.8v
30.8v
LB
RK
NN
0.20
0.04
-1.94
0.78
'Variance
0.16
0.47
1.96
0.33
Amplitude
27.Ov
26.7v
ean
Error
Variance
Amplitude
3)
Preferred Amplitude:
JD
EMean
Error
23. lv
Means in tenths of inches
Variances in hundredths of square inches
Table V-b-3
Area Boundary Localization
21.2v
-184Two Area Transitions - Amplitude
Difference
Experiment b-IV.
In order to obtain an initial measurement of the
ability of a subject to locate the boundary between
two areas excited by signals which were identical except
for differing amplitudes, an experiment was performed
using the results of experiment b-I.
Using the same
stimuli as in the "best amplitude" case of that experiment,
with the amplitudes differing by two times the measured
JND, the procedures of experiment b-III were repeated.
At this level of difference all of the subjects
felt that the difference in amplitudes was easily
detectable.
Any smaller difference, however, would
elicit complaints that the difference was not always
clear.
Those subjects utilizing the texture sensation
reported that unlike the situation of the previous experiment which provided a distinct boundary, the boundary in
this case was more like an indistinct region than like a
sharp transition.
This remained true even for very large
amplitude differences, only disappearing when the border
was emphasized by grounding a row of pins.
No experiment
was performed to test this technique.
The results appear in table V-b-4.
The amnlitude
values used are recorded as well as the mean and variance
of the errors.
As might be expected, these results show
7
-185-
Localization of the Boundary Between Two Areas
Whose Excitation Differs by Two JND's in
Amplitude
Error
Center
Subject
Amplitude
Mean
Variance
JD
39.6v
-0.166
1.47
LB
43.4v
-0.024
1.56
RK
38.6v
0.106
1.88
NN
37. Ov
0.177
1.56
Mean error in tenths of inches
Error variance in hundredths of square inches
Table V-b-4
Two Area Transition Localization!is.!tude Differences
-186a somewhat larger variance than the pnevious experiment,
but it is not excessively large.
Two Area Transitions - Free-uencv
Difference
Exoeriment b-V.
The previous boundary localization experiment
was also carried out with a two JND difference in frec-uency.
The procedures were the same as was the method
of data analysis.
In this case the frequencies and
amplitude were taken from the results of the free-iuency
JND experiment,
b-II.
The results appear as table V-b-5.
With the notable
exception of the results of JD which show an exceedingly
small variance, these results are similar to those of
the amplitude boundary experiment.
The subjects again
reported that the boundary was again not a distinct thing,
but rather an indistinct region between two areas of
distinctly different texture.
Subject N did not
participate in this experiment since she was unable to
make distinctions based on frequency alone, as recuired.
Summary:
The experiments reported in this section indicate
that both amplitude and frequency provide useful variables
for use in a textured area display.
The JND and boundary
localization experiments should provide a starting place
-187-
Localization of the Boundarv Between Two Areas
Whose Excitation Differs b
Two JND's in
Fre uency
Error
Sub 'ect
Amolitude
Mean
Variance
JD
39.6v
0.71
0.078
L-
43.4v
0.311
2.70
RK
38. 6v
0.74
3.67
Mean Error in tenths of an inch
Error Variance in hundredths of a square inch
Table V-b-5
Two Area Transition Localization FrecuencyjDifferences
I
-188for work with larger displays, and it
is hoped, will
allow future experimenters to derive a better measure
for the complexity needed in such a display.
The fact
that the ability of the subject to use the texture
sensation itself is not a prerequisite for the use of
the display in the amplitude mode is worth noting, though
it is expected that in this respect a blind user would
suffer some degradation of his ability to localize boundaries.
Blindness is not, however, expected to hamper
the uJse of the display in the texture mode.
Complex presentations, particularly of figures with
complex boundaries, were not presented.
It is thought
that the next step in this direction is to constrlct
a large display which can be used to examine such cuestions,
and with which we can explore ways of enhancing
the boundaries between regions.
V-c.
Experiments Relating to Point-and-Line Displays
The experiments relating to displays for use in
Dresenting line drawings covered three areas.
The first
is the separation which must be maintained between two
excited areas in order to be certain that there are in
fact two figures and not one.
The second is the ability?
of the subject to localize a point in a large area.
-189The final measurement is the minimum size which a figure
must have in order to distinguish between relatively
simple shaPes.
As was pointed out in section V-a, the size of the
exIcited electrode group can have an apreciable effect
on the touch threshold voltage.
In the experiments
which follow, there was no attempt to account for this
difference in threshold or for accompanying difference
in apparent strengths at higher levels.
The subject
was simply told to adjust the amplitude of the single
source until the presentation was comfortable and he
could feel adequately all parts of the presentation.
The subjects never complained that this was difficult
to do.
In order to provide the reader with a feel for the
magnitude of this difference in the presentations reported
below, a set of threshold measurements is dfered in table
V-c-l.
These measurements were made in the same manner
as those in section V-a.
These thresholds are offered
for comparison, and the absolute value is unimportant.
The reader is referred to section V-a for a more thorough
discussion of the reason behind the difference in
thresholds.
Subject
JD
T
AA
RA
AD
RD
N
LB
T
AA
RA
AD
RD
RK
16.9v
50.lv
67.2v
14.8v
22.2v
16.4v
22.5v
10. lv
5.9v
6.75v
20
14.6v
19.4v
4.5v
7. 2v
4.5v
28.6v
19.8v
25.4v
3. 4v
16
36. 7v
9.5v
17.lv
15.5v
14
55.6v
61.4v
16.6v
49.Ov
ll.7v
15
85.2v
92.2v
6.4v
73. Ov
5.4v
9
85.5v
93.9v
9. 78v
76.5v
12.2v
14
22.4v
6.8v
8. 3v
16.9v
18
17.lv
15
T
49.6v
63.4v
13.7v
30. Ov
N
T
AA
RA
RD
N
Double Width
Line
41.lv
52.4v
9.lv
25.2v
9.lv
12
N
AA
RA
AD
RD
NN
Single Point
-190Single Width
Line
14.9v
Table V-c-i
Point-and-Line Thresholds
13. lv
16.9v
3.4v
7. 7v
6.8v
18
8
ll.6v
.4.5v
11
-191Experiment c-I.
Form Separation Measurements
In each of the several measurements listed below
the subject was presented with two figures whose separation was variable.
The figures were all simple points,
lines and rectangular areas.
The unexcited areas of the
array were connected to ground.
The source pulse, the
same for all excited portions of the display, was symmetrical, with a half-width of 0.60 milliseconds.
The source
pulse reoetition rate was 200 pulses per second, and the
subjects were allowed to select the amplitude that they
felt most comfortable with, as they would in using a
point-and-line display.
The subject was informed of the shape of the pattern,
and was allowed to adjust the amplitude to a comfortable
level.
He was then instructed to report that he could
feel two distinct forms or that the,) were merged.
It
was impressed on the subject that a report of "separate"
was to be given only if they were felt to be separate,
not if they"might" be separate.
It was expected that
this instruction would result in a larger measurement,
but that the result would be more useful in judging the
usefulness of a display.
The data c, llected have been organized to show the
percentage of responses "separate" which were elicited
for each number of grounded pins separating the two
-192and an average over the subject set using the texture
sensation is also given.
Since the measurement is relative-
ly coarse, due to the constraint of pin size in the
available electrode array, the results are given in
tabular form.
Each set of results represents 75
presentations to each subject with separation distances
ranging from zero to five pins.
The presentations were
made in a random order, with no attempt to constrain the
Response times averaged about 10 seconds
response time.
for all but NN who, still using a tingle sensation,
recuired an average of 20 seconds.
The results are presented in figures V-c-1, through
V-c-5.
Each figure consists of a drawing of the
nresentation, and a table of the results.
The results
are essentially the same for all the patterns.
For the
most part, a separation of three points will guarantee
distinctness.
In some cases,
roints separation.
NN reqiiired four or more
In some instances, particularly with
two-point and point-and-line figures, a value of two
noints separation is sufficient for most subjects.
Exoeriment c-II.
Localization of Points in a Grounded
Field of Electrodes
In order to determine how well a user might be able
to determine the position of a figure in the field of
-193-
1X=2
000000
000
-9f
F+-QIin
Separation X.
(in points & spacing)
Subject Responses (M)
RK
LB
0
0
1
0
1606
0
16.6
5.5
2
12.5
33.3
58
91.6
34.6
3
97
88.2
94
100
4
100
100
100
100
100
5
100
100
100
100
100
Necessary Separation:
JD
8.3
NN
0
3 points
Figure V-c-1
The Separation of Two Points
I
Texture Subi.
2.7
93.1
-194-
000000e00
000000000
0060000000
00600000
OOOOOOO
00900000
000800:00
0090006 00
000000*00
0090000O
X=3 ;
Separation X
(in point spacings)
Subiect Responses
RK
LB
JD
NN
(%)
Texture Subj.
0
0
0
0
7.6
0
1
0
0
0
0
0
2
16.6
68.6
58.3
42.4
3
91
100
94
100
95
4
100
100
100
100
100
5
100
100
100
100
100
Necessary Separation:
42
3 points
Figure V-c-2
The Seoaration of Two Lines
-195-
000000000
OOO OO OOO
00000000
OOOOOO
000000*00
000000000
000000
00000
00 000
000000000
oooooooo
X=
Separation x
(in poin t spacings)
Subject
___Responses _ (/) _
Texture Sb.
RK
0
0
33
0
0
91
98
100
100
3
100
100
100
100
100
4
100
100
100
100
100
5
100
100
100
100
100
0
17
1
83
2
0
Necessary Separation:
2 points
Figure V-c-3
The Separation of a Point From a Line
5.5
39
96.5
-196-
HX2A-
Seraration X
(in point spacings)
u!J2pct Resronses (%)
RK
LB
JD)
0
2
0
0
6
0.7
1
6
0
1
49
2.3
2
27
31
39
87
32.3
3
94
51
94
87
79.7
4
100
83
95
100
92.3
5
100
100
100
100
Necessary Separation:
Texture Sul '.
100
4 points
Firrre _V-c-4
The Enc-to-nd Senaration of Two Lines
-197-
09000 0
0..o000
**O*:*
00080.oO
X0000
0
:90009000*
0:00000909
9000009
000000000
rSeparation X
(in point spacings)
Subject Responses
RK
JTD
LB3
(o)
t:'Th
S
Texture _S
3.3
0
0
10
0
10
1
29
19
9
38
19
2
100
69
46
31
71.7
3
100
91
92
67
94.3
4
100
100
100
75
100
5
100
100
100
100
100
Necessary Separati.on:
3 points
irl
e 3V-c-5
The Senaration of Two Filled Areas
-198-
electrodes, subjects were presented with single points
in the right half of the array of electrodes, and asked
to specify the row and column in which they believed the
Since the area used was considerably
point to be located.
larger than the finger pad area, the subject was requited
to find the excited electrode as well as to precisely
determine its position.
The stimulus pulse was symmetrical, with a half-width
of 0.60 milliseconds and a pulse repetition rate of
200 pulses per second.
mine the amplitude.
The subject was allowed to deter-
Ten pins were selected at random
from the right half of the display area as presentation
points.
Fifty presentations were made to each subject,
and the response of row and column were recorded.
The task was described to the subjects using a point
not included in the test set.
The presentaton time was
not constrained, and response times averaged30 seconds
for subjects using the texture sensation and one minute
for NN.
Subject NN again recquired close visual examina-
tion to make any decisions, in fact she felt that she
could not perform the task without it.
The other subjects
used visual cues only after the point had been located.
LD preferred to work with his eyes closed, only opening
them to locate his finger as a whole.
-199-
The resulting data were analyzed to determine the
means and variances of the errors.
the basis of the data reduction,
tation points.
Figure V-c-6 shows
and the set of presen-
Table V-c-2 contains the important
results, the mean and variance of the errors in each
direction, and the mean and variance of the error distance.
For reference purposes the amplitude chosen by
the subjects is also recorded.
Results do not appear
for JD who was ill during the period .when this exneriment
was run.
Lacking the third subject using the texture sensation, no generalizations will be made about the results.
It is interesting to note that NN did extremely well,
in spite of the painstaking effort taken.
On the basis
of subject responses it is felt that the ability of those
subjects using the texture sensation will carry over to
a situation where vision is excluded, but the experimenter
is certain, and the results of the next experiment confirm, that the tingle sensation is not really sufficient,
even given sight as a guide, to handle complex patterns.
Experiment c-III.
Small Pattern Discriminability
The last of the experiments performed was done with
the view of getting a measure of how small a figure can
be made and still be recognizable for the figure it is.
-
200-
0000 OOoo®
0000 000®0
0oooo
0000
ooo Oo0oo
0000 0ooo
0000
0oooo
0000 ooo o
0®oo0®000
0000 00000
0000 00000
®
PRESENTATION
POINT
ELECTRODE CENTER
RESPONSE
ERROR DISTANCE
0
0
0
0
PRESENTATIO N
Y ERROR
e
0
x
ERRO R
F IGURE V-c-6
POINT LOCALIZATJON
PRESENTATION
-20'-
Variable
Subject
LB
RK
NN
(Mean
0.595
0.148
0.510
fVariance
0.794
0.254
0.420
IMean
(
0.319
-0.085
0.893
Variance
0.940
0.092
0.139
Mean
1.040
0.306
1.234
Variance
1.024
0.287
0.094
29.5v
51.6v
90v
d
Source Amplitude
(volts
(peak))
Table V-c-2
Point Localization Results
-202display includes effects
the
This characteristic of
relating not only to the resolution of the user's finger
tips, but also relating to the kinds of patterns being
used, and the relationships which exist between the
shape of the pattern and the characteristics grain of the
dot matrix which must be used to approximate the Cigure
for presentation.
mated on a three
Thus if the patterns are to be approxiby three matrix, a circle and a
rhombus must be indistinguishable, while a square may
be distinctly different.
The figures presented for identification were
circles, squares and triangles, or appropriate approximations thereto, each realized in sc-uare dot matrices
of six different sizes.
The figure patterns actuallyv
used are shown in figure V-c-7, along with the designations used for them.
A total of 150 presentations
were made to each subject, each presentation consisting
of one of the patterns shown in the figure,
and centered
on a fixed location in the display array at a known
orientation.
The source pulse used was again syrmmetrical with a
half width of 0.60 milliseconds and a repetition rate
of 200 pulses per second.
The subjects were permitted
to set the source amolitude.
In each presentation, all
of the electrodes used in the pattern were connected
-203CIRCLE
SIZE
00
0
0
4
000
0
5
0
0
0
00
0
0
000
0
00
0
0
0
0 0
TRIANGLE
0000
00
0000
0 0
0000
0
0
0
0
0
0
SQUARE
0
0
0
00000
0
0
0
0
0
0
0 0
00000
00000
000000
0
0
0
0
0
0
0
0
000000
00
0
0
000000
00
7
00 00
0
0
00
00
00 00
0
00
0
0
0g 00O 0
0
9
0
0
0
0
0
0
0
0
00 00
00 00
0
0
0000000
0
0
0
0
0
0
0
0
0
0
0000000
0
0
00
0
0
0
0
0
00
0
0
0
0
00000000
000000000
0
0
0
0
0000000
00000000
0
0
0
0
0
0
0
0
0
0
0
0
00000000
0
0
0
0
0
0
0
0
0
0
0
0
0
0
000000000
0
0
0 0
0 0
0
0
0
0
0
0 0
0 0
0
0
0
000000000
1.25 SCALE
FIGURE V-c-7
PATTERNS FOR
IDENTIFICATION TEST
-204to the same source, and the remaining electrodes were
grounded.
In order to be certain that the subjects understood
the task, and they had an opportunity to develop a
strategy for making the decisions prior to the beginning
of the experiment, they were first presented with examples
of the largest patterns to be used in the experiment and
asked to draw pictures of them, not knowing what they
were.
They were then shown visual versions of the patterns,
and were told what the process of approximation on the
grid pattern entailed.
When it was clear that they
understood what shjpes the patterns would have, they
were again presented with the largest patterns and allowed
to practice on them, primarily to develop strategies for
feeling the shapes.
When the subject reported that he
was confident that he understood the task, he was
instructed that he was to respond by naming the shape,
and was not to be concerned about the size.
He was
told that he would be given an unlimited time, but that
he was to work as fast as possible.
Most of the subjects took advantage of the symmetries
in the patterns to develop strategies which avoided
scanning the entire figure, usually concentrating on
the corners.
Response times, for those subjects using
the texture sensations ranged from 20 seconds to 30
-205Subject NN, in those few trials attemPted,
seconds.
recuired times in excess of five minutes.
The reason
for this is the necessity to make a painstakinq point
by point man of the critical portions of the disrlav.
That is, she had to feel each point individually, using
vision to guarantee that she was feelinq one and only
one point.
Due to the fact thatshe absolutely recuired
vision and that such a long period of time was necessary,
it was felt that for her at least, add probably for all
users of the tingle sensations, the display would be of
minimal use for patterns of this type, and that the test
recruired more effort than it was worth for comoarison
purposes.
The experiment was terminated, and no results
appear for her below.
The responses for each subject have been reorganized
into the form of confusion matrices.
For each subject
and for each size of figure there appears a three by
three matrix, the rows of which correspond to the figure
actually presented, and the columns of which correspond
to the subject's response.
Thus each entry in the matrix
is the number of times that the subject identified the
presentation of that row with the figure labeling that
column.
At the right hand edge is the percentage of
correct identifications of each figure presented.
The
results for the three subjects appear as figures V-c-8
-206-
through V-c-10 , and the sum of the three appears as f igure
V-c-11.
DATA ANALYSIS:
The responses show several cases of
repeated error, though in each case it seems to be confined to one subject and to one experimental session.
It is believed that these errors are due primarily to
the peculiarities of the methods of examination used by
the subjects, and the the errors vary because the method
varies.
The particular errors of interest are a general
confusion of squares with circles and the converse, which
might have been expected on the basis of the patterns
presented, and the unexplained failure of RK during one
session to make appropriate distinctions between triangles
and circles when triangles were presented, and to recognize circles of size 7 at all.
It is interesting also
that the circle-square confusions cause most of the difficulty in the larger patterns.
Observation of the subject's hand motions indicate
that the primar7 cue in most cases was the existence or
absence of corners on the upper and lower right of the
figures, all of the subjects being right handed.
This
could explain circle-s-fuare confusions if the subjects
did not first determine the size of the figure, since
large circles may have false corners as can be seen in
the patterns.
-207Presentation
Responses
Percent
Correct
Size .'Figure
4A
0
5
6
7
8
9
5
3
0
62.5
0
6
2
75
3
0
5
62.5
4
3
50
3
37.5
6
75
60
1
0
A
l
0
A
LI
0
4
0
4
37.5
75
57.2
80
12.5
7
0
87.5
120i
0
80
4
50
0
0
100
80
Pattern Discrimination: Subject RK
Presentation
Size
Percent
Correct
Fiqure
A
4
-208Responses
0
0
A
0
A
0
0
A
0
2
2
4
4
3
0
1
8
4
50
67
50
2
37.5
62.5
6
75
5
25
1
3
80
37.5
0
75
5
50
5
62.5
1
1
7
87.5
1
87.5
3
85.8
70
0
100
8
80
Firnre-V-c-9
Pattern Discrimination:
SL
ct T,7
-209Percent
Presentation
Size
Resnonses
Correct
Figwfe
62.5
0
1
87.5
1
12.5
3
12.5
5
25
0'
5
62.5
A
1
75
0
7
90
5
0
0
A
0
A
0
A
n
0
A
01
0
A
0-
0
3
7
7
87.5
2
3
70
5
62.5
1
87.5
75
0
5
100
0
0
100
8
80
50
100
Fi ure V-c-10
Pattern Discrimination: Subject JD
-210Percent
Presentation
S ize
4
5
Correct
Firgre
A
12
8
4
50
l
4
13
3
65
6
8
10
41.6
A
8
6
10
33.3
F
5
10
9
5
5
14
41.6
58.3
14
3
7
r
3
23
4
0
1
7
16
67
16
0
3
67
20
5
10
0
7
6
11
67
45.8
A
LI
21
0
3
87.5
1
95.4
0
0
A
6
Responses
0
20
0
0
10
19
1
4
0
0
21
0
6
24
20
58.3
70.6
67
79.1
100
80
PiguD
rit V-c-T
Pattern Discrimination: Total Reso~onse
-211-
The data would seem to indicate that a pattern
size of seven electrodes by seven electrodes is sufficient
in this display to present patterns of the sort used
here.
However, an examination of the patterns used
indicated that this result may be more a measure of the
capabilities of this electrode array than of the subject
himself.
It should be noted that the size six patterns
are the smallest which can be said to have a "good" form
even visually.
The smaller patterns tend to become ob-
scured by the pin matrix properties.
The experiments of this section are intended to provide the basis for a point-and-line display.
Three basic
results are provideo, one for a minimum figure size,
one for a minimum separation of figures, and one for
the localizability of an object.
These results and their
implications will be discussed in chapter VI.
-212CHAPTER VI
A Summary and Some Conclusions
The basis for an explorable electrotactile display
has been demonstrated.
It was found during an examina-
tion of the sensations elicited by electric currents
applied through an explorable electrode array that a
hitherto neglected form of electrotactile.stimulus was
commonly used by most subjects who were not instructed
to do otherwise.
The sensation elicited is one of texture.
The
cualities of the sensation have been examined, and a
model for the production mechanism has been proposed.
The proposed mechanism is based on m electrostatic
force applied to the skin by a low current, relatively
high voltage stim'ulus pulse.
All of the experimental
findings to date appear to be consistent with this
mechanism, and it has been demonstrated that the force
generated by the mechanism is of the correct order of
magnitude to cause perceptions of touch if an analogy
is made to the normal methods of causing tactile sensations by mechanical means.
-213A large number of short experiments have been run,
designed to explore three areas:
the sensations and
their properties, the characteristics of the phenomena
governing the use of these sensations in a display based
on the presentation of textured areas and the characteristics relating to their use in a point-and-line display.
During these experiments, a comparison was acquired
between the classical electrotactile effects and the
texture effects by the use of one subject who did not
prefer the texture effect, in addition to three who did
use the texture effect, in the majority of the experiments.
The experiments relating to the sensations themselves provide preliminary measurements of the scales
for amplitude and frequency, and provide some results
on the effects of the available stimulus variables on
the sensations.
In particular, linear and square law
scales were developed for stimulus amplitude, and it is
conjectured that the linear scale is related to the texture sensation, and the square law scale to the classical
sensations, though the recorded data do not force this
conclusion.
A linear scale is indicated for frequency
in the case of the texture sensation, while the dlassical electrotactile sensations do not apparently allow
differences in frequency to be distinguished in the
region of interest.
-214The shape of the stimulus pulse
was shown to have little effect on the perceptions
elicited, though a subject preference was demonstrated
for a wide symmetrical pulse of a bipolar rectangular
The effects of all stimulus variables, including
form.
amplitude and frequency, on the apparent spatial extent
of a stimulus, and on other variables relating to spatial resolution was shown to be small though the effects
of amplitude and frequency on texture quality were indeed
large.
The stimulus requirements in either case are not
very stringent in general, though a failure of the texture sensation apparently related to a reduced skin
resistance poses a problem which must be overcome.
The
source characteristics leading to the two sensations
are similar, in that approximately the same pulse shape
and repetition rate are required for both.
The neces-
sity for high skin resistances in the texture mode is
ordinarily overcome simply by drying out the fingers on
the display surface and keeping the display surface
clean.
There is however an instance in which this is
not a satisfactory solution, and in that case,
the
reason for the low resistance may be physiological rather
than environmental.
-21.5The parameters peculiar to a textured area type
display were examined in terms of the just-noticablem
differences for amplitude and frequency, and the ability
of the subject to localize the boundary of a region,
both with the adjacent area excited, and with it unexcited.
The results show JNDs of 5 to 10% for ampli-
tude differences and 30 to 40% for frequency differences
when the subject uses the texture effect.
The variance
of the localization error when searching for a boundary
between two areas is on the order of three electrode
spacings, that is 0.3 inches for areas differing in only
one variable, and by two JNDs, and less than one electrode spacing, 0.1 inch, when the adjacent area is
unexcited.
The exoeriments relating to the use of the stimuli
in a point-and-line display examined the spacing required
between figures, the minimum size a figure could have
and still be recognizable, and the ability of the subject to localize isolated points.
The figure separation
experiments show that a separation of more than 0.2
inches or more should be allowed between any two figures.
The pattern size experiment shows that with the given
array of electrodes, the minimum size for very simple
patterns is on the order of 0.7 by 0.7 inches.
point will be discussed further.
This
The error variance
-216encountered in the point localization was on the order
of 0.1 inches or less.
There are yet several points to be discussed.
The
first, and the most important in terms of the experiments
presented herelis the question of the response times
in these experiments.
Inmany instances they seem at
first glance to be quite long.
It is believed that the
primary reason for their length is the need to "restart"
the finger for each presentation, and even more than
once within a presentation when two areas are to be
compared.
By restart is meant the process of reacauiring
the sensation after loss of contact with the surface, and
it involves among other things adjusting the finger
pressure to an optimum.
It is felt that this process
hampers experiments with short presentations much more than
it will hamper the process of actual display use.
In the case of the small dot pattern figures used
in section V-c, it is believed that the evils of the
fixed-array-of-dots type presentation mentioned in
chapter I have returned.
This experimenter is inclined
to believe that the primary difficulty is with the
patterns themselves, and not with the user's resolution,
though it is evident that the users were unable to distinguish between two adjacent electrodes in the array
used.
-21 7-
It appears that in this case, as in many others, the
real constraint is imposed by technological limits, and
not by physiological limits.
It would be hoped that in
the future we might be able to devise a sufficiently
clever presentation so that the complexity caused by
the technology does not overshadow the complexity of
the figure presented itself.
This difficulty seems to
have plagued almost all of the work with tactile stimuli
so far.
The postulate is that we can avoid this problem
by increasing the resolution of the display sufficiently,
but as was shown there are adverse physiological consecuences to this course which must be considered.
It
would appear, however, that some research with smaller
electrodes with appropriate subject protection is in
order.
The primary differences between the two sensations
seem to be limited to threshold differences, the annoying
qualities of the tingle sensations, the inability to make
frequency distingtions on the basis of the tingle
sensations and the necessity for a spatial reference
not associated with the skin when using the tinglesensations.
While additional differences may exist in
the data presented, they appear to be small.
Since
however the tingle sensations result in the upper limit
of amplitude for texture users, and since the low resis-
-218tance failure mode does recur often enough to be annoying, it is suggested that serious thought be given to
methods of eliminating the tingle effects altogether
through the use of a very thin insulating surface
coating the electrodes.
Consideration might also be
given to gaining a favorable coefficient of friction in
this manner.
The last question to be addressed is the relationship between these results and what is known about
tactile picture presentations of other sorts.
Actually
we are interested in two distinctly different aspects.
First, we are interested in the kinds of tactual discriminations which can be made in ordinary tactual
experience.
Very little work has been done in the area,
and what work has been done is not easily related to
either the work presented here or to other work in the
field.
The real difficulty seems to be that we do not
as yet have a consistent way of describing the complexity
of a picture, and correspondingly we have no good way'
to measure subject performance at so complex a task as
form discrimination unless the forms are all simple and
all of a very narrow class.
A second difficulty arises in that the interest of
those doing all of the work to date has been in the
training of blind children.
Such people are quite
-219naturally more interested in the changes which occur in
a child's ability to make discriminations as he grows
older than in making absolute measurements of discriminan
bility itself.
These people also tend to be interested
in the relationships between the perceptions of the
blind and of the sighted.
Thus the majority of the
results cannot be compared with one another.
In line with certain interests, particularly the
construction of maps for the blind, some useful information has been produced in recent years.
Since the
interpretation of these recent results in light of
the experimental work presented in this thesis cannot
be done without raising unanswerable questions, the
references are offered, and the interested reader is
asked to draw his own conclusions.
Aside from the
qualitative work by J. J. Gibson cited earlier, the
most useful work in this area has been by Nolan (1964).
Recent work on texture descrimination has been done
well by Culbert and Stellwagen (1963), by Gliner (1966)
and by Heath (1958).
Gliner (same work) also presents
the results of a form discrimination experiment which
avoids the issue of form complexity by limiting the set
to elipses.
Foulke and Warm (1966) address the question
of complexity directly, but with a rather contrived
set of figures, resulting in an interaction between
figure size and complexity for which they have not
accounted.
-220Finally, the most universally applicable
study seems to be a measurement of the JND for length
performed by Duran and Tufenkjian (1967).
The other area of interest is the direct comparison
of this display with embossed paper displays.
In spite
of the fact that such paper displays have existed for
many years, little in the way of objective experimentation has been done.
Many schemes have been tried,- but
few have been studied extensively, none has received wide
acceptance, and almost nothing has been published.
The American Foundation for the Blind is just now attempting to develop standards for embossed paper displays.
The net result is that we have essentially nothing
with which to compare the results obtained here.
Appar-
ently we must wait for that time when blind users can
be given extensive experience with such a device, so
that they who have had experience with both forms can
give a subjective evaluation, and perhaps tell us how
to go about evaluating tactile displays of the explorable
kind.
In the meantime, the evidence presented in chapter V
leaves room for hppe that explorable electrotactile
displays will be useful both in the textured area
form and the line drawing form.
Sufficient resolution
has been shown in either case to warrent further study,
-221and to make the system directly useful for certain classes of two-dimensional presentation.
In addition, the
very existence of the texture effect provides a new
line of investigation for those interested in perceptual psychology.
It is hoped that this will lead to more
detailed and objective work, much needed by the blind
population, on the abilities of the human being exploring
his environment by touch and on explorable displays.
-222APPENDIX
A more detailed description is given here of the
two types of sources developed for use in these experiments.
Each of the schemes is based on the use of a
transformer for impedance conversion.
The first system
provides many electrically isolated sources of independently adjustable amplitude and output impedance, but
identical timing parameters.
It also allows nearly
arbitrary waveforms to be applied.
The second system
is designed to produce rectangular pulses of bipolar
form only.
low cost.
It was desiqned primarily for simplicity and
It provides a means of constructing many
iddependent sources with different timing parameters,
which can be made easily adjustable by the system, but
with either fixed or similar amplitudes.
A-1.
An Amplifier Based System
The basic structure of this system is shown in
figure A-1.
The principle used is that a high power
audio amplifier can be used to generate relatively
large amplitude signals, which are then applied to a
large number of output networks each consisting of an
artificial system output impedance and a transformer.
older vacuum tube amplifiers have an advantage in this
POWER
AMPLIFIER
CLOCK
PUL SE
INPU T
FPUNGE
NET WORK
LOAD
NETWORKS
T
0
---
D
I
2
--
-
T
R
S
U
3
15
_ .y
0
N
N
E
T
w
0
R
K
FIGURE A-I
THE AMPLIFIER BASED SYSTEM
I
N
04
-224use since they tend to have the characteristics of a
voltage source over quite a wide range of load impedances,
particularly at less than the rated load.
The system
described was designed around such an amplifier, although
a current mode equivalent could certainly be designed to
use a modern transistor amplifier.
The system used in these experiments consisted of
a 50-watt amplifier built by Macintosh Laboratories
using two type 6L6 tubes in its output stage, and fifteen
of the load networks shown in figure A-2.
The amplifier
possessed a 600 ohm putput, as many of this type do,
and so byusing a load network consisting of a fifteen
kilohm variable resistor, normally set at 7000 ohms,
and a so-called "universal" output traisformer, an
output impedance of 200 kilohms was obtained with a
system which would generate source pulses in excess of
200 volts peak value.
The transformer operated between
nominal impedances of 600 ohms at the amplifier side and
20 kilohms at the electrode side.
The transformer system has the disadvantage that
it is relatively intoleraftt of capacitive loads, such
as long cables.
The pulse shape distortions which result
can be corrected by either preemphasizing the amplifier
drive signal to account for the loss in rise time, or
by using some of the extra power in a resistive preload.
FROM
AMPLIFIER
SOURCE
IMPEDANCE
CONTROL
7 kfl NOMINAL
1\)
>*TO
TRANSFORMER
(TA PPED)
600n
2Ok x't-
FIGURE A-2
A LOAD NETWORK
U,
DISTRIBUTION NETWORK
In this method, the entire control over the source
pulse is exercised at the amplifier input.
With proper
correction for the capacitive portion of theload, any
source pulse can be used whose shape can be produced
at the low voltage amplifier input.
A-2.
A Switch Based System
If a bipolar rectangular pulse is sufficient to
cause the- desired sensation, as it seems to be, then
a system based not on an amplifier, but rather on a
high voltage switch is sufficient.
source is shown in figure A-3.
The form of this
The pulse produced by
this scheme is not quite as "clean" as that of the
amplifier system in that correction for load characteristics is possible only by changing the load, and in
that the current reversal transient in the transformer
can significantly reduce the rise time of the second
half-pulse.
The operation of this system is straightforward.
Each of the transistors acts as asqitch.
When both are
off, that is the switches are open, both ends of the
transformer primary are at supply voltage and no current
flows in it.
If either switching transistor is saturated
when the other is not, current will flow through the
transformer toward the saturated transistor.
Again, if
both are saturated, there is no current in the transfor-
TO D.C. SUPPLY VOLTAGE
tVs
5kn.
5k .n.
600--
Cl 20k -a
SIGNAL B, 0
-T,
SIGNAL B,
T
~~
~+1,
T,,ARE 2N3441
TO DISTR[BUTION NEWORK
A
F IGURE A-3
SWITCHING SOURCE
N
N
-228mer.
Thus if we wish to generate a bipolar pulse, we
saturate first one then the other transistor, the order
determining the polarity of the pulse.
The signals
applied to the transistor bases and the resulting waveforms are shown in figure A-4.
The transistors used were 2N3441's, an inexpensive
high voltage type.
If the full voltage range is to be
used at the current levels used, it is necessary to
operate them in the switched mode, though at lower
voltages or currents an amplifier mode might be possible.
These transistors have demonstrated remarkable ability to
withstand over-voltage operation and thermal overloads.
The transformer used in this system is the output transformer used in the load network of the amplifier based
system.
The source pulse parameters are controlled in a
slightly different manner with this system.
The timing
parameters are contained in the repetition rate and pulse
widths of the two base signals.
Control of the stimulus
amplitude is exercised by controlling the power supply
voltage Vs.
Thus while this system provides better
control over the timing parameters, the amplitude becomes more difficult to adjust.
T
SiGNAL
T
1
T,
.........
................
1
o.,.
U
U
N)
Q0
I
s
C.
+'
OUTPUT
I
0
U
-VFIGURE A-4
WAVEFORMS
-, ,I
11:11;
--
-
No
.
j
-230-
BIBLIOGRAPHY
-231Experimental Determination of Some
Alles, David S,
Properties of the Skin.
Doctoral Dissertation,
Department of Mechanical Engineering, Massachusetts
Institute of Technology, 1968.
Bliss, James C. and Hewitt D. Crane, 9 periments in
Tactual Perception. National Aeronautics and
Space Administration, Contractor Report NASA
No. CR-322, November, 1965.
Culbert, Sidney S. and William T. Stellwagen, "Tactual
Perceptual and Motor
Discrimination of Textures".
Skills, Vol. 16, 1963, pp. 545-552.
Duran, P. and S. Tafenkjian, The Measurement of Length
Blind Children and a uasi-formal
by Connitally
Unpublished Research
Aqoroach for Snatial Concents.
Report, Sensory Aids Evaluation and Development
Center,
Massachusetts
Cambridge,
Foulke,
E.
Institte
Massachusetts, 1967.
and J. Warm,
of Technology,
"The Effects of Pattern Complexity
and Redundancy on Factual Recognition of Metric
Figures".
In Sensory2Aids Evaluation and Development
Center, Proceedinrs of the Braille Research and
Development Conference, Cambridge, Massachusetts,
Massachusetts Institute of Technology, 1966, pp. 29-39.
Franke, Ernst K.,
"Mechanical Impedance of the Surface of
the Human Body".
Journal of Aonlied Physiology,
Vol. 3, 1950-51, pp. 582-590.
-232Geldard, F. A., "Adventures in Tactile Literacy".
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Geldard, F. A., "Some Neglected Possibilities of
Communication". Science, Vol. 131, No. 3413,
1960, pp. 1583-1588.
Geldard, F. A., "The Language of the Human Skin".
Proceedings _of the XIV International Congress of
Applied Psycholoqgy, Vol. 5, 1962, pp. 26-39.
Geldard, F. A. and C. E. Sherrick, "Multiple Cutaneous
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Patterns". The Journal of the Acoustical Society
of America, Vol. 37, 1965, pp. 797-801.
Gibson, James J., "Observations on Active Touch".
Psychological Review, Vol. 69, No. 6, November,
1962, pp. 477-491.
Gibson, James J., The Senses Considered as Perceptual
Boston, Houghton-Mifflin, 1966.
Systems.
Gibson, Robert H., "Requirements for the Use of Electrical
Stimulation of the Skin". Proceedings of the
International Congress on Technology and Blindness,
Vol. II, 1963, pp. 183-207.
Gibson, Robert H., Electrical Stimulation of the Skin
Senses. Progress Report No. 4, Department of
Psychology, University of Pittsburgh, February 1, 1967.
Gibson, Robert H., Private Communication, April, 1967.
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Alternating Electric Currents". American Journal
of Psychology Vol. 49, 1937, pp. 444-449.
Gliner, C. R., A Psychophysical Study of Tactual Perception. Doctoral Dissertation, University of
Minnesota, 1966.
Hahn, J. F., ''Cutaneous Vibratory Thresholds for SquareWave Electrical Pulses". Science, Vol. 127, 1958,
pp. 879-880.
Hawkes, G. R.,
"Cutaneous Discrimination of Electrical
Intensity". American Journal of Psychology,
Vol. 24, 1961, pp. 45-53.
Heath, W. R., Maps and Graphics for the Blind:, Some
Aspects of the Discriminability of Textural Surfaces for Use in Areal Differentiation. Doctoral
Dissertation, University of Washington, 1958.
Mallinckrodt, Edward, A. L. Hughes, and William Sleater,
Jr., "Perception by the Skin of Electrically Induced
Vibrations".
Science, Vol. 18, September 4, 1953,
pp. 277-278.
Miller, George A., "The Magical Number Seven, Plus or
Minus Two: Some Limits on our Capacity for
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Vol. 63, No. 2, March, 1956, pp. 81-97.
-234of Patterns:
Nolan, C. Y., Cues in the Tactual
Progre s Report. Louisville, Kentucky, American
Printing House for the Blind, 1964. (WIH Grant
Number NB 03129-04)
Peterson, David L., An Eight-Finrer Tactile Display.
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BIOGRAPHICAL HOTE
Robert Michael Strong was born in Pittsburgh, Penns-lvania, in 1943.
He received his bachelor's degree
cum laude in electrical enrineering from Villanova University in June, 1965.
He entered the Massachusetts Institute
of TechnolorTw in the fall
of 1965 with the support of
The Fanni.e and John Hertz Foundation, and received the
degree of Master of Science in September, 1966.
While
at M.I.T., he assisted with Course 6.05, Circuits, Signals,
and Sustems.
He is a member of Tau Beta Pi Association
and Eta Kappa Nu Association, and is an associate member
of The Society of the Sigma Xi.