The basis of perceptual constancy and
perceptual illusion
R. H. Day
A wide range of perceptual illusions, including many of size, orientation, and movement,
can be explained in terms of the mechanisms which normally initiate and maintain perceptual
constancy. Perceptual constancy is the relative stability of the apparent value of object properties (size, shape, orientation, movement, etc.) when the representation at the eye (retinal
image) is variant with change in observer position, posture, and movement. These constancies
are consequent on stimuli for object distance and observer posture and motion. When the
retinal image is invariant and these stimuli are manipulated, perceptual illusions occur. That
is, the mechanisms which normally preserve constancy can be invoked to cause illusions.
able criticism, mainly on the grounds that
they cannot cope with numerous specific
geometrical illusions.
Despite the inadequacies of earlier attempts, a general explanation of visual
spatial illusions will be proposed here in
terms of visual constancy, the tendency
for apparent values of object properties
such as their size, shape, and orientation
to remain relatively stable despite considerable variation in their retinal projection, as the individual changes his position, bearing, and posture relative to the
object. It will be suggested that the mechanisms which under normal conditions
serve to initiate and maintain visual constancy of object size, orientation, and movement, to name three classes, under specified
conditions also initiate visual illusions of
these properties. Thus the problem of illusion reduces essentially to the problem
of perceptual constancy. Indeed it can
be argued that consistent failure to explain
illusory phenomena either in psychophysiologic or neurophysiologic terms is due in
large part to a failure to recognize their
basis in perceptual constancy. The main
justification for another attempt to explain
perceptual illusion in terms of perceptual
constancy is that previous attempts have
hile the problem of perceptual illusion has not aroused quite the same degree of empirical or theoretical interest
among neurophysiologists as among experimental psychologists there has nevertheless
been a continuing concern with the neural
correlates of illusory phenomena. Both
Motokawa1 and Burns and Pritchard2 have
recently attempted to establish the neural
interactions associated with these distortions. However, the problem has proved
extraordinarily intractable and the neural
structures and processes associated with
illusory effects remain quite obscure. Furthermore, it cannot be said that attempts
to explain perceptual illusions at the behavioral level have so far met with much
greater success, although efforts to do so
have been more frequent and sustained
and the body of data far more substantial.
Recent explanations in terms of perceptual
constancy by Gregory,3"5 Kristof,G and
Tausch7 have been subjected to consider-
From the Department of Psychology, Monash
University, Clayton, Victoria 3168, Australia.
Preparation of this paper was undertaken while
the author was on sabbatical leave at the University of Exeter, United Kingdom.
525
Investigative Ophthalmology
June 1972
526 Day
10
30
«O
SO
73"
IOO
10 cm
100 cm.
s
3
10
25 cm.
IS 1*
//
21
SO
IOO
too
+OO cm.
s
too
«oo
Equivalent Distance (cm)
Fig. 1. Convergence and accommodation of the
eyes gives rise to stimuli for distance which
normally contribute to size constancy when the
object image is variant with distance. Size illusions occur when convergence and accommodation are varied with object distance and object
image invariant. For a fixed distance the degree
of convergence-accommodation was optically adjusted to equivalence with that for greater and
less distances (equivalent distance). For actual
distances of 10, 25, 50, and 100 cm., apparent
size corresponded with expectancies based on convergence-accommodation (diagonal lines). The
horizontal lines represent expectancies for constant apparent size. (From Leibowitz, H., and
Moore, D.: J. Opt. Soc. Am. 56: 1120, 1966.)
been restricted to a small group of geometric illusions and have not taken into
account illusions (and constancy) of orientation and motion involving body tilt and
motion stimuli.
It will be useful to make four introductory points. First, although visual illusions, which are defined as consistent
discrepancies between the apparent and
physical value of an object property, are
usually associated with geometric diagrams,
these represent only a small proportion of
illusory phenomena. Second, there are numerous classes of illusion, each deriving
from a particular class of perceptual constancy; the geometric illusions are drawn
from more than one of these classes. Third,
although there are different classes of visual
constancy (and derived illusions) the general principles in terms of which they are
explained are essentially the same for all
classes. Finally, in recent years there has
been dramatic success in isolating cells
specifically tuned to single object features
such as movement, orientation, contour,
and color. It can be argued (and this is
the point of the paper) that the next level
of complexity are analyzers which respond
to the constancy of object size, shape,
orientation of movement despite retinal
image changes. These analyzers may well
be those responsible for illusions when
they are invoked under conditions which
will be generally described below.
The visual constancies
As the organism changes its position relative to environmental objects either by
locomotion or postural change the projection of the object at the eye, i.e., the retinal
image, undergoes change. Such changes
occur with variation in the observer's distance, orientation, angular bearing, and
movement relative to the object. However,
even though the image changes at the eye
are often large as, for example, when the
observer moves to a great distance or
changes posture through 90 degrees, the
apparent values of object properties remain remarkably stable. Apparent size with
distance variant is relatively very stable,
as is apparent shape with change in bearing, apparent orientation with change in
lateral body tilt, and apparent stationariness with movement of the head and
eyes. This relative stability of apparent
size, shape, orientation, and movement (including in the limit stationariness) are
instances of visual constancy. The biological significance of these constancies is obvious; the observer sees object properties
in terms of their physical characteristics
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Perceptual constancy and illusion 527
Fig. 2. Examples of size illusions due to the manipulation of projected stimuli for distance
such as perspective, texture, and elevation.
rather than of their varying representation
at the eye. Such a perceptual achievement
has obvious biological utility.
In order to show how illusory phenomena derive from the mechanisms controlling perceptual constancy it is convient to consider first the case of size constancy if for the only reason that it is
the best known and most intensively
studied.
Visual size constancy and illusion
As the observer recedes from an object
(or vice versa) the retinal image of the
latter diminishes but its apparent size is
relatively constant. Therefore information
for distance must be involved in the maintenance of constant apparent size. That
this is so was made clear in the classic
experiments of Holway and Borings in
which distance stimuli were progressively
reduced. Since then data from three separate experiments9"11 have shown that when
distance stimuli are entirely eliminated an
object's apparent size decreases as a linear
function of observer-object-distance, i.e.,
apparent size follows the "law of the visual
angle." "Cues" or stimuli for distance fall
into five classes: (1) retinal disparity (or
binocular parallax; (2) muscular stimuli
(convergence and accommodation); (3)
monocular parallax; (4) atmospheric stimuli (aerial perspective and the Tyndall ef-
fect); and (5) projected stimuli (perspective, texture, overlay, elevation in
field, element and interspace size, and element and interspace frequency. Normally
all or most of these stimuli for distance
are present and visual size constancy is
perfect. However, although size constancy
falls off as distance stimuli are systematically reduced (e.g., when binocular parallax is eliminated by using one
eye and monocular parallax by holding the
head stationary8) some degree of constancy obtains, i.e., apparent size does not
follow the law of the visual angle, as observer-object distance increases. The principle of visual size constancy can be simply stated: With retinal image size variant
consequent upon change in observer-objectdistance apparent size is relatively invariant
when distance stimuli are present. Even
in a picture some degree of size constancy
obtains when perspective, texture, and elevation stimuli for distance are present.12
It follows from this principle that when
the retinal image is invariant and the stimuli for distance are manipulated, apparent
size will also vary. That is, when the distance stimuli which normally preserve size
constancy are varied with the image fixed
an illusion of object size must occur. Three
examples will serve to make this point
clear. First, Gogel, Wist, and Harker13
systematically varied retinal disparity with
528
Investigative Ophthalmology
June 1972
Day
1-0 G gravitational force
I -414 G resultant force
\
gravitoinertial
vertical
10 G
centrifugal
force
'
L
h
centrifugal centre post
t
\
10 G gravitational (contact) force
1-414 G resultant
(contact) force
Fig. 3. The human centrifuge. Vestibular stimuli for observer orientation relative to the
gravitational axis and for observer motion can be manipulated by centrifugation at constant
angular velocity and by acceleration, respectively.
image size fixed. This was achieved by
effectively varying the interpupillary distance with object distance (152.4 cm.) invariant. For the three interpupillary bases
of 12.4 (double), 6.5 (normal), and 3.2
(half), the apparent size of a 30.5 cm.
object was 28.5, 31.9, and 40.0 cm., respectively. Second, Leibowitz and Moore14
varied convergence-accommodation taking
care to preserve their normal relationship
and obtained matches to a standard object at observation distances of 10, 25, 50,
100, or 400 cm. At each of these distances
accommodation was adjusted to 0.25, 0.50,
1.00, 2.00, and 3.00 diopters with convergence in normal correspondence. Apparent size varied with convergence-accommodation following a linear function
up to about 1 m. as shown in Fig. 1.
Third, if the retinal image of an object
is invariant and projected stimuli such as
those of perspective, texture, and elevation
are varied, the apparent size of the object
undergoes change. This change in apparent size is the well known Ponzo illusion of which some examples are shown
in Fig. 2. The point to be made is that
when the retinal image varies with distance
apparent size is relatively constant when
some or all classes of distance stimuli
are present. This is size constancy. When,
however, the image does not vary, but
distance stimuli do, changes occur in apparent size. These are size illusions. That
is, the principle governing size constancy
can be invoked to cause a size illusion.
Geometric size illusions
The projected stimuli for distance, stimuli which derive essentially from the projection of a three-dimensional space on to
the two-dimensional retinal surface, are
more frequent and subtle than is usually
realized. In addition to the well-known
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stimuli such as perspective, texture, overlay, and elevation, the size and frequency
of elements other than the focal object
and the size and frequency of interspaces
between them vary with distance and provide information for distance. The trees
on the near side of a lake are bigger and
fewer per unit visual angle than the smaller
and more frequent trees on the far side,
as indeed are the interspaces between
them. Many geometric optical illusions involve such distance stimuli.15 Of two objects one is usually located in the context
of larger and less frequent elements or
spaces consonant with nearness and the
other in the context of smaller and more
frequent elements corresponding to greater
distance. The former object is judged smaller than the latter. The Oppel-Kundt, Delboeuf, and Miiller-Lyer illusions are examples of size illusions in which the retinal
image of an object, usually a line or simple figure, is invariant but projected stimuli for distance are varied. It should be
noted, however, that as had been pointed
out elsewhere15 the Miiller-Lyer illusion as
it is classically sbown represents two separate effects. The "short" version with "inboard" elements is probably a different illusion than the "long" version with "outboard" elements. Evidence for this difference has been adduced by Erlebacher and
Sekuler.10
Visual orientation constancy and illusion
When the observer's head is tilted laterally as posture is changed, the retinal
orientation of the object's image relative
to the normally vertical meridian of the
eye changes. When the observer is recumbent this change is nearly 90 degrees.
However, under conditions of normal illumination and when only the object itself is visible in a dark room, its apparent
tilt is relatively stable, a phenomena called
visual orientation constancy.17 In normal
illumination the bar remains perceptually
invariant even for large lateral body tilts.18
In darkness its apparent slant varies in
Perceptual constancy and illusion 529
direction and magnitude as a function of
tilt but is always less than retinal tilt.
The complete function is referred to as
the Aubert-Miiller effect.17
Since apparent orientation is relatively
stable with image orientation variant, it
follows that information for body tilt must
be involved in this perceptual outcome.
That is, information for body tilt must
be combined with that for retinal image
orientation to preserve constancy. There
are at least three classes of stimuli for
body orientation: vestibular involving the
utricular statocysts, visual involving the
contours of the visual field, and kinesthetic.17 It is likely that interoceptive stimuli are also involved. It follows that if
the observer is upright and the orientation
of the retinal image is invariant while the
stimuli for body orientation are manipulated either singly or together, an illusion
or orientation must occur, e.g., an upright
object will appear tilted. The vestibular
stimulus for body tilt, i.e., gravity, can be
independently manipulated in direction by
rotating the observer at constant angular
velocity in the darkened cabin of a human
centrifuge as shown in Fig. 3. The resultant gravitoinertial force acting on the
statocysts modifies information for body
tilt so that a gravitationally vertical bar
in an otherwise dark cabin is apparently
tilted. This is the oculogravic illusion noted
by Mach19 and investigated in detail by
Graybiel.20'21 The normally near-vertical
and near-horizontal contours of the natural
and man-made environment which also
serve as stimuli controlling lateral body
tilt can be changed simply by tilting them
while the observer himself remains upright. Even though the vestibular (gravitational) stimulus is present a considerable illusion of object orientation occurs.22' 23 The "tilting room" effect, the rodframe effect, and the tilted line illusion are
visual orientation illusions which include
many of the Zollner group. In general then,
as is the case with size when distance
stimuli are varied, apparent orientation,
530
Day
which is normally constant as the body
tilts, varies when the image is invariant
and body orientation stimuli are manipulated. If kinesthetic and interoceptive
stimuli could be similarly varied visual
orientation illusions would also be expected to occur.
Visual movement constancy and illusion
When the image of a stationary object
sweeps across the retina as the body and
head move, the object remains apparently
stationary. This constancy of position (and
movement, if the object is moving) occurs
even when the object alone is visible in
an otherwise dark space.24 In a manner
essentially similar to visual orientation constancy, visual movement and position constancy must depend on information for
body motion and stationariness. This information is carried in large part by vestibular and visual stimuli such that when
the image of a stationary object moves
it remains perceptually stationary. If, however, the retinal representation of a stationary object is invariant and body movement stimuli changed, illusions of movement occur. The oculogyral illusion occurs when vestibular stimuli are manipulated by accelerating the observer in a
human centrifuge thus activating semicircular canal receptors-5'2G (Fig. 3) and
induced movement occurs when the usually
stationary visual field itself is made to
move.27'2S
Mechanisms of visual constancy
and illusion
Visual constancy, a biologically important
feature of space perception, refers to the
relative stability of the apparent value of
an object property as its retinal representation changes in various ways. Visual illusion, for long a curiosity of the psychological laboratory, refers to a consistent
discrepancy between the apparent and
physical values of an object property. The
second derives from the first for a wide
range of conditions and separate classes
Investigative Ophthalmology
June 1972
of constancy; if the stimuli which preserve constancy when the image is variant
are independently manipulated when it is
invariant, an illusion occurs. Such is the
case with visual size, orientation, and
movement. A central question is: Are there
neural mechanisms which serve to maintain perceptual constancy and which when
invoked under "inappropriate" conditions
give rise to illusory effects?
There is already a small body of data
which suggests that such mechanisms do
exist. These structures continue to respond
in a stable manner even though the stimulus at the eye varies, and it can be concluded that input to the system includes
that from sources additional to the visual
representation of the object property itself.
Wiersma29 has reported "space constant"
fibers in the neural system of the crayfish
Procamberus clarki such that when the
animal is rotated about either its transverse or longitudinal axis so that the projection at the eye varies in orientation the
space constant fibers continue to respond.
Wiersma has suggested that positional information which preserves this stability
of response might derive from the statocysts. More recently Horn and Hill30 have
reported unchanging output from units in
the visual cortex of the cat. When the
head axis of the animal was tilted through
30 degrees, the meridian of maximum response remained more or less stable. In
all, 57 units responding in this manner
were studied. It is worth noting that in
this experiment there was sometimes an
"overshoot" of the axis when the animal
was returned to the normal orientation
suggesting a correlate for the visual orientation aftereffect reported recently.31 If the
observer is tilted for about two minutes
with closed eyes and then returned to the
upright, a gravitationally vertical line of
line in a dark room is apparently tilted
in the opposite direction to body tilt. This
finding, together with Horn and Hill's
data, suggests that the aftereffect represents
a breakdown of spatial constancy due to
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adaptive processes. Of course, it has long
been recognized that vestibular input affects activity in the visual system.32
A means for isolating those mechanisms
associated with visual illusions follows from
the suggestion that perceptual constancy
and illusion are related. A neural unit
which continues to respond in a uniform
fashion to a variable stimulus at the eye,
e.g., as the animal is tilted, would be
expected to show a highly variable discharge pattern if the stimulus is invariant
at the eye and the stabilizing stimulus
changed. If, for example, the cat were
subjected to a gravitoinertial force with
the retinal image fixed, a change in the
neural pattern correlated with the oculogravic illusion would be expected. Similarly, a unit which responds uniformly
when the size of the stimulus changes
would be expected to exhibit a different
pattern with fixed image and manipulation of one or more of the distance stimuli
described above (providing, of course, the
other stimuli for distance are eliminated.
To put the point succinctly: Where there
exist units which normally respond with
a constant excitation pattern to a variable
stimuli ("constancy units"), it can be
expected that the pattern will change
when the stimuli are invariant and the
stabilizing stimuli (e.g., distance, body
orientation, body movement stimuli) are
varied.
Assistance in the preparation of this paper by
the Department of Psychology, University of
Exeter, is gratefully acknowledged.
REFERENCES
1. Motokawa, K.: Physiology of color and pattern vision, New York, 1970, Springer-Verlag.
2. Burns, B. D., and Pritchard, R.: Geometrical
illusions and the response of neurones in
the cat's visual cortex to angle patterns, J.
Physiol. 213: 599, 1971.
3. Gregory, R. L.: Distortion of visual space
as inappropriate constancy scaling, Nature
199: 678, 1963.
4. Gregory, R. L.: Visual illusions, in Foss, B.
M., editor: New horizons in psychology,
Perceptual constancy and illusion 531
Harmondsworth, 1966, Penguin Books, Inc.
5. Gregory, R. L.: Eye and brain, London,
1966, Weidenfeld and Nicholson.
6. Kristof, W.: Ober die Einordnung geometrisch-optischer Tauschungen in die Gesetzin assigkeiten der visuellen Wahrnehmung,
Arch. Gesamte Psychol. 113: 1, 1961.
7. Tausch, R.: Optische Tauschungen als artifizielle Effekte der Gestaltungs prozesse von
Grossen und Formenkonstantz in der niiturlichen Raumwahrnehmung, Psychol. Forsch.
24: 299, 1954.
8. Holway, A. H., and Boring, E. G.: Determinants of apparent visual size with distance
variant, Am. J. Psychol. 54: 21, 1941.
9. Hastorf, A. H., and Way, K. S.: Apparent
size with and without distance cues, J. Gen.
Psychol. 47: 181, 1952.
10. Lichen, W., and Lurie, S.: A new technique for the study of perceived size, Am.
J. Psychol. 63: 280, 1950.
11. Over, R.: The effect of instructions or sizejudgments under reduced conditions, Am. J.
Psychol. 73: 599, 1960.
12. Boring, E. G.: Size constancy in a picture,
Am. J. Psychol. 77: 494, 1964.
13. Gogel, W. C., Wist, E. R., and Harker,
G. S.: A test of the invariance of the ratio
of perceived size to perceived distance, Am.
J. Psychol. 76: 537, 1963.
14. Leibowitz, H., and Moore, D.: Role of
changes in accommodation and convergence
in the perception of size, J. Opt. Soc. Am.
56: 1120, 1966.
15. Day, R. H.: Visual spatial illusions: A general explanation, Science 75: 1335, 1972.
16. Erlebacher, A., and Sekuler, R.: Explanation
of the Miiller-Lyer illusion: confusion theory
examined, J. Exp. Psychol. 80: 462, 1969.
17. Day, R. H., and Wade, N. J.: Mechanisms
involved in visual orientation constancy, Psychol. Bull. 71: 33, 1969.
18. Neal, E.: Visual localization of the vertical,
Am. J. Psychol. 37: 287, 1926.
19. Mach, E.: Beitrage zur Analyse der Empinfindungen, Jena, 1886, Fischer.
20. Graybiel, A.: Oculogravic illusion, Arch. Opthalmol. 48: 605, 1952.
21. Graybiel, A.: The importance of the otolith
organs in man based on a specific test for
utricular function, Ann. Otol. Rhin. Laryngol.
65: 470, 1956.
22. Asch, S. E., and Witkin, H. A.: Studies in
space orientation. I. Perception of the upright with displaced visual fields, J. Exp.
Psychol. 38: 325, 1948.
23. Austin, M., Singer, C., and Day, R. H.:
Visual orientation illusion following judg-
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Wallach, H., and Kravitz, J. H.: The measurement of the constancy of visual direction
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Graybiel, A., and Hupp, D. I.: The oculogyral illusion; a form of apparent motion
which may be observed following stimulation of the semicircular canals, J. Aviat. Med.
17: 3, 1946.
Craybiel, A.: The effect on vision produced
by stimulation of the semicircular canals
by linear acceleration, in White, C. S., and
Benson, O. O., editors: Physics and medicine
of the upper atmosphere, Mexico, 1952,
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Duncker, K.: Induced motion, in Ellis, W.
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201: 56, 1959.
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Discussion
PETTIGREW: There have been a number of attempts including my own to confirm the Horn
and Hill experiment but they have been unsuccessful. It is very difficult to keep the eyes from
shifting under these circumstances.
DAY: It is of course disappointing to learn
that that is so. However, one mentions the experiment merely to emphasize the sort of experimental approach to perceptual constancy and,
therefore, to illusion that is suggested by my
argument. The lesson is: Look for the mechanism
of constancy first and then you will have that
of the illusion.
Investigative Ophthalmology
June 1972
:' I would like to make a pedagogic
comment to neurophysiologists here. We have
to appreciate two categories of perceptual constancies. One category occurs when the nervous
system has information available about bodily
changes, and every engineer knows if he wants
to build a stabilized gun site platform on a tank
that it is not difficult to do so since he has all
the information available on position and by
proper feedback it can be stabilized. On the other
hand to make devices which would know now
that a coin in different perspective is still a coin
when, for example, it looks like an ellipse is a
completely different and much more involved
problem. It can be solved by some advanced
ideas in mathematics, but it is several orders
of magnitude more difficult. For example when
we speak about size constancy, this again is a
fantastically different thing if we consider Emmert's law and convergence movements which
occur within about one meter where stereopsis
is handy and helps you to get almost perfect
size constancy from the problems involved in
seeing someone begin to shrink as a train moves
away. These points came out in your talk, but
I would like to emphasize it for neurophysiologists
who, whenever they listen to size constancy,
should ask if it is type I (the simple one) or
type II. Most optical illusions belong to the
latter class and consequently are very controversial.
DAY: I do not wish to make such an unequivocally clear distinction between constancies based
on stimuli for observer posture and movement
and those based on observer distance and bearing from the object. In both situations the same
principle applies; stimuli which serve to preserve perceptual constancy of an object property
can be manipulated to produce an illusion. To
me it matters little that those stimuli activate
proprioceptive or exteroceptive systems.
BURKE : Is there any demonstration that an
animal can have an illusion?
DAY: Yes, there is good evidence that they do.
MACKAY: Don't you think some illusions are
due to the fact that we view a three-dimensional
world in two-dimensional array and are not merely a problem of constancy?
DAY: It is conceivable. I do not claim that
all illusions derive from activating those mechanisms which normally maintain constancy, merely that there is a wide range of such effects.