Bimanual Input and Patterns of User Behavior
G.E. Evreinov
Department of Computer Sciences, University of Tampere, Tampere, Finland
Abstract
The assumption that two-handed manipulation save
time is not always the right way for interface design.
This paper describes the results of the study of using
two analog buttons in a target acquisition task
illustrating behavioral principles for two-handed
interfaces. The results showed that despite the
apparent contradiction of the motion artifact to the
one-to-one mapping principle, Y-axis changed with
the left hand whereas the X-axis was controlled by
the right hand, the subjects normally did not notice
the artifact as such. This can be explained by the
nature of the input mapping designed to
accommodate the genuine structure of bimanual
manipulation.
Keywords
Two-handed interfaces, integral behavioral pattern.
1 Introduction
The most of input techniques were designed to
support one-hand interaction. As it was shown in [2],
the preferred hand articulates its motion relative to
the dynamic frame-of-reference determined by the
non-preferred hand. This has an immediate effect
upon both the behavioral strategy and the way the
subjects could perform the task efficiently in twohanded interaction. Indirect pointing on a computer
screen with an input device may or may not follow
the patterns found in direct hand pointing at physical
targets [1]. The mapping between cursor motion and
input device displacement is often a complex transfer
function of the control-display ratio, which may
further increase the complexity of the hand eye
coordination in a target acquisition task. In the case
of computer mice, most of them have non-linear
acceleration schemes. The complexity of two-handed
cooperation grows exponentially when a pointing
task has to be implemented in 3D environment [4].
Thus, there is a challenge to explore a number of
research questions on the behavioral level with
subsequent application of the results in the design of
dynamic multimodal interfaces.
2 Method Design
To simulate spatial-distributed input in bi-manual
interaction and facilitate interpreting the data
gathered in a target acquisition task a special input
device, analog buttons, were designed. The
distribution of X-Y coordinates is the simplest way to
estimate the behavioral features in two-handed
coordination. As such a way can be considered as an
“asymmetric dependent” task [2], and according to
Kabbash [3], is more suitable and efficient way to
simultaneously operate independent controls.
2.1 Analog buttons
The pointing device used in the experiment was a set
of two analog buttons which were developed using of
silicon tube. It provided stability of mechanical parameters
whereas dynamic displacement was similar to a normal
digital key (not less than 4 mm); the current amplifier
compensated nonlinear characteristics of optical forcedisplacement transducer. An overall view and construction
features of the buttons are shown in Figure 1.
5
4
2
1
3
Figure 1. Overall view and construction features of the
analog buttons. 1 - silicon tube; 2 - photodiode; 3 - light
emitting diode; 4 - box; 5 - nominal displacement.
2.2 Apparatus and Procedure
The experiment was conducted on a Pentium III 800MHz
desktop PC with a 19" monitor having a screen resolution of
1024 by 768 pixels. The software tools were written in
Microsoft Visual Basic 6.0 SP6 under Windows 2000. The
experimental software was used to present the task and
capture the data from the pointing device. The pointing
device comprised of two identical analog buttons connected
to joystick port. The button in the left hand was used to
control the Y-axis of the cursor position to move the cursor
downwards, whereas the button in the right hand controlled
the X-axis to move the cursor rightwards. This setup was
used for all the subjects. The nominal displacement of the
analog buttons was equal to 4 mm (Figure 1). It was
translated to 256 pixels of the test-window and scaled by
three times to fit to the monitor screen. Thus, ±5-pixels of the
test-window corresponded to ±200 microns of physical
button displacement. This implies that the low resolution and
accuracy of button displacement could be enhanced and
compensated by means of visual feedback loop.
Twenty unpaid volunteers (15 males and 5 females) took part
in the test. Two participants were left-handed and two
reported they used both hands equally. Other 16 participants
considered themselves right-handed. All participants used
computers on a daily basis. None had prior experience with
the analog buttons.
The task consisted of capturing one of 49 circular targets
arranged in a square grid 7 by 7. A screen distance between
the centers of adjacent targets in the grid was 36 mm. Only
one target was visible on the screen at a time. The targets
were presented in a random order. The task was explained
and demonstrated to participants and a warm-up block of
trials was given. In each trial, a circular target of diameter 4
mm appeared on the display. The cursor had a shape of a
cross-hair tracker. Participants moved the cross-hair tracker
by manipulating with two analog buttons. The goal was to
select the target by moving the tracker over it. The selection
3 Results and Conclusion
The results showed that despite the apparent
contradiction of the motion artifact with the spatialdistributed input to the one-to-one mapping principle,
the subjects normally did not notice the artifact as
such. This can be explained by the nature of the input
mapping designed to accommodate the genuine
structure of bimanual manipulation.
along the dimension controlled by the dominant hand. The
second phase is the closed-loop phase during which the
subjects manipulate the buttons to get the cursor over the
target. When either of the coordinates gets as close to the
target as 6 pixels away from its center, the third phase begins,
the final target acquisition phase. During this phase, the other
coordinate should be caught up, so that the cursor finally gets
over the target.
X-axis
Y-axis
160
Distance to target, pxls
occurred only when the tracker remained within the
target area for an uninterrupted period of 300 ms
(dwell time). Only one trial was performed on each
target, so the session consisted of 49 trials. In each
trial, participants were given a limit of 10 seconds to
complete selection of the target. If they did not
acquire the target within the time limit, an error was
recorded for the trial. An error was accompanied by a
negative sound beep. The next target was presented
after a delay of 1 second. Participants were instructed
to accomplish the task as quickly as possible.
120
80
40
32
0
6
0
1
2
3
4
5
6
Time, s
1
2
3
Figure 3. Generic behavioral pattern: 1–motor programming
phase; 2 – aiming on target phase; 3 – target acquisition.
The most of people prefer to employ their dominant hand and
the most of input techniques were designed to support onehand interaction. This paper describes the results of the study
of using two analog buttons in a target acquisition task
illustrating behavioral principles for two-handed interfaces.
The analog buttons developed allow the user to manipulate
the cursor position across the entire area of the screen with a
resolution of 256 by 256 pixels by having only 4 mm
displacement. Visual feedback loop compensates both the
lack of low resolution of the input device and the low
accuracy in finger dexterity.
The precision of pointing in a spatial-distributed task with bimanual input for novices is quite difficult task which may
contradict or may not with previous user experience. Still,
cooperation between the various motor systems might be
facilitated when the input mapping would be designed to be
adaptive to the personal features in bimanual interaction. The
integral behavioral pattern can be recorded in a simple gamelike testing procedure. The results can be used to adapt
parameters of the input technique in appropriate way.
Figure 2. Plots of 49 trials and integral behavioral
patterns (white lines) of the left-handed subject
(middle pictures), right-handed subject (bottom
pictures) and the case of ambidexter (upper pictures).
The time scale (grid on X-axis) is 300 ms. The
displacement scale (grid on Y-axis) is 12.8 pixels.
Our observations also revealed that the integral
behavioral pattern (Figure 2) averaged on 49 trials
could be divided into three phases. The first is the
motor programming phase (Figure 3). It is the time
span between the onset of the movement and the
time, when either of the coordinates cross the aimingon-target zone (32 pixels in the current study). The
final point of the motor programming phase coincides
with the end of the sudden movement of the cursor
Supported by the Academy of Finland (grant 200761 and 107278).
References
1. Chatty, S. (1994) Issues And Experience In Designing
Two-Handed Interaction. CHI’94, Boston, Massachusetts
US, ACM SIGCHI, 253-254.
2. Guiard, Y. (1987) Asymmetric Division of Labor in
Human Skilled Bimanual Action: The Kinematic Chain
as a Model. Journal of Motor Behavior, 19 (4), 486-517.
3. Kabbash, P, Buxton, W. (1994) A. Sellen. Two-Handed
Input in a Compound Task. CHI’ 94, Boston,
Massachusetts US, ACM SIGCHI, 417–423.
4. Wang, Y. and MacKenzie, C.L. (1999). Effects of
orientation disparity between haptic and graphic displays
of objects in virtual environments. INTERACT '99,
Edinburgh, Scotland, 391-398.