Size-contrast illusions deceive the eye but not the hand
Salvatore Aglioti*, Joseph F.X. DeSouzat and Melvyn A. Goodale*
*Dipartimento di Scienze Neurologiche, e Della Visione, Sezione di Fisologia Umana, UniversitA Degli Studi di Verona, 37134 Verona,
Italy. Department of Physiology, University of Western Ontario, London, Ontario N6A 5C1, Canada. *Department of Psychology,
University of Western Ontario, London, Ontario N6A 5C2, Canada.
Background: When we reach out to pick up an object,
not only do we direct our moving limb towards the location
of the object, but the opening between our fingers
and thumb isscaled in flight to the object's size. Evidence
obtained from patients with neurological disorders has
shown that the visual processing underlying the calibration
of grip aperture and other movement parameters
during grasping is mediated by visual mechanisms located
in the cerebral cortex that are quite distinct from those
underlying the experiential perception of object size and
other object features. Under appropriate conditions, such
dissociations can also be observed in individuals with normal
vision. Here we present evidence that the calibration
of grasp is quite refractory to pictorial illusions that have
large effects on perceptual judgements of size.
Results: We used a variation of the familiar 'Titchener
circles' illusion in which two target circles of equal size,
each surrounded by a circular array of either smaller
or larger circles, are presented side by side. Subjects typically
report that the target circle surrounded by the array
of smaller circles appears to be larger than the target
surrounded by larger circles. In our test, two thin 'pokerchip'
discs were used as the target circles. The relative size
of the two discs was randomly varied so that on some
trials the discs appeared perceptually different but were
physically equivalent in size, and on other trials they were
physically different but appeared perceptually equivalent.
The perceptual judgements made by the 14 subjects in
our experiment were strongly affected by this size-contrast
illusion. However, when asked to pick up a disc, the
scaling of the subjects grip aperture (measured opto-electronically
before contact with the disc) was largely determined
by the true size of the target disc and not its
illusory size.
Conclusions: It would seem that the automatic and
metrically accurate calibrations required for skilled
actions are mediated by visual processes that are separate
from those mediating our conscious experiential perception.
Earlier studies on patients with neurological deficits
suggest that these two types of processing may depend on
quite separate, but interacting, visual pathways in the
cerebral cortex.
Current Biology 1995, 5:679-685
Background
Our perception of the visual world is by its very nature
relative. As we gaze across a room, some of the objects
within our field of view will be perceived, in an obligatory
fashion, as larger or closer than others. But although
we might perceive that one object is larger or closer than
another, such relative judgements of size and distance are
not enough to calibrate the grasping movement that
might be directed at that object - to grasp the object
accurately, our visuomotor system must access information
concerning its exact size and distance. In fact, it is
possible to show that normal subjects often make perceptual
judgements about the location (or apparent change
in location) of a visual stimulus that are at odds with the
motor acts that they direct towards those stimuli.
Bridgeman et al. [1] have shown, for example, that even
though a fixed visual target surrounded by a moving
frame is perceived to drift in a direction opposite to that
of the frame (which seems to be stationary), when asked,
the subjects persist in pointing to the actual location of
the target. Wong and Mack [2] obtained similar dissociations
between a perceptual judgement and the control
of a motor act, but monitored saccadic eye movements
rather than pointing. In their experiments, a small target
was presented within a surrounding frame and, after a
500 millisecond blank period, the frame and target reappeared,
but now the frame was displaced a few degrees to
the left or right. The target itself was presented in exactly
the same location as before. Yet instead of perceiving the
frame as having changed position, subjects had the strong
illusion that it was the target that had changed position,
in a direction opposite to that of the actual displacement
of the frame. This illusion was maintained even when the
target was displaced in the same direction as the frame,
but by only one third the distance. In this latter condition
the perceived change in target position, after the blank
period, was still in the direction opposite to the change
in frame position, and thus, even more remarkably, in the
direction opposite to the actual change in position of the
target. Yet despite the presence of this strong illusory displacement
of the target, subjects consistently directed
their saccades to the true location of the target (in other
words, to its location with respect to the eye rather than
to its location with respect to the frame).
The complementary dissociation was observed when a
target was moved unpredictably during a saccadic eye
movement [3]; here it was found that subjects were unable
Correspondence to: Melvyn A. Goodale. E-mail address: goodale@uwo.ca
© Current Biology 1995, Vol 5 No 6 679
680 Current Biology 1995, Vol 5 No 6
to report, even in a forced-choice situation, whether or
not the target had changed position, even though the correction
saccades and manual-aiming movements directed
at the target showed near-perfect adjustments for the
unpredictable target shift. In other words, an illusory perceptual
constancy of target position was maintained in the
face of large amendments in visuomotor control.
The results of all these experiments suggest that the visual
mechanisms mediating the perception of object location
operate largely in allocentric coordinates, whereas those
mediating the visual control of object-directed actions
operate in egocentric coordinates. In other words, visual
perception seems to use a coordinate system that is
world-based, in which objects are seen as changing location
relative to a stable or constant world; the systems
controlling actions, however, cannot afford these kinds of
constancies and must compute the location of the object
with respect to the effector that is directed at that target.
Thus, in the experiments by Bridgeman et al. [1] and
Wong and Mack [2], the target within the moving or displaced
frame was perceived as moving relative to the
frame, whereas the frame itself, the only large visible feature
in the field of view, was perceived as a stable background.
The visuomotor systems computing the saccadic
(or aiming) eye movements directed at the target simply
ignored the movement of the frame and computed the
actual position of the target in retinocentric (and perhaps
also in head- and/or shoulder-centred) coordinates.
In the experiments by Goodale et al. [3] in which the
position of the target was sometimes changed during a
saccade, the subjects' failure to perceive the displacement
of the target was probably a reflection of the broad tuning
of perceptual constancy mechanisms that preserve the
identity of a target as its position is shifted on the retina
during an eye movement. When no other reference
points are available in the field of view, the perceptual
system assumes that the position of the target, which was
stable at the beginning of the saccade, has not changed.
Such an assumption has little consequence for perception
and is computationally efficient. But the visuomotor systems
controlling saccadic eye movements and manual
aiming movements cannot afford that luxury. At the end
of the first saccade, they must re-compute the position of
the target (within egocentric frames of reference), so that
the appropriate correction saccade and amendment to
the trajectory of the moving hand can be made. In short,
different types of visual computation are required for
visual perception and visuomotor control.
Just as the perception of object location seems to operate
within allocentric frames of reference, so does the perception
of object size. Although we often make subtle
relative judgements of object size, we rarely make absolute
judgements. Indeed, our judgements of size appear
to be so inherently relative that we can sometimes be
fooled by visual displays in which visual stimuli of the
same size are positioned next to comparator stimuli that
are either much smaller or much larger than the target
Fig. 1. The Titchener circles illusion. (a)The standard version of
the illusion. The target circles in the centre of the two arrays
appear to be different in size even though they are physically
identical. For most people, the circle in the annulus of smaller
circles appears to be larger than the circle in the annulus of
larger circles. (b) A version of the illusion in which the target
circle in the array of larger circles has been made physically
larger than the other target circle. The two target circles should
now appear to be perceptually equivalent insize.
stimuli. Such size-contrast illusions are popular examples
used in many introductory textbooks in psychology and
perception. One such illusion is the 'Titchener circles'
(or 'Ebbinghaus') illusion, in which two target circles of
equal size, each surrounded by a circular array of either
smaller or larger circles, are presented side by side (Fig.
la). Subjects typically report that the target circle surrounded
by the array of smaller circles appears larger than
the one surrounded by the array of larger circles, presumably
because of the difference in the size contrast between
the target circles and the surrounding circles. Although
the illusion is usually depicted as we have just described,
it is also possible to make the two target circles appear
identical in size by increasing the actual size of the target
circle surrounded by the array of larger circles (Fig. lb).
Although perception is clearly affected by these manipulations
of the stimulus array, there is good reason to
believe that the calibration of size-dependent motor outputs
- such as grip aperture during grasping - would
(a)
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Size-contrast illusions Aglioti eta!. RESEARCH PAPER 681
not be. After all, when we reach out to pick up an
object, particularly one we have not seen before, our
visuomotor system must compute its size accurately if we
are to pick it up efficiently, that is, without fumbling or
re-adjusting our grip. It is not enough to know that the
target object is larger or smaller than surrounding objects;
the visuomotor systems controlling hand aperture must
compute its real size. For this reason, one might expect
the calibration of grip aperture to be refractory to sizecontrast
illusions. It was this idea that we tested in the
present study.
Results
We designed a three-dimensional version of the Titchener
circles illusion, in which two thin 'poker-chip' discs
were used as the target circles (Fig. 2). The discs were
arranged as pairs on a standard Titchener annular circle
display drawn on a white background and positioned
directly in front of the subject. Before being tested in the
grasping paradigm, each of the 14 subjects in the study
was presented with a range of different discs (27-33 mm
in diameter) in order to find a pair that would be reliably
identified as identical in size. To be judged equivalent in
size, the disc in the array of larger circles had to be, on
average, 2.5 mm wider than the disc in the array of
smaller circles.
During the actual testing, trials in which the two discs
appeared perceptually identical but were physically different
in size were randomly alternated with trials in
which the discs appeared perceptually different but were
physically identical; for half of these latter trials a pair of
small discs was used, and for the other half a pair of
larger discs was used. On each trial, the two discs were
presented for 3 seconds by illuminating the table with
an overhead lamp. Subjects were given the following
instructions: 'if you think the two discs are the same size,
pick up the one on the left; if you think they are different
in size, pick up the one on the right'. In a subsequent
test session, these instructions were reversed. All
subjects were asked to pick up the disc with the index
finger and thumb of their right hand.
The trajectory of the grasping movement was recorded
with two cameras that tracked three infrared light-emitting
diodes (IREDs), which were attached to the ends of
the index finger and thumb as well as to the wrist (Fig.
2). Using these recordings, we were able to reconstruct
the change in grip aperture - the opening between the
index finger and thumb - as subjects reached out and
picked up the target disc. The main kinematic variable
we measured was the maximum grip aperture, which is
typically achieved about 70 % of the way through a
grasping movement and has been shown to be well-correlated
with the size of the goal object [4-6]. The calibration
of maximum grip aperture has also been shown
to be largely refractory to visual information that is available
during the execution of the movement, relying
instead on the motor programming that occurs before
the movement begins [4-6].
Our version of the Titchener circles illusion proved to be
quite effective, and all the subjects remained sensitive to
the illusion throughout testing. In other words, they
Fig. 2. A photograph of our threedimensional
version of the Titchener
circles illusion. Note the infrared lightemitting
diodes (IREDs) attached to the
finger, thumb and wrist of the subject.
Size-contrast illusions Aglioti et al. RESEARCH PAPER 681
682 Current Biology 1995, Vol 5 No 6
treated discs that were actually physically different in size
as perceptually equivalent, and discs that were physically
identical were judged to be perceptually different. Indeed,
these observations are reflected by the finding that the
subjects took significantly longer to initiate their movement
on trials in which they judged the two discs to be
identical - a familiar finding in 'same/different' judgement
tasks - even though, in this case, the discs were
actually physically different in size (Fig. 3).
fact they were identical. On some of these trials, some
subjects did open their fingers slightly more for the disc
surrounded by the small circles than they did for the disc
surrounded by the large circles. Nevertheless, as Figure 5
Remarkably, however, the scaling of the subjects grasp
was affected very little by these illusory perceptions.
Instead, the maximum aperture of their grip was largely
determined by the true size of the disc. Thus, on trials in
which the two discs were perceived as being the same
size, subjects opened their hand wider for the larger disc
than they did for the smaller one (Fig. 4a). Moreover, as
illustrated in Figure 4b, the relationship between true size
and maximum grip aperture was as evident on trials in
which the two discs were judged to be the same size (but
were physically different) as it was on trials in which the
two discs were judged to be different (but were actually
both small or both large). In short, the calibration of grip
size seemed to be largely impervious to the effects of the
size-contrast illusion.
Of course, the control of skilled movements is clearly not
isolated from perceptual information. The perceived
function of an object (such as a hammer or telephone for
example) has clear effects on the nature of the grasp we
adopt when we pick it up. Even in this task, one could
detect some influence of the perceptual judgements on
grip scaling - at least on those trials in which subjects
perceived the two discs to be different in size when in
Fig. 3. The mean movement-onset times for trials in which subjects
judged the two discs to be perceptually different in size (even
though the discs at the center of each annulus of circles were
actually of the same diameter), and trials in which they perceived
the discs to be the same size (even though the discs at the center
of each annulus of circles were actually of different diameters).
Fig. 4. Graphs illustrating grip aperture under different testing conditions.
(a) Representative grip-aperture profiles for one subject
picking up alarge disc and asmall disc on separate trials in which
he judged the two discs to be identical in size (even though, of
course, the two discs were physically quite different). In both
cases, the disc was located on the left hand side of the display. (b)
The mean maximum grip aperture for the 14.subjects in different
testing conditions. The grey bars indicate the maximum aperture
on trials inwhich the two discs were judged to be perceptually the
same, even though they were physically different in size. The open
bars indicate the mean maximum aperture on trials in which the
two discs were judged to be perceptually different, even though
they were physically the same size (either two large discs or two
small discs). The line to the extreme right of the graph indicates
the average difference in disc size that was required to achieve
perceptual equivalence in the pre-test. The difference between the
maximum grip aperture achieved for large discs was significantly
greater than the maximum grip aperture achieved for small discs,
independent of whether or not the subject believed the discs were
the same or different sizes (p< 0.001). No other comparisons
were significant. Error bars indicate the standard error of the mean.
Size-contrast illusions Aglioti et al.
Fig. 5.The effect of the size-contrast illusion on the calibration of
the grasp and on perceptual matching of disc size. The open bar
shows the average difference between maximum grip aperture
for discs surrounded by an array of small circles, and the maximum
grip aperture for identical discs surrounded by an array of
large circles; that is,the effect of the illusion on grip size. For half
these trials, the two discs were both large and for the other half
they were both small. The difference score was an average of all
trials - that is,when both discs were large, and when both were
small. This effect was significantly smaller (p <0.02) and more
variable than the average difference indisc size that was required
to obtain a judgement of perceptual equivalence with such arrays
(the grey bar). Error bars indicate the standard error of the mean.
illustrates, the effect of the perceptual judgement on grip
aperture was quite variable, and was significantly smaller
than the size difference that was required to achieve perceptual
equivalence between the two discs in the matching
test that had been carried out earlier. In other words,
the effect of the illusion on grip size was much smaller
and more variable than the effect of the illusion on perceptualjudgements
of size. In contrast, as we have already
seen, the calibration of grip was strongly influenced by
the real object size in all subjects - independent of
whether or not the grasp was initiated on perceptually
different or perceptually identical trials.
Discussion
The results of these experiments make it clear that
although size-contrast illusions have a powerful effect on
the perceptual judgements of an object's size, grasping
movements directed at that same object remain metrically
accurate. Thus, the very act by which subjects indicated
their susceptibility to the illusion (that is, picking up one
of the two target discs) was itself unaffected by the visual
information driving that illusion.
It should be emphasized that the accurate scaling of the
grasp was not due to the fact that the subjects were
somehow comparing their grip aperture with the diameter
of the target disc during the grasping movement.
Firstly, as can be seen in Figure 4, the maximum grip
aperture was actually much larger than the diameter of
the disc - more than twice as large on most trials. But,
as reported in many previous studies (see for example
[4-6]), maximum grip aperture still correlated well with
the size of the target. Secondly, subjects remained susceptible
to the illusion throughout the experiment, despite
the fact that they were scaling their grasp to the real size
of the target disc. If the subjects were adjusting their
grasp on the basis of information delivered by the same
visual networks that they used to make their perceptual
judgements, then one might expect to see some weakening
of the illusion over the course of the experiment. No
such weakening was observed. Thirdly, as mentioned
earlier, the calibration of maximum grip aperture is
largely determined by motor programming that is carried
out before the hand has actually started to move [4-6]. In
fact, on a few occasions, subjects expressed surprise when
they handled the disc after they had picked it up claiming
that it seemed larger or smaller than they had
expected - even though their grasp had been calibrated
accurately in flight.
But why should perception be so susceptible to the illusion
while the calibration of grasp is not? It is possible
that the illusion arises from a straightforward relative-size
scaling mechanism, whereby an object that is smaller
than its immediate neighbours is assumed to be smaller
than a similar object that is larger than its immediate
neighbours [7]. It isalso possible that some sort ofimagedistance
equation is contributing to the illusion, in which
the array, of smaller circles is assumed to be more distant
than the array of larger circles; as a consequence, the target
circle within the array of smaller circles will also be
perceived as more distant (and therefore larger) than the
target circle of equivalent retinal-image size within the
array of larger circles. In other words, the illusion may be
simply a consequence of the perceptual system's attempt
to make size-constancy judgments on the basis of an
analysis of the entire visual array [8].
Mechanisms such as these, in which the relationships
between objects in a visual array play a crucial role in
scene interpretation, are clearly central to perception. In
contrast, the execution of a goal-directed act, such as
prehension, depends on metrical computations that are
centred on the target itself. As a consequence, computation
of the retinal-image size of the object coupled with
an accurate estimate of distance will deliver the true size
of the object for calibrating the grip - such computations
may be quite insensitive to the kinds of pictorial
cues that drive our perception offamiliar illusions.
The distinction between visual perception and the visual
control of skilled motor acts has also been observed in
patients who have sustained selective damage to different
visual areas of the cerebral cortex. Thus, patients with
damage in the superior regions of the posterior parietal
cortex are often unable to use information about the
size, shape, orientation and location of an object to
control the posture of the hand and fingers and/or the
RESEARCH PAPER 683
684 Current Biology 1995, Vol 5 No 6
trajectory of their moving limb during a grasping movement
[5,9-12]. Yet these same patients can usually identify
and describe the very objects they cannot grasp. The
opposite dissociation has been observed in another
patient (D.F.) who can direct accurate and well-formed
grasping movements towards objects that she cannot
identify or discriminate [11-13]. Although the damage
in D.E's brain is quite diffuse, the ventrolateral regions of
her occipital lobe are particularly compromised. These
neuropsychological data, together with the results of
electrophysiological and behavioural studies in the monkey,
have led to the proposal that the ventral stream of
visual projections from primary visual cortex to inferotemporal
cortex is critical for the visual perception of
objects, whereas the dorsal stream, which projects from
primary visual cortex to the posterior parietal region,
mediates the required sensorimotor transformations for
visually-guided actions directed at those objects [11,14].
It is possible that the dissociation between perceptual
judgements of object size and the calibration of grasp
that we have reported here is a reflection of the different
computations carried out by these two visual processing
streams in the normal brain.
Conclusions
By recording the kinematics of a grasping movement
directed at discs presented in a three-dimensional version
of the Titchener circles illusion, we have shown that the
calibration of grip aperture is quite refractory to the
compelling size-contrast illusion induced by the display.
This result suggests that the automatic and metrically
accurate calibrations required for skilled actions may
depend on visual computations that are different from
those driving our perceptual judgements about objects in
the world. In short, what we think we see may not
always be what guides our actions.
Materials and methods
The target discs were constructed of 3 mm thick white plastic
with a thin black line drawn around the circumference on their
top surface (Fig. 2). The discs ranged in size from 27-33 mm
(in 1 mm steps). During presentation, two discs were positioned
on a Titchener circles display, one disc in the centre of
an array of small circles (each 10 mm in diameter) and the
other in an array oflarge circles (each 58 mm in diameter). The
overall diameter of the array of small circles (through the centre
of circles) was 47 mm; the overall diameter of the array of
large circles was 110 mm (Fig. 2). The centres of the two arrays
were 120 mm apart. The entire display was mounted on a
turntable so that the left/right position of the arrays could be
easily changed.
The 14 subjects (9 males and 5 females) ranged in age from 19
to 41 years (mean 22.2 years) and were all strongly righthanded
as assessed by a modified version of the Edinburgh
Handedness Inventory [15]. Their vision was normal or
corrected-to-normal and their stereoacuity was at least 40"
of arc as measured by the Randot Stereotest (Stereo Optical,
Chicago).
During testing, subjects stood in front of the table on which
the Titchener circles display was placed. This gave them a
bird's-eye view of the display that was very nearly orthogonal
to the display surface. They were instructed that they could
move their eyes but should try to keep from moving their
head. During a pre-test phase, subjects were systematically
tested with different pairs of discs in order to establish which
pairs would be reliably judged as equivalent in size. On average,
the disc centered in the array of large circles had to be
2.5 mm larger than the disc centered in the array of small circles
in order to achieve perceptual equivalence. For 8 subjects
the required difference was 2 mm, for 5 subjects it was 3 mm,
and for 1 subject it was 4 mm. Each subject was then tested
with his or her particular disc pair during subsequent testing.
During the testing we used two types of trial. In the first type,
which was repeated 36 times, the large disc was placed in the
array of large circles, and the small disc in the array of small circles,
to create the illusion that the discs were in fact the same
size. The second type of trial, in which two discs of the same
size were used to create the illusion that the discs were different
sizes, was subdivided into two further types - each
repeated 18 times - in which both discs were either small or
large. The left/right position of the arrays was randomly varied.
At the beginning of each trial, subjects placed the tips of their
index finger and thumb of their right hand on a start button
positioned on the table at their midline about 160 mm from
the centre of each of the discs. The display on each trial was
arranged out of the subject's field of view and before each trial
began the room lights were extinguished. An overhead lamp
was then turned on, thus illuminating the Titchener circles display
for 3 seconds. The subjects were instructed that on each
trial they were to pick up the disc on the left if they thought
the discs were the same size, and the disc on the right if they
thought the discs were different. These instructions were
reversed halfway through the testing.
The movements of the hand and fingers during grasping were
tracked by conventional opto-electronic recording. Infrared
light-emitting diodes (IREDs) were attached to the index finger,
thumb, and wrist with small pieces of adhesive tape (Fig.
2). The position of each IRED was tracked with two infraredsensitive
cameras and this information was stored on a WATSMART
computer (Northern Digital, Waterloo, Canada) at a
sampling rate of 100 Hz. The kinematics of the grasping movement
on each trial were reconstructed off-line at the end of the
experiment. Movement onset was defined as the point at
which the velocity of the wrist IRED first reached a resultant
velocity of more than 5.0 cm's- l.
Maximum grip aperture was
defined as the maximum vectored distance-between the index
finger and thumb.
Differences in maximum grip aperture across conditions were
analyzed using standard repeated-measures analysis of variants.
Variable factors included perceptual condition (same/different),
physical size (large disc/small disc), position (left/right), and
array type (large circles/small circles). When appropriate, posthoc
tests were carried out by applying paired t-tests using Bonferroni
corrections.
Acknolledgements: This research was supported by a grant from the
Medical Research Council of Canada to M.A.G.
Size-contrast illusions Aglioti et al. RESEARCH PAPER 685
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Received: 20 February 1995; revised: 22 March 1995.
Accepted: 4 April 1995.