Neuropsychologia Visual illusions, delayed grasping, and memory: V.H. Franz

Neuropsychologia Visual illusions, delayed grasping, and memory: V.H. Franz
Neuropsychologia 47 (2009) 1518–1531
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Neuropsychologia
journal homepage: www.elsevier.com/locate/neuropsychologia
Visual illusions, delayed grasping, and memory:
No shift from dorsal to ventral control
V.H. Franz a,∗ , C. Hesse b , S. Kollath a
a
b
Justus-Liebig Universität, Giessen, Germany
Ludwig-Maximilians-Universität, München, Germany
a r t i c l e
i n f o
Article history:
Received 4 March 2008
Received in revised form 31 July 2008
Accepted 31 August 2008
Available online 11 September 2008
Keywords:
Perception
Action
Müller-Lyer
Dorsal
Ventral
a b s t r a c t
We tested whether a delay between stimulus presentation and grasping leads to a shift from dorsal to
ventral control of the movement, as suggested by the perception–action theory of Milner and Goodale
(Milner, A.D., & Goodale, M.A. (1995). The visual brain in action. Oxford: Oxford University Press.). In this
theory the dorsal cortical stream has a short memory, such that after a few seconds the dorsal information
is decayed and the action is guided by the ventral stream. Accordingly, grasping should become responsive
to certain visual illusions after a delay (because only the ventral stream is assumed to be deceived by these
illusions). We used the Müller-Lyer illusion, the typical illusion in this area of research, and replicated the
increase of the motor illusion after a delay. However, we found that this increase is not due to memory
demands but to the availability of visual feedback during movement execution which leads to online
corrections of the movement. Because such online corrections are to be expected if the movement is guided
by one single representation of object size, we conclude that there is no evidence for a shift from dorsal to
ventral control in delayed grasping of the Müller-Lyer illusion. We also performed the first empirical test
of a critique Goodale (Goodale, M.A. (2006, October 27). Visual duplicity: Action without perception in the
human visual system. The XIV. Kanizsa lecture, Triest, Italy.) raised against studies finding illusion effects
in grasping: Goodale argued that these studies used methods that lead to unnatural grasping which is
guided by the ventral stream. Therefore, these studies might never have measured the dorsal stream, but
always the ventral stream. We found clear evidence against this conjecture.
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
It has been reported repeatedly that the effects of certain
visual illusions on motor behavior increase if a delay is introduced
between stimulus presentation and execution of the movement
(e.g. Gentilucci, Chieffi, Daprati, Saetti, & Toni, 1996; Hu & Goodale,
2000; Westwood, Heath, & Roy, 2000; Westwood, McEachern, &
Roy, 2001; Westwood & Goodale, 2003). In the perception–action
framework (Milner & Goodale, 1995) this was interpreted as a
shift between two completely different neuronal control systems:
vision-for-action and vision-for-perception. The vision-for-action
system is assumed to reside in the dorsal cortical stream and
to be refractory to certain visual illusions (as, for example, the
Ebbinghaus/Titchener illusion; Aglioti, DeSouza, & Goodale, 1995,
or the Müller-Lyer illusion; Hu & Goodale, 2000, p. 858; Goodale
∗ Corresponding author at: Justus-Liebig-Universität Giessen, FB 06/Abt. Allgemeine Psychologie, Otto-Behaghel-Strasse 10F, 35394 Giessen,
Germany. Tel.: +49 641 99 26112; fax: +49 641 99 26119.
E-mail address: [email protected] (V.H. Franz).
0028-3932/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.neuropsychologia.2008.08.029
& Westwood, 2004, p. 208; Goodale, Westwood, & Milner, 2004, p.
137). In addition, the vision-for-action system is assumed to have
an extremely short memory (“certainly less than 2 s”, Milner &
Goodale, 1995, p. 173).
According to this hypothesis, it is easy to explain the increase
of illusion effects if a delay is introduced between stimulus presentation and execution of the movement: the vision-for-action
system has forgotten the exact parametric values of the target
object and therefore has to rely on the stored visual information
from the vision-for-perception system. This information, however,
is affected by the illusion and therefore the illusion effect increases
with the delay.
Recently, an even stronger version of this hypothesis has been
proposed: the “real-time view of action” (Goodale et al., 2004;
Westwood & Goodale, 2003). According to this view, the visionfor-action system only computes the exact parametric values of the
movement at the very moment the movement is initiated. Consequently, introducing even a very brief delay between stimulus
presentation and movement initiation should force the motor system to use ventral information and thereby lead to an illusion effect
in motor behavior.
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
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1.1. Critique of the perception–action interpretation of grasping
However, recent research has shown that some of the keyassumptions of the perception–action hypothesis might be based
on problematic empirical evidence. For example, a number of
researchers have argued that grasping is affected by the Ebbinghaus/Titchener illusion to a similar degree as perception (Franz,
Gegenfurtner, Bülthoff, & Fahle, 2000; Pavani, Boscagli, Benvenuti,
Rabuffetti, & Farnè, 1999; for reviews see: Franz, 2001 and Franz &
Gegenfurtner, in press). They argued that the apparent dissociation
between perception and grasping is mainly due to methodological
problems, as for example a mismatch in task demands (Franz et
al., 2000), or the fact that different responsiveness of the dependent measures were not taken into account (see below and Franz,
2003). This view is consistent with the results of van Donkelaar
(1999) who found that pointing-movements also were affected by
the Ebbinghaus/Titchener illusion to a similar degree as perception.
Other researchers showed similar problems for the classic
studies (Bridgeman, Kirch, & Sperling, 1981; Bridgeman, Peery,
& Anand, 1997) on the induced Roelof’s effect and showed that
these findings can better be explained by a common representation of space, thereby corroborating our results regarding
the Ebbinghaus illusion (Dassonville, Bridgeman, Bala, Thiem, &
Sampanes, 2004; Dassonville & Bala, 2004a, 2004b). Similarly,
Schenk (2006) questioned whether the dissociation in the famous
patient D.F. is really between perception and action as suggested by Milner and Goodale (1995). Specifically, the notion that
object size is calculated twice, once in the ventral stream for
perception (deceived by certain visual illusions, but with long
memory) and once in the dorsal stream for action (not deceived
by certain visual illusions, but with short memory) seems problematic.
For these reasons, we decided to test the empirical evidence for
the differential effects of delay on illusions in perception and action.
We used the Müller-Lyer illusion, as the illusion which was used
first in this area of research (Gentilucci et al., 1996) and very often
subsequently (e.g. Daprati & Gentilucci, 1997; Franz, Fahle, Bülthoff,
& Gegenfurtner, 2001; Heath, Rival, & Binsted, 2004; Heath, Rival, &
Neely, 2006; Westwood et al., 2000, 2001). We employed grasping
as motor response because grasping is the typical response used
in studies investigating the dissociation between perception and
action (e.g. Aglioti et al., 1995) and because grasping likely minimizes the problem that position and extent might be dissociated
in the Müller-Lyer figure (Gillam & Chambers, 1985; Mack, Heuer,
Villardi, & Chambers, 1985).
1.2. The critical role of visual feedback
Besides replicating the earlier studies, we were interested in two
potential methodological problems: the first issue is related to the
use of visual feedback. The condition with minimal memory load
would be a full-vision condition. That is, the participants grasp the
shaft of the Müller-Lyer figure with full vision of hand and stimulus (following the tradition in the motor literature, we will call this
“closed-loop” condition). In this closed-loop condition, visual information is available all the time such that there is no need to employ
memory mechanisms. While that seems to make the closed-loop
condition an ideal baseline for the memory conditions, there is one
serious limitation of this condition: during execution of the movement, feedback mechanisms (e.g. Woodworth, 1899) could detect
the “error” introduced by the illusion and lead to online corrections.
These online corrections, however, could hide an illusion present
in the motor system (Post & Welch, 1996). Therefore, we took great
care to disentangle the effects of visual feedback and of memory
demands. For this, we systematically varied the amount of visual
Fig. 1. Viewing conditions used in our experiments. In all conditions, participants
viewed the stimulus for 1 s (preview-period) and an auditory go-signal indicated
when the movement should be initiated. In the CL condition, participants had full
vision of hand and stimulus during the movement (as indicated by the gray bar).
In the OL-Move-2/3 (1/3) condition, participants only had vision until the hand had
traveled 2/3 (1/3) of the way to the target object. In the OL-Move condition, vision
was suppressed as soon as the hand started to move. In the OL-Signal condition
vision was suppressed after the preview-period and when the go-signal started. In
the OL-Delay condition, an additional delay of 5 s was introduced between end of
the preview and the go-signal.
feedback and the memory demands using a large number of visual
conditions (cf. Fig. 1).
1.3. Correcting illusion effects for comparisons across action and
perception
The second issue is related to the potentially different responsiveness of each of the dependent measures to a physical variation
of object size. Because this issue sometimes leads to confusion, we
will discuss it in some detail here. The perceptual and motor measures must respond to a physical variation of object size. Otherwise
we would not be able to evaluate their response to an illusionary
variation of size. But, this is not enough. We need to know, how
exactly each measure responds to a physical change of, say, 1 mm.
Only if we know this, we can say that an illusion had a corresponding effect of, say, 1 mm. Luckily, most dependent measures used in
this area of research are linearly related to physical size. This simplifies things. For example, in grasping the standard measure is the
maximum grip aperture (MGA; i.e. the maximum aperture between
index finger and thumb during the reach phase of the grasp movement). The MGA is a linear function of physical size (Jeannerod,
1981, 1984): it has a certain intercept, such that the MGA is always
larger than the object allowing for a certain safety margin. And it has
a certain slope. This slope tells us, how much the MGA will change
if we change physical size by 1 mm. In a meta-analysis, Smeets and
Brenner (1999) determined an average slope of 0.82 for MGA. That
is, if we increase the physical size by 1 mm, then MGA will increase
by approximately 0.82 mm. This implies that, if we measured an
increase of MGA of 0.82 mm in response to an illusionary change
of size, we can conclude that the illusion had an effect that corresponds to a 1 mm increase of the physical size. More generally, if we
measured an illusion effect of X mm in MGA then we can conclude
that this corresponded to an X/0.82 mm change in physical size. In
the following we will call this ratio (illusion-effect divided by slope)
the “corrected” illusion effect (Franz, 2003; Franz et al., 2001; Franz,
Scharnowski, & Gegenfurtner, 2005). Some authors also call it the
“scaled” illusion effect (Glover & Dixon, 2002).
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V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
Now, consider another measure. For example, manual size estimation (participants indicate the size of an object with index finger
and thumb) which is very often interpreted as a perceptual measure (Haffenden & Goodale, 1998). This measure can have a larger
slope. For example, Franz (2003) found a slope of 1.57 for this
measure (other studies found even larger slopes; e.g. Haffenden,
Schiff, & Goodale, 2001). Now, if we increased the physical size of
the object by 1 mm, then manual size estimation will increase by
approximately 1.57 mm. Similarly, if we induce an illusion of 1 mm,
then manual size estimation will also increase by approximately
1.57 mm. This effect might look larger as the corresponding effect of
0.82 mm in grasping. However, both effects are created by the same
illusion. Therefore, it is erroneous to compare the illusion effects of
different measures if they have different slopes. Only the corrected
illusion effects allow an adequate comparison. Unfortunately, the
vast majority of studies comparing manual size estimation to grasping did not perform this correction—and thereby systematically
overestimated the perceptual illusion in manual size estimation
(for a review see Franz & Gegenfurtner, in press).
We avoided these problems by using classic perceptual measures, like an adjustment procedure and a comparison with a
graded series (e.g. Coren & Girgus, 1972). These measures are known
to produce slopes close to 1, such that the difference of the slopes
between grasping and these perceptual measures is not as problematic as for manual size estimation. In addition, we measured
the slopes for each condition and calculated the corrected illusion effects. This is especially important because we did not know
whether the delay might change one of our measures. It could well
be that after a delay the slopes in grasping or perception change. For
example, because the information starts to decay, the slopes might
get shallower. This could lead to the same problems as described
above. Interestingly, we will see that this is not the case: the slopes
in grasping (as well as in perception) do not change with increasing
memory demands. This result and its implications will be discussed
further in Section 5.
1.4. Overview of this study
Before describing our experiments in detail, we want to give an
overview of the experiments and our main conclusions: in Experiment 1 we replicated the basic effect: in grasping, we found a
clear increase of the illusion if a delay of 5 s is introduced between
stimulus presentation and execution of the movement. This corresponds well to the literature (e.g. Gentilucci et al., 1996; Westwood
et al., 2000, 2001). In addition, we show, that the perceptual
effect of the illusion is not changed by the delay and that the
illusion effects in perception and grasping are similar after the
delay.
In the Experiments 2 and 3 we tried to disentangle the factors which might be responsible for the increase of the illusion
effects in grasping. For this, we independently varied the memory
demands imposed by the delay and the amount of visual feedback available during movement execution: in Experiment 2 we
show that the memory demands do not change the illusion effect
in grasping. This shows that the motor illusion is not changed by
the delay and thereby contradicts the perception–action hypothesis
and the real-time view of action. In Experiment 3 we systematically varied the amount of visual feedback available during grasping
and found that the availability of visual feedback can explain the
relatively small illusion effect under closed-loop conditions. We
conclude that (at least for the Müller-Lyer illusion) there is no evidence for two separate representations of object size that guide
actions. Instead, grasping behaves exactly as we would expect based
on the classic notion of online correction of errors (Woodworth,
1899) and the idea that perception and grasping are guided by a sin-
gle representation of object size that is deceived by visual illusions
(common-representation model, Franz et al., 2000).
Finally, we took the opportunity to test in Experiment 3 an objection (Goodale, 2006, in press) that has been raised against all our
studies on a potential dissociation between perception and action
in visual illusions (e.g. Franz, 2003; Franz, Bülthoff, & Fahle, 2003;
Franz et al., 2000, 2005). The main idea of this critique is that due to
some specifics of our setup participants might have grasped in an
unnatural and awkward way and that this grasping was guided by
the ventral stream. Therefore, we might never have been measuring
the vision-for-action system, but always the vision-for-perception
system. In consequence it would be no surprise that we found
effects of visual illusions in our grasping tasks. We performed the
first empirical test of this conjecture by directly comparing our
method with the method used by Goodale and co-workers (e.g.
Aglioti et al., 1995; Haffenden & Goodale, 1998; Haffenden et al.,
2001). We found no difference in the illusion effects on grasping,
thereby clearly refuting this conjecture.
2. Experiment 1: replicating the increase of the motor
illusion
In Experiment 1 we attempted to replicate the increased effect of
the Müller-Lyer illusion on grasping if a delay is introduced between
stimulus presentation and execution of the movement. For this, we
tested two extreme cases: in the closed-loop (CL) condition, full
vision of hand and stimuli is available and no memory component
is involved. In the open-loop delay (OL-Delay) condition, no vision
of hand and stimuli is available during performance of the task and
the visual information has to be stored for 5 s. According to the
literature, we expected a drastic increase of the illusion effect from
CL to OL-Delay.
2.1. Methods
2.1.1. Participants
Twenty-eight volunteers (16 female, 12 male) participated in the experiment,
ranging in age from 17 to 33 years (mean: 23.9 years). In return for their participation,
they either received course-credit or were paid 8 EURO (app. 11.5 US$) per hour.
Participants had normal or corrected-to-normal vision and were right-handed.
When setting up the experiment, we had a number of participants perform
only the perceptual task in order to test the setup (we used these participants to
test a graded series method versus an adjustment method). Therefore, twelve of the
participants performed only the perceptual task and sixteen participants performed
the perceptual task as well as the grasping task. Because the results were essentially
identical, we pooled their data in the perceptual task (see below in Section 2.1.4).
2.1.2. Stimuli
We used three-dimensional versions of the Müller-Lyer illusion (Fig. 2a). The
shaft of the Müller-Lyer figures were black plastic bars of different length (39 mm,
41 mm, 43 mm) and constant width (8 mm) and height (5 mm). For each bar we
individually printed a fin-in (FinIn) and a fin-out (FinOut) version of the Müller-Lyer
figure. In the FinIn figure the angle between shaft and fins was 35◦ and in the FinOut
figure it was 145◦ . The fins were positioned such that the edges of the bar were
clearly discriminable (Fig. 2a). The fins were 21 mm long.
2.1.3. Apparatus
Participants sat on a chair and used a chin rest to keep the position of the head
constant. They looked down at a 21-inch CRT monitor (Sony, Trinitron flat screen,
resolution 1280 × 1024 pixels, refresh rate 85 Hz, effective screen diagonal: 48.5 cm)
as if looking at the top of a table. The monitor was positioned at a distance of approximately 50 cm from the eyes. The screen of the monitor served as table for the
presentation of the Müller-Lyer figure, which were positioned 390 mm away from
the start position at which the participants rested their hand before each grasp.
The screen was tilted to be oriented perpendicular to gaze direction (angle relative to horizontal: 45◦ ). Participants wore liquid-crystal (LC) shutter glasses (Plato,
Translucent Technologies Inc., Toronto, Ontario, Canada; cf. Milgram, 1987) which
allow to efficiently suppress vision. The grasp trajectories were recorded using an
Optotrak 3020 system (Northern Digital Inc., Waterloo, Ontario, Canada) at a sampling rate of 100 Hz. Six infrared light-emitting diodes (LEDs) were mounted on two
small, lightweight flags (three LEDs per flag). The flags were attached to the finger
nails of thumb and index finger (Fig. 2a) using adhesive pastels (UHU-patafix, UHU
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
1521
closed for 5 s. During this time, the experimenter removed the Müller-Lyer stimuli
and prepared the comparison bars. Then the goggles opened and the tone sounded,
and the participant either selected a matching bar from the graded series or adjusted
the comparison bar (again without time limit).
Each participant performed in each of the two conditions 36 trials
(3 bar-lengths × 2 fin-orientations × 6 repetitions) in randomized order. A repeatedmeasures ANOVA with the between-subjects factor response (graded-series vs.
adjustment) and the within-subjects factors: condition (Perc-CL vs. Perc-OL-Delay),
bar (bar-lengths of: 39, 41, 43 mm), and illusion (FinIn vs. FinOut) showed that the
response had no differential effects (main effect response: F(1, 26) = 0.46, p = .51, all
seven interactions of response: p > .11). We therefore pooled the two groups for all
further analyses.
Fig. 2. (a) The 3-marker method traditionally used in our experiments on grasping
visual illusions (e.g. Franz et al., 2000). This method allows to calculate the trajectories of the typical grasp-points on the finger tips (using mathematical rigid-body
transformations) and ensures that the finger tips are completely free to receive tactile feedback. (b) The 1-marker method traditionally used in experiments of Goodale
and co-workers (e.g. Aglioti et al., 1995). Goodale (2006, in press) argued that this
method interferes less with the grasping movement, such that it might be better
suited to tap the dorsal stream. We tested this notion in Experiment 3.
GmbH, Bühl, Germany). Before the experiment, the typical grasp points on the fingers were determined and measured relative to the markers on the flags. Employing
mathematical rigid-body transformations on the three markers, this enabled us to
determine the trajectories of the grasp points for each finger.
All experiments were programmed in Matlab (MathWorks Inc., Natick, MA, USA),
using the Psychophysics Toolbox (Brainard, 1997; Pelli, 1997) and our custom build
Optotrak Toolbox (URL: http://www.allpsych.uni-giessen.de/vf/OptotrakToolbox).
Data–analysis was performed in Matlab and R (R Development Core Team, 2008).
2.1.4. Procedure
2.1.4.1. Grasping. Participants performed two motor conditions: Grasp-CL and
Grasp-OL-Delay (cf. Fig. 1). The conditions were performed in two blocks, thereby
employing the feedback schedule with maximum effect of delay (Heath et al., 2006).
The order of the blocks was counterbalanced across participants. Each trial started
with the experimenter preparing the Müller-Lyer figure. Then the LC-goggles opened
for a preview period of 1 s. In the Grasp-CL condition a tone (sine-tone, 1000 Hz,
100 ms) followed, indicating that the participant should grasp with a precision grip
the central bar of the Müller-Lyer figure with full vision of hand and stimuli. In
the Grasp-OL-Delay condition, the goggles closed after the preview period for 5 s.
Then the tone sounded, and the participant grasped the central bar without vision of
hand or stimuli. Participants were instructed to grasp natural and fast. For the whole
grasp (from the tone until removing the bar further than 50 mm away from the
Müller-Lyer figure) they had a total time of 4 s. Each participant performed in each
of the two conditions 48 trials (3 bar-lengths × 2 fin-orientations × 8 repetitions) in
randomized order.
2.1.4.2. Perception. When setting up the experiment, we were worried whether a
graded series would lead to different results than an adjustment method. To test this,
we had twelve participants perform a grades series and sixteen participants perform
an adjustment task (these sixteen participants also performed the grasping task).
In the graded series, participants selected a matching stimulus from a graded series
of bars (22 bars; lengths from 30 mm to 51 mm; stepsize: 1 mm; bar-widths: 8 mm)
that was printed on paper and presented 145 mm below the Müller-Lyer figure. In the
adjustment task, participants adjusted a comparison bar that was displayed on the
monitor to match the length of the shaft of the Müller-Lyer figure. The comparison
bar was presented 50 mm to the right of the target, randomly at one of two positions
(vertical offset ±25 mm) and had a random initial size between 20 mm and 51 mm
(step-size: 0.285 mm). We will see below that graded series and adjustment method
gave similar results.
Using either the graded series or the adjustment method, participants performed two perceptual conditions: Perc-CL and Perc-OL-Delay. The conditions were
performed in two blocks (with the order counterbalanced across participants). In the
Perc-CL condition, each trial started with the experimenter preparing the MüllerLyer figure and the comparison stimuli. Then the LC-goggles opened for a preview
period of 1 s, followed by a tone indicating that the participant should either select
a matching bar from the graded series or adjust the comparison bar (no time limit
was imposed on these responses).
In the Perc-OL-Delay condition, the experimenter first prepared the Müller-Lyer
figure (but not the comparison stimuli). After the 1 s preview period the goggles
2.1.5. Data analysis
From each grasp trajectory, we determined the following parameters: reaction
time (RT) was defined as the time between start of the auditory go-signal and movement onset (the first frame in which index finger or thumb exceeded a velocity
threshold of 0.025 m/s). Movement time (MT) was defined as the time between
movement onset and end of the movement (the first frame in which index finger or thumb came closer than 3 mm to the plane in which the grasp object was
placed). MGA was the maximum distance between thumb and index finger during
MT. Relative time to MGA was the relative time when MGA occurred within the MT.
If not specified otherwise, repeated measure ANOVAs were run on these parameters with the within-subject factors: condition (CL vs. OL-Delay), bar (lengths of
39, 41, 43 mm), and illusion (FinIn vs. FinOut).
To calculate corrected illusion effects we divided the mean illusion effects by
the mean slopes of the linear functions that relate physical size to the dependent
measure (MGA or perceived size). Standard errors for these corrected illusion effects
were calculated using the Taylor-approximation:
i
S.E.M. =
s
s2
s2
+
i2
i2
−2
is
is
with, i: mean illusion effect, s: mean slope, i2 : S.E.M. of the illusion effect, s2 : S.E.M.
of the slope, is : covariance of illusion effect and slope. This approximation is valid
because the slopes were highly significant different from zero. The statistical rationale for this procedure is discussed in Franz et al. (2005) and Franz (submitted;
preprint at arXiv:0710.2024); see also Buonaccorsi (2001).
A significance level of ˛ = .05 was used for all statistical analyses. p-Values above
.001 are given as exact values. For parameters which are given as A ± S.E.M., S.E.M.
is the standard-error of the mean.
2.2. Results
2.2.1. Grasping
MGA depended linearly on bar length with slopes of:
0.54 ± 0.098 (CL) and 0.48 ± 0.192 (OL-Delay); which is also
reflected in a main effect of bar length in the ANOVA (F(2, 30) = 20,
p < .001). Participants grasped overall larger in the OL-Delay condition than in the CL condition (main effect condition: F(1, 15) = 35,
p < .001), as can be seen in the left panel of Fig. 3. Participants
showed a reliable illusion effect (main effect illusion: F(1, 15) = 17,
p = .001). The illusion effect depended strongly on the condition
(interaction illusion × condition: F(1, 15) = 38, p < .001), All three
other interactions were not significant (all p > .17). Separate analyses showed that the illusion effect was non-significant in the CL
condition (t(15) = 1.1, p = .27) and significant in the OL-Delay condition (t(15) = 5.2, p < .001).
We also calculated temporal aspects of the grasping movement
(RT, MT). These were in a normal range and are summarized for all
experiments in Table 1.
In summary, participants grasped after the delay with larger
MGA, but with a similar slope. This result conforms well to the literature (cf. Hesse & Franz, submitted for publication). The illusion
effect was much larger in the OL-Delay condition (4.1 ± 0.78 mm)
than in the CL condition (0.4 ± 0.39 mm), as can be seen in the upper
right panel of Fig. 3. The corresponding corrected illusion effects
are shown in the lower right panel and show the same pattern of
results.
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V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
Fig. 3. Experiment 1: we replicated the increase of the Müller-Lyer illusion after a delay in grasping. The left panels depict the adjusted size in the perceptual task and the
MGA in the grasping task as functions of object size. The upper right panel depicts the illusion effect (calculated as the mean difference between FinOut and FinIn conditions).
The lower right panel depicts the corrected illusion effects (calculated by dividing the illusion effects by the slope). Errorbars depict ±1 S.E.M.
2.2.2. Perception
Perceived size depended linearly on bar length with slopes of:
0.79 ± 0.054 (CL) and 0.77 ± 0.064 (OL-Delay); see also the left panel
of Fig. 3.
Participants showed a reliable illusion effect (main effect
illusion: F(1, 27) = 157, p < .001). This effect was modulated by
bar-length (interaction bar × illusion: F(2, 54) = 10, p < .001). Fig. 3
shows, however, that this interaction was small in comparison to
the illusion effect and the effect of bar-length (main effect bar: F(2,
54) = 181, p < .001). The condition (CL vs. OL-Delay) had no effects
(main effect condition: F(1, 27) = 0.54, p = .47; all three interactions
with condition: p > .08). In summary, the illusion effect was quite
similar in the CL and the OL-Delay conditions (3.9 ± 0.25 mm and
3.9 ± 0.40 mm, respectively), as is also shown in the upper right
Table 1
Temporal parameters of grasping
Condition
RT
MT
t(MGA)
Mean
S.E.M.
Mean
S.E.M.
Mean
S.E.M.
Experiment 1 (N = 16)
CL
OL-Delay
412
440
22
28
908
1207
39
49
82
74
1.5
1.7
Experiment 2 (N = 8)
OL-Move
OL-Delay
549
474
37
45
1064
1182
87
96
79
80
2.1
3.2
Experiment 3 (N = 40)
CL
OL-Move-2/3
OL-Move-1/3
OL-Move
OL-Signal
301
304
335
354
304
10
11
13
16
12
755
748
803
813
829
21
22
32
31
28
80
81
81
80
78
1.4
1.3
1.2
1.2
1.0
Note: RT and MT are in ms, t(MGA) is the relative time to MGA in percent of MT.
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
1523
panel of Fig. 3. The corresponding corrected illusion effects are
shown in the lower right panel and show the same pattern of
results.
2.3. Discussion
We found a strong increase of the effect of the Müller-Lyer illusion on grasping in the OL-Delay condition as compared to the
CL condition. This replicates an effect that has traditionally been
counted as evidence for a transition from dorsal to ventral control
of the movement and for the notion that the undeceived dorsal
stream has a too short memory to bridge the 5 s delay imposed in
the OL-Delay condition (Goodale et al., 2004; Milner & Goodale,
1995; Westwood & Goodale, 2003).
However, in this traditional design there is a confound between
memory demands and the availability of visual feedback during
execution of the movement because the OL-Delay condition differs from the CL condition in two respects: participants have to
store the visual information for 5 s and they don’t see their hand
during execution of the movement. It is to be expected from the
motor literature that visual feedback during execution of the movement leads to online corrections which will reduce the measured
illusion effect, but are not indicative of a switch from dorsal to ventral control (Post & Welch, 1996). To disentangle these possibilities
we varied the availability of visual feedback independent of the
memory demands in the Experiments 2 and 3.
A second finding is that our classic perceptual measure of the
Müller-Lyer illusion gives similar illusion effects in the CL and the
OL-Delay conditions. This indicates that the perceptual illusion is
fairly constant, a fact which greatly simplifies the interpretation of
the data. In the Experiments 2 and 3 we can therefore concentrate
on grasping.
3. Experiment 2: the increase of the motor illusion is not
due to delay
In Experiment 2 we tested the influence of memory on the illusion effects while matching the amount of visual feedback available
during execution of the grasping movement. For this, we used the
same OL-Delay condition as in Experiment 1 but replaced the CL
condition with a condition, in which participants had full vision of
hand and stimuli during programming but not during execution of
the movement. That is, vision of hand and stimuli was prevented
as soon as the participants started to move their hand (OL-Move
condition, cf. Fig. 1). This condition ensures that participants cannot perform online corrections during execution of the movement
and can therefore be seen as the “standard” condition for studies on
the effects of visual illusions on grasping (as argued by numerous
researchers, e.g. Haffenden & Goodale, 1998; Post & Welch, 1996).
The availability of full visual information during programming of
the movement should, according to the perception–action hypothesis and to the real-time view of action, lead to an accurate,
undeceived programming of the movement in the dorsal stream.
Therefore, both theories predict that the illusion effect should
increase from the OL-Move condition to the OL-Delay condition.
If, on the other hand, the increase of the motor illusion found in
Experiment 1 is due to online corrections performed in the CL condition then the illusion effects should be similar in the OL-Move
and OL-Delay conditions.
3.1. Methods
Eight volunteers (3 female, 5 male) participated in the experiment, ranging in
age from 21 to 29 years (mean: 27.6 years). The methods were identical to the grasping task of Experiment 1, except that we now replaced the CL condition with an
OL-Move condition (cf. Fig. 1): the LC-goggles closed as soon as the participant had
Fig. 4. Experiment 2: increasing the memory demands did not change the effect
of the Müller-Lyer illusion on grasping. The perception–action theory and the realtime view of action both predict a large increase of the motor illusion between the
OL-Move and OL-Delay conditions. This was, however, not the case. The left panel
depicts the MGA in the grasping task as function of object size. The upper right
panel depicts the illusion effect (calculated as the mean difference between FinOut
and FinIn conditions). The lower right panel depicts the corrected illusion effects
(calculated by dividing the illusion effects by the slope). Errorbars depict ±1 S.E.M.
moved the hand away by 20 mm from the start position, thereby preventing vision
as soon as the movement had started.
3.2. Results
Results are shown in Fig. 4. MGA depended linearly on bar length
with slopes of: 0.48 ± 0.104 (OL-Move) and 0.69 ± 0.168 (OL-Delay).
This is also reflected in a main effect of bar length in the ANOVA
(F(2, 14) = 19, p < .001). Participants also showed a reliable illusion
effect (main effect illusion: F(1, 7) = 7.7, p = .027), which did not differ
between the OL-Delay and OL-Move conditions (interaction illusion × condition: F(1, 7) = 0.017, p = .9), All other main effects and
interactions were not significant (all p > .45).
Because the OL-Delay condition is identical to Experiment 1, we
compared the results of this condition across experiments. For this,
we calculated an ANOVA with the between-subjects factor experiment and the within-subjects factors bar length and illusion. As in
the separate analyses for each experiment, we found main effects of
bar length (F(2, 44) = 9.3, p < .001) and of the illusion (F(1, 22) = 32,
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V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
p < .001), while none of the other main effects or interactions were
significant. Most importantly, all effects involving the factor experiment were not significant (all p > .14), suggesting that we succeeded
in replicating the condition. As in the other experiments, we also
calculated the temporal aspects of the grasping movement (RT, MT).
These are shown in Table 1.
In summary, participants showed similar illusion effects in
the OL-Move and the OL-Delay conditions (2.0 ± 0.75 mm and
2.1 ± 0.92 mm, respectively), as can be seen in the upper right panel
of Fig. 4. The corrected illusion effects showed a similar pattern of
results (lower right panel of Fig. 4).
3.3. Discussion
We found no difference between the illusion effects in the
OL-Move and OL-Delay conditions. That is, the memory demands
imposed by the 5 s delay in the OL-Delay condition did not lead
to a strong increase of the illusion. In Section 5 we will present
a summary of all our experiments and of other studies and
show that the result of Experiment 2 is consistent with these
other data and therefore likely not due to a lack of statistical
power.
Taken together, this result suggests that the availability of visual
feedback during execution of the movement led to the difference
between CL and OL-Delay conditions in Experiment 1 (and not the
memory demands). In the next experiment we explored the effects
of visual feedback further.
4. Experiment 3: the increase of the motor illusion is due to
visual feedback
In Experiment 3 we attempted to further test our interpretation that the availability of visual feedback during execution of the
grasping movement is the critical factor for the relatively small
illusion effects in the CL condition of Experiment 1. For this, we
systematically decreased the amount of visual feedback in five
conditions (see also Fig. 1): (i) CL: full vision during execution
of the movement (this is identical to the CL condition in Experiment 1); (ii) OL-Move-2/3: full vision until the hand had been
transported 2/3 of the distance to the grasp object; (iii) OL-Move1/3: full vision until the hand had been transported 1/3 of the
distance; (iv) OL-Move: full vision until the hand had started to
move (this is identical to the OL-Move condition in Experiment
2) and (v) OL-Signal: full vision until the auditory start signal. If
our interpretation is correct, then we expect the illusion effects
to vary systematically with the amount of visual feedback, such
that the illusion effects should increase from CL, OL-Move-2/3,
OL-Move-1/3, OL-Move, OL-Signal. In addition, the OL-Signal condition allowed us to test the central assumption of the real-time
view of action (Goodale et al., 2004; Westwood & Goodale, 2003).
According to this variant of the perception–action hypothesis, there
should be a large difference between the illusion effects in the
OL-Move and OL-Signal conditions. The real-time view of action
assumes that the dorsal vision-for-action system only computes
the exact parametric values of the movement if the target object
is visible at the moment of movement programming. This is the
case in the OL-Move condition (because vision is available until the
hand starts to move, i.e. during programming of the movement),
but not in the OL-Signal condition (because vision is suppressed
as soon as the go-signal comes up, i.e. before programming of the
movement). Consequently, there should be no illusion effect in the
OL-Move condition (controlled by the dorsal stream) and the full
illusion effect in the OL-Signal condition (controlled by the ventral
stream).
4.1. Did we make a fundamental mistake in all our studies?
Before presenting the results, we need to explain one more
experimental manipulation used in Experiment 3. As discussed
in the Introduction, we had repeatedly found effects of certain
visual illusions on grasping in recent studies. Typically, these motor
illusions were of similar size as the perceptual illusions if the
task demands of motor task and perceptual task were carefully
matched (for reviews see: Franz, 2001; Franz & Gegenfurtner, in
press). This contradicts Milner and Goodale’s (1995) interpretation
of grasping, because they argued that grasping is immune to these
illusions.
To explain our results, Goodale (2006, in press) suggested that
we might have used a problematic method to assess the motor
illusion—and that this method might not tap the vision-for-action
system, but only the vision-for-perception system. Consequently, it
would be no surprise that we found similar effects of visual illusions
on grasping and on perception.
This argument is based on the fact that we used a different
method to attach the infrared markers to the fingers than was used
in the studies of Goodale and co-workers (e.g. Aglioti et al., 1995;
Haffenden & Goodale, 1998; Haffenden et al., 2001). The idea is that
our method led to unnatural grasping which might make the right
hand behave like an unskilled left hand. Because Gonzalez, Ganel,
& Goodale, 2006 suggested that the left hand is always controlled
by the vision-for-perception system, this would mean that we actually never measured vision-for-action with our grasping task, but
always vision-for-perception.
An example of our method can be seen in Fig. 2a: we attached to
the finger-nails of index finger and thumb small, lightweight flags,
each holding three infrared markers. Goodale and co-workers on
the other side attached only one marker to each finger (Fig. 2b). Our
3-marker method has two advantages: (a) employing mathematical
rigid-body transformations on the three markers, we determined
the trajectories of the grasp points for each finger. This is not possible with the methods of Goodale and co-workers. Therefore they
always had an additional measurement error; depending on the
thickness and orientation of the fingers (b) with our method the
finger tip is completely free, allowing for full tactile feedback. This
is not guaranteed with the method of Goodale and co-workers
because the tape they used to attach the single marker could cover
parts of the finger tip (as can, for example, be seen in Fig. 2 of Aglioti
et al., 1995 and Fig. 3 of Haffenden & Goodale, 1998).
But, maybe we traded these advantages for the disadvantage
of missing the dorsal vision-for-action system altogether—as suggested by Goodale (2006, in press)? We decided to perform the first
direct, empirical test of this notion. For this, we split the participants
in two groups. Both groups performed exactly the same task, except
that for one group we used our 3-marker method and for the other
group the 1-marker method. If the conjecture of Goodale (2006, in
press) is correct, then there should be large differences between
these groups.
4.2. Methods
Forty volunteers (33 female, 7 male) participated in the experiment, ranging in
age from 19 to 34 years (mean: 23.2 years). The methods were almost identical to
the grasping task of the other experiments, except for the following modifications:
As visual conditions we used, CL: full vision of hand and stimuli during grasping
(identical to the CL condition of Experiment 1). OL-Move-2/3 (OL-Move-1/3): full
vision until the hand had traveled 2/3 (1/3) of the way to the target; that is until it
had approached the target object by 120 mm (240 mm). OL-Move: the LC-goggles
closed as soon as the movement had started (identical to the OL-Move condition
of Experiment 2). OL-Signal: the LC-goggles closed when the start tone sounded,
thereby preventing vision during the RT-phase of the movement.
As in the other experiments, participants were instructed to grasp natural and
fast. For the whole grasp (from the tone until removing the bar further than 50 mm
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
1525
Fig. 5. Experiment 3: we split our sample in two groups and tested whether 3-marker method and 1-marker method (cf. Fig. 2) lead to different illusion effects, as hypothesized
by Goodale (2006, in press). This was not the case. The upper panel depicts the illusion effect (calculated as the mean difference between FinOut and FinIn conditions). The
lower panel depicts the corrected illusion effects (calculated by dividing the illusion effects by the slope). Errorbars depict ±1 S.E.M.
away from the Müller-Lyer figure) we reduced the total allowed time from 4 s (as
was used in the other experiments) to 3 s. This was done because we now had a
much larger number of conditions and the previous experiments had shown that
participants were much faster than 4 s to complete the movement. Each participant
performed 36 trials in each visual conditions, resulting in a total of 180 trials (3 barlengths × 2 fin-orientations × 6 repetitions × 5 visual conditions).
We split the forty participants in two groups. One group performed the experiment with our traditional 3-marker method (Fig. 2a) and the other group with the
1-marker method of Goodale and co-workers (Fig. 2b).
4.3. Results
First, we compared the illusion effects as determined by the two
methods to measure grasping (1-marker vs. 3-marker methods).
These results are shown in Fig. 5. We found a highly significant
overall effect of the illusion on grasping (main effect illusion: F(1,
38) = 76, p < .001). This effect did neither depend on the method
used to measure grasping (main effect 1- vs. 3-marker methods:
F(1, 38) = 0.042, p = .84) nor was there an interaction of the method
with the visual condition (interaction method × condition: F(4,
152) = 1.3, p = .28). The visual condition (CL, OL-Move, OL-Move2/3, OL-Move-1/3, OL-Move, OL-Signal) had a highly significant
effect on the illusion effect (main effect condition: F(4, 152) = 12,
p < .001).
Due to the lack of difference in illusion effects between the
1-marker and 3-marker methods, we pooled the data for further
analyses. A summary of these pooled data is shown in Fig. 6.
In all conditions, MGA depended linearly on bar length with
slopes of CL: 0.62 ± 0.087 mm, OL-Move-2/3: 0.60 ± 0.067 mm,
OL-Move-1/3: 0.53 ± 0.072 mm, OL-Move: 0.44 ± 0.073 mm, OLSignal: 0.46 ± 0.097 mm, as shown in the left panel of Fig. 6.
As shown by the ANOVA above, the illusion effects depended
strongly on the visual feedback condition. The mean illusion
effects were CL: 0.54 ± 0.27 mm, OL-Move-2/3: 1.08 ± 0.29 mm, OLMove-1/3: 1.73 ± 0.28 mm, OL-Move: 2.22 ± 0.26 mm, OL-Signal:
2.61 ± 0.32 mm, as shown in the upper right panel of Fig. 6. The
corresponding corrected illusion effects show the same pattern of
results and are shown in the lower right panel of Fig. 6. For the
temporal aspects of the grasping movement (see Table 1).
4.4. Discussion
We found two things: first, the illusion effects depended
strongly on the availability of visual feedback. Second, the illusion
effects as measured by the 1-marker and the 3-marker methods did
not differ. We will discuss these findings successively.
The main objective of Experiment 3 was to determine whether
the availability of visual feedback can explain the relatively small
illusion effect in the CL condition. And indeed, this is the case: the
more visual information about hand and stimuli was available during execution of the movement the smaller were the illusion effects.
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V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
Fig. 6. Experiment 3: changing the amount of visual feedback during movement execution changed the effect of the Müller-Lyer illusion on grasping. This indicates that
visual feedback is the critical factor for the increase of the illusion. The left panels depict the MGA in the grasping task as function of object size. The upper right panel depicts
the illusion effect (calculated as the mean difference between FinOut and FinIn conditions). The lower right panel depicts the corrected illusion effects (calculated by dividing
the illusion effects by the slope). Errorbars depict ±1 S.E.M.
This suggests that the availability of visual feedback is the critical
factor (and not memory demands) that leads to a modulation of the
illusion effects in grasping.
We also tested the real-time view of action by using the OLMove and OL-Signal conditions. The real-time view of action would
predict that between these two conditions the shift from dorsal to
ventral control happens. Therefore the main variation of the illusion effects should also happen between these two conditions. This,
however, was not the case. The illusion effects in OL-Move and
OL-Signal conditions were very similar, thereby providing evidence
against the real-time view of action.
Finally, we compared the illusion effects as measured by the
1-marker method and the 3-marker method. Goodale (2006, in
press) had suggested that in all our studies on grasping visual illusions (e.g. Franz, 2003; Franz et al., 2000, 2001, 2003, 2005) we
erroneously had measured vision-for-perception instead of visionfor-action and therefore it would be no surprise that we consistently
found illusion effects on grasping. We performed the first empirical test of this notion and found that our 3-marker method leads
to similar illusion effects as the 1-marker method used by Goodale
and coworkers, thereby refuting this conjecture.
Note, that there is a second reason why we think the conjecture
of Goodale (2006, in press) is not valid: if it were correct, then our
results should be “atypical”. That is, we should have obtained larger
illusion effects for grasping than other studies. But this is not the
case. In fact, the grasping data are surprisingly consistent across all
laboratories, as has been demonstrated recently in a detailed review
by Franz and Gegenfurtner (in press). We will show in Section 5 that
this is also true for the present data on the Müller-Lyer illusion.
5. General discussion
We tested whether a delay between stimulus presentation and
response leads to an increase of the effects of the Müller-Lyer illusion on grasping. In Experiment 1 we found that this is indeed the
case. Similar findings of an increased illusion effect on grasping
have been counted as evidence for a shift from a dorsal representation of object size (non-deceived, short memory) to a ventral
representation of object size (deceived, long memory).
However, in Experiment 2 we found that this increase of the
motor-illusion is not due to memory because removing visual
information about hand and stimulus during movement execution
restored the motor-illusion to about the same level as the motor
illusion after 5 s delay. In Experiment 3 we tested this notion further by removing visual information about hand and stimulus at
different times during movement execution.
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
1527
Fig. 7. Summary of all experiments. The data show that the perceptual illusion is not affected by delay and that the motor illusion depends on the time the hand is visible
during grasping. If vision of the hand is suppressed during grasping (in OL-Move, OL-Signal, and OL-Delay), then the corrected motor illusion is as strong as the corrected
perceptual illusion. Therefore the decrease of the illusion effect in grasping is due to the availability of visual feedback during the movement and not to memory demands.
For statistics on the difference between OL-Move (the standard grasping condition) and all other conditions see Table 2. Of course, the error correction by visual feedback is
not perfect such that we find even in the CL-condition some residual illusion effect (t(55) = 2.3, p = .024). Errorbars depict ±1 S.E.M.
This can also be seen in Fig. 7 which gives a summary of
the results of all three experiments: in the OL-Move condition
the corrected motor illusion is already at the level of the corrected perceptual illusion. In this condition, participants perform
their motor programming under full vision such that according
to the perception–action hypothesis and to the real-time view
of action the dorsal stream should control the movement and
there should be hardly any illusion effect. However, we find
a clear illusion effect in this condition. If we now introduce
delays relative to the OL-Move condition (in the OL-Signal and
OL-Delay conditions), these theories predict a switch to ventral
control, such that only now the motor illusion should emerge.
But, again, this is not the case: the corrected motor illusion is
already at the level of the corrected perceptual illusion and stays
about constant at this level (for statistics of these differences, see
Table 2).
On the other hand, if we allow more visual feedback during
movement execution (OL-Move-2/3, OL-Move-1/3, and CL conditions), we find a reduction of the motor illusion. This reduction is
to be expected according to classic notions of online correction of
errors (Woodworth, 1899; Post & Welch, 1996).
Based on these results, we conclude that it is the availability
of visual feedback and not a switch from dorsal to ventral control
that leads to the change of the illusion effects. But what about all
the other studies reporting evidence for an increase of the effects
of visual illusions on grasping after a delay? We will discuss these
studies in the following sections and argue that, surprisingly, our
data are not inconsistent with these studies and that the evidence
for a switch from dorsal to ventral control in most of these studies
is weak.
5.1. Other studies on delayed grasping of the Müller-Lyer illusion
Westwood et al. (2001) and Westwood et al. (2000) also investigated delayed grasping of the Müller-Lyer illusion. The conditions
used by them are a subset of the conditions used by us. Fig. 8 summarizes their results (using for consistency our terminology to label
the conditions). Comparing their results to our results shows that
the data for grasping are very consistent: both studies found essentially the same pattern of results for the motor illusion. We even
Table 2
Differences in corrected illusion effects between OL-Move and all other conditions
Condition
Lower
Mean
Upper
Sign
Perc-CL
Grasp-CL
Grasp-OL-Move 2/3
Grasp-OL-Move-1/3
Grasp-OL-Signal
Grasp-OL-Delay
Perc-OL-Delay
−2.0
−5.4
−4.7
−3.5
−2.2
−2.7
−2.0
0.1
−4.0
−3.0
−1.6
0.9
1.4
0.2
2.1
−2.6
−1.3
0.3
3.9
5.5
2.4
ns
*
*
ns
ns
ns
ns
Note: Differences are in mm and are relative to the OL-Move condition of grasping
(positive value: larger than in OL-Move). For an overview of the conditions, see
also Fig. 7. The column “Mean” is the mean difference; “Lower” and “Upper” are
95% confidence limits as calculated by a Taylor-approximation; “Sign” denotes the
significant differences: *p < .05.
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V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
Fig. 8. Results of Westwood et al. (2001) and Westwood et al. (2000). These results are very similar to our results: The motor illusion increases from CL to OL-Move and
OL-Signal and the additional memory demands in OL-Delay do not further change the motor illusion. Note that the full motor illusion in OL-Move clearly contradicts both,
the perception–action theory and the real-time view of action. In the lower panel, we performed a rough estimate of the corrected illusion effects for Westwood et al. (2001).
This was done because the perceptual task (manual size estimation) had a larger slope than grasping and therefore the perceptual illusion cannot be compared to the motor
illusion without correction (Franz, 2003). After correction, the illusion in manual size estimation is similar to the motor illusion in grasping. Data are from: Westwood et al.
(2001); Table 1, Figs. 3 and 4. Slopes for the conditions OL-Move, OL-Signal, and OL-Delay were estimated as being equal to the slope in CL (these slopes were not reported, but
no big difference in the slopes is to be expected between these conditions, cf. Hesse and Franz (submitted). Westwood et al. (2000); data are from a personal communication
with D. Westwood (June, 18th 2001). Errorbars depict ±1 S.E.M. For the corrected illusion effects these errorbars underestimate the size of the S.E.M. because we needed to
assume a S.E.M. of zero for the slopes (because these S.E.M. were not reported).
found a larger suppression of the motor illusion in the CL condition
than these studies.
At first sight, there seems to be only one slight inconsistency:
Westwood et al. (2001) found a somewhat larger perceptual illusion than motor illusion (the perceptual illusion is even larger than
the motor illusion in the OL-Delay condition). This can, however,
easily be explained by the fact that they used manual size estimation as perceptual measure, but did not correct for the larger slope
that is present in this measure (see Section 1 why this is important).
Because we know from other studies that manual size estimation
has often a larger slope than grasping (Franz, 2003), it can lead to
unusual large illusion effects as long as no correction is performed.
In the lower panel of Fig. 8 we performed a rough estimate of the
corrected illusion effects. This shows that after correction the illusion effect in manual size estimation likely is similar to the illusion
effect in grasping.
Therefore, even the data of Westwood et al. (2001) and
Westwood et al. (2000) do not provide strong evidence for the
perception–action theory and the real-time view of action. Also,
the fact that Westwood et al. (2001) found a strong illusion effect
in the OL-Move condition is clearly at odds with these theories,
because both theories predict that there should be no illusion effect
in this condition (the illusion should only emerge in the OL-Signal
condition).
Now, one might also argue that the Müller-Lyer illusion is a bad
test-case for the perception–action hypothesis, because this illu-
sion might be created very early, before the split of dorsal and
ventral streams. Therefore, it would be no surprise to not find a
perception–action dissociation for this illusion (Milner & Dyde,
2003). This would, however, not be a counter-argument against
our position, because all we are saying is that the Müller-Lyer illusion does not provide positive evidence for the perception–action
hypothesis. Why this is the case (because of an early split, or
because the perception–action hypothesis is wrong) we cannot
decide yet. But, our study provides important information, given
that the same authors did count studies on the Müller-Lyer illusion as positive evidence for the perception–action hypothesis. For
example, Goodale et al. (2004) write with respect to the study of
Hu and Goodale (2000; we will discuss this study in detail in the
next section): “the participants are presumably scaling their grasp
on the basis of their perceptual memory of the target’s size, which
was originally encoded in scene-based relative metrics. Similar
increases have been demonstrated in a variety of pictorial illusions
in which relative metrics and scene-based frames of reference drive
the illusion” (p. 137)—and then cite the studies of Gentilucci et
al. (1996) on pointing in the Müller-Lyer illusion and the study of
Westwood et al. (2000) which we just discussed. Similar citations
can be found, for example, in Goodale & Westwood (2004, p. 206).
In short, the data on delayed grasping the Müller-Lyer illusion
are surprisingly consistent across laboratories and do not provide
positive evidence for the perception–action hypothesis or the real
time view of action.
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
5.2. Other studies on delayed grasping of visual illusions
What about studies on delayed grasping of other visual illusions? It turns out that there are currently only two such studies.
The first and most prominent study on delayed grasping of visual
illusions was performed by Hu and Goodale (2000). Participants
grasped virtual cubes that were accompanied either by a larger
or a smaller second cube. This constitutes a simple size-contrast
illusion. In two experiments, Hu and Goodale (2000) found that
in their OL-Move condition (we again use our terminology for
consistency) grasping was not significantly affected by the illusion, while it was significantly affected in their OL-Delay condition.
They concluded from this pattern of results (no significant effect
in one condition vs. significant effect in the other condition) that
there is a difference between the conditions and interpreted this
as evidence for a shift from dorsal to ventral control. However,
this conclusion is statistically not valid. To come to the conclusion
that the two conditions are affected differently one would have to
test the difference of the effects in these conditions. This statistical problem is discussed in detail in Franz and Gegenfurtner (in
press) and the same issue has already been raised earlier by Cantor
(1956).
This is not a negligible problem. For the study of Hu and Goodale
(2000) it is possible to perform a recalculation of the correct
analysis from the published data, testing the difference of the illusion effects between OL-Move and OL-Delay conditions (cf. the
appendix of Franz & Gegenfurtner, in press). This analysis shows
that neither in Experiment 1 the difference of the illusion effects
between OL-Move and OL-Delay is significant (t(24) = 0.81, p = .42)
nor in Experiment 2 (t(26) = 1.68, p = .10). Even if we pooled the
two experiments to increase power, the combined difference is not
significant (t(52) = 1.12, p = .27). Therefore, we should not count Hu
and Goodale (2000) as strong evidence for an increase of the motor
illusion after a delay.
The second study on grasping other illusions than the MüllerLyer illusion with a delay was performed by Westwood and Goodale
(2003). This study used a size-contrast illusion similar to Hu
and Goodale (2000). And indeed, this study did find a significant
increase of the motor illusion when going from an OL-Move to an
OL-Signal condition. After this, there was no increase when going
to the OL-Delay condition.
In short, we are left with one study showing the increase of the
illusion effect (as suggested by the perception–action hypothesis
and the real-time view of action) and a number of studies on the
Müller-Lyer illusion not showing the increase. Therefore, the data
on delayed grasping of visual illusions are not as strong as they are
often presented in the literature and it seems necessary to replicate
the results of Westwood and Goodale (2003) if a strong argument
in favor of the perception–action hypothesis or the real-time view
of action shall be made.
Here is one reason why we tend to be skeptical that such an
endeavor will be successful: Westwood and Goodale (2003) used
a simple size contrast illusion (one object was accompanied by
a larger or smaller object). This is very similar to the Ebbinghaus illusion (one object is surrounded by a number of larger or
smaller objects). For the Ebbinghaus illusion, however, we and other
researchers argued that the motor illusion in OL-Move conditions is
already at the level of the perceptual illusion (Franz, 2001; Franz et
al., 2000, 2003; Franz & Gegenfurtner, in press; Pavani et al., 1999).
If this is true (not every researcher is convinced by this view, cf.
Haffenden et al., 2001; Goodale, in press), it already contradicts
both, the perception–action hypothesis and the real-time view of
action because both theories assume that in the OL-Move condition there should be hardly any motor illusion. In addition, if the
motor illusion is already at the level of the perceptual illusion, then
1529
introducing a delay cannot increase the illusion further, even if we
assume that during the delay a shift from dorsal to ventral control
would happen.
5.3. Relation to the planning-control model of Glover and Dixon
(2001)
We argued that the availability of visual feedback leads to the
changes of the illusion effects reported in most of the literature on
delayed grasping of the Müller-Lyer illusion—and not a switch from
dorsal to ventral control due to memory demands. But, if visual
feedback is important, how does this relate to the planning-control
model of Glover & Dixon (2001; Glover, 2004) which stresses the
online-control during a movement? Although the planning-control
model was not in the focus of this study (we tested and criticized
this model in detail in Franz et al., 2005), we want to shortly discuss
the implications of the current results for this model. We will argue
that the data do not support the planning-control model, but are
better explained by a common representation of object size for perception and action which is deceived by the illusion and corrected
if visual feedback is available during movement execution (i.e. the
common representation model, Franz et al., 2000).
Glover and Dixon proposed that motor acts are guided by two
different processes, first by a planning process and later by a control process (Glover & Dixon, 2001, 2002; Glover, 2002, 2004). They
assume that the planning process is ventral and deceived by visual
illusions, while the control process is dorsal and not deceived by
visual illusions. Therefore, they argue that a late movement parameter as the MGA will be little affected by visual illusions, because
at the time of the MGA the control system has already corrected
the “error” introduced by the illusion (e.g. Glover, 2004, p. 5, 11).
Note, that Glover and Dixon specify two sources for the correction:
the non-deceived representation in the dorsal control system and
visual feedback.
However, the use of visual feedback is not specific to their model
but is also assumed by classic motor control theories (Woodworth,
1899) and by the common-representation model (Franz et al.,
2000). Therefore, the predictions of the planning-control model do
not differ from the common-representation model as long as visual
feedback is available. Both models assume that the error introduced
by the illusion can be corrected by the use of visual feedback and
that therefore the illusion effect in MGA can be reduced if visual
feedback is available.
The predictions differ only in an open-loop condition without
visual feedback: the common-representation model assumes that
the illusion cannot be corrected because there is no visual feedback
available. Therefore the MGA should be affected by the illusion to
a similar degree as perception. The planning-control model, on the
other hand, assumes that the illusion might still be corrected due
to the switch to the non-deceived dorsal representation during late
phases of the movement (Glover & Dixon, 2002).
However, the corrected illusion effect in MGA in the OL-Move
condition was similar to the corrected illusion effect in perception
(Fig. 7). This is exactly what we expect from the commonrepresentation model. Therefore, we don’t need to assume a switch
from ventral to dorsal control and a non-deceived representation
in the dorsal system, as suggested by Glover and Dixon in their
planning-control model.
In short, our data do not support the planning-control model
and are better described by the common-representation model.
This is consistent with other studies which also came to a negative appraisal of the planning-control model (e.g. Franz et al.,
2005; Handlovsky, Hansen, Lee, & Elliott, 2004; Meegan et al.,
2004). We now return to our discussion of the perception–action
hypothesis.
1530
V.H. Franz et al. / Neuropsychologia 47 (2009) 1518–1531
6. Conclusions
We found that delayed grasping of the Müller-Lyer figure does
not provide evidence for a switch from one internal representation
(dorsal, not-deceived) to the other (ventral, deceived). The changes
found in the motor effects of the illusion can easily be explained by
the availability of visual feedback during movement execution. This
removes one piece of evidence that has traditionally been counted
as positive evidence for the perception–action hypothesis.
This result is consistent with Hesse and Franz (submitted for
publication), where we tested other evidence that has been put
forward for a shift from dorsal to ventral control of grasping after
a delay (Hu, Eagleson, & Goodale, 1999). We found the well-known
exponential decay of visual information for grasping—and also no
indication for a shift between two qualitatively different neuronal
control systems that control actions.
This fits well to other, similar critique of the evidence for
the perception–action hypothesis. For example, the finding that
the Ebbinghaus illusion should not affect grasping (Aglioti et al.,
1995) has been criticized seriously (for summaries of the critique, see Franz & Gegenfurtner, in press; Smeets & Brenner, 2006).
Similarly the classic distinction between cognitive vs. sensorimotor maps (Bridgeman et al., 1981, 1997) which is a predecessor
of the perception–action hypothesis has also been challenged
(Dassonville & Bala, 2004a, 2004b). These studies are intriguing
because Bridgeman had a similar notion that illusion effects on
pointing movements should increase after a delay—which should
also be indicative of a shift from one representation (motor map, not
deceived) to the other representation (cognitive map, deceived).
However, in a collaborative study Dassonville and Bridgeman found
that these findings can be better explained by a unitary representation of space (Dassonville et al., 2004). Also, Schenk (2006)
questioned whether the dissociation in the famous patient D.F.
is really between perception and action as suggested by Goodale
and Milner or maybe between different task demands. Other
researchers raised further concerns against Goodale and Milner’s
interpretation of the patient data (e.g. Pisella, Binkofski, Lasek, Toni,
& Rossetti, 2006).
In summary, this criticism might indicate that the division of
labor in the brain is not as suggested by the perception–action
hypothesis. Specifically, the notion that object size is calculated
twice, once in the ventral stream for perception (deceived by visual
illusions and with long memory) and once in the dorsal stream
for action (non-deceived and with short memory) seems problematic, given our results on the effects of delay on visual illusions and
grasping.
Acknowledgments
We wish to thank Tim Klucken for his careful help with the data
collection. This work was supported by grants DFG/FR 2100/1-2/3
and the research unit DFG/FOR 560 ‘Perception and Action’ by the
Deutsche Forschungsgemeinschaft (DFG).
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