Available online at www.sciencedirect.com
Vision Research 48 (2008) 63–69
www.elsevier.com/locate/visres
What does the illusory-flash look like?
David McCormick *, Pascal Mamassian
Laboratoire Psychologie de la Perception, CNRS & Université Paris Descartes, 45 rue des Saints-Pères, 75270 Paris Cedex 06, France
Received 21 February 2007; received in revised form 8 October 2007
Abstract
In the illusory-flash effect (Shams, L., Kamitani, Y., & Shimojo, S. (2000). Illusions. What you see is what you hear. Nature, 408, 788),
one flash presented with two tones has a tendency to be seen as two flashes. Previous studies of this effect have been ill-equipped to establish whether this illusory-flash is the result of a genuine percept, or that of a shift in criterion. We addressed this issue by using a stimulus
comprising two locations. This enabled contrast-threshold measurement by means of a location detection task. High-contrast white or
black flashes were presented simultaneously to both locations, followed by threshold contrast flashes of the same contrast polarity at the
two locations in half of the trials; observers reported whether or not the low-contrast flashes had been present. Irrelevant to the task, half
of the trials contained one tone, the other half contained two tones. In this way, we were able to compute the change in sensitivity and
shift in criterion between illusory and non-illusory trials. We observe both a decrease in visual sensitivity and a criterion shift in the illusory-flash conditions. In a second experiment, we were interested in determining whether this change in visual sensitivity gave rise to
measurable visual attributes of the illusory-flash. If it has a contrast, it should interact with a spatio-temporally concurrent real flash.
Using a similar two-location stimulus presentation, we found that under certain conditions, we were able to infer the polarity of the perceived illusory-flash. We conclude that the illusory-flash is indeed a perceptual effect with psychophysically assessable characteristics.
2007 Elsevier Ltd. All rights reserved.
Keywords: Multisensory perception; Audio–visual; Illusory-flash
1. Introduction
It is well established that what we perceive visually is
readily influenced by our auditory perception, and vice
versa. If cues from both modalities are temporally synchronous, the nervous system has been shown to categorise the
cues as emanating from the same external event at both
neural (Meredith, Nemitz, & Stein, 1987) and behavioural
(Bertelson & Radeau, 1981; Sekuler, Sekuler A. B., & Lau,
1997; Slutsky & Recanzone, 2001; Watanabe & Shimojo,
2001) levels. Low level audio–visual integration has also
been shown to be influenced by the spatial location of the
cues from the two modalities (Alais & Burr, 2004; Meyer,
Wuerger, Röhrbein, & Zetzsche, 2005). Multisensory
‘binding’ has been demonstrated in the presence of a temporal disparity between visual and auditory signals
*
Corresponding author. Fax: +33 142863322.
E-mail address: [email protected] (D. McCormick).
0042-6989/$ - see front matter 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.visres.2007.10.010
(McDonald, Teder-Salejarvi, & Ward, 2001; Meredith
et al., 1987). The ‘multisensory temporal integration window’ defines the potential extent of such a disparity that
can still facilitate such cross-modal perception. A robust
and highly persuasive demonstration of how audition and
vision influence each other can be seen in the McGurk
effect (McGurk & MacDonald, 1976). Here, lip and tongue
movements presented with incongruous auditory information can bring about a misperception of the auditory signal.
This interaction between hearing and vision in speech perception indicates that it involves information from more
than one sensory modality. Perhaps the most striking evidence for sensory trading in healthy individuals comes
from synaesthesia. In this condition, involuntary conscious
sensations from one sensory modality can be induced by
stimuli from another modality. For example, in sound–colour synaesthesia, individuals experience colours in
response to some aspect of a musical stimulus, such as
pitch or timbre. Furthermore, Ward, Huckstep, and Tsak-
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D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
anikos (2006) showed that there are consistent trends
among such synaesthetes to experience higher pitched
notes as more brightly coloured. In light of this and the
presence of similar patterns of pitch-brightness matching
in non-synaesthetic subjects, they posit that this form of
synaesthesia shares mechanisms with non-synaesthetes.
It is clear that different sensory modalities do not necessarily process perceptual information separately and it is of
interest to establish how, and under what circumstances,
such multi-modal interactions take place.
A useful strategy for disambiguating the contributions
of individual modalities in multimodal perception is to
put sensory modalities in conflict with one another in situations where they would normally concur (as with the
McGurk effect). Shams et al. (2000) demonstrated that a
single flashed disc, presented with two brief tones, has a
tendency to be perceived as two flashes. This finding
illustrated that vision does not always win when vision
and audition are in conflict. They went on to investigate
this effect using the brain imaging techniques EEG (Bhattacharya, Shams, & Shimojo, 2002; Shams, Kamitani,
Thompson, & Shimojo, 2001) and MEG (Shams, Iwaki,
Chawla, & Bhattacharya, 2005). The MEG study showed
that activity in the visual cortical areas could be modulated
by sound, with modification of activity in occipital, parietal and anterior areas. The EEG studies indicated significantly higher oscillatory and induced gamma band
responses in illusory than non-illusory trials, as well as
significant supra-additive audio–visual interactions in
illusory trials. Furthermore, the brain potentials for the
illusory-flashes were similar to those for a genuine flash.
These studies connote neurophysiological correlates to
the perception of the illusion; the processing of information in visual cortical regions can be modulated by sound.
It is not clear, however, that a visual cortical area response
to an auditory stimulus implies a perceptual event having
been ‘seen’.
Other research in this area has demonstrated that, under
certain conditions, an equivalent but opposite ‘fusion’
effect is also possible; one tone presented with two flashes
can give rise to the perception of only one flash (Andersen,
Tiippana, & Sams, 2004). This ‘illusory-flash effect’, plus
analogous visuo-haptic (Ernst, Banks, & Bülthoff, 2000)
and audio-haptic (Violentyev, Shimojo, & Shams, 2005)
effects, have recently received considerable attention in
the literature, both from experimental and modelling
(Andersen, Tiippana, & Sams, 2005; Ernst & Banks,
2002; Shams, Iwaki, Chawla, & Bhattacharya, 2005a;
Shams, Ma, & Beierholm, 2005b) perspectives. Much of
the research into this effect has purported to show that
the illusion is the result of a genuine perceptual effect, suggesting that the illusory-flash has characteristics of appearance akin to those of real flashes. As yet, however, no
psychophysical study has sought to answer questions pertaining to the specifics of what one might perceive at the
time of the illusion. Here, we are interested in establishing
whether the sound merely creates confusion, or whether it
evokes illusory-flashes with visual characteristics similar to
those resulting from genuine visual stimulation—a perceptual event.
We created a new experimental set-up in which we
wanted to present two simultaneous illusory-flashes at
two separate screen locations. In our first experiment,
we were interested in two aspects of the illusory-flash as
they pertained to our design. First, to measure the efficacy
of the method in generating illusory-flashes. Second, and
more importantly, to ascertain whether the effect’s incidence was the product of more than a shift in observers’
criteria alone. That is, did the presence of two tones presented with one flash bring about a change in visual sensitivity? We found that not only was our method
proficient in generating illusory-flashes, but that trials in
which illusory-flashes were reported were associated with
both a shift in observers’ criterion and a decrease in visual
sensitivity.
If the illusory-flash has a measurable contrast, it should
interact with a real flash presented at the same time and
location. If this is the case, and if real and illusory flashes
were to occur simultaneously, we would expect to see an
enhanced detectability of a real flash with the same contrast
polarity as that of an illusory-flash. Conversely, a flash of
opposite contrast to the illusory-flash would diminish in
detectability. With this premise in mind and the confirmed
effectiveness of our presentation method in experiment 1,
we devised a second experiment. In this second experiment,
one location contained only one high-contrast flash, while
the other contained a high-contrast flash followed by a
low-contrast flash. Observers were then required to indicate which of the two locations contained two flashes, that
is, which of the two locations contained a low-contrast
flash. We were then able to infer what, if any, influence
the presence of an illusory-flash had on the perceived contrast of a real flash by determining the difference in observers’ thresholds in the one-tone (non-illusory) and two-tone
(illusory) conditions.
2. Experiment 1
2.1. Methods
2.1.1. Subjects
Eight observers (5 male, 3 female, aged 23–35) with normal or corrected-to-normal vision participated without
payment in experiment 1.
2.1.2. Apparatus
Stimuli were presented via an Apple PowerMac G4
computer on a 2100 Sony FD Trinitron CRT monitor at a
refresh rate of 75 Hz. The spatial resolution of the display
was 1024 · 768 pixels. Observers were positioned at a distance of 57 cm from the monitor, maintained by a chinrest. The experiment was conducted in a dark room. Stimuli were generated using MATLAB and the Psychophysics
D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
Toolbox (Brainard, 1997; Pelli, 1997). Tones were presented through Sennheiser HD280 headphones.
2.1.3. Design
Experiment 1 comprised two parts: first, two preliminary sessions were conducted for each observer to obtain
their contrast detection thresholds. Second, each observer
ran 10 blocks of trials in the main experiment. Two square
locations comprising mid-level grey, each measuring four
degrees of visual angle across, were embedded in a 25%
contrast grey-level checkerboard pattern. The inner edges
of the locations were separated horizontally by 10 visual
angle. Observers were required to maintain fixation on a
cross midway between the two locations. In the two preliminary sessions, observers’ thresholds for determining the
location (left or right) of a low-contrast flash presented
after a high-contrast flash were measured. These sessions
comprised two-interleaved staircases of stochastic approximation (Robbins & Monro, 1951, as cited in Treutwein,
1995) converging on 75% correct location detection. White
and black first flashes were separated into these two sessions and the low-contrast flashes had the same contrast
polarity as the high-contrast flashes. First, a high-contrast
flash of 2 frames (26 ms) was presented simultaneously to
each location. Second, an inter-stimulus interval (ISI) of
4 frames (52 ms) was presented (mid-level grey at both
locations). Third, a low-contrast flash of 2 frames (26 ms)
was presented at one of the two locations (left or right)
while the other location remained mid-level grey. The location of the low-contrast flash was determined pseudo-randomly. The background outside of the checkerboard
pattern was mid-level grey. In these sessions, one tone
(3750 Hz, 26 ms) was always presented simultaneous with
the low-contrast flash. The task in these preliminary sessions was to determine at which of the two locations the
low-contrast flash occurred; observers were required to
press the left cursor key if it occurred at the left location
or the right cursor key if it occurred at the right location.
No feedback was given.
These 75% correct location detection threshold values
were then used as the low-contrast flashes in the main
experiment. Observers ran 10 blocks of trials and alternate blocks contained white and black high-contrast
flashes. High-contrast flashes were presented at both
screen locations simultaneously. These were followed by
low-contrast flashes (of the same contrast polarity) in half
of the trials. When they occurred, the low-contrast flashes
were present in both screen locations. Half of the trials
contained one tone, half contained two tones (3750 Hz,
26 ms). When only one tone was present, it occurred
simultaneous with the low-contrast flashes. When two
tones were presented, they occurred simultaneous with
the two flashes. Fig. 1 depicts a diagrammatic representation of the stimulus set-up. Observers reported whether
low-contrast flashes had been present on a trial using
the computer keyboard. d’ and criterion (c) values were
65
computed for the four combinations of high-contrast flash
colour and number of tones.
2.2. Results
In signal detection terms, a false alarm is an instance
where an observer reports having seen something which
was not physically present. In this experiment, an illusory-flash is evidenced by the presence of a false alarm.
That is, when observers reported having seen two flashes
when only one was present. Our data contain two sets of
such false alarms: the first when only one tone was present
(FA1); the second when two tones were present (FA2). The
illusory-flash as described in the literature is the result of an
audio–visual interaction when tones outnumber flashes.
Therefore, an accurate measurement of the proportion of
illusory-flashes for any given observer is the difference
between these two sets of false alarm rates:
FA2 FA1 = D.
Using this method, we find a higher false alarm rate in
the two-tone than one-tone trials, but that the colour of
the initial flash (white or black) has no bearing on the number of illusory-flashes reported: D = 0.15 in white-first-flash
trials and 0.14 in black-first-flash trials. We used an exact
binomial test (Hollander & Wolfe, 1973) to establish that
FA2 was significantly greater than FA1 in the white first
and black first flash conditions. In both instances, exact
binomial p (one-tailed) < .05.1 This illustrates that our
method was indeed effective in generating illusory-flashes
in approximately 15% of trials.
Signal detection analysis reveals no differences in sensitivity (d 0 ) or criterion (c) between black- and white-firstflash trials. However, in both of these conditions, d 0 was
reduced—from 1.6 to 0.8 across observers—when a flash
was accompanied by two tones relative to when only one
tone was presented: F(1, 30) = 18.8, p < .05. This indicates
that the presence of illusory-flash inducing tones reduces
visual sensitivity. We also see an accompanied shift in criterion—from 1.1 to 0.5 across observers—between these
two conditions indicating the illusion pertains to more than
just visual sensitivity: F(1, 30) = 45.1, p < .05. Fig. 2 is a
receiver operating characteristic (ROC) diagram with hit
rates plotted against false alarm rates for black/white first
flashes with one/two tones. This figure indicates that d 0 is
reduced by approximately 1 in the trials containing two
tones. It also shows that there is very little difference in
the proportions of hits and false alarms between the whiteand black-first-flash trials. ROC diagrams of the individual
observers’ data are available as supplementary material,
online.
We thus established the efficacy of our method in generating illusory-flashes. We also witnessed the second tones’
1
We report the result of an exact binomial test because there is no
guarantee that data are normally distributed. t-Test results for the same
data are: white first flash, t(14) = 3.7, p < .05; black first flash, t(14) = 3.3,
p < .05.
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D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
Auditory
Visual
1000ms
26ms
26ms
52ms
26ms
26ms
>300ms
Fig. 1. Stimulus set-up and timeline for the main part of experiment 1. One tone was always presented simultaneous with the low-contrast flashes. Half of
the trials also contained a tone simultaneous with the high-contrast flashes. Low-contrast flashes had the same contrast polarity as the preceding highcontrast flashes: white high-contrast flashes with lighter than mid-level grey low-contrast flashes are shown.
1
3. Experiment 2
N=8
d' = 2
3.1. Methods
d' = 1
0.8
3.1.1. Subjects
Twelve graduate student observers (7 male, 5 female,
aged 23–35) with normal or corrected-to-normal vision
participated without payment in the two conditions of
experiment 2.
d' = 0
Hit Rate
0.6
3.1.2. Apparatus
Apparatus for experiment 2 was the same as experiment 1.
0.4
2 Tone, White 1st Flash
2 Tone, Black 1st Flash
1 Tone, White 1st Flash
1 Tone, Black 1st Flash
0.2
0
0
0.2
0.4
0.6
0.8
1
False Alarm Rate
Fig. 2. A receiver operating characteristic (ROC) diagram with hit rates
plotted against false alarm rates, averaged across observers with standard
errors. Filled symbols represent black-first-flash trials, open symbols
represent white-first-flash trials with one (triangles)/two (squares) tones.
Dashed lines represent iso-sensitivity curves for d 0 values of 0, 1 and 2.
effect on observers’ visual sensitivities and criteria. It
should be noted that signal detection theory does not make
any claims on a responder’s conscious state and hence is
not qualified to permit an interpretation of the data in
terms of what a responder has ‘perceived’. Subsequently
we were interested to know if this lowered visual sensitivity
would be manifested in a perceptual event. That is, is there
any extent to which we can measure what observers see at
the time of the illusory-flash?
3.1.3. Design
Stimulus presentation (potential locations and timing)
remained similar to the preliminary sessions in experiment
1. A low-contrast flash always occurred after two initial
simultaneous high-contrast flashes, and was presented to
one location only (varying from trial to trial, determined
pseudo-randomly) while the opposite location remained
at mid-level grey. The low-contrast flash could have either
contrast polarity. White and black first flashes were separated into two conditions—condition 1: white; condition
2: black. Observers ran 10 blocks of trials per condition,
each of which comprised two-interleaved staircases converging on 75% (threshold) correct. Alternate blocks contained one or two tones with the same timing and
frequency as those of experiment 1. Threshold detection
values were averaged over the 10 blocks for the four combinations of low-contrast flash polarity and number of
tones. In each condition (white first flash/black first flash)
there were four types of trial; the number of beeps (1/2)
was blocked, while the contrast polarity (above/below
mid-level grey) of the low-contrast flash varied pseudo-randomly from trial to trial.
D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
a
0.35
White 1st Flash
Contrast Threshold
TA
2Tones
1 Tone
0.3
0.25
Difference in Light Flash Contrast Thresholds
b
67
0.1
White 1st Flash
0.05
0
-0.05
-0.1
0.2
Dark
Light
Low-Contrast Flash Polarity
-0.1
-0.05
0
0.05
0.1
Difference in Dark Flash Contrast Thresholds
Fig. 3. (a) Mean absolute threshold contrast detection values for a typical observer with white first flashes—split into light/dark low-contrast flashes with
standard errors across blocks. Filled discs represent two-tone thresholds, open discs represent one-tone thresholds. (b) Summary plot of the 12 observers’
data with white first flashes. Each observer’s mean thresholds for the one-tone trials are subtracted from those of the two-tone trials for light/dark lowcontrast flashes. The diagonal (bottom left to top right) therefore represents a dimension along which there is no difference between the change in light/
dark low-contrast flashes when in the presence or absence of two tones. Means/standard errors across observers shown.
3.2. Results
In condition 1, the high-contrast flashes were white.
Fig. 3(a) shows absolute threshold contrast detection values for a typical observer. These data are split into
instances where the low-contrast flash was either lighter
or darker than mid-level grey.
Fig. 3(b) is a summary plot of the 12 observers’ data.
This was generated by subtracting the threshold values
for the one-tone trials from those of the two-tone trials
for both the light and dark low-contrast flashes. The diagonal (bottom left to top right) therefore represents a dimension along which there is no difference between the change
in light and dark second flashes when in the presence or
absence of two tones. The observers’ data tend to lie below
this diagonal, indicating that the presence of two tones
(and thus an illusory-flash) facilitates the detection of a real
light flash, but not that of a dark one. We conducted an
exact binomial test of the probability of this configuration
of points around the diagonal.2 The test indicates that we
can discount the null hypothesis that this arrangement happened by chance and that a significantly greater proportion
of points lie below the diagonal (exact binomial p (twotailed) < .05). From this result, we infer that when illusory-flashes were preceded by high-contrast white flashes,
they had a tendency to be perceived as light flashes.
2
t-Test results for the same data are: condition 1, t(22) = 2.7, p < .05;
condition 2, t(22) = 0.1, p = .89.
In condition two, the high-contrast flashes were black.
Fig. 4(a) again shows data for a typical observer, and
4(b) shows a summary of all observers’ data. In condition
one, where initial flashes were white, the results point to the
perception of light illusory-flashes. In condition two, where
the initial flashes were black, we might expect to see the
opposite effect—dark illusory-flashes. In this condition,
however, there is little deviation of the data from the diagonal and the exact binomial test indicates an insignificant
deviation in either direction (p = .77).
4. Discussion
The experiments reported here lend further support to
the assertion that the illusory-flash is at least partly
explained in terms of a genuine perceptual effect and not
simply the product of artefacts (Shams, Kamitani, & Shimojo, 2002). It should be noted, however, that a significant
proportion of the effect, classically described, is almost certainly attributable to an increased willingness (in a nonvisual decision) to report having perceived a greater number
of flashes than were presented when they occurred in the
presence of a greater number of concomitant tones (McCormick & Mamassian, 2005). This can be explained in terms of
the multi-sensory integration window—illusory-flashes will
tend to occur when flashes and tones are not separated by
more than approximately 100 ms (Shams et al., 2000)—coupled with the fact that the auditory system has a higher temporal resolution than that of the visual system. We have
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D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
a
0.35
Black 1st Flash
Contrast Threshold
TA
2 Tones
1 Tone
0.3
0.25
Difference in Light Flash Contrast Thresholds
b
0.1
Black 1st Flash
0.05
0
-0.05
-0.1
0.2
Dark
Light
Low-Contrast Flash Polarity
-0.1
-0.05
0
0.05
0.1
Difference in Dark Flash Contrast Thresholds
Fig. 4. (a) Mean absolute threshold contrast detection values for a typical observer with black first flashes—split into light/dark low-contrast flashes with
standard errors across blocks. Filled discs represent two-tone thresholds, open discs represent one-tone thresholds. (b) Summary plot of the 12 observers’
data with black first flashes; details as per Fig. 3(b).
illustrated here, however, that this criterion shift cannot be
exclusively responsible for the effect; a change in visual
sensitivity must also be taken into account.
In experiment 1, we have demonstrated that the illusoryflash effect is attributable to both a decrease in visual sensitivity when tones outnumber flashes and a simultaneous
shift in observers’ criterion as to what constitutes a flash.
This is in line with results from the haptic illusory-flash
study of Violentyev et al. (2005).
Previous studies of the illusory-flash have tended to be
susceptible to response bias. Because we are interested in
genuine visual attributes of this effect, by using a location
detection task, experiment 2 measures along a different
dimension than ‘present/absent’ and overcomes biases
inherent in ‘yes/no’ tasks. Using this paradigm, we did
not measure the prevalence of the illusory-flash directly,
but were able to infer its properties nevertheless.
In the two-tone trials in both experiments, we envisaged
there being an effect of the first tone cueing the timing of
the second tone, and hence the onset of the low-contrast
flash. This was a necessary asymmetry between the two
types of trials (one tone/two tone). The absence of a sensitivity change between one- and two-tone trials in experiment 2, condition 2, however, renders it unlikely that
cueing affected the results in either experiment. In experiment 2, our results elucidate a trend suggesting that the
illusory-flash can have a measurable contrast: lighterthan-grey low-contrast flashes have a tendency to increase
in salience in the presence of simultaneous tones when they
are preceded by high-contrast white flashes. This indicates
that, in this instance, the illusory-flash is of the same
contrast polarity of contrast as the flash which precedes
it. However, this effect is evident only when the low-contrast flash was preceded by a high-contrast white flash (darker-than-grey flashes are not better detected following a
high-contrast black flash).
We witnessed a relatively small proportion of illusoryflashes in the data from experiment 1 compared to previous
studies of this effect. This is most likely attributable to two
factors. First, the inherent assumption that illusory-flashes
will be created at both locations. While we have no reason
to believe that this is not the case, it may have increased
doubt for observers and hence caused them to choose a
more conservative estimate of whether or not two flashes
were indeed present. Second, observers had perceptual
access to a genuine (albeit threshold) real flash in half of
the trials. It is likely that this point of reference would also
reduce willingness to report having seen two flashes in oneflash trials.
Our signal detection data from experiment 1 indicate
that illusory-flashes preceded by real white or black flashes
involve the same perceptual processes. The asymmetry in
the data from experiment 2 could be attributable to differences between the physical symmetry (on a calibrated
screen) and the perceptual symmetry of what constitutes
the mid-point between black and white; this seems unlikely,
however, given the lack of asymmetry witnessed between
black first flash and white first flash trials in experiment 1. Furthermore, experiment 2 was not attuned to
establishing which two-tone trials were perceived as indeed
containing illusory-flashes (experiment 1 indicates that
approximately only 15% did). Thus, a genuine effect in
D. McCormick, P. Mamassian / Vision Research 48 (2008) 63–69
the data could well have been contaminated by the necessary inclusion of those trials which were not perceived as
illusory.
We have illustrated that the sound induced illusory-flash
is a product of both a shift in observers’ criteria and a
change in visual sensitivity. Furthermore, we have shown
that under certain conditions, inferences regarding the perceptual nature of the illusory-flash can be made. Our data
reject a model whereby the illusory-flash is the results of a
lower threshold for after-image detection (because this
would predict perceived illusory-flashes of the opposite
contrast polarity to the real high-contrast flash). They also
appear to reject a model in which the illusory-flash is
always the same contrast polarity as the initial inducing
flash. Our paradigm is attuned to measuring the perceived
contrast of the illusory-flash; further study using a similar
technique could establish whether other visual attributes
(for example shape and orientation) are equally susceptible
to influence via the illusory-flash.
Acknowledgements
We would like to thank Tobias Andersen, Simon Barthelmé and Andrei Gorea for their help. This research was
funded by a chaire d 0 excellence from the French Ministry
of Research.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/
j.visres.2007.10.010.
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