psycho.belg.2011_2.book Page 177 Tuesday, June 28, 2011 3:18 PM
Psychologica Belgica
2011, 51-2, 177-192
DOI: http://dx.doi.org/10.5334/pb-51-2-177
177
TRANSSACCADIC MEMORY FOR THE POSITION OF
STATIONARY AND TRANSLATING BIOLOGICALMOTION WALKERS
Goedele VAN BELLE(1, 2), Peter DE GRAEF(1), & Karl VERFAILLIE(1)[1]
(1) Katholieke Universiteit Leuven & (2) Université Catholique de Louvain
Previous research demonstrated an advantage for translating objects over stationary objects in transsaccadic displacement detection. However, in some
studies, this benefit was absent. The current study was designed in order to
clarify the basis of these contradictory findings. To this end, the procedure of
an experiment with a clear motion benefit was combined with the stimuli of a
study in which the motion benefit was absent. Participants saccaded towards
either a stationary or a translating point-light walker and had to detect the
intrasaccadic displacement of either the saccade target or the saccade flanker.
Intrasaccadic displacements of the translating walker were found to be easier
to detect than displacements of the stationary walker. Furthermore, displacements of the saccade target walker were better detected than displacements of
the flanking walker. Implications for the previously contradictory observations
are discussed and an explanation is proposed emphasising the differential
importance of spatiotopic coding when a viewer is engaged in smooth object
pursuit rather than having a stable fixation before making a saccade towards a
translating object.
Introduction
High-acuity vision is restricted to the small central foveal and parafoveal
part of the visual field (e.g., Anstis, 1974; Loschky, McConkie, Yang, &
Miller, 2005). Therefore, during scene exploration, the eyes constantly
alternate between short periods of relatively stable eye position (fixations)
and very fast oculomotor jumps (saccades), in order to project new objects
of the scene onto the high acuity foveal region of the retina (e.g., De Graef,
Christiaens, & d’Ydewalle, 1990; Einhäuser, Spain, & Perona, 2008; Henderson & Hollingworth, 1998; Rayner, 1998). However, eye movements also
1.
Goedele Van Belle, Laboratory of Experimental Psychology, Katholieke Universiteit Leuven, and Institute of Psychological Sciences, Université Catholique de Louvain; Peter De
Graef, Karl Verfaillie Laboratory of Experimental Psychology, Katholieke Universiteit
Leuven.
The research was supported by a grant from the Research Foundation Flanders FWO
(G.0583.05N Transsaccadic perception of moving objects in an interactive environment).
Correspondence concerning this article should be addressed to Karl Verfaillie, Katholieke
Universiteit Leuven, Laboratory of Experimental Psychology, Tiensestraat, 102, B-3000
Leuven. E-mail: [email protected]
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TRANSSACCADIC MEMORY FOR THE POSITION OF STATIONARY
create new problems for the visual system. Indeed, saccades shift the image
on the retina, so that information enters the visual system in a temporally
distributed and spatially fragmented way. Nevertheless, in everyday life,
human observers perceive the world as stable and unified. Vision scientists
are therefore confronted with the question whether and how transsaccadic
integration is achieved: is image information gathered on presaccadic fixation n-1 carried across the saccade in transsaccadic memory and then integrated with information acquired on postsaccadic fixation n in order to
achieve a stable percept (e.g., Demeyer, De Graef, Wagemans, & Verfaillie,
2009; Germeys, De Graef, Van Eccelpoel, & Verfaillie, 2010; Irwin, 1991;
Martinez-Conde, Krauzlis, Miller, Morrone, Williams, & Kowler, 2008;
Melcher & Colby, 2008; Van Eccelpoel, Germeys, De Graef, & Verfaillie,
2008; Verfaillie & De Graef, 2001)?
One experimental paradigm that has proven to be a useful tool for
investigating the content and the spatial extent of transsaccadic memory
and transsaccadic integration is the transsaccadic change detection paradigm (e.g., Henderson & Hollingworth, 1999; Rayner, McConkie, & Zola,
1980; Verfaillie, De Troy, & Van Rensbergen, 1994). In this paradigm, an
aspect of the stimulus is changed during the saccade (intrasaccadically) on
a proportion of the trials. The subjects’ task is to judge whether or not a
change occurred. Because the information entering the visual system during the saccade is not useful due to retinal ‘smearing’, intrasaccadic
changes normally can only be noticed by comparing the postsaccadic
image with information about the presaccadic image, stored in transsaccadic memory. The ability to notice a certain type of change is therefore
interpreted as evidence that the type of information that was changed is
actually stored in transsaccadic memory. Vice versa, if changes in an
object’s features go unnoticed, then these features are assumed either not
to be stored in transsaccadic memory or not to be integrated with postsaccadic information.
Research with the transsaccadic change detection paradigm has shown
that even very large intrasaccadic changes in contrast, luminance, colour,
location, and shape often remain unnoticed, a phenomenon known as transsaccadic change blindness (e.g., Grimes, 1996; Henderson, Brockmole, &
Gajewski, 2008; McConkie & Currie, 1996). Particularly intrasaccadic
changes of object position are extremely hard to detect (e.g., Bridgeman,
Hendry, & Stark, 1975; Li & Matin, 1990; Mack, 1970; Stark, Kong,
Schwartz, Hendry, & Bridgeman, 1976; Verfaillie, 1997; Verfaillie & De
Graef, 2000; Verfaillie et al., 1994; Wallach & Lewis, 1966), suggesting
that an object’s position is not accurately stored and integrated across saccades.
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GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
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However, Deubel, Schneider, and Bridgeman (1996) discovered that
externally interrupting the visual input after the saccade (by inserting a
postsaccadic blank before the reappearance of the visual stimulus) results in
much more accurate detection of intrasaccadic position changes (see
Deubel, Bridgeman, & Schneider, 1998; Deubel, Koch, & Bridgeman,
2010; Deubel, Schneider, & Bridgeman, 2002, for related research on the
blanking effect). These findings suggest that position information is in fact
stored in transsaccadic memory, but, under normal circumstances, the information is unavailable to conscious perception. One possible reason, originally advanced by Deubel and colleagues (also see Demeyer, De Graef,
Wagemans, & Verfaillie, 2010), is that the visual system assumes that the
visual world remains stable across saccades. This assumption is plausible in
real-life vision, where the brief interruption in the input caused by a saccade
is unlikely to coincide with unpredictable and sudden object displacements
in the outside world. As a result of this assumption of visual stability, only
large discrepancies between presaccadic and postsaccadic object locations
are consciously noticed by observers. When the object is absent after the
saccade (due to postsaccadic blanking), the assumption of stability is falsified and conscious processes regain access to information that initially was
stored only implicitly, resulting in more accurate detection of intrasaccadic
displacements.
In most experiments providing evidence that the visual system assumes
a stable position of the saccade target during a saccade, the position change
occurred in stationary objects. It can be expected, however, that, in the case
of a saccade to a moving object, the visual system no longer holds the
assumption that the position of the moving object will remain unchanged
during the saccade. The position of moving objects might therefore have a
special status during transsaccadic integration. This was demonstrated by
Gysen, De Graef, and Verfaillie (2002; also see Gysen, Verfaillie, & De
Graef, 2002a). In a study in which subjects had to make a saccade to one of
two objects, one of which translated towards the other which remained stationary, it was shown that transsaccadic displacement detection was more
accurate for the translating object than for the stationary object, suggesting
that translating objects indeed have a special transsaccadic status (see Pollatsek & Rayner, 2002, for similar findings).
Moreover, using the same paradigm, Gysen, De Graef, and Verfaillie
(2002b) replicated Deubel et al’s (1996) finding that postsaccadic blanking
improves displacement detection for stationary objects, but also found that
blanking eliminates the transsaccadic benefit for moving objects. In fact,
when the moving object was blanked after the saccade (even for as short as
60 ms), the detection of intrasaccadic displacements of the moving object
was much worse than the detection of comparable displacements of station-
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TRANSSACCADIC MEMORY FOR THE POSITION OF STATIONARY
ary objects. This suggests that, when the visual system plans a saccade to a
moving object, it does not make the default assumption of visual stability.
Instead, fast and precise pre- and postsaccadic spatial processing seems to
be necessary. When the object is blanked postsaccadically, this information
is no longer available, making transsaccadic change detection more difficult. In sum, the detection of intrasaccadic displacements of stationary
objects is difficult, presumably because the visual system assumes stability
across saccades, whereas displacements of translating objects are much
more detectable, suggesting that, under these circumstances, the visual stability assumption is no longer retained.
Note that overall motion per se is not a sufficient condition for a transsaccadic benefit for the moving object. Gysen et al. (2002) did not find
enhanced displacement detection for objects rotating in depth compared to
stationary objects. Dahlstrom-Hakki and Pollatsek (2006) observed no benefit in the detection of intrasaccadic depth orientation jumps in a foveally
presented rotating vs. stationary cube. Apparently, the assumption of visual
stability is relaxed and even abandoned only in the case of motion with a
component of translation. This seems plausible given that translation
involves a change in the spatiotopic position of objects and the visual stability hypothesis mainly pertains to the spatiotopic position of objects.
There is, however, one study from our lab that seems to contradict this
line of reasoning. In Experiment 2 of Verfaillie et al. (1994), participants
viewed point-light walkers: stimuli in which the available information is
confined to a number of point lights attached to the major joints of a human
figure. At a designated time, participants had to make a saccade within the
point-light walker and judge whether the walker was displaced during the
saccade or not. In one condition, the walker did not translate, i.e., moved as
if walking on a treadmill. In another condition, the walker translated across
the screen. It was found that transsaccadic memory for the global position
of the walker was very poor in both conditions (in fact, even slightly worse
for a translating than for a non-translating walker). This is at odds with
Gysen et al.’s (2002) finding of superior transsaccadic memory for translating objects. The main purpose of the present article is to better understand
these contradictory observations and in this way help to further uncover the
mechanisms underlying transsaccadic integration.
One difference between the studies showing a transsaccadic benefit for
moving objects and Verfaillie et al.’s (1994) study is that in the former studies the stimuli consisted of inanimate objects, whereas in the latter an animate figure in motion was shown. It has been suggested that the perception
of animate actions might be “special”, in the sense that actions are processed in a qualitatively unique way, supported by a specialised architecture
hard-wired in the brain (e.g., Grossman, Donnely, Price, Morgan, Pickens,
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GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
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Neighbor, & Blake, 2000; Lestou, Pollick, & Kourtzi, 2008; Peuskens,
Vanrie, Verfaillie, & Orban, 2005; Reed, McGoldrick, Shackelford, &
Fidopiastis, 2004; Vaina, Solomon, Chowdhury, Sinha, & Belliveau, 2001).
However, it is hard to envision how the animacy of the moving object could
moderate the transsaccadic benefit for moving objects over stationary
objects (but see Orban de Xivry, Coppe, Lefèvre, & Missal, 2010, for a
report on the influence of biological motion on smooth pursuit).
A more plausible explanation has to do with the dynamics of the eye
movements involved. In the experiments demonstrating a transsaccadic
benefit for moving objects, participants were fixating a fixation cross prior
to making a saccade to the moving object. In Experiment 2 of Verfaillie et
al. (1994), subjects were pursuing the translating walker both before and
after making a saccade within the figure. One could argue that, in the former
case – making a saccade to a moving object after a stable fixation –, fast and
precise processing of the spatiotopic position of the object is necessary, to
allow accurate saccade targeting and object tracking. Under these circumstances, the visual system might not stick to the assumption of visual stability. Gysen et al. (2002b) discuss several reasons why this might be the case.
Moreover, as already mentioned before and in support of this hypothesis,
Gysen et al. (2002b) showed that briefly blanking the moving object postsaccadically eliminated the transsaccadic benefit for moving objects. In
fact, after postsaccadic blanking of the moving object, displacement detection was much worse than without blanking (whereas the opposite effect
was observed for stationary objects). In contrast, when the observer is pursuing the moving object, both before and after the saccade, the precise spatiotopic location of the object is less relevant. Indeed, one of the primary
aims of the visual system during ocular pursuit is to keep the eyes lagging
behind the moving object to a minimum (e.g., Thier & Ilg, 2005; but see
Orban de Xivry & Lefèvre, 2007, for a discussion of the commonalities
between the saccade and the pursuit system). This is primarily based on the
measurement of retinal slip, i.e., judgment of the retinal projection of the
moving object in relation to the fovea, rather than on the spatiotopic position of the object. Less accurate coding of the objects’ spatiotopic position
during pursuit could therefore result in poorer detection of intrasaccadic
displacements of the pursued object.
If the latter hypothesis holds (rather than something being special about
animate vs. inanimate object motions), then having participants saccade
from a stable fixation position either to a translating biological-motion
walker or to a non-translatory walker moving on a treadmill, should result
in a benefit for the translating walker. This is precisely what we investigated
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in the present study. We combined the paradigm used in Gysen et al. (2002)
with the stimulus employed in Verfaillie et al. (1994).[2]
Method
Subjects
Six University of Leuven students participated in the experiment. All had normal or corrected-to-normal vision.
Apparatus
Stimuli were displayed in a 60-Hz noninterlaced mode on a Sony GDMW900 Trinitron Colour Graphic Display with a 756 × 468 resolution. Eye
movements were monitored with a Generation 5.5 dual-Purkinje-image eye
tracker (Fourward Technologies, San Diego, CA) with an accuracy of 1 min
of arc and a 1000-Hz sampling rate. It was interfaced with a PC, storing every
sample of the left eye’s position. For each sample, the computer made an online decision about the eye state: fixation, saccade, blink, or signal loss. This
on-line classification algorithm enables detection of a saccade within 4 ms of
the onset of a saccade. Eye state and position were fed into a second PC in
control of stimulus presentation.
Procedure
Participants were seated at 80 cm from the stimulus display, with their head
stabilised by a headrest and a bite bar with dental impression compound.
Once the eye tracker was successfully calibrated for nine points along the
2.
Note that, in a way, our experimental design was incomplete, because it did not include a
condition in which participants have to pursue an extrafoveal object before making a saccade. Indeed, in the present experiment subjects (after steady fixation) made a saccade to an
extrafoveal object, whereas in Verfaillie et al’s (1994) study subjects (during pursuit) made
a saccade to the foveal object. In order to test our hypothesis, an experiment in which the
manipulation of the state of the subject’s eyes prior to making a saccade (fixation vs.
smooth pursuit) and the manipulation of the dynamic status of the extrafoveal saccade target object (translating vs. stationary) are fully crossed in a factorial design, would be ideal.
However, the main problem is that a condition in which observers pursue an extrafoveal
object is extremely hard, if not impossible, to realise. It might be possible to implement an
approximation of such a condition (e.g., asking participants to pursue a point moving above
a walker translating in the periphery), but the resulting situation would be very artificial and
probably will not advance our understanding of the dynamics of visual perception under
natural circumstances. This is not surprising. After all, the main function of smooth pursuit
is precisely to keep a moving (attended) object in the fovea!
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GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
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diagonals of the stimulus field, a block of 48 practice trials was initiated followed by an experimental session.
As illustrated in Figure 1 (p. 184), each trial consisted of the following
events. (Note that, for reasons of clarity, the point-light walkers in Figure 1
are depicted relatively larger than during the actual experiment, thus overrepresenting their size in relation to the distance between the walkers and the fixation cross.) First a fixation cross (0.5° × 0.5°) appeared. Participants were
instructed to fixate the cross and calibration accuracy was checked. The trial
proceeded once eye position had continuously been within a box of 1° × 1°
surrounding the cross for 1000 ms. Two dots (0.15° in diameter) were then
presented at the locations where the point-light walkers would appear. The
dots stayed on for 250 ms, after which they were replaced by two point-light
walkers. On every trial, both a stationary (i.e., walking on a treadmill) and a
translating walker (which always moved towards the stationary walker) were
shown. 1023 ms after the appearance of the point-light walkers, the fixation
cross was replaced by an arrow, directed towards one of the walkers. This was
the signal for the viewer to initiate an eye movement towards the indicated
walker. If the gaze position moved outside the virtual fixation box before the
appearance of the arrow, the trial was interrupted and a new trial began.
During the saccade towards the target point-light walker, the arrow was
erased and a position change in one of the walkers could take place. In one
third of the trials, the position of the stationary walker was shifted, in another
third of the trials, the position of the translating walker was shifted, and in the
remaining third of the trials, no position shift took place. In half of the trials
with a position shift, the saccade target walker shifted position; in the remaining half, the other walker (i.e., the saccade flanker) shifted position. Displacement size was 1.2° for stationary and 0.7° for translating walkers. Pilot work
showed that displacements of 0.7° for stationary walkers were almost impossible to detect. In order to avoid floor effects and similarly to Gysen et al.
(2002), we therefore decided to increase the displacement size for stationary
objects to 1.2°.The position change was 15.2% and 9.1% of the distance (i.e.,
8°) between fixation cross and the stationary and translating walkers respectively. The displacement did not disrupt the natural walking cycle of the
point-light figures.
Following the critical saccade, viewers had to indicate whether one of the
point-light walkers had been displaced and if so, which walker was displaced.
Viewers responded with a single left or right button press when, respectively,
the walker presented on the left or right side of the screen was perceived as
having been displaced. When no displacement was perceived, a combined left
and right response was required. Following the response, a new trial began.
Each participant completed one practice block and 32 experimental
blocks. Each block contained 48 trials which were produced by the factorial
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TRANSSACCADIC MEMORY FOR THE POSITION OF STATIONARY
Figure 1
Temporal course of a trial. Upon good fixation on a central fixation cross, two dots
indicating the position of the point-light walkers appear. After 250 ms, the dots
change into point-light walkers, one of them stationary (walking on a treadmill), the
other one translating towards the stationary walker. After 1023 ms., the fixation cross
changes into an arrow indicating which point-light walker is the saccade target.
During the saccade, the fixation cross disappears and in two thirds of the trials, one
of the point-light walkers is horizontally displaced. Subjects then indicate whether
they noticed a displacement, and if so, in which of the two point-light walkers
combination of displaced walker (stationary walker, static walker, and no displaced walker), position of the stationary and translating walker (i.e., stationary left / translating right vs. translating left / stationary right), saccade direction (left vs. right), orientation of the static walker (i.e., faced towards vs.
away from the translating walker), and displacement (towards vs. away from
other walker). The order of the trials was randomized for each subject and
block separately.
Stimulus displays
The point-light figures consisted of 13 dots (0.15° in diameter) attached to the
ankles, knees, hips, shoulders, elbows, wrists, and the head. The point-light
figure performed a walking motion that was designed using motion capture
data from a real walker and a 3D animation technique (Dekeyser, Verfaillie,
& Vanrie, 2002; Vanrie & Verfaillie, 2004). Point-light walkers were 2.7° in
height and were dark gray on a light gray background. On each trial, both a
stationary and a translating point-light walker were present. While the station-
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GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
185
ary point-light figure walked on a virtual treadmill, the translating walker
walked towards the centre of the screen at a speed of 3.14° s-1.
The horizontal starting position of the translating walker differed from
that of the stationary walker by 3.20°. This was done in order to have a similar
horizontal position for both walkers (i.e., 5.5° left or right of centre) at the
time of the saccade go-signal (i.e., this signal to initiate a saccade was presented 1023 ms after the appearance of the walkers, which corresponds to a
moving distance of 3.20° for the translating walker), ensuring that saccadic
amplitude and duration in the conditions with translating and stationary walkers were comparable.
Results
Unsuccessful trials were excluded from the analyses. A trial was considered
to be unsuccessful when the subject lost his or her fixation before the saccade
go-signal, when saccade latency (measured from arrow onset) was smaller
than 80 ms or above 1430 ms (because, at 1430 ms, the distance between the
translating point-light walker and the middle of the screen was only 1°), when
the saccade was directed towards the flanker instead of the target, or when the
intended displacement of the saccade target did not complete within the time
period of the saccade because of computer processing delays or gaze position
signal loss. This procedure resulted in an elimination of 28.7% of all trials.
Saccade latencies measured from central arrow-cue onset were 420 ms on
average.[3] A repeated measures analysis of variance with saccade direction
(left vs. right) and dynamic status of the saccade target (stationary vs. translating) as within-subject variables only yielded a marginally significant main
effect of the latter variable, F(1, 5) = 6.01, p = .06, MSE = 0.020. On average,
saccades towards translating walkers were initiated 8 ms faster compared to
saccades towards stationary walkers. This means that, given the temporal and
spatial parameters of the critical events making up a trial (especially the differential horizontal starting positions depending on the dynamic status of the
walkers), translating and stationary walkers were approximately at the same
retinal eccentricities at the time the eyes were launched.
First, hits (i.e., correct identifications of the object that shifted position)
were combined with false alarms (i.e., false reports of a displacement of that
3.
The mean saccadic latency was relatively long. Note, however, that saccadic latency was
measured from the moment the fixation cross changed into an arrow and the two potential
saccade targets were already present at that time. Therefore, saccade latencies included the
time needed to process the directional cue and decide where to move the eyes next and the
time needed to plan and launch the saccade (including attention disengagement from the
arrow, the presaccadic movement of attention to the selected saccade target, and the start of
the actual saccadic movement).
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particular object when no object was displaced) to derive d’ values. To obtain
d’ we followed the constant ratio rule (MacMillan & Creelman, 1991, pp.
243-245; see Gysen et al., 2002b, Appendix A, for a detailed example). From
the overall contingency table produced by the three stimulus types (shift
translating, shift stationary, no shift) × 3 response types (translating shifted,
stationary shifted, nothing shifted), we extracted two 2x2 tables, one for the
translating walker (2 stimulus types, shift translating vs. no shift × 2 response
types, translating shifted vs. nothing shifted) and one for the stationary walker
(2 stimulus types, shift stationary vs. no shift × 2 response types, stationary
shifted vs. nothing shifted). This was done separately for each subject both for
the conditions in which the saccade target was displaced and the conditions
in which the saccade flanker was displaced. Sensitivity estimates (d’) were
entered in a repeated measures analysis of variance (ANOVA) with dynamic
status (stationary vs. translating) and saccadic status (saccade target vs.
flanker) as within-subject variables.
The analysis revealed significant main effects of both dynamic status, F(1,
5) = 8.03, p < 0.04, and saccadic status, F(1, 5) = 11.41, p < 0.03. In addition,
the interaction between the two variables was significant, F(1, 5) = 15.24, p
< 0.02 As shown in Figure 2, for translating walkers, sensitivity was much
higher when the translating walker was the saccade target than when it was
the flanker (d’ of 1.80 and 0.94, i.e., a significant saccade target advantage of
0.86, t(5) = 4.89, p < 0.02). For stationary walkers, however, this saccade target advantage was much smaller (d’ of 0.93 and 0.76, i.e., a non-significant
saccade target advantage of 0.17, t(5) = 0.93, p > 0.75). Comparison of translating and stationary walkers shows that, for saccade targets, sensitivity was
much higher for translating walkers, t(5) = 4.32, p < 0.03. The same comparison for flankers, shows no difference, t(5) = 0.7, p > 0.85. However, displacements of stationary walkers were almost 60% larger than displacements
of translating walkers, indicating that sensitivity for displacement was still
better for translating walkers.
Second, we analysed the proportion of misattributions (trials in which participants reported that they saw a displacement, but incorrectly attributed the
displacement to the walker that was not displaced; see Figure 3). There was
no effect of the saccadic status (F(1, 5) = 1.15, p > 0.3). However, while
detection of stationary walker displacements was almost at chance level (52%
correct), subjects almost never attributed changes of a translating walker to
the stationary figure (92% correct), F(1, 5) = 12.4, p < 0.02. This preference
for attributing displacements to translating compared to stationary point-light
walkers was also reflected in the observation that, in trials without displacement but in which subjects reported a displacement (false alarms), the
reported displacement was attributed to the translating walker (68%) more
often than to the stationary (32%) point-light walker, t(10) = 2.96, p < 0.02.
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GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
187
Figure 2
Sensitivity (d’) as a function of dynamic and saccadic status of the point-light walker
in which the change occurred. The error bars represent standard errors
Discussion
Contrary to Verfaillie et al. (1994) and in line with Gysen et al. (2002) and
Gysen et al. (2002a, 2002b), intrasaccadic displacements of the translating
walker were easier to detect than displacements of the stationary walker. This
supports our hypothesis that the absence of the motion benefit in Experiment
2 of Verfaillie et al. (1994) is due to the fact that the critical saccade in that
study occurs during pursuit of the translating figure.[4] During smooth pursuit, the visual system tries to keep the moving object of interest in the fovea
as accurately as possible. To achieve this, the retinotopic position of the
4.
Obviously, apart from the fact that participants had to maintain fixation before making a
saccade in the Gysen et al. (2002) study whereas participants had to pursue the walker
before the saccade in the Verfaillie et al. (1994) study, there are other procedural differences between the two studies (e.g., one object in Verfaillie et al., 1994, vs. two objects in
Gysen et al., 2002, but see Experiment 2 in Gysen et al., 2002). Future studies should further disentangle the contribution of these factors.
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1
Proportion correct attributions
0.9
0.8
0.7
0.6
0.5
target
0.4
flanker
0.3
0.2
0.1
0
stationary
translating
Figure 3
Proportion of correct displacement attributions in trials in which the displacement
was detected as a function of dynamic and saccadic status of the point-light walker in
which the change occurred. The error bars represent standard errors
object is most important, at the expense of memory for the spatiotopic position of the objects. It has been shown before that the remembered location of
translating objects is shifted in the direction of the motion (e.g., Hubbard,
1995) but that this mislocalisation decreases when the eyes remain stationary,
instead of tracking the object’s path of motion (Kerzel, 2000; Kerzel, Jordan,
& Müsseler, 2001).
When a displacement was detected, there was a large bias to attribute it to
the translating object. This could follow from the visual system’s proven
assumption that stationary objects available immediately after the saccade
remained spatially stable during the saccade and can be used as a reference
object to recalibrate the position of the whole scene. Indeed, Deubel et al.
(1998) showed that when observers saccade to one of two objects and one of
the objects is blanked postsaccadically (either the target or flanker object)
while one object is displaced intrasaccadically (again either the target or the
flanker), the blanked object is generally perceived as being displaced and the
nonblanked object is seen as spatially stable (regardless of which object was
actually displaced).
psycho.belg.2011_2.book Page 189 Tuesday, June 28, 2011 3:18 PM
GOEDELE VAN BELLE, PETER DE GRAEF & KARL VERFAILLIE
189
The present study also underlines the importance of the saccadic status for
displacement detection. Displacements of the saccade target walker were better detected than displacements of the flanking walker. This is in line with
many studies showing that the spatial extent of transsaccadic memory is primarily focused on the saccade target region (Currie, McConkie, Carlson-Radvansky, & Irwin., 2000; Deubel et al., 1998; McConkie & Currie, 1996).
However, since performance was well above chance level even for flanker
objects, despite the small displacement sizes, the data also confirmed the findings of Verfaillie & De Graef (2000; see also De Graef, Verfaillie, & Lamote,
2001; Germeys, De Graef, Panis, Van Eccelpoel, & Verfaillie, 2004) that,
given appropriate control for extrafoveal preview quality, transsaccadic
memory for position information is not strictly limited to the saccade target
region.
In the work of Gysen et al. 2002, we controlled for the fact that the moving
object might attract attention more than the stationary object. One could claim
that such an attentional effect might be stronger for biological motion, given
that a translating walker is more familiar and natural than a walker moving on
a treadmill. Some observations are in favour of an explanation of the benefit
for the translating walker in terms of attention. First, saccadic latencies were
marginally longer for saccades directed towards the stationary walker. Second, when no intrassaccadic displacement occurred, false alarms were biased
towards the translating walker. However, in other (unpublished) experiments
with nonbiological objects, we repeatedly observed that saccadic latencies to
translating objects were shorter than saccadic latencies to stationary objects.
And, without intrasaccadic displacement, jump reports indeed were biased
towards the translating walker, but this is again something we observed
before with nonbiological objects.
In sum, we showed that the transsaccadic position displacement benefit
for translating over stationary objects as reported by Gysen et al. (2002) and
the absence of the motion benefit in the studies of Verfaillie et al. (1994) was
most probably related to the fact that, in the latter study, observers were pursuing the figure before and after the saccade and spatiotopic position coding
is less relevant during smooth pursuit.
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Received September 16, 2010
Revision received April 12, 2011
Accepted May 10, 2011