Mislocalization of a target toward subjective contours: attentional modulation of location signals.

Abstract

This study examined whether a briefly presented target was mislocalized toward a subjective contour. Observers manually reproduced the position of a briefly presented peripheral target circle above a central fixation cross. A luminance contour, a subjective contour, or a no-contour stimulus was presented in either the left of right visual field, and a no-contour control was presented in the opposite visual field. After these stimuli vanished, a target circle was then presented. Consequently, the degree of mislocalization toward the subjective and luminance contours was the same; this indicated that image integration at a coarse spatial scale cannot explain mislocalization. Experiment 2 revealed that the mislocalization in Experiment 1 was not a result of eye movements. Experiment 3 found that the spatial attention allocated at the location of the luminance and subjective contours was more than that allocated at the no-contour stimulus. An attentional shift toward the task-irrelevant stimulus resulted in a mislocalization of the target.

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Yamada, Kawabe, & Miura (2008)
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Mislocalization of a target toward subjective contours:
Attentional modulation of location signals
Yuki Yamada, Takahiro Kawabe, Kayo Miura

Abstract
This study examined whether a briefly presented target was mislocalized toward a subjective contour.
Observers manually reproduced the position of a briefly presented peripheral target circle above a central
fixation cross. A luminance contour, a subjective contour, or a no-contour stimulus was presented in either
the left or right visual field, and a no-contour control was presented in the opposite visual field. After these
stimuli vanished, a target circle was then presented. Consequently, the degree of mislocalization toward the
subjective and luminance contours was the same; this indicated that image integration at a coarse spatial scale
cannot explain mislocalization. Experiment 2 revealed that the mislocalization in Experiment 1 was not a
result of eye movements. Experiment 3 found that the spatial attention allocated at the location of the
luminance and subjective contours was more than that allocated at the no-contour stimulus. An attentional
shift toward the task-irrelevant stimulus resulted in a mislocalization of the target.

Introduction
The remembered location of a briefly-presented
target is often mislocalized toward task-irrelevant
stimuli (e.g., Nelson & Chaiklin, 1980; Schmidt,
Werner, & Diedrichsen, 2003).
mislocalization was attributed to landmark
attraction effect (Bryant & Subbiah, 1994) or
spatial memory averaging (Hubbard, 1995;
Hubbard & Ruppel, 1999). In this study, we refer
to this kind of biased memory distortion toward
task-irrelevant stimuli as memory averaging.
There are two explanations for the existence of
memory averaging. First is the perceptual
integration of the target and irrelevant objects at a
coarse scale. As suggested by the Gestalt
psychologists, proximal visual elements are
perceived as a group (e.g., Wertheimer, 1923).
Such perceptual grouping is partly attributed to
image integration at a coarse spatial scale (Watt,
1988; Watt & Morgan, 1985). In this case, a
low-pass filtering of proximal dots results in a
large blob. It is believed that early vision
decomposes the visual image into various scales,
and thus, it is likely that proximal items can be
integrated at early vision. We believe that the
perceived position of each item in the blob is
biased toward the center of the blob and that this
may be the origin of memory averaging. Actually,
the spatial distance between two perceptually
grouped stimuli was perceived as closer than that
between ungrouped stimuli (Coren & Girgus,
1980). Thus, the Gestalt law of “proximity”
seems connect to spatial distortion and it may
stem from coarse image integration.
The second possible explanation for memory
averaging is an attention shift toward the
task-irrelevant stimuli. In previous studies, an
attentional shift along a bistable apparent motion
This
path displaced the perceived position of a flash,
which was on the midway of the path, in the
direction of the attentional shift (Shim &
Cavanagh, 2004, 2005). Furthermore, a briefly
flashed target was reproduced with a greater bias
toward the peripheral flashed landmark than the
non-flashed landmark (Uddin, Kawabe, &
Nakamizo, 2005a). They argued that a flashed
landmark with an abrupt onset and offset resulted
in an attentional shift toward it (Yantis & Jonides,
1984); moreover, this attentional shift caused a
spatial distortion around the flash (see also Uddin,
Kawabe, & Nakamizo, 2005b). Likewise, the
final position of a moving object was reproduced
with a bias toward the abrupt onset objects
(Kerzel, 2002). Thus, the perceptual and memory
mislocalizations of the target appear to occur in
the direction of the attentional shift.
However, previous studies did not distinguish
whether the mislocalization of a target toward
task-irrelevant stimuli originated in image
integration at a coarse scale or as a result of an
attention shift. In this study, we examined
memory averaging by using a subjective contour
(e.g., Kanizsa, 1976). Using a subjective contour
is advantageous in the following manner. First, it
is not integrated with the target at a coarse scale
(Figure 1). Second, using the subjective contour
serves to test the relationship between memory
averaging and visual attention. It is known that
Kanizsa’s subjective contour can be detected in
parallel (Davis & Driver, 1994) and that such a
popout item attracts attention (Joseph, Chun, &
Nakayama, 1997; Wolfe, 1997). Moreover, the
subjective contour captures visual attention
(Ricciardelli, Bonfiglioli, Nicoletti, & Umiltà,
2001; Senkowski, Röttger, Grimm, Foxe, &
Herrmann, 2005). Therefore, using a subjective
This is a preprint version
of the article published
at Psychological
Research 2008; 72:
273-280

Correspondence to:
Yuki Yamada, Faculty
of Human-Environment
Studies, Kyushu
University,
6-19-1 Hakozaki
Higashi-ku, Fukuoka
812-8581, Japan
yamadayuk@gmail.com
ORIGINAL ARTICLE

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Yamada, Kawabe, & Miura (2008)
2
contour enables us to directly investigate the role
of visual attention in memory averaging and
eliminates image integration at a coarse scale.

Experiment 1: Does mislocalization
toward subjective contours occur?

Method
??
Observers Nine students from Kyushu
University as well as the author (YY) voluntarily
participated in this experiment. All participants
had normal or corrected-to-normal visual acuity,
and with the exception of YY, they were naive to
the purpose of the study.
?? Apparatus and stimuli Stimuli were displayed
on a CRT monitor (EIZO FlexScan T761, Japan)
with a resolution of 1024 × 768 pixels; the
vertical refresh rate was 75 Hz. Further, the
viewing distance was 60 cm. A PC/AT
compatible computer controlled the presentation
of the stimuli and the collection of data. The
luminance of the stimuli (a fixation cross,
inducers, the luminance contour, a target circle,
and the mouse cursor) was 33.0 cd/m2 and the
luminance of the background was 99.7 cd/m2.
The fixation cross was presented at the center of
the screen. The target stimulus was a small circle
with a radius of 0.17°. The inducer disks’ visual
angle of radius was 1°. Each of the four disks
was centered at the corner of an imaginary
rectangle with a height and width of 6° and 2°,
respectively. As indicated in Figure 1, the
following three types of stimuli were employed:
(a) luminance contour, (b) subjective contour,
and (c) without contour. Further, the luminance
contour stimulus consisted of four inducers with
an inside notch and a luminance contour with a
width of 0.33°. The subjective contour stimulus
consisted of four inducers with an inside notch
that resulted in the perception of a subjective
contour lacking luminance
stimulus without any contours consisted of four
inducers with an outside notch that did not yield
any subjective contours. On a given trial, two of
the three stimuli were simultaneously presented
in the upper right and left visual fields,
respectively. In the no-contour condition, the
same stimulus without any contour was presented
on each side. In other conditions, either the
luminance contour or the subjective contour
stimulus was presented in the left visual field and
the no-contour stimulus was presented in the
right visual field and vice versa. In other words,
the luminance and subjective contours were
never simultaneously presented on any given trial.
The centers of the left and right stimuli were
separated from one another by a visual angle of
contours. The
10°. The central position of each stimulus was
presented 3.33° above the fixation cross. The
overall position of the stimulus was horizontally
varied within 3.33°, although the distance
between them was maintained. The target was
presented on the middle position between the two
stimuli. The following five trial types were
conducted: (a) luminance contour stimuli
presented on the left or right (LL or LR condition,
respectively), (b)subjective
presented on the left or right (SL or SR condition,
respectively), and (c)
presented on both the sides (no-contour
condition).
?? Procedure and design The experiment was
conducted in a dark room. Figure 2 presents a
diagram of the temporal sequence on each trial.
The observers were asked to maintain their gaze
on the fixation cross and initiate each trial by
pressing the spacebar. First, four filled disks
without any notches were presented for 500 ms:
This is a time required to shift attention to the
disks. Immediately after, the disks were replaced
by inducers with notches (and a luminance
contour in LL and LR conditions) for a period of
300 ms. After a 200 ms inter-stimulus interval
(ISI), the target was presented for 50 ms.
Subsequently, after a 500 ms ISI, the mouse
cursor—identical to the target circle—appeared
at a random position on an imaginary circle with
a radius of 3.3° that was centered at the same
position as that of the target. The fixation cross
was present throughout the trial. The observers
were required to view the display and reproduce
contour stimuli
no-contour stimuli
Figure 1: Examples of the stimuli used in this experiment.
(a) Left panel: A square with a luminance contour. Center: A square with a subjective
contour. Right panel: The control stimulus. (b) Images output by the low-pass filter. In
this scale, the small target image disappears.

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Yamada, Kawabe, & Miura (2008)
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the target position by clicking the left button of
the computer mouse, while maintaining their
gaze on the fixation cross; in other words, they
were expected to move the mouse and position
the cursor where they believed the circle had
appeared. LL, LR, SL, and SR conditions were
repeated 10 times, and the no-contour condition
was repeated 20 times; thus, a total of 60
experimental trials were conducted in a
pseudo-randomized order.

Results and Discussion
The bias and degree of mislocalization of the
target position were calculated for each observer
in each condition. The mean and standard errors
are indicated in Figure 3. In further analysis, only
the data of the x-coordinate was analyzed
because the purpose of the experiment was to
observe whether or not mislocalization occurred
toward the peripheral contour (Sheth & Shimojo,
2004; Uddin et al., 2005a, b). A one-way analysis
of variance (ANOVA) of the degree of biased
mislocalization with trial type (LL, LR, SL, SR,
and no-contour) as a factor revealed a significant
main effect, F(4, 36) = 17.58, MSE = 0.08, p
< .001.
Multiple comparisons using Ryan’s method1
(Ryan, 1960) indicated that the target positions in
the LL and SL conditions were mislocalized
significantly leftward
no-contour conditions, t(36) = 2.58, p < .02, t(36)
= 3.37, p < .002, respectively; further, the target
positions in the LR and SR conditions were
mislocalized significantly rightward than those in
the no-contour conditions, t(36) = 2.48, p < .02,
t(36) = 3.33, p < .003, respectively. However,
neither mislocalizations in LL and SL conditions
nor mislocalizations in LR and SR conditions
differ from each other. In addition, we conducted
a comparison against zero to observe whether the
reproduced target position was different from the
actual position. An analysis of 95% confidence
intervals revealed significant mislocalization in
the LL, LR, SL, and SR conditions (-0.35 deg ±
0.29 [average displacement ± 95% confidence
intervals], 0.30 ± 0.17, -0.45 ± 0.23, 0.40 ± 0.21,
respectively). However, the reproduced target
position in the no-contour conditions did not
differ from the actual one (-0.02 ± 0.15).
The results clearly indicated an obvious
mislocalization of the target location toward the
task-irrelevant Kanizsa’s subjective contour as
well as toward the luminance contour, regardless
of the image features defining the contour. This
supports the hypothesis that memory averaging is
caused by attentional shifts resulting from the
presence of the subjective contour; further, it
than those in the
Figure 2: Schematic representation of the time course of a trial with a subjective
contour in Experiment 1.
Figure 3: Observed localization bias.
The LL, LR, SL, SR, and NO represent the Luminance-Left, Luminance-Right,
Subjective-Left, Subjective-Right, and No-contour conditions, respectively. The positive
and negative values represent the rightward and leftward biases, respectively. Error bars
denote standard errors.
1. Ryan’s test adopts nominal significant level given as follows: p = 2*0.05 / (n*(m-1)),
where n means the number of group to be compared, and m means the distance defined
as the number of group Xp satisfying Xi ≤ Xp ≤ Xj. Here, Xi and Xj are pair in a
concerned hypothesis. On the other hand, the significant level in Bonferroni’s test is set
at p = 0.05/nC2, where n means the number of group to be compared.

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Yamada, Kawabe, & Miura (2008)
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eliminates the involvement of the outputs of
coarse spatial filtering that did not result in a bias
in each condition.
Further discussion is required as to whether or
not the effect of the inducers with an inside notch
was necessarily based on the presence of a
subjective contour. Bryant and Subbiah (1994)
found that the empty region of a figure could
serve as a subjective landmark; consequently, it
appears that the central
inducer-cluster could have perhaps functioned as
a landmark even if a subjective contour itself was
not perceived by the observers. In this regard, we
would like to emphasize the effect of attention on
the selection of empty regions. In the conditions
with subjective contours (SL and SR conditions),
two empty regions with or without a subjective
contour always existed. Hence, if our results had
stemmed merely from the effects of empty
regions, the significant attraction of the targets
toward the center of the inducers with subjective
contours would not have been observed because
both the empty regions would competitively
attract the target’s position. However, that was
not the case. Therefore,
interpretation that a subjective contour that drew
attention attracted the target’s position. To
further strengthen our argument, in the next
experiment, we first dismiss the possibility of
mislocalization caused by eye movements
(Experiment 2), and then, we verify that the
subjective contours used in Experiment 1
engaged visual selective attention (Experiment
3).

Experiment 2: Is mislocalization
caused by eye movements?
In this experiment, we address the role of eye
movements in memory averaging. In the previous
experiment, observers might have reflexively
gazed at the subjective contours because these
figures attention. Moreover, it should be noted
that eye movements are commonly known as a
causative factor of the mislocalization of a briefly
presented target (e.g., Honda, 1989; Schlag &
Schlag-Rey, 1995). Accordingly, we measured
the observers’ eye movements in conditions
identical to those in Experiment 1.

Method
?? Observers Three people who were naive to the
purpose of the study voluntarily participated in
this experiment. All participants had normal or
corrected-to-normal visual acuity.
?? Apparatus, stimuli, procedure, and design This
experiment was identical to Experiment 1 in all
aspects, with the exception that in Experiment 2,
region of the
we favor the
the observers’ eye movements were monitored.

Results and Discussion
?? The data with a radial fixation error larger than
1.25° from the initial eye position was excluded
from the analysis. As a result, on an average,
8.3% of the data were excluded as failed trials.
Figure 4(a) is a representative example of
successful eye movement trials and Figure 4(b)
indicates the individual data. A one-way
ANOVA revealed a significant main effect of
trial type, F (4, 8) = 14.17, MSE = 0.08, p < .002.
Multiple comparisons using Ryan’s method
indicated that the target positions in the LR
condition was mislocalized
rightward than those in the no-contour conditions,
t (8) = 3.01, p < .02.
Albeit slightly, the significant mislocalization
was observed even in only three observers
employed in this experiment. Regardless of the
precise control of eye movements, a localization
bias similar to that in the first experiment was
observed in this experiment as well. Therefore,
we believe that the oculomotor factor did not
contribute to the results obtained in Experiment
1.

Experiment 3: Attentional allocation in
the contours
This experiment aimed to confirm whether or not
attention was directed to the inside of the
significantly
Figure 4
(a) Representative scan-path of eye movements drawn from an experimental trial by
DK. Sixty eye positions, which were plotted as red circles, were sampled during a trial
for 4 s at 15 Hz. All observers exhibited a similar tendency in the successful trials. (b)
The observed localization bias from each observer (DK, SG, and SS). The LL, LR, SL,
SR, and NO represent the Luminance-Left, Luminance-Right, Subjective-Left,
Subjective-Right, and No-contour conditions, respectively. Values larger than 0
represent the rightward biases, and vice versa.

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Yamada, Kawabe, & Miura (2008)
5
subjective contour used in Experiment 1. An
illusory line motion (ILM) (Hikosaka, Miyauchi,
& Shimojo, 1993) was employed as an index of
the presence of attention (Kawahara, 2002). ILM
is a kind of motion illusion in which a horizontal
line appears to gradually unfold from a pre-cued
location, while all parts of the horizontal line is
presented simultaneously. In this experiment, a
horizontal line between the two clusters of
inducers was presented instead of the circular
target used in Experiment 1. ILM is perceived
from the subjective contours in the SL and SR
conditions, when visual attention is selectively
allocated at that location.

Method
??
Observers Eight students from Kyushu
University voluntarily participated in this
experiment. All participants had normal or
corrected-to-normal visual acuity and were naive
to the purpose of this study.
?? Apparatus, stimuli, procedure, and design This
experiment was identical to Experiment 1 in all
aspects, with the exception that in Experiment 3,
the target circle was replaced with a horizontal
white line with a height and width of 1° and
8.33°, respectively. The observers were required
to report—by pushing an assigned key—whether
the perceived motion direction of the line was
leftward or rightward in a two alternative forced
choice procedure. Further,
performed a total of 60 trials that included 10
repetitions of the four experimental contour
conditions (LL, LR, SL, and SR) and 20
repetitions of the no-contour condition.

Results and Discussion
The proportion rightward motion was calculated
for each observer in each contour condition
(Figure 5). Using these data, we conducted a
one-way ANOVA with trial type as a factor; the
results revealed a significant main effect, F (4,
28) = 31.69, MSE = 0.03, p < .001. Multiple
comparisons using Ryan’s method indicated that
the proportion rightward motion in the LL and
SL conditions were significantly higher than
those in the no-contour condition, t (28) = 3.32, p
< .003, t (28) = 2.63, p < .02, respectively;
further, those in the LR and SR conditions were
significantly lower than those in the no-contour
condition, t (28) = 4.71, p < .001, t (28) = 5.12, p
< .001, respectively. However, neither the
proportions rightward motion in LL and SL
conditions nor the proportions rightward motion
in LR and SR conditions differ from each other
(p > .05). This indicates that the horizontal line
appeared to unfold from the position of the
luminance and subjective contours; subsequently,
this suggests that the mislocalization observed in
Experiment 1 may have stemmed from
attentional allocation to the luminance and
subjective contours.

General Discussion
This study aimed to clarify the contribution of an
attentional shift to memory averaging between a
target and the subjective
distinguishing it from a perceptual factor. In
order to achieve this, we employed Kanizsa’s
subjective contours that were presented prior to
the target and demonstrated robust memory
averaging (Experiment 1). Given that the target
and inducers did not cause any blob at a coarse
scale (Figure 1), it can be stated that this effect
did not appear to stem from perceptual grouping.
Furthermore, the involvement of eye movements
was ruled out in mislocalization (Experiment 2).
Additionally, Experiment 3 provided indirect
evidence that the mislocalization in Experiment 1
can be attributed to an attention shift toward the
the observers
contours by
Figure 5: Proportion rightward motion denoting the observed localization bias in
each condition in Experiment 1.
The LL, LR, SL, SR, and NO represent the Luminance-Left, Luminance-Right,
Subjective-Left, Subjective-Right, and No-contour conditions, respectively. Values
larger than 0.5 represent the rightward biases, and vice versa. Error bars denote standard
errors.

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Yamada, Kawabe, & Miura (2008)
6
luminance and subjective contours.
Our results concur with Tsal and Bareket’s
(2005) concept of attentional modulation of
neural location signals. Several adjacent neural
units (i.e., the cells in V1) are activated
coincidentally at various intensities depending on
the degree of spatial coincidence with the target.
However, due to an overlap, the precise location
of the target cannot be determined; this results in
a coarse spatial distribution of the location
signals. Thus, in nature, raw location signals are
coarse. However, an attention shift toward the
target location leads to a fine localization
performance (Atkinson & Braddick, 1989).
Therefore, it can be stated that attention spatially
sharpens the location signals and shifts or skews
the distribution of location signals toward the
receptive field, which considerably overlaps the
actual target position (Suzuki & Cavanagh, 1997).
In Experiment 1, this function of attention might
be the cause of the mislocalization.
Other phenomena pertaining
distortion can be discussed in terms of attentional
modulation of location signals. For example, a
localization bias toward the fovea (foveal bias)
(e.g., Müsseler, van der Heijden, Mahmud,
Deubel, & Ertsey, 1999; Sheth & Shimojo, 2001;
Uddin et al., 2005a, b) can also be explained by
an attentional modulation of the distribution of
location signals toward or around the fovea
wherein visual attention dwells (Wolfe, O’Neil,
& Bennet, 1998). Likewise, the fact that the ILM
caused the endpoint of a horizontal line to be
displaced in its direction (Yamada, Kawabe, &
Miura, submitted) suggests the existence of an
attentional modulation of location signals. In
particular, an attention shift along the ILM
(Hamm & Klein, 2002) appears to bias the
location signal of a line edge toward its direction.
It can be stated that the results obtained in
Yamada et al. originate in a manner similar to the
mechanism for attentional repulsion effect
(Suzuki & Cavanagh, 1997) in which briefly
presented vertically-aligned bars perceptually
shift in the opposite direction of the pre-cued
positions. Suzuki and Cavanagh proposed a
model indicating that the attentional distortion of
visual receptive fields leads to the repulsion
effect. We propose that the attentional repulsion
may result from the overshoot of an attentional
shift from the cue to the bar; consequently, the
bars appear to repulse the cue. A recent study
using neural recording reported that visual
receptive fields in MT in the primate visual
cortex dynamically shifted in the direction of an
attention shift (Womelsdorf, Anton-Erxleben,
Pieper, & Treue, 2006); this finding is in
to spatial
accordance with the model suggested by Suzuki
and Cavanagh.
In contrast, the results of our experiment
revealed that the presentation of an irrelevant
object near the target resulted in a mislocalization
with an attraction toward—and not repulsion
from—the irrelevant object. We suggest that this
apparent contradiction
between the results of the previous studies and
our results might stem from the dynamics of
attention. In Suzuki and Cavanagh, it appeared
that attention was more likely to shift from the
cue to the probe because the cue and the probe
were very similar in shape and size. On the other
hand, in our study, the probe was dissimilar to
the cue (irrelevant objects); moreover, the cue
was more salient than the probe in shape and size.
This might have been the cause for the delay in
the disengagement of attention from the salient
cue. Thus, in our study, the mislocalization of the
target toward the cue occurred resulting in a
contradictory bias of the mislocalization (i.e.,
attraction). Therefore, we speculate that the dwell
time of attention at the cue location determines
the attraction-repulsion.
discrepancy of spatial distortions, attraction, and
repulsion, appear to have a common mechanism
underlain with attentional modulation of location
signals.

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