Journal of Experimental Psychology: General
1994, Vol. 123, No. 3, 316-320
Copyright 1994 by the American Psychological Association. Inc.
0096-3445/94/S3.00
COMMENT
Grouped Locations and Object-Based Attention:
Comment on Egly, Driver, and Rafal (1994)
Shaun P. Vecera
Recently, R. Egly, J. Driver, and R. D. Rafal (1994) provided evidence for an object-based
component of visual orienting in a simple cued reaction time task. However, the effects of objects
on visual attention can be due to selection from either of two very different types of representations: (a) a truly object-based representation that codes for object structure or (b) a grouped array
representation that codes for groups of spatial locations. Are Egly et al.'s results due to selection
from an object-based representation or from a grouped array representation? This question was
addressed by using a variant of Egly et al.'s task. The findings replicated those of Egly et al. and
demonstrated that the selection in this task is mediated through a grouped array representation. The
implications of these results for studies of attentional selection are discussed.
In the past, the study of attentional selection has primarily
focused on how visual attention selects stimuli on the basis
of spatial location in the visual field. However, in recent
years there has been an increase in the study of how visual
attention selects stimuli on the basis of shape or structure.
The former approaches have come to be known as spatial or
location-based models of attention, and the latter have come
to be known as object-based models of attention. (The
literature concerning these two types of selection is quite
large and continually growing, so it will not be reviewed
here. The reader is referred to the following for reviews and
characteristic positions: Duncan, 1984; Egly, Driver, &
Rafal, 1994; Eriksen & Eriksen, 1974; Kramer & Jacobson,
1991; Posner, 1980; Posner, Snyder, & Davidson, 1980;
Vecera & Farah, 1994)
Egly, Driver, and Rafal (1994) used a clever procedure
that was reported to show both location-based and objectbased components of visual selection in normal subjects
as well as differential impairments in these attentional
components in parietal-damaged patients. The results
from the normal subjects are of primary interest here.
Subjects were shown two rectangles that appeared either
above and below fixation or to the left and right of fixation. One of the corners was cued by brightening, and a
target followed. Subjects made a simple reaction time
(RT) response to the onset of the target, a procedure similar to Posner's classic spatial cuing paradigm (see Posner
& Cohen, 1984). The cues were either valid (cue and target at same corner) or invalid (cue and target at different
corners). Not surprisingly, Egly et al. found a validity effect: Subjects were faster to respond to validly cued trials
than to invalidly cued trials.
The finding of interest, however, comes from an analysis
of the invalid trials. These trials involved movements of
attention either within a rectangle or across rectangles.
Although the spatial separation of these two conditions was
identical, Egly et al. found that subjects were faster to
respond to an invalidly cued target when that target was in
the rectangle that had previously been cued (a withinrectangle shift) as compared with a target that appeared in
the other, uncued rectangle (an across-rectangles shift).
Egly et al. referred to this nonspatial component of orienting
as object-based attentional selection. They then showed that
these two aspects of attentional selection were dissociable in
brain-damaged subjects. Patients with damage to the left
parietal lobe showed a deficit in moving attention between
objects in the contralesional visual field, whereas patients
with damage to the right parietal lobe showed a deficit in
moving attention between spatial locations in the contralesional field relative to the ipsilesional field.
Although Egly et al.'s data convincingly showed a nonspatial aspect of attentional selection, is the term objectbased attention warranted? Recently it has become clear
that the effects of objects in the visual field on attentional
selection can be explained within a modified location-based
framework (see Kramer & Jacobson, 1991; Vecera & Farah,
1994). It might be that these effects are due to selection
from a location-based representation in which the locations
have been parsed or grouped according to whether the
locations belong to one object or another. This grouped
array selection would not be object-based selection; rather,
it would be a modified location-based selection in which
groups of locations are selected. The underlying representational medium would remain spatial (or location-based).
Kramer and Jacobson (1991) provided empirical evidence
This work was supported in part by a Sigma Xi Grants-in-Aid of
Research. I thank Martha Farah for her comments and encouragement on this work, and I thank Marlene Behrmann, MaryLou
Cheal, Roberta Klatzky, David Plaut, and Bob Rafal for further
discussion of these ideas. Thanks also to Kendra Gilds for her
assistance in running subjects and analyzing data.
Correspondence concerning this article should be addressed to
Shaun P. Vecera, Department of Psychology, Carnegie Mellon
University, Pittsburgh, Pennsylvania 15213-3890. Electronic mail
may be sent to vecera+ @cmu.edu.
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COMMENT
for selection from a grouped array representation. Subjects
performed a response competition task in which the target
and distractors were embedded in the same object or in
different objects. Subjects showed more competition from
the distractors when targets were embedded in the same
object as the distractors as compared with when the target
and distractors were grouped in different objects. However,
this effect was reduced if the target and distractors were
moved farther away from one another, although grouping
within the same object produced more interference than
grouping within different objects. That is, the amount of
response competition was a function not only of object
grouping (same object vs. different object) but also of
spatial position (distractors near the target vs. distractors far
from the target).
Although this grouped array account might explain all of
the so-called object-based effects, Vecera and Farah (1994)
recently demonstrated that certain types of selection may
not be mediated through a grouped array. Using a variant of
Duncan's (1984) object-selection task, they found a decrement in selecting from two objects as compared with selecting from a single object (as Duncan originally found),
but this effect was independent of whether the objects were
spatially adjacent (i.e., superimposed) or spatially separated.
This finding suggests that selection in Duncan's task is truly
object-based in that object structure, independent of spatial
location, is selected.
Given the findings of both grouped array selection and
object-based selection, an important question is whether
Egly et al.'s results were due to selection from a grouped
array representation or from an object-based representation.
This question is addressed in the experiment below. Egly et
al.'s paradigm was used, with one minor modification—
317
namely, a spatial manipulation. That is, the corners of the
rectangles could be equidistant, as in Egly et al.'s work (the
far condition), or the corners of the two different rectangles
could be closer than the two comers of a single rectangle
(the near condition). The grouped array and object-based
accounts make differing predictions as to subjects' performance in these two conditions. If selection in this task is
truly object based, then the rectangle itself is being selected.
When the cue is invalid, subjects should always be faster if
the target is on the same (cued) rectangle (a within-rectangle
shift of attention) than when the target is on the other
(uncued) rectangle (an across-rectangles shift of attention).
This finding would appear as a main effect for type of
attention shift (within rectangle vs. across rectangles). The
grouped array account, however, suggests that the underlying representation is location based. This account would
predict that moving the rectangles closer together should
reduce the amount of cost in switching attention from one
rectangle to the other. This would appear as an interaction
between rectangle separation (near vs. far) and type of
attention shift (within rectangle vs. across rectangles).
Method
Subjects
Subjects were 15 Carnegie Mellon University undergraduates.
All reported having normal or corrected-to-normal vision.
Stimuli
The displays used in this experiment were similar to those used
by Egly et al. and are shown in Figure 1. Two rectangles appeared
A. Large Separation
("Far" Condition)
B. Small Separation
("Near" Condition)
Horizontal
Orientation
Vertical
Orientation
Figure 1. Examples of stimuli used in the experiment.
318
COMMENT
either above and below or to the left and right of fixation. There
were two amounts of spatial separation between the rectangles;
they could be far from one another or they could be near one
another. The far condition was a replication of Egly et al.'s
experiment, and the near condition was used to test whether
selection was modified in part by location. Note that the name of
the "far" condition is a bit of a misnomer, because the corners of
the rectangles were actually equidistant. Finally, the cues could be
either valid (cue and target appearing at same corner) or invalid
(cue and target appearing at different corners). Also note that there
were two types of invalid cues, those in which the cue and target
were within the same rectangle (within-rectangle shifts of attention) and those in which the cue and target were across rectangles
(across-rectangle shifts of attention).
All figures (e.g., the fixation point and rectangles) were white
drawn on a black background (i.e., the inverse of Figure 1). The
fixation point measured 0.5 cm X 0.5 cm (0.48° X 0.48° of visual
angle, respectively). The horizontal and vertical rectangles were
identical except for orientation. Each of the rectangles measured
10 cm X 3.5 cm (9.46° X 3.34° of visual angle). The lines that
composed the rectangles were 3 pixels wide. The cue consisted of
the brightening of one of the ends of a rectangle. This brightening
was achieved by having the width of the line increase from its
original 3 pixels to 8 pixels. Note that this cuing procedure is
slightly different from the one used by Egly et al., in which the
corner switched from a gray line to a white line. This procedural
difference does not have any theoretical implications for the
present article, assuming that Egly et al.'s results are replicated
in the far condition. Finally, the target was a gray box that appeared at one of the corners. The target measured 1.9 cm X 2.3 cm
(1.81° X 2.2° of visual angle).
When the rectangles were far from one another, the center of the
rectangle was 2.95 cm (2.81°) from the center of fixation. In this
condition the four corners of the two rectangles were equidistant
from one another; this distance measured 6.1 cm (5.81° of visual
angle) from the center of the target in each corner. When the
rectangles were near one another, the center of the rectangle was
2.05 cm (1.96°) from the center of fixation. In this condition the
ends of the two rectangles were closer to one another (3.2 cm or
3.05° from the center of the target in each corner) than were the
ends of a single rectangle (6.1 cm; see Figure 1). Thus, in the near
condition, subjects would move attention a shorter spatial distance
when switching from one rectangle to the other, whereas in the far
condition the spatial distance was the same when moving either
within or across rectangles.
Procedure
All stimuli were presented on a Macintosh Plus computer.
Subjects sat approximately 60 cm from the monitor. Subjects first
participated in 80 practice trials in which their eye movements
were monitored by the experimenter through a mirror. Subjects
were told that they should not make any eye movements. By the
end of the practice, subjects made no eye movements. After this
practice, subjects received 640 experimental presentations that
were divided into 8 blocks of 80 trials each. Subjects were allowed
to rest between blocks.
An individual trial began with a 1,000 ms fixation display that
contained the fixation point and the two rectangles. After this
display the cue was presented for 100 ms. The fixation display was
then presented for another 200 ms (i.e., the interstimulus interval
between the cue and target was 200 ms). Finally, the target
appeared and remained visible until the subject responded. Subjects responded by pressing the space bar on a standard keyboard.
Half of the subjects responded with their left hand and half
responded with their right hand. After each response the screen
was blank for 500 ms before the next trial began.
The trials were distributed as follows. Half of the time the
rectangles were horizontal and half of the time they were vertical.
RTs to horizontal and vertical rectangles were averaged because
Egly et al. found no effect for horizontal versus vertical presentation. Half of the time the rectangles were far from one another and
half of the time they were near one another. Finally, 20% of the
640 trials were catch trials in which no target appeared. On these
trials the fixation display followed the cue for 2,000 ms. Subjects
were told to withhold responses on the catch trials. They were also
asked to say "error" if they made a false alarm; all subjects
complied with this request. For the remaining trials in which a
target was presented, 75% of the time the cue was valid, and 25%
of the time it was invalid. Half of the invalid trials involved
shifting attention within the same rectangle and half of the time
they involved shifting attention across the two rectangles. Presentation of the trials was random.
Results
First, any RTs that were over 1,000 ms or less than 100
ms were excluded from the analyses. This trimming eliminated less than 1.5% of all of the data. For each subject the
median RT for each condition was calculated; these median
RTs were then analyzed with a within-subject analysis of
variance (ANOVA). All analyses were conducted using the
SuperANOVA software on the Macintosh. The RTs for the
far condition were analyzed first. Any subject who did not
show faster responses to invalid cues within a rectangle as
compared with those across rectangles was excluded from
further analyses. This exclusion was done because these
subjects showed a spatial selection strategy; that is, they did
not show the nonspatial component described by Egly et al.
Excluding these subjects should make it more difficult to
find spatial effects in the near condition. Three subjects
were excluded on the basis of this criterion. These subjects
may have explicitly used a spatial strategy to predict the
onset of the target; however, it is also possible that there are
individual differences in this task.
The mean RTs for cue type and spatial separation for 12
subjects appear in Figure 2. Subjects made false alarms on
less than 1.5% of the total number of catch trials. As is
evident from Figure 2, subjects were faster to respond to
validly cued trials as compared with all invalidly cued trials,
F( 1,11) = 19.74, p < .001. (Note that the invalid trials have
not been separated by within-rectangle vs. across-rectangle
shifts. The invalid RTs are due to all invalidly cued targets.)
The main effect for spatial separation was not significant,
F(l, 11) = 2.17, p > .15, suggesting that the slight spatial
distance difference between the near and far conditions did
not reliably influence attentional selection (see also Cheal &
Lyon, 1989; Reuter-Lorenz & Fendrich, 1992, for consistent findings). Finally, the interaction between these two
variables was not significant, F(l, 11) < 1, suggesting that
the validity effects in the near and far conditions did not
differ significantly from one another.
Next, the invalid cues were divided into within-rectangle
shifts and across-object shifts. The mean RTs for invalid cue
319
COMMENT
Valid Versus Invalid Cues
370
I
I
Near
Distance
Figure 2. Mean reaction times to valid and invalid cues for the
near and far conditions. Note that the error bars contain betweensubject variability.
type and spatial separation appear in Figure 3. The main
effect for the shift of attention was significant, F(l, 11) =
45.33, p < .0001, with within-rectangle shifts resulting in
faster RTs than across-rectangle shifts. The main effect for
separation was not significant, as in the first analysis, F(l,
11) = 1.37, p > .25. Finally, and most important, the
interaction between the two variables was significant, F(l,
11) = 9.50, p < .02, suggesting that the cost in shifting
from one rectangle to another was influenced by the separation of the two rectangles.
Planned comparisons were conducted on the invalid trials
to test within- versus across-rectangle shifts for the near and
far conditions separately. For the far condition, the difference was significant, f(ll) = 115.54, p < .0001, replicating
Egly et al.'s findings. Similarly, the difference for the near
condition was also significant, f(ll) = 45.83, p < .0001.
This finding suggests that the structure that the rectangles
imposed on the visual field still influenced the allocation of
visual attention. This finding is discussed further below.
The second important finding calls into question the use
of the term "object-based" in reference to this task. When
the rectangles were moved closer to one another (the near
condition), subjects were again faster to respond to validly
cued targets than to invalidly cued targets, and this validity
effect did not differ from the validity effect in the far
condition. However, within the invalid trials, the movement
of attention (within a rectangle vs. across rectangles) interacted with the distance of the two rectangles. That is, the
cost in shifting between the two rectangles was significantly
reduced when the two rectangles were moved closer to one
another. This finding demonstrates that the nonspatial finding reported by Egly et al. can indeed be influenced by a
spatial manipulation.
What implication does this interaction have for Egly et
al.'s findings? The major implication of this result is that
it suggests that the object-based selection argued for by
Egly et al. is not object-based selection, assuming that
object-based selection is unaffected by location manipulations (Vecera & Farah, 1994). If selection in this task were
truly object-based, then manipulating the distance between
the ends of the rectangles should have little if any effect on
the movement of attention within and across rectangles. In
particular, the advantage for within-rectangle shifts relative
to between-rectangles shifts should have been the same in
the far and near conditions, resulting in a main effect of
attention shift but no interaction. Given this, what type of
selection is occurring in this task? The present results are
consistent with attentional selection from a grouped location-based (or grouped array format) representation. This
type of representation contains features or locations (or
both) that have been grouped according to whether they
belong to the same shape or not (see Kramer & Jacobson,
Shifts Within Versus Between Rectangles
370
360
350
Within Rectangle
Across Rectangles
340-
Discussion
There are several findings of theoretical relevance from
this study. First, the results of the far condition replicate
Egly et al.'s results. Subjects were faster to respond to
targets that followed a valid cue than to those that followed
an invalid cue. This finding confirms a spatial component of
attentional selection. However, on the invalid trials alone,
subjects were faster to respond when the cue and target were
located within the same object than when they were located
across different objects. This finding confirms a nonspatial
component of attentional selection. Recall that Egly et al.
referred to this component as object-based.
Figure 3. Mean reaction times for the invalid cues, broken
down by whether the shift of attention was within a rectangle or
between rectangles. Note that the error bars contain betweensubject variability.
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COMMENT
1991; Vecera & Farah, 1994). However, this type of representation is still location based, because if the shape
changes ocations, the group of locations also changes.
Selection from this type of representation would still be
location based (and thus would show spatial effects, as in
the present experiment), although it would not be simple
location-based selection, as with a spotlight, zoom lens,
or other mechanism that does not respect perceptual
groupings. It should also be noted that location-invariant
object-based selection is not merely a theoretical construct. Recent results have supported object-based selection irrespective of object location (Vecera & Farah,
1994). Thus, a simple spatial manipulation can be diagnostic in determining whether selection is occurring from
a grouped array or from an object-based representation.
Next, it is important to note that even in the near condition there was a significant difference between moving
attention within an object as compared with moving attention between objects: Movements of attention within a
rectangle were faster than movements across rectangles.
(Note that this finding does not compromise the above
finding and conclusion, because an object-based theory
would predict no interaction between distance and movement of attention within and between rectangles.) This
finding does, however, suggest that perceptual groupings
are extremely strong in the influence they have on locationbased attentional selection. In the near condition, even
though a movement across objects was spatially smaller
than a movement within an object, there was still an advantage for moving within an object. This suggests that the
perceptual groups established by early and intermediate
vision can dramatically influence the processing of subsequent visual mechanisms, such as spatial attention.
Finally, what implications do the present results have for
the work that Egly et al. conducted with neurological patients? With respect to this work, the present findings suggest that left-parietal-damaged subjects may not have a
deficit in object-based attention per se, but rather they may
have a deficit in attending to grouped array representations.
Right-parietal-damaged subjects may have deficits in allocating location-based attention to spatial locations, indepen-
dent of the structure in the visual field, as Egly et al.
suggested. The present experiment does not address these
issues, but it offers testable predictions based on the
grouped array-object-based distinction.
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Received December 1, 1993
Revision received January 30, 1994
Accepted February 2, 1994