Brain and Cognition 50 (2002) 95–115
www.academicpress.com
Visual field asymmetries and allocation
of attention in visual scenes
Dell L. Rhodesa,* and Lynn C. Robertsonb,1
a
Reed College, 3203 S.E. Woodstock Boulevard, Portland, OR 97202, USA
b
Veterans Administration Medical Research, Martinez, CA 94553, USA
Accepted 25 February 2002
Abstract
Single items such as objects, letters or words are often presented in the right or left visual
field to examine hemispheric differences in cognitive processing. However, in everyday life,
such items appear within a visual context or scene that affects how they are represented and
selected for attention. Here we examine processing asymmetries for a visual target within a
frame of other elements (scene). We are especially interested in whether the allocation of visual
attention affects the asymmetries, and in whether attention-related asymmetries occur in
scenes oriented out of alignment with the viewer. In Experiment 1, visual field asymmetries
were affected by the validity of a spatial precue in an upright frame. In Experiment 2, the same
pattern of asymmetries occurred within frames rotated 90° on the screen. In Experiment 3,
additional sources of the spatial asymmetries were explored. We conclude that several left/
right processing asymmetries, including some associated with the deployment of spatial
attention, can be organized within scenes, in the absence of differential direct access to the two
hemispheres. Ó 2002 Elsevier Science (USA). All rights reserved.
Keywords: Hemispheric lateralization; Visual attention; Endogenous cuing; Frames of reference; Visual
scenes
1. Introduction
It is not news that there is functional asymmetry within the human brain. Motorically one hand, eye or foot is preferred over the other, and cognitively the
hemispheres differ in a number of ways. The latter processing differences are often
described in terms such as linguistic/spatial, part/whole, time/space, or local/global.
Although the nature of the cognitive processes underlying hemispheric asymmetry
may be debated, it is a fact that asymmetric function exists. Most of us are righthanded, would likely become aphasic following left hemisphere damage and would
likely suffer hemineglect following right hemisphere damage. The neuropsychological
*
Corresponding author.
E-mail addresses: dell.rhodes@reed.edu (D.L. Rhodes), lynnrob@socrates.berkeley.edu (L.C. Robertson).
1
Department of Psychology, Tolman Hall, University of California, Berkeley, CA 94720, USA.
0278-2626/02/$ - see front matter Ó 2002 Elsevier Science (USA). All rights reserved.
PII: S 0 2 7 8 - 2 6 2 6 ( 0 2 ) 0 0 0 1 4 - 3
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literature as a whole makes it undeniable that the hemispheres process information
differently.
Perhaps the strength of the clinical literature was one of the reasons psychology
was fast to embrace the study of hemispheric differences in normals. Studies exploiting the decussation of the optic pathways, which directs the visual input of the
left and right visual fields initially to the right and left hemispheres, respectively,
produced an overall picture congruent with the clinical literature. For instance, when
lateralized presentations of words produced a right visual field (left hemisphere)
performance advantage (Mishkin & Forgays, 1952), it was hard to resist linking this
finding to language deficits observed following left hemisphere damage. Such convergence produced a plethora of studies aimed at understanding functional hemispheric differences in normal participants through controlled visual field
presentations.
As successful as this method has been, there remain questions regarding when an
obtained visual field difference reflects a difference in entry-level hemispheric function (i.e., in input-dependent processing) and when it reflects a lateralization of more
controlled processing such as executive control or allocation of attention (e.g.,
Bryden & Mondor, 1991). The present studies were designed to examine the role of
various components of automatic and controlled visual processing in producing
visual field asymmetries by manipulating spatial attention and the orientation of
spatial reference frames.
A spatial reference frame refers here to a global coordinate system that defines the
spatial layout of local parts (Ivry & Robertson, 1998). From studies of normal
perception, we know that reference frames are necessary to account for how we
process stimuli in the real world (see Palmer, 1999). For example, consider a photograph of a family dinner. The spatial coordinates of the scene that includes the
table, chairs and diners are defined by the orientation of the photograph, not by
the coordinates of the room in which we are standing, by our own orientation, or by
the orientation of individual objects within the scene.2 Instead, the global spatial
frame of the photograph defines the spatial relationships and orientations of objects
within that scene. We see where the family portrait is hung in relation to the landscape painting, or that the chair in the corner has been tipped over. Consistent with
many formal arguments about spatial reference frames (Hinton & Parsons, 1988;
Schyns & Oliva, 1994; Quinlan, 1995), as well as with common sense, we use ‘‘scenebased’’ to refer to this type of environmental, contextual reference frame.
Other types of environmental reference frames, such as ‘‘object-based’’ and ‘‘environment-based’’ (gravity-bound), as well as a variety of ‘‘viewer-centered’’ frames
have been invoked to account for the influence of different types of information on
performance. Particularly relevant to the present experiments are studies of attention-related deficits in patients with visual neglect (often called hemineglect) resulting
from unilateral brain damage. Several investigators have dissociated viewer-,
gravity-, and object-centered frames of reference, and, in general, hemineglect has
been found to occur within all of these frames. For instance, Calvanio, Petrone, and
Levine (1987) dissociated environment- and viewer-centered reference frames by
turning patients on their sides while presenting stimuli upright as usual in environmental coordinates. Under such conditions, neglect was observed in both environment- and viewer-centered frames of reference. A more recent study by Behrmann
2
However, the spatial coordinates of the scene within a photograph could be defined by the
configuration of the objects within the scene, if, for example, the vertical axis of the camera was not aligned
with the vertical axis of the environment when the photograph was taken and the objects in the
photograph unambiguously define gravitational up/down. A ‘‘scene’’ thus established by the configuration
of a group of individual objects may be similar to the type of reference frame studied, for example, by
Palmer and his colleagues (e.g., Palmer, 1980).
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
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and Tipper (1999) showed that left-sided neglect occurred both in object-based and
in environment/viewer-based frames (viewers were always upright). Importantly in
this study, neglect of the left side of an object (i.e., the side initially located in the
neglected left visual field) rotated with the object, even when this side ended up in the
patient’s right visual field. What is surprising about these findings is that a deficit
resulting from damage in one hemisphere was not confined to visual information
projected directly to the damaged hemisphere. In other words, even a processing
deficit with an almost certain hemispheric basis was not confined to viewer-centered
coordinate systems.
In the present work, we demonstrate that a scene-based reference frame such as
that provided by the display itself can have powerful influences on visual processing
asymmetries in normal viewers. We examine the sources of the affected asymmetries
by rotating the display and by using spatial precues to direct attention. The studies
follow up earlier studies (Robertson, 1995; Robertson & Lamb, 1988, 1989), which
showed that some visual field asymmetries found when viewers are upright shift with
transformations of display orientation (although certainly not all do—see for example, Hellige, Cowin, Eng, & Sergent, 1991). Specifically, Robertson (1995) showed
that participants were faster to make a reflection judgment about letters presented in
the right (vs left) visual field when a display orientation congruent with viewer upright was primed. Then, when the primed display was oriented 90° orthogonal
to viewer-centered coordinates (with participants themselves remaining upright),
the advantage continued to occur for targets presented on the right side of the display, whether this location was in the upper or lower hemifield in viewer-centered
coordinates.
The rightward advantage in Robertson’s 90° displays could not be attributed to
differential access of the target stimuli to one hemisphere or the other. Moreover, the
size of this rightward advantage did not vary as a function of display orientation,
supporting the conclusion that it was occurring nearly exclusively in the scene-based
frame. In an additional experiment, Robertson showed that participants were not
mentally rotating the 90 scenes back to environment/viewer upright before making
the response. If anything, they were mentally rotating a viewer-centered frame into
alignment with the display.
The cognitive mechanisms generating the rightward advantage in Robertson’s
studies were not investigated. Robertson (1995) speculated that her scene-based
rightward advantage was due to a directional advantage in covertly orienting attention. There is support for the existence of one or more rightward attention-related
spatial advantage(s) in the literature (Heilman, 1995; Posner & Cohen, 1984; ReuterLorenz, Drain, & Hardy-Morais, 1996; Reuter-Lorenz, Kinsbourne, & Moscovitch,
1990). Further, the studies of hemineglect patients described above demonstrate that
an attention-related spatial advantage can occur in object-based as well as in viewercentered coordinates. Finally, a large number of studies demonstrate that attention
can be allocated in object-based frames in normal viewers (e.g., Egly, Driver, &
Rafal, 1994; Gibson & Egeth, 1994; Olson & Gettner, 1996). Thus, it seems reasonable to propose that attention-related spatial advantages can occur within scenebased frames in normal participants.
Nonetheless, the allocation of attention was not explicitly manipulated in Robertson’s experiments, and the rightward advantage could instead have resulted from
asymmetries in target-related perceptual processes or in post-discrimination response
selection. Knowing the source(s) of spatial advantages that are not tied to viewercentered coordinates, such as those described by Robertson, can help us to decide
which lateralized processes require differential direct access to the hemispheres and
which do not.
In the present studies, a spatial precuing method was used to investigate possible
attention-related sources of the scene-based rightward advantage in the letter
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reflection task obtained by Robertson. Specifically, we used a precuing procedure
generally thought to affect the endogenous (voluntary) allocation of attention, i.e., a
central arrow (the precue) occurred before a letter target and predicted the target
location with 75% accuracy (Posner, 1980). The precue-induced focusing of attention
should help to distinguish among several different possible sources of the rightward
advantage.
1. If the rightward advantage is due to an automatic process that is immune to the
effects of endogenous attention, it should occur in all trials, independent of precue
validity (e.g., Hardyck, Chiarello, Dronkers, & Simpson, 1985).
2. If the rightward advantage is due to a global asymmetry in the activation of attentional resources, such as that accompanying differential strategic engagement
of the left hemisphere (Kinsbourne, 1970), it should be eliminated altogether,
or at least reduced, by precuing (e.g., Mondor & Bryden, 1992).
3. The rightward advantage could reflect an asymmetry in the ability to move attention to letter targets appearing on the right as a result of a learned strategy, such
as scanning from left to right during reading (Posner & Cohen, 1984). If so, it
ought to be diminished following valid precues when there is a long enough stimulus onset asynchrony (SOA) between precue and target for attention to be engaged at the precued location before the target appears. However, the
rightward advantage should still be present following invalid precues, because
the appearance of the target itself will serve as the ‘‘cue’’ that activates the biased
strategy.
4. If the rightward advantage is due to an attention-sensitive asymmetry in the ability to perceive or process the targets, precues should have a greater influence on
targets presented on the disadvantaged side. In other words, left valid precues
should counteract the superiority (producing a larger benefit for validly precued
left targets) and right invalid precues should exacerbate the superiority (producing
a larger cost for invalidly precued left targets). Such a pattern was obtained by
Nicholls and Wood (1998), who reported a larger effect of precue validity for left
visual field targets in a word recognition task for which there was a rightward advantage in neutral precue trials.
5. If the rightward advantage is due to an asymmetry in the consequences of focusing endogenous attention on the left and right sides, there should be a different
asymmetry in the effect of precue validity. If there is a larger facilitatory effect
(benefit) of endogenous attention on the right, there might be a rightward advantage in valid precue trials compared to neutral and invalid precue trials if focusing
attention in response to the precue shortens the time to maximum benefit. Such a
pattern was seen, for example, in a letter localization task in Egly and Homa
(1984, see their Fig. 5). If there is also a larger inhibitory effect (cost) on the right
of focusing attention on the left (i.e., a bi-directional asymmetry in the effects of
attention), there should be a leftward advantage in invalid precue trials, compared
to both neutral and valid precue trials.
Thus, the pattern of spatial advantages obtained in the presence of predictive
spatial precues should help to characterize the bases of the rightward advantage
found in the letter reflection task. In addition, rotation of the display will determine which components are mapped in the scene-based frame. In Experiment
1, left/right asymmetries were examined with the display aligned with viewer/environment upright to determine the effects of endogenous spatial precues at short
and long SOAs. In Experiment 2, we showed that the pattern of asymmetries
observed in Experiment 1 occurred in scenes visibly rotated through 90° on the
screen (before the precuing procedure), as predicted by Robertson’s (1995)
results with primed scene orientations. Finally, in Experiment 3, a more detailed
examination of possible sources of the scene-based spatial advantages was
undertaken.
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
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2. Methods: general
The primary task adopted in the present experiments was the letter reflection task
employed by Robertson (1995). Studies of mental rotation suggest that object representations rarely specify the object’s left/right sides (Hinton & Parsons, 1981;
McMullen & Jolicoeur, 1990; Rollenhagen & Olson, 2000; Tarr & Pinker, 1990), so
most handedness judgment tasks require rotation of the object into viewer-centered
coordinates. However, several investigations have shown that an informative contextual frame can also be used to determine the left/right sides (Corballis, Nagourney, Shetzer, & Stefanatos, 1978; Hinton & Parsons, 1981). Thus, a handedness
judgment can be used to induce reliance on a scene for the purposes of comparing the
use of this type of environmental frame with that of the viewer-centered frame.
In the present experiments, three letter targets were used: E, K, and G. Because
two of these letters (E and K) are symmetrical around their horizontal axes, participants were required to use their knowledge of the scene orientation to do the
letter reflection task in Experiment 2, when frames rotated to orientations 90° with
respect to the viewer. However, previous studies (Robertson & Lamb, 1989) found
no difference in response times between symmetric and asymmetric letters as a
function of side or of frame orientation, and Robertson (1995) obtained a strong
scene-based rightward advantage using only asymmetric letters. So, there’s no reason
to expect that the effects we obtained in these studies depend on the specific letters
used.
Participants reported the reflection of the target letter by pressing left and right
response keys with two fingers (index and middle) of one hand. Differential hemispheric activation associated with the use of one hand to respond could influence the
rightward advantage. Thus, half the participants in each study used their right hand,
the other half used their left hand, and response hand was included as a between
subjects variable in the data analyses. For all participants, however, the right key
press indicated a ‘‘normally-oriented’’ (henceforth: normal) target letter and the left
key press indicated a ‘‘mirror-reflected’’ (henceforth: reflected) target letter. This
mapping seemed much more natural to participants, presumably because the informative side of a normal letter is on its right and the informative side of a reflected
letter is on its left.3 It did, however, set up the possibility that we would obtain a
stimulus/response compatibility effect known as the Simon Effect (normal letters on
the right side would be responded to faster than reflected letters on the right side and
than normal letters on the left side). This possibility is examined in Experiment 3.
Spatial precuing was accomplished using a standard Posner-type endogenous
precuing paradigm (Posner, 1980). Specifically, a centrally located arrow indicated
the side of the upcoming target with 75% accuracy. In Experiments 1 and 2, the
arrow formed the arms of a stick figure with head and feet, to emphasize the orientation of the frame. In Experiment 3, more traditional arrow cues were used. In
each experiment, two or three SOAs were used for three reasons: (1) to reduce the
temporal predictability of target onset, (2) to allow us to confirm that we were
obtaining the normal SOA-dependent validity effects with our precuing procedure,
3
This mapping could produce a response hand target side interaction, as the index finger might be
faster than the middle finger. This pattern was in fact obtained in Experiment 2 (p < :05). However, for the
right hand, the index finger signaled a reflected letter, whereas for the left hand, the index finger signaled a
normal letter. Thus, this mapping should help to prevent asymmetries in stimulus/response compatibility
effects. Furthermore, in Experiment 1, the response hand target side interaction was in the opposite
direction, reflecting a weak hand-based spatial compatibility effect (p < :10), and in Experiment 3, the
response hand target side interaction was not significant (p ¼ :61). Note also that because the
informative sides of normal and reflected letters occur closer to and farther from fixation, depending on
target side, possible effects of stimulus feature eccentricity on each target side should be controlled for.
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and (3) because some sources of the spatial asymmetries were predicted to vary
with SOA.
Head position was constrained in Experiment 2, when display orientation was
dissociated from viewer/environment upright. In all Experiments, the instructions
emphasized the importance of maintaining fixation at the center of the display.
Although eye movements were not monitored, it is unlikely that our findings are due
to eye movements.4
Although most studies of hemispheric laterality employ brief stimulus presentations to insure that participants do not fixate the target, in these studies both the
precue and the target remained on the screen until the participant responded. We
adopted longer stimulus presentation times for two reasons: (1) precue disappearance might have increased the likelihood of eye movements from central fixation,
and (2) briefly presented targets can produce across-trial inhibition of return (Klein,
2000). The ways in which these procedures may have affected our findings are addressed in Section 6.
3. Experiment 1: Spatial asymmetries in upright scenes
3.1. Participants
Six men and 10 women, aged 18–47 (mean ¼ 23) participated. All but one were
naive to the goals of the study. Participants were right-handed and had normal or
corrected-to-normal vision. All gave written consent and were paid for their participation.
3.2. Stimuli and apparatus
Stimuli were white on a black background. Each trial began with a 500 ms fixation
display of a centered ‘‘A’’, 1.5° high. The ‘‘A’’ was replaced by ‘‘arrowperson,’’ a
stick figure with a head, two legs, and an arrow for arms. The arrow precue predicted
the upcoming target side with 75% accuracy. There were no target location markers.
Following randomized SOAs of 150, 400, or 650 ms, a 1.5° target letter (E, K, or
G) appeared 6° to the left or right of fixation. The target letter was normally oriented
(with respect to the computer screen/display) or mirror-reflected around its vertical
axis. Arrowperson and the letter remained on the screen for 1000 ms. Incorrect responses were followed by auditory feedback. Intertrial intervals were 517 ms.
3.3. Procedure
Participants were seated 54 cm from the computer in low level illumination.
Sample trials were provided, and the following aspects of the task emphasized:
1. fixate the center of the screen, i.e., look at the fixation ‘‘A’’ and then arrowperson,
without moving your eyes, throughout each trial;
2. the arrow predicts the target location;
4
We have obtained spatial asymmetries of comparable directionality and magnitude in experiments in
which eye movements have been monitored (Rhodes & Montgomery, 2000). It is also unlikely that the
asymmetries we report here were due to eye movements to the target locations. Eye movement
asymmetries cannot easily explain the dissociation from viewer-centered coordinates obtained in
Experiment 2 and in Robertson (1995). Additionally, the target stimuli were presented for only 100 ms
in Robertson’s studies, and the rightward advantage was about the same size as those obtained in the
present studies.
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
101
3. report whether the target letter is normally oriented or mirror-reflected (backward) by pressing the right or left response buttons, respectively;
4. respond as rapidly as possible while not making many errors.
There were one block of 24 practice trials and three blocks of 144 trials each.
3.4. Data analyses
Reaction times (RTs) were not included if the response was incorrect, or if the RT
was longer than the display duration or more than three standard deviations from
the participant’s mean. Mean RTs were calculated collapsed over letter identity and
letter reflection, as there were not enough invalid precue trials (18/cell combined over
letter identity and letter reflection) to support inclusion of these variables in the
overall analysis. Mean percent errors (incorrect responses/total responses) were also
calculated.
Mean RTs and percent errors were analyzed in separate 4-factor mixed effects
ANOVAs with responding hand as a between groups variable, and precue validity,
target side, and SOA as within subjects variables.
3.5. Results
The mean RT over all conditions was 563 ms. Precue validity and SOA effects
conformed to those normally obtained in an endogenous spatial precuing task.
Responses were 25 ms faster to validly than invalidly precued targets,
F ð1; 14Þ ¼ 25:9, p < :001, and RTs were inversely related to SOA, F ð2; 28Þ ¼ 21:2,
p < :001. Responses were faster to validly than to invalidly precued targets at all
SOAs, but the difference was largest at the 650 ms SOA (validity SOA interaction,
F ð2; 28Þ ¼ 5:3, p ¼ :01Þ.
The main effect of target side was not significant, F ð1; 14Þ ¼ :6, but the target
side precue validity interaction was highly reliable, F ð1; 14Þ ¼ 34:6, p < :001. For
validly precued targets, responses were 21 ms faster to right than to left targets,
tð15Þ ¼ 2:6, p < :05. For invalidly precued targets, this spatial advantage was reversed, but the leftward advantage was only 8 ms, tð15Þ ¼ 1:0, n.s.
As can be seen in Fig. 1, the pattern of spatial asymmetries was similar at all SOAs
(target side precue validity SOA interaction, n.s.). On valid precue trials there
was a right side advantage at all SOAs; on invalid precue trials, the mean RT advantage either reversed to a left side advantage or was absent. The pattern of spatial
asymmetries was weakly related to responding hand (p < :10).5
The mean error rate was low (3.8%). In general, longer RTs were accompanied
by higher error rates, arguing against a speed-accuracy trade-off. Errors did not
vary significantly as a function of precue validity, target side, or precue validity target side (valid right 2.9%, valid left 3.8%, invalid right 3.9%, invalid left
4.9%).
3.6. Discussion
The rightward spatial advantage observed by Robertson (1995) appeared only for
validly precued targets in this study. For invalidly precued targets, the spatial advantage was leftward overall. This pattern of spatial asymmetries is not compatible
with some of the options described in Section 1. First, because the rightward advantage was not eliminated by valid precues, it is unlikely to be due to a global
5
The rightward advantage in valid precue trials was due primarily to participants using their right hand
to respond, and the leftward advantage in invalid cue trials was due primarily to participants using their
left hand to respond. However, this pattern was not replicated in Experiments 2 and 3.
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Fig. 1. Mean reaction times to report letter reflection as a function of target side (right/left) and cue
validity (valid/invalid) at each SOA (150/400/650 ms) in Experiment 1 with all frames of reference upright.
asymmetry in the activation of attentional resources, such as that accompanying
differential strategic engagement of the left hemisphere (option #2; Kinsbourne,
1970). Second, the obtained pattern was the converse of that predicted by hypotheses
of more rapid orienting of attention to targets on the right side (option #3; Posner &
Cohen, 1984). Finally, the most consistent effect that can be seen in Fig. 1 is a larger
difference between valid and invalid precue trials for targets on the right (40 ms) than
for those on the left (11 ms). Theories suggesting that there are more attentional
resources available for targets on the right side would predict a larger effect of
precues on the left (option #4; Nicholls & Wood, 1998).
The larger effect of precue validity on the right could reflect a larger benefit of
right valid precues, a larger cost of left invalid precues (i.e., a larger cost for right
targets appearing after left precues), or both (option #5). However, the rightward
advantage in valid precue trials in the present study is of approximately the same
magnitude as the rightward advantage obtained by Robertson (1995), who used no
predictive precues. It thus seems likely that there is a rightward advantage that occurs whenever a target appears on the right, that is counteracted in invalid precue
trials by a leftward advantage associated with a larger cost of invalid left precues.
Whether the rightward advantage is attention-insensitive (option #1) or is instead a
consequence of focusing attention on a right target (a version of option #5) cannot
be determined from these data.
Two other explanations of the findings in Experiment 1 are possible. First,
rightward precues might produce an RT advantage for whatever target follows
them.6 Such an advantage could be due, for example, to greater arousal following
right precues (or, in the absence of precues, greater arousal in response to right
targets). Second, there could be an interaction between the direction of cued attentional shifts and the relative strengths of stimulus–response spatial compatibility
effects on the right and left sides of the display. For example, responses to normal
letters could be especially fast on the right following valid (right) precues, whereas
responses to reflected letters might be especially slow on the right following invalid
(left) precues.
These alternative explanations of the precue validity target side interaction
will be explored in more detail in Experiment 3. Before considering these possibilities further, we first establish that the pattern of asymmetries observed in
Experiment 1 occur following rotation of the scene out of alignment with viewer
upright.
6
We thank Ray Klein for this suggestion.
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
103
4. Experiment 2: Spatial asymmetries in rotated frames
Only the methods differing from those of Experiment 1 are described below.
4.1. Participants
Nine men and seven women, aged 17–33 (mean ¼ 22.5) participated. All were
naive to the goals of the study.
4.2. Stimuli and apparatus
Fig. 2 shows the stimulus displays used in this experiment. In addition to the ‘‘A’’
at fixation, two vertical columns of ‘‘A’’s and ‘‘V’’s located 6.2° right and left of
center emphasized the scene orientation. After 500 ms, the display equiprobably
rotated around its center 90° clockwise (+90, top at right), rotated around its center
90° counterclockwise ()90, top at left), or remained upright. When rotation of the
display occurred, it took about 230 ms and was perceptually smooth. Five hundred
ms following rotation (or 730 ms after onset for upright displays), the fixation A was
replaced by arrowperson, oriented upright in the scene.
Either 150 or 650 ms after arrowperson appeared, the target letter appeared 4° to
the left or right of arrowperson, i.e., in the upper or lower visual fields in the 90°
displays. The top of the target letter was always oriented in the same direction as the
top of the display frame and arrowperson’s head. Arrowperson and the letter remained on the screen for 1500 ms or until the participant responded. Intertrial intervals were 548 ms.
4.3. Procedure
Participants’ heads were held upright by a chin rest, forehead bar, and side rings.
Instructions emphasized that the reflection of the letter must be determined with
Fig. 2. The stimulus displays in Experiment 2 for a +90° frame, validly cued, left target side, and normal
target orientation trial. Drawing not accurately scaled.
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respect to the orientation of the display and that fixation should be maintained at the
center throughout each trial. There were one block of 72 practice trials and six blocks
of 144 trials each.
4.4. Data analyses
Mean RTs and percent errors were analyzed in separate 5-factor mixed effects
ANOVAs with responding hand as a between groups variable, and frame orientation
(upright/+90/)90), precue validity, target side (in scene-based coordinates) and SOA
as within subjects variables.
4.5. Results
Overall RTs in the upright frame were comparable to those in Experiment 1
(572 ms). RTs were 30 ms longer in rotated than upright frames, F ð2; 28Þ ¼ 12:4,
p < :001. This was expected, because in rotated frames the viewer/environment and
scene-based frames were in conflict.
RTs indicated successful precuing of spatial attention. Responses were 28 ms
faster to validly than invalidly precued targets, F ð1; 14Þ ¼ 32:6, p < :001, and
were faster at the long than short SOA, F ð1; 14Þ ¼ 38:4, p < :001. The validity
effect (invalid–valid precue trial RTs) was significant at both SOAs, but was
larger at the 650 ms SOA (precue validity SOA interaction, F ð1; 14Þ ¼ 14:0,
p < :005Þ.
As in Experiment 1, the main effect of target side (left/right in scene-based coordinates) was not significant, F ð1; 14Þ ¼ :2, but the target side precue validity
interaction was highly reliable, F ð1; 14Þ ¼ 31:9, p < :001. The scene-based spatial
asymmetries were very similar to those in Experiment 1. In valid precue trials, responses were 25 ms faster to right than left targets, tð15Þ ¼ 3:9, p < :005. For invalid
precue trials, responses were 13 ms faster to left than right targets, tð15Þ ¼ 2:2,
p < :05. Furthermore, these asymmetries were independent of frame orientation
averaged over SOA and of SOA averaged over frame orientation.
However, there was also a rightward advantage that was independent of precue
validity at the short SOA in this experiment, target side SOA, F ð1; 14Þ ¼ 13:5,
p < :005. Averaged over precue validity and frame, responses were faster to targets
on the right than left side at the short SOA (right ¼ 605 ms, left ¼ 620 ms),
F ð1; 15Þ ¼ 2:5, p < :05, but not at the long SOA (right ¼ 573 ms, left ¼ 570 ms).
Because a rightward advantage occurred in the absence of precues in Robertson
(1995), this precue validity-independent rightward advantage will be referred to as a
precue-independent rightward advantage. The asymmetries that depend on precue
validity (i.e., that are precue-dependent) will be referred to as cue-validity dependent
spatial advantages.
The only influence of frame orientation also occurred at the short SOA,
frame target side precue validity SOA, F ð2; 28Þ ¼ 3:6, p < :05. The target
side precue validity RTs are plotted separately for the 150 and 650 SOAs for each
frame orientation in Fig. 3. At long SOAs, the target side precue validity interactions were remarkably similar across frame orientations, frame target
side precue validity interaction, F < 1. In each frame orientation, RTs were faster
to validly precued right than left targets and slower to invalidly precued right than
left targets. At the short SOA, however, the precue-independent rightward advantage enhanced the rightward advantage for targets on valid precue trials (i.e., the
rightward advantage in valid precue trials was larger at the short SOA than at the
long SOA). It also reduced the leftward advantage for invalid precue trials, but only
in the upright and +90 frames (frame target side precue validity interaction,
F ð2; 28Þ ¼ 3:9, p < :05 at the short SOA).
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
105
Fig. 3. Mean reaction times to report letter reflection as a function of target side and cue validity at each
SOA (150/650 ms) and in each frame orientation (upright, rotated þ90°, rotated )90°) in Experiment 2.
Target side is identified in scene-based coordinates. For example, the ‘‘right target’’ in the +90° frame was
in the participants’ lower visual field and in the )90° frame it was in the upper visual field.
The mean error rate was low (2.8%). As in Experiment 1, shorter RTs were
generally associated with fewer errors. Error rates were lower in the upright frame
(1.6%) than in the rotated frames (+90: 3.2%, )90: 3.5%), F ð2; 28Þ ¼ 8:32, p < :005.
The main effects of precue validity and target side and the precue validity target
side interaction were not significant (valid right 2.0%, valid left 2.8%, invalid right
3.1%, invalid left 3.1%), nor did the effects of these variables interact with frame
orientation.
4.6. Discussion
The pattern of cue validity-dependent spatial asymmetries obtained in this experiment was very similar to that obtained in Experiment 1. There was a consistent
rightward RT advantage for target discrimination in valid precue trials. Although
less consistent than the rightward advantage, there was a leftward advantage for
target discrimination in invalid precue trials. The overall effect of precue validity
(benefit of valid precues plus cost of invalid precues) was larger and more consistent
for right targets than for left targets across both SOAs and all frame orientations.
The mechanisms underlying these asymmetries were clearly organized within
scene-based coordinates. If the left/right asymmetries included components mapped
in viewer/environment coordinates, at the very least the target side precue validity
interaction would have been reduced in the rotated scenes. Further, if an additional
viewer/environment asymmetry (such as an upward advantage) had been exposed, a
new set of frame orientation dependencies would have appeared.
The one difference in the asymmetries obtained in Experiments 1 and 2 was the
appearance of a precue-independent rightward advantage at the short SOA in Experiment 2. The leftward advantage for invalidly precued targets was numerically
larger at the short than at the long SOA in Experiment 1, but was larger at the long
than at the short SOA in Experiment 2. These findings at least tentatively suggest
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that a precue-independent rightward advantage is replaced over time by cue-validity
dependent asymmetries. In Experiment 1, the ‘‘short’’ SOA might not have been
short enough to reveal the precue-independent advantage. In Mondor & Bryden’s
(1992) experiments, for example, a rightward advantage in letter identification was
overcome by predictive peripheral precues at SOAs as short as 67 ms. In Experiment
2, the addition of rotation to the displays may have added temporal and/or spatial
uncertainties that extended the time course of events, such that the precue-independent rightward advantage was still strong at the 150 ms SOA. A precue-independent rightward advantage at short but not long SOAs could be explained by
mechanisms such as a rightward advantage in the speed of orienting attention (option #3 in Section 1).
As in Experiment 1, the cue validity-dependent spatial asymmetries could reflect
larger effects of attention (facilitation or benefits and/or inhibition or costs) on the
right side of the scene. In other words, the rightward advantage could reflect an
enhanced facilitatory effect of attention directed to the right on valid precue trials
and/or an enhanced inhibitory effect on the right when attention was directed to the
left by the precue. A model that could account for both of these effects is a bi-directional gradient model that operates both to facilitate an attended location and
inhibit an unattended location (option #5; e.g., Handy, Kingstone, & Mangun,
1996).
To assess the cost/benefit predictions of this model and to compare it with the
alternative explanation that right precues speeded RTs overall, in Experiment 3 we
added neutral precue trials to a version of Experiment 1. The neutral precue trials
allowed us to evaluate costs and benefits of cuing independently. In addition, we
increased the number of trials to assess stimulus/response spatial compatibility effects
(often called the Simon Effect). Hommel & Lippa (1995) demonstrated a Simon
Effect within rotated visual contexts, so an examination of this spatial asymmetry is
interesting both in its own right and as a possible source of the scene-based spatial
asymmetries.
5. Experiment 3: Cost/benefit analysis
Only the methods differing from those of Experiment 1 are described below.
5.1. Participants
Eight men and nine women, aged 18–23 (mean ¼ 20.3) participated in Experiment
3. Three female participants were eliminated from the data analyses because the
number of acceptable trials in one or more cell(s) of the design fell below 80% (see
Section 5.4). Of the remaining participants, seven used their right hand and seven
used their left hand to respond.
5.2. Stimuli and apparatus
Each trial began with a 525 ms fixation display of a centered ‘‘A’’, 1° high. After
60 ms, the ‘‘A’’ was replaced by a diamond of the same height. In 80% of the trials,
the two right or the two left lines of the diamond were white, forming an arrow that
predicted the target location with 75% accuracy. The two lines forming the opposite
side of the diamond were dark gray. In the remaining 20% of the trials, the entire
diamond was light gray (neutral precue).
Following randomized SOAs of 150 or 650 ms, the target appeared 5° to the left
or right of the precue. Intertrial intervals were 555 ms. There was no feedback for
incorrect responses.
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
107
5.3. Procedure
The experiment was run at Reed College on a Macintosh rather than on the PCbased computer system used in the previous experiments. Participants reported
whether the target was normal or reflected by pressing the K or J keys, respectively,
on a computer keyboard. There were one block of 36 practice trials and eight blocks
of 120 trials each.
5.4. Data analyses
Trials were eliminated if the response was incorrect (2.1%), or if the RT was
longer than 1000 ms (0–3% of all trials, mean ¼ .7%). Participants were excluded
from analysis if the percentage of trials in any cell of the design fell below 80% (<20/
24 for invalid and neutral precue trials, <58/72 for valid precue trials).
Mean RTs and mean percent errors (incorrect responses/total responses) were
calculated collapsed over letter identity and analyzed in separate 5-factor mixed
effects ANOVAs with responding hand as a between groups variable, and precue
validity, target location, SOA, and letter reflection as within subjects variables. The
first analysis included only validly and invalidly precued trials, to confirm the effects
obtained in Experiments 1 and 2. The second analysis included all three precue
validity conditions, and separate analyses of the benefits (valid/neutral) and costs
(neutral/invalid) of precuing were performed where indicated. There were no significant effects of responding hand in these analyses.
5.5. Results
The mean RT over all conditions was 517 ms. Fig. 4 shows the RTs to targets on
each side following valid, neutral and invalid precues at short and long SOAs, collapsed over reflection. Fig. 5 displays the RTs to normal and reflected target letters
separately. Percent errors are shown above each bar.
Valid/invalid trials. RTs to validly precued targets were 21 ms faster than to invalidly precued targets, F ð1; 12Þ ¼ 43:4, p < :001, comparable to the effects obtained
Fig. 4. Mean reaction times to report letter reflection as a function of target side (right/left) and cue validity (valid/neutral/invalid) at each SOA (150/650 ms) in Experiment 3 with all frames of reference upright.
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Fig. 5. Mean reaction times to report letter reflection as a function of target side and cue validity at each
SOA (150/650 ms) for normal and reflected letters in Experiment 3. Numbers above each bar are mean
percent errors.
in Experiment 1 (25 ms) and Experiment 2 (28 ms). As expected, the validity effect
was larger at the long SOA, F ð1; 12Þ ¼ 9:7, p < :01.
RTs to targets on the right side were 10 ms faster overall. Although this difference
was only 3 ms larger than in Experiments 1 and 2, it was more consistent, leading to a
reliable main effect of target side, F ð1; 12Þ ¼ 15:6, p < :005. Most importantly, the
target side precue validity effect resembled that obtained in the previous experiments, F ð1; 12Þ ¼ 14:0, p < :005. Specifically, there was a 16 ms rightward advantage
for valid precue trials, tð13Þ ¼ 15:7, p < :001, but only a 4 ms rightward advantage
for invalid precue trials (n.s.). This pattern of asymmetries was independent of SOA.
Valid/neutral/invalid trials. Confirming the existence of a precue-independent
rightward advantage, there was an 18 ms rightward advantage for neutral precue
trials when analyzed alone, tð13Þ ¼ 17:6, p < :001. Costs and benefits were evaluated
relative to this neutral condition. The main effect of precue validity, F ð2; 24Þ ¼ 27:6,
p < :001 consisted of a 7 ms benefit, F ð1; 12Þ ¼ 5:8, p < :05 and a 15 ms cost,
F ð1; 12Þ ¼ 27:5, p < :001. The benefit of valid precues was approximately the same
for targets on the right (6 ms) and left (8 ms) sides, while the cost of invalid precues
was much larger for targets on the right (22 ms) than on the left (8 ms), precue validity target side, F ð1; 12Þ ¼ 15:3, p < :005. The precue validity target side interactions for benefits and costs did not vary with SOA. Thus, the asymmetrical
validity effects obtained in Experiments 1 and 2 were due entirely to a larger cost of
invalid left precues (i.e., longer RTs to right targets following left precues).
Not surprisingly, RTs to normal letters were 20 ms faster than to reflected letters
overall, F ð1; 12Þ ¼ 7:7, p < :05. An important question is whether an asymmetry in
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
109
stimulus–response spatial compatibilities (i.e., the Simon Effect) can account for
either the precue-independent or the cue validity-dependent spatial asymmetries. A
Simon Effect should produce a significant target side letter reflection interaction, in
which right keypresses to normal letters are faster when the letters appear on the
right side and left keypresses to reflected letters are faster when the letters appear on
the left side of the display. This interaction was not significant overall, but the
SOA target side letter reflection interaction was highly reliable, F ð1; 12Þ ¼ 20:0,
p < :001.
At the short SOA, there was no evidence for a Simon Effect. There was a rightward advantage for both normal (12 ms) and reflected (16 ms) letters (target
side letter orientation n.s.). These data make it particularly clear that the precueindependent rightward advantage was not due to an asymmetry in the Simon Effect.
At the long SOA, there was a Simon Effect, target side letter reflection,
F ð1; 24Þ ¼ 5:5, p < :05. The rightward advantage increased to 25 ms for normal
letters, and reversed to a 3 ms leftward advantage for reflected letters. Thus, the
precue-independent rightward advantage seen at the short SOA for both normal and
reflected letters was symmetrically modulated by the Simon Effect at the long SOA,
confirming that the precue-independent rightward advantage did not result from an
asymmetry in the Simon Effect.
Perhaps more important is the possibility that an interaction between the precuing
procedure and the response mapping used in these studies could account for the
precue validity target side interaction. Critically, the precue validity target
side letter reflection interaction was not significant (p ¼ :38). However, consistent
with the presence of a Simon Effect only at the long SOA, the precue validity target
side letter reflection SOA interaction was significant, F ð2; 24Þ ¼ 5:5, p ¼ :01.
This interaction was complicated, but the data in Fig. 5 make it reasonably clear that
the longer RTs for invalidly precued right targets was not an artifact of the Simon
Effect, as it occurred for normal letters at the short SOA as well as for reflected
letters at the long SOA.7
The mean error rate was low (2.1%). The main effects of precue validity and target
side and the precue validity target side interaction were not significant. However,
the 4-way interaction (SOA precue validity target side letter reflection) was
significant, F ð2; 24Þ ¼ 7:6, p < :005. As can be seen in Fig. 5, response accuracy
varied across SOA especially for invalidly precued normal letters on the right, for
which error rates were high at the short SOA and low at the long SOA. At the short
SOA, errors were lowest for validly precued normal letters on the right side and
highest for validly precued normal letters on the left side. At the long SOA, errors
were lowest for validly precued reflected letters on the left, and were highest for
invalidly precued normal letters on the left. Thus, the interaction reflects precue
validity and SOA modulation of the Simon Effect, which just barely achieved significance averaged over SOA and precue validity, target side letter reflection,
F ð1; 11Þ ¼ 4:8, p ¼ :05.
7
At the short SOA, costs and benefits of precuing were virtually identical for reflected targets on the
right and left sides (validity main effect F ð2; 24Þ ¼ 17:1, p < :001, target side precue validity, n.s.). For
the normal letters, however, there were no effects of precue validity on the left side, and a large cost of
invalid precuing on the right (validity effect n.s., target side precue validity, F ð2; 24Þ ¼ 4:1, p < :05). At
the long SOA, it was the normal letters that showed identical costs and benefits of precuing on the right
and left sides (validity main effect, F ð2; 24Þ ¼ 29:3, p < :001, target side precue validity n.s.). For the
reflected letters, one can see both the influence of the Simon Effect (rightward advantage reduced for
neutral precue trials and reversed for valid precue trials compared to the rightward advantage for normal
letters) and the exaggerated cost of invalid precuing of right targets (27 ms cost for right reflected letters,
no cost for left reflected letters, target side precue validity, F ð2; 24Þ ¼ 6:1, p < :01). The detailed sources
of this interaction are not understood.
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5.6. Discussion
Analysis of the valid and invalid precue trials in Experiment 3 revealed the same
target side precue validity interaction observed in Experiments 1 and 2, i.e., a
rightward advantage in valid precue trials that was reduced or eliminated in invalid
precue trials (Fig. 4). The cost/benefit analysis afforded by the addition of neutral
precue trials revealed that the benefit of valid precues (neutral—valid precue trial
RTs) was about the same for right and left targets. Confirming the findings of
Robertson (1995), who used no spatial precues, there was also a substantial rightward advantage following neutral precues (18 ms). Further, a smaller (10 ms), but
significant, rightward advantage that was independent of precue validity was obtained in this study. Thus, there was strong evidence for a precue-independent
rightward advantage that might reflect either an attention-insensitive mechanism
(option #1 in Section 1) or an attention-related asymmetry engaged by all targets
appearing on the right side (a version of option #5). These alternatives will be discussed in more detail in Section 6.
Given that there was no spatial asymmetry in the benefit of valid precuing, the
target side precue validity interaction was due entirely to an asymmetrical cost of
invalid precues (invalid-neutral precue trial RTs). The cost of invalid precues was
much larger for targets on the right than on the left, and this effect was present at
both short and long SOAs.
In Experiment 2, the rightward advantage in valid precue trials and the asymmetrical validity effect in upright displays was essentially unchanged in displays
rotated 90° out of alignment with the viewer-centered frame. Thus, both the precueindependent rightward advantage and the larger inhibitory effect of invalid left
precues on right targets were programmed in scene-based coordinates. Neither of
these effects could be due to differential direct access of the target stimuli to the two
hemispheres.
There was no evidence of a Simon Effect at the short SOA in this study, but an
appropriate target side letter reflection interaction occurred at the long SOA.
Because both the precue-independent and cue-validity dependent spatial asymmetries were obtained at both short and long SOAs, it is unlikely that they result from
asymmetrical stimulus/response compatibility effects. Mean RTs for normal and
reflected letters in the invalid precue conditions of Experiments 1 and 2 must be
calculated from a very small number of trials (9/cell), but the pattern of effects was
quite similar to that seen in Experiment 3.
In summary, the results of Experiment 3 suggest that there are three spatial
asymmetries affecting performance in the task used here. (1) A rightward advantage
that occurs when there is no directional precue and that is independent of precue
validity when precues are provided. (2) Greater inhibition of the right than of the left
side of the display by invalid spatial precues, revealed in greater costs for targets
appearing on the right after left precues than for targets appearing on the left after
right precues. (3) A Simon Effect evident only at longer SOAs. Most importantly, it
seems likely that all three of these asymmetries occur in scene-based coordinates.
6. General discussion
When participants discriminated the reflection of a target letter whose location
was predicted by a central spatial precue, we obtained a pattern of spatial asymmetries that included a larger precue validity effect (invalid–valid precue trial RTs)
on the right than the left side of the display. This pattern occurred at all SOAs across
three experiments. Importantly, in Experiment 2 the pattern was unaltered when
right and left target locations were defined within displays that had been rotated
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
111
90° out of alignment with the viewer and the environment. This is strong evidence
that the spatial asymmetries generated by the cue-induced allocation of spatial attention occurred in scene-based coordinates and not in viewer- or environmentbased coordinates. Indeed, there was no evidence for a viewer-centered component,
and thus no evidence that the cue validity-dependent spatial asymmetries were due to
differential direct access of visual information to one or the other hemisphere.
The cost/benefit analysis of Experiment 3 indicated that the cue validity-dependent component of the pattern of asymmetries was due specifically to a larger cost of
invalid left precues. This effect is not easily explained by existing theories of attention-related asymmetries, such as those outlined as options #2–4 in the Introduction.
However, it is consistent with a gradient theory of attention, in which the focusing of
attention facilitates processing at the attended location and inhibits processing at
non-attended locations (option #5 in Section 1; e.g., Handy et al., 1996).
In its simplest form, an attentional gradient model might predict that both the
facilitatory and the inhibitory effects of precuing attention would be amplified on the
right side of a frame. In this case, one would expect to see a rightward advantage in
RTs to validly precued targets and a leftward advantage in RTs to invalidly precued
targets (because the inhibition of left targets by right invalid precues would be less
than the inhibition of right targets by left invalid precues). This is close to the pattern
we obtained, with one important exception—the rightward advantage we obtained
was not dependent on precue validity. Although we obtained a rightward advantage
in valid precue trials, it was not consistently larger than the rightward advantage
obtained in neutral precue trials (Experiment 3) and in no precue trials (Robertson,
1995). Further, there was no asymmetry in the benefit of valid precues. And, finally,
although the leftward advantage in invalid precue trials was consistently present in
comparison to the rightward advantage in valid precue and neutral precue trials, it
appeared to occur ‘‘on top of’’ a rightward advantage. As a result, the leftward
advantage was not always apparent numerically, and, in some cases (Experiment 3
and short SOA in Experiment 2), a rightward advantage that was independent of
precue validity (main effect of target side) appeared in the statistical analyses.
Thus, it is not clear that the rightward advantage obtained in valid and neutral
precue trials in this study, and in Robertson’s (1995) no precue trials, is related to an
asymmetry in the effects of attention. It could, instead, be due to an attention-insensitive process (option #1 in the Introduction). However, an attention-related
explanation seems more likely. One reason is that the benefits of precuing in Experiment 3 were numerically rather small (7 ms). This may have been because the
effect of the valid precues was ‘‘masked’’ by a comparable process occurring in the
neutral precue trials. Neutral precue trials (and no precue trials) can be thought of as
trials in which the appearance of the target itself serves as the ‘‘cue’’ that initiates the
movement and subsequent focusing of attention. If focusing attention on the right
side of a scene facilitates processing more than focusing attention on the left side, this
‘‘benefit’’ should occur in neutral precue trials (and in no precue trials) as well as in
trials with valid precues, and it does. The prediction that there ought to be a
rightward advantage in only valid precue trials depends upon an assumption that
precues speed up the benefit of focusing attention.
In addition, the attention-insensitive option seems unlikely. There is little reason
to expect stimulus-related or task-related processing asymmetries in this task, in
which the target stimuli differed only in their reflection around their vertical axes.
Further, an attention-insensitive account for the cue-independent rightward advantage would predict its disappearance in rotated displays as was observed by
Hellige et al. (1991) in studies of CVC nonsense syllable identification. Instead, the
precue-independent rightward advantage occurred in scene-based coordinates, as it
appeared across frame orientations at the short SOA in Experiment 2, and in noprecue trials across frame orientations in Robertson’s experiments.
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Further, we obtained no evidence to suggest that response processes produced the
precue-independent rightward advantage. In general, responding hand did not affect
the asymmetries we obtained, and when it did, it did not do so in a consistent way
across experiments3 . Our examination of stimulus/response compatibility effects (i.e.,
the Simon Effect) in Experiment 3 provided no evidence that an asymmetry in these
effects could account for either the cue-validity dependent or the precue-independent
spatial advantages.
Thus, an especially appealing explanation of the spatial advantages obtained in
these experiments is that both the facilitatory effect of focusing attention on an
upcoming target location and the inhibitory effect of focusing attention on a contralateral (invalidly cued) location were larger on the right side of a scene than on the
left. The inhibitory component was activated by an invalid precue. The facilitatory
component, however, was activated whenever the target stimulus was attended, i.e.,
following a valid precue, but also following invalid and neutral precues. This explanation conforms to bi-directional gradient theories of the effects of attention
(Handy et al., 1996), and attributes both the cue validity-dependent and the precueindependent spatial advantages obtained in these experiments to processes associated
with the allocation of attention within scenes. It also suggests that the deployment of
attention in environmental or scene-based spatial reference frames may account for
some of the spatial asymmetries previously attributed to the lateralized projections
of the retinal hemifields.
Across experiments, the strength of the precue-independent rightward advantage
was variable. Comparing procedures across experiments, one possible explanation is
that the larger cost of invalid left precues (i.e., the leftward advantage in invalid
precue trials) was reduced when the locations of upcoming targets became less predictable overall. In Experiment 1, there was a (not statistically significant) leftward
advantage in invalid precue trials and no cue validity-independent rightward advantage. In Experiment 2, trial-by-trial frame rotation added a degree of temporal,
and perhaps spatial,8 uncertainty to the process of localizing the predicted target
location, especially at the short SOA. In this Experiment, there was a smaller leftward
advantage in invalid precue trials at the short (than at the long) SOA, and a significant
precue validity-independent rightward advantage at the short SOA. Finally, in Experiment 3 the overall proportion of valid precue trials was reduced (from 75% to
60%) by the presence of neutral precue trials, and the precue-independent rightward
advantage dominated the leftward advantage in invalid precue trials at both SOAs.
The notion is that, under conditions of greater temporal/spatial uncertainty,
participants were less likely to focus all their attention at the precued location, thus
reducing the asymmetry in the cost of invalid precues. The impact of these small
differences in the predictive value of the precues may have been enhanced by the fact
that the precue, and especially the target, remained on the screen until the participant
responded.
In addition to the attention-related spatial asymmetries, the findings in Experiment 3 suggest that stimulus/response compatibility effects can be added to the list of
spatial asymmetries influenced by scene-based frames. As there were not enough
trials in the invalid conditions to analyze the letter reflection variable as a function of
scene orientation in Experiment 2, we cannot be certain that this spatial asymmetry
occurred within scene-based coordinates. However, the consistency of the spatial
asymmetries obtained across experiments is very suggestive, and support for the idea
that the Simon Effect can occur in scene-based coordinates is found in the literature.
For instance, Hommel & Lippa (1995) showed quite convincingly that a stimulus/
response compatibility effect can occur in the functional equivalent of a scene rotated
8
This problem may have been amplified by the lack of target location markers, especially in the 90°
display orientations.
D.L. Rhodes, L.C. Robertson / Brain and Cognition 50 (2002) 95–115
113
out of alignment with the viewer and environment. Although it might seem counterintuitive that an asymmetry related to response-selection processes might rely on
representations of space independent of the viewer, there is considerable evidence
that the division into left/right of the stimulus representations underlying the Simon
Effect is either ‘‘attention-centered’’ or relies on the programming of attentional
movements through space (Nicoletti & Umilta, 1989, 1994 Stoffer & Yakin, 1994).
The evidence reported here showing that multiple attention-related processing
mechanisms are organized in scene-based frames does not mean that the hemispheres
are not asymmetrically involved in the computation or use of the spatial coordinate
systems that define these frames. In fact, the literature on hemispatial neglect reviewed in the Introduction suggests that they are (see also Behrmann & Hamison,
1999). Spatial locations in higher-order frames ultimately must be calculated on the
basis of retinal coordinates in combination with other coordinate systems, and the
left and right sides of these higher-order frames may remain preferentially represented in the right and left hemispheres, respectively (see Driver, 1999). However,
our findings demonstrate that it would be a mistake to draw conclusions regarding
the roles of the hemispheres on the basis of left/right visual field asymmetries alone,
as the asymmetries we obtained do not appear to be due to differential direct access
of visual information to the two hemispheres. Electrophysiological and other functional imaging methods will be beneficial in assessing both the temporal and spatial
aspects of differential hemispheric function in the future.
In addition, our findings require that studies of environment-based spatial representations take into account not only gravity-bound reference frames and reference
frames provided by individual objects, but also the influence of contextual reference
frames that encompass objects and are themselves more global objects, or scenes. The
present findings show that even when there are multiple spatial influences on response
times to task-relevant objects, the mappings underlying these influences can operate
together in scene-based frames, dissociated from both the viewer and the gravitybound environment. Much work remains to be done in order to determine the conditions in which scenes are utilized and how the brain contends with multiple spatial
reference frames, selecting the one(s) most relevant for the task at hand. Nevertheless,
there is increasing evidence that the perceptual organization of a display and the
spatial frames that define different levels of organization within it are critical for
object perception, response mapping and the control of attention (Humphreys, 1983;
Lamberts, Tavernier, & d’Ydewalle, 1992; Palmer, 1989; Quinlan, 1995; Robertson &
Kim, 1998; Rock, 1990; Schendel & Robertson, submitted), and that each of these can
influence the visual field asymmetries observed in normal performance.
Acknowledgments
The research was supported by a Veterans Administration Medical Research
Scientist Award and National Science Foundation Award SBR-9222118 to Lynn C.
Robertson, supplemented by a Research Opportunity Award to support Dell
Rhodes. We thank John Lackey for the development of computer software, and
Dana Myers for help in running Experiment 3. Harold Pashler, Ray Klein, Bill
Yund, Krista Schendel, and three anonymous reviewers provided helpful comments.
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