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Right hemisphere control of visuospatial attention:line-bisection judgments evaluated with high-density electrical
mapping and source analysis
John J. Foxe,a,b,c,* Mark E. McCourt,d and Daniel C. Javitta,e
a The Cognitive Neurophysiology Laboratory, Nathan S. Kline Institute for Psychiatric Research, Program in Cognitive Neuroscience and Schizophrenia,
140 Old Orangeburg Road, Orangeburg, NY 10962, USAb Department of Neuroscience, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USA
cDepartment of Psychiatry and Behavioural Science, Albert Einstein College of Medicine, 1300 Morris Park Avenue, Bronx, NY 10461, USAd Department of Psychology, North Dakota State University, Fargo, ND 58105, USA
e
Department of Psychiatry, New York University School of Medicine, 550 1st Avenue, New York, NY 10016, USA
Received 20 December 2001; revised 22 November 2002; accepted 18 December 2002
Abstract
The line-bisection task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients.
Here, we investigated the neural processes involved in performing this task by recording high-density event-related potentials from 128 scalp
electrodes in normal observers. We characterized a robust net negative potential from 170 400 ms poststimulus presentation that correlates
with line-bisection judgments. Topographic mapping shows three distinct phases to this negativity. The first phase (170190 ms) has a
scalp distribution exclusively over the right parieto-occipital and lateral occipital scalp, consistent with generators in the region of the right
temporo-parietal junction and right lateral occipital cortices. The second phase (190240 ms) sees the emergence of a second negative
focus over the right central parietal scalp, consistent with subsequent involvement of right superior parietal cortices. In the third phase
(240400 ms), the topography becomes dominated by this right central parietal negativity. Inverse source modeling confirmed that right
hemisphere lateral occipital, inferior parietal, and superior parietal regions were the likeliest generators of the bulk of the activity associated
with this effect. The line stimuli were also presented at three contrast levels (3, 25, and 100%) in order to manipulate both the latency of
stimulus processing and the relative contributions from magnocellular and parvocellular inputs. Through this manipulation, we show that
the line-bisection effect systematically tracks/follows the latency of the N1 component, which is considered a temporal marker for object
processing in the ventral visual stream. This pattern of effects suggests that this task invokes an allocentric (object-based) form of
visuospatial attention. Further, at 3% contrast, the line-bisection effect was equivalent to the effects seen at higher contrast levels, suggesting
that parvocellular inputs are not necessary for successful performance of this task.
2003 Elsevier Science (USA). All rights reserved.
Introduction
Line-bisection tasks are a commonly used metric for the
clinical assessment of visuospatial neglect syndrome, a condi-tion that predominantly results from vascular lesions to the
right inferior parietal or temporoparietal cortex (e.g., Vallar
and Perani, 1987; Cappa et al., 1991; Mesulam, 2000; Na et al.,
2000; Kerkhoff, 2001;seeKarnath, 2001for a comprehensive
treatment). Patients with visuospatial neglect syndrome gener-
ally bisect lines significantly to the right of veridical center
(Robertson and Halligan, 1999), due either to a decrement inthe ability to allocate attention to left visual space or to hyper-
attention to rightward space. Clinical findings from neglect
patients allied to the scores of neuroimaging studies showing a
predomination of right parietal activity during tasks requiring
visuospatial attention (e.g., Corbetta et al., 2000; Coull et al.,
2001) have led to the formulation that while both left and right
parietal areas are involved in attention to the right visual field,
right parietal areas alone may control attention to the left visual
Supplementary data associated with this article can be found at
doi: 10.1016/S1053-8119(03)00057-0.
* Corresponding author. Fax: 1-845-398-6545.
E-mail address:[email protected] (J.J. Foxe).
NeuroImage 19 (2003) 710 726 www.elsevier.com/locate/ynimg
1053-8119/03/$ see front matter 2003 Elsevier Science (USA). All rights reserved.
doi:10.1016/S1053-8119(03)00057-0
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hemifield (e.g., Heilman and van den Abell, 1980; Weintraub
and Mesulam, 1987). A measure of support for this conjecture
is derived from the finding that neurologically normal subjects
demonstrate a phenomenon known as pseudoneglect (Bow-
ers and Heilman, 1980), in which a significant and systematic
misbisection of lines (or space) occurs that is leftward of
veridical center. This leftward tendency in normal observershas been theorized to result from a profound parietal asymme-
try in attentional control, which gives rise to some degree of
hyper-attentiveness to the left visual field (in space-based at-
tention), or to the left-hand side of individual objects (in object-
based attention) (see Jewell and McCourt, 2000; McCourt,
2001; McCourt and Jewell, 1999; McCourt et al., 2000).
Functional imaging (fMRI) studies have confirmed a
central role for right parietal cortices in performance of both
line-bisection tasks (Weiss et al., 2000; Fink et al., 2000a,
2000b, 2001, 2002; Galati et al., 2000) and judgments of
object location relative to the midsagittal plane (Vallar et
al., 1999; Galati et al., 2000). There have, to date, been no
electrophysiological investigations of line bisection; conse-
quently the timing of right parietal involvement relative to
the timing of ongoing stimulus processing is as yet un-
known. In the current study, we performed high-density
electrical mapping (from 128 scalp electrodes) of the visual
event-related potential (ERP) while subjects engaged in
either a tachistoscopic forced-choice line-bisection task
(McCourt and Olafson, 1997) or a control task in which they
simply judged whether or not lines were transected. Our
main objectives were to both confirm the involvement of
and define the timecourse of right parietal activity in per-
formance of the line-bisection task. In particular, we wished
to assess the relationship of parietal attentional processes inthe dorsal visual processing stream to the processing of the
line stimuli by object recognition areas of the ventral visual
stream. An open question is whether line midpoint judg-
ments can only be made upon completion of object recog-
nition processes for the object that is to be bisected. In the
present study, we systematically varied the timing of object-
recognition processes by varying the contrast level at which
lines were displayed. We used the well-characterized N1
component of the visual evoked potential (VEP) as an index
of the timing of object recognition processes, since this
component has repeatedly been shown to be correlated with
the processes involved in object recognition (e.g., Allison etal., 1999; Bentin et al., 1999; Doniger et al., 2000, 2001).
Systematically varying the latency of this component al-
lowed us to assess whether bisection processes tracked this
latency manipulation.
Materials and methods
Subjects
Nine (4 male) neurologically normal, paid volunteers
(ages 1945 years, mean 29.2 years) participated. All
subjects provided written informed consent, and the Insti-
tutional Review Boards of the Nathan Kline Research In-
stitute and North Dakota State University approved all pro-
cedures. All subjects possessed normal or corrected-to-
normal vision and were right-handed, as measured using the
Oldfield (1971) laterality inventory (mean score 58.3, SD
22.7).
Instrumentation and stimuli
Subject responses were sensed and collected, and stimuli
were presented as 640 480 pixel images on a 21-in
flat-screen monitor; frame refresh rate was 60 Hz and mean
display luminance was 22 cd/m2. The generation and se-
quencing of stimuli and the collection of subject responses
were accomplished using the ERTS software package
(Beringer, 1995).Luminance and contrast calibrations were
made using a photometer.
Stimuli were line segments (29 cm wide by 0.5 cm high)presented for 150 ms. At the viewing distance of 108 cm,
lines subtended 15.3 (width) by 0.27 (height) of visual
angle. All lines were horizontally and vertically centered
within the display. Seventy-five percent of the lines were
transected at 1 of 25 different locations ranging from 0.6
of visual angle relative to veridical line midpoint. This range
of transector locations is sufficient to determine perceived
line midpoint in most observers (McCourt, 2001). The re-
maining 25% of lines were nontransected. Lines were pre-
sented at three levels of Michelson contrast: 3, 25, and
100%. This range of stimulus contrast was employed for
both methodological and theoretical reasons. Methodologi-cally, the presence of low contrast lines ensured that the
visual discriminations in the feature detection task
(transected vs nontransected) would not be trivially easy,
and that subjects would remain alert and vigilant throughout
the blocks of trials. Theoretically, it was of interest to
determine whether there would be any unique behavioral or
electrophysiological signature regarding the lowest contrast
stimulus that, at 3% contrast, is well below the operating
range of the parvocellular division of the geniculostriate
projection, and thus preferentially engages the magnocellu-
lar stream(Shapley and Perry, 1986).
Fig. 1illustrates examples of the line stimuli. Both mem-bers of the upper pair of lines (Figs. 1A, and B) possess
100% contrast, and are transected to the left of center, by
0.33 (2.17%) and 0.60 (3.91%), respectively.
Both members of the lower pair of lines (Figs. 1E and F)
possess 3% contrast and are transected to the right of center,
by 0.47 (3.04%) and 0.17 (1.09%), respectively. Lines
C and D possess 25% contrast; C is veridically transected
and line D lacks a transector. Pairs (Figs. 1A and B) and
(Figs. 1E and F) differ in contrast polarity. Lines of varying
contrast and contrast polarity appeared with equal fre-
quency. The order of appearance of all line types was
quasi-randomized within blocks of trials.
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Procedures and tasks
Subjects were seated in a comfortable chair with their
midsagittal plane aligned with the display monitor. Stimuli
were viewed binocularly through natural pupils.
Each block of trials consisted of 200 line presentations.
On alternate blocks subjects performed one of two tasks. In
the Control Task subjects performed an oddball (feature
discrimination) task in which they judged whether lines
were transected (75% of trials) or not (25% of trials). Thenonbisected lines thus served as target stimuli. Use of the
oddball task as the control condition ensured that subjects
were actively engaged in an attentionally demanding task in
both conditions and that equivalent button push responses
were made during both tasks. If a simple passive viewing
task were to be used, changes in general arousal level might
have accounted for any effects seen.
In the Line-Bisection task subjects made judgments re-
garding transector location (left vs right) relative to per-
ceived line midpoint by depressing the appropriate mouse
button left or right, respectively. Subjects were in-
structed to respond to nontransected lines by depressing
either mouse button.
Subjects used their dominant (right) hand to depress
mouse buttons. Subjects were instructed to delay their re-
sponses for at least 1 s following stimulus presentation to
obviate motor artifacts in the EEG signal. Intertrial intervals
were approximately 2 s, since subsequent trials were initi-
ated 750 ms following the previous response. Each subject
completed three or four blocks of both Line-Bisection and
Control Task trials. At each level of line contrast subjectsmade either six or eight leftright judgments at each
transector location, such that estimates of perceived line
midpoint in each line contrast condition were determined
based on 150200 bisection trials. Each block of trials was
completed in approximately 9 min.
Measurements and analyses
Behavioral measures
For behavioral analysis the dependent measure was the
proportion of trials on which subjects indicated that transec-
tors appeared to the left of perceived line midpoint. The
Fig. 1. Line stimuli used in the experiment. Both members of the upper pair of lines (A and B) possess 100% contrast, and are transected to the left of center,
by 0.33 (2.17%) and 0.60 (3.91%), respectively. Both members of the lower pair of lines (E and F) possess 3% contrast and are transected to the
right of center, by 0.47 (3.04%) and 0.17 (1.09%), respectively. Lines C and D possess 25% contrast; C is veridically transected and line D lacks a
transector. Pairs (A and B) and (E and F) differ in contrast polarity. Lines of varying contrast and contrast polarity appeared with equal frequency. The order
of appearance of all line types was quasirandomized within blocks of trials.
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method of constant stimuli was used to derive psychometric
functions, and nonlinear least-squares regression was used
to fit a cumulative Gaussian distribution to the psychometric
functions (see McCourt and Jewell, 1999). Based on these
fits, transector locations corresponding to a 50% left re-
sponse rate were determined. The transector location for
which left and right responses occur with equal fre-quency is called the point of subjective equality (pse) and
is an objective measure of perceived line midpoint. Infer-
ential statistical analyses were performed on the pse values.
Electrophysiological measures
Continuous EEG was acquired from 128 scalp electrodes
(impedances 5k), referenced to nose, band-pass-filtered
from 0.05 to 100 Hz, and digitized at 500 Hz. Data were
epoched off-line from 100 prestimulus to 500 ms post-
stimulus and baseline-corrected from 100 to 0 ms. There-
after, trials with blinks and eye movements were rejected on
the basis of horizontal and vertical electrooculogram. An
artifact rejection criterion of 60V was used at all other
scalp sites to reject trials with excessive EMG or other noise
transients. The average number of accepted trials per con-
dition across subjects was 476 (SD 84.2). Responses to
the nontransected lines, which served as target stimuli in
the control task, were not included in the averages nor
analyzed further. This latter point is important, as during the
control task the N2/P3 component complex associated with
infrequent target stimuli was generated for these stimuli
(see, e.g., Ritter and Vaughan, 1969; He et al., 2001). Thus,
use of these responses as constituents of the control av-
eraged response would have been problematic. Also, false
alarms (which were very few) were also excluded from thisanalysis.
The latencies and scalp topographies of the standard ERP
componentry over posterior scalp were identified from
group-averaged waveforms collapsed across the Control
and Line-Bisection conditions. Repeated measures analyses
of variance (ANOVA) was used to test for significant dif-
ferences between conditions for four preselected time win-
dows, centered at the peak amplitude of the P1 (92100 ms),
the N1 (174182 ms), the P2 (230240 ms), and the fol-
lowing negative deflection over occipitotemporal regions,
N280ot (270280 ms). For these time windows, a measure
of integrated amplitude was derived between the responseand the 0-V baseline. ANOVAs possessed a repeated-
measures 2 2 4 design with the following factors:
condition (control, line-bisection), hemisphere (left, right),
and electrode (4 electrode pairs over lateral occipitoparietal
scalp).
Topographic mapping
Scalp topographic maps in the present study represent
interpolated potential distributions, derived from the 128
scalp measurements and based on the computation of a
common average reference. These interpolated potential
maps are displayed on the 3-D reconstruction of an average
rendered scalp surface (derived from anatomical MRIs),
using the boundary element method (BEM; e.g., Fuchs et
al., 1998) and as implemented in the CURRY multimodal
neuroimaging analysis software package (Version 4.0,
Philips Research, Hamburg, Germany).
Exact electrode locations were assessed for each subject
on the day of testing by 3-D digitization of the locations ofthe scalp electrodes with respect to fiduciary landmarks (i.e.,
the nasion and preauricular notches) using a magnetic digi-
tization device (Polhemus Fastrak and 3DspaceDX soft-
ware, Neuroscan, Inc.). Electrode placement was highly
consistent across subjects due to the use of a custom-de-
signed electrode cap that constrained interelectrode spacing
and placement. An averaged version of these electrode lo-
cations was projected onto the averaged rendered head for
computation of the group topographic data.
Last, one obvious constraint of the printed page is that
only a limited number of discrete maps can be shown to
represent a given topographic distribution, and such static
maps fail to depict the full spatiotemporal dimensionality of
the data. This can make it particularly difficult for the reader
to assess the extent of contribution to the maps of back-
ground noise. In determining the display gain to be used for
the maps in the current study, we followed the topography
over its entire timecourse (through observing animated time
series). Observation of these maps in the baseline period
(from 100 ms up to the onset of significant activity at
about 50 ms) allowed us to determine the relative contribu-
tion of noise to the topographic maps. The gain was then set
so that background noise during this baseline period ac-
counted for less than 12 topographic lines of potential in
the maps.This is more readily seen in the animation appendix to
this paper, which can be viewed or downloaded on Science-
Direct.
Dipole source analyses
Information about the intracranial generators contribut-
ing to effects seen in the data was also obtained through
dipole source analysis using electromagnetic source estima-
tion as implemented in CURRY (Version 4.0) software.
This method assumes that there are a limited and distinct
number of active brain regions over the evoked potential
epoch, each of which can be approximated by an equivalentdipole. Dipole generators are placed within a three-shell
spherical volume conductor model and overlaid on and
adjusted to a BEM-segmented structural MRI (in this case,
an averaged brain). The forward solution to this dipole
configuration is tested against the observed experimental
data. When not fixed, the positions and orientations of the
dipoles are iteratively adjusted to minimize the residual
variance between the forward solution and the observed
data. The upper bound of the number of modeled dipole
sources is determined using a test dipole (Scherg and Picton,
1991). If the number of modeled sources, m, is adequate,
then addition of another source (test dipole) and solving for
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m 1 sources would not be expected to further reduce the
residual variance, above that attributable to noise. The
reader is referred to the following papers for comprehensive
treatments of source localization procedures (Simpson et al.,
1995a,b; Scherg and Berg, 1996; Michel et al., 2001).
Results
Behavioural results
Fig. 2 presents group-averaged psychometric functions
obtained in the Line-Bisection condition at each level of line
contrast. Open symbols plot the mean percentage left
responses ( 1 SEM) against transector location (in degrees
relative to veridical line midpoint). The data are well be-
haved, and the range of transector locations sampled is
observed to encompass perceived line midpoint in this sam-
ple of subjects. Solid lines depict cumulative Gaussian dis-tributions fitted to the data by nonlinear least-squares re-
gression. The cumulative Gaussian function is described by
the equation
fx,,, 50 50 erf((x / 20.5 )) ,
where xis transector location (in degrees relative to veridi-
cal line midpoint), is an overall gain parameter, is the
x-axis location corresponding to the mean of the underlying
Gaussian density function (i.e., pse, the transector location
at which leftright responses occur with equal frequency),
andis its standard deviation. The error function (erf) is an
approximation to the cumulative Gaussian distribution, forwhich there is no closed-form analytical expression. The
horizontal dashed line in each panel indicates the 50% left
response rate; the transector location for which the cumu-
lative Gaussian intersects the dashed line is one measure of
perceived line midpoint (pse).
The solid symbols (and vertical dotted lines) in each
panel plot mean pse (1 SEM) based on the analysis of
psychometric functions from individual observers. The
agreement with pse estimates from the group-averaged fits
is excellent, and reveals the systematic leftward error
(pseudoneglect) in perceived line midpoint that typifies the
performance of neurologically normal right-handed observ-ers (Jewell and McCourt, 2000; McCourt, 2001). A one-way
repeated-measures ANOVA conducted on the pse values in
each line contrast condition revealed no significant effect of
line contrast, F(2, 16) 0.74, P 0.49. While single-
sample t tests comparing mean pse values in each line
contrast condition against veridical fail to achieve signifi-
cance (100%,t(8) 1.63,P 0.14; 25%,t(8) 1.73,
P 0.12; 3%, t(8) 1.68, P 0.13), increasing statis-
tical power by collapsing across levels of line contrast
revealed, however, that grand mean pse is shifted signifi-
cantly leftward of veridical by approximately 0.2, or 1.4%,
t(26) 2.98, P 0.006.
Electrophysiological results
The analysis of behavioral results revealed that line con-
trast had no significant effect on bisection performance, and
we therefore collapsed across the three levels of line con-
trast in the initial statistical analysis of the electrophysio-
logical data. Inspection of group-averaged visual evoked
potentials for both the Line-Bisection and Control Task
conditions revealed the traditional series of ERP compo-
Fig. 2. Behavioral data. Group-averaged psychometric functions obtained
in the Line-Bisection condition at each level of line contrast. Open symbols
plot the mean percentage left responses (1 SEM) against transector
location (in degrees relative to veridical line midpoint). Solid lines depict
cumulative Gaussian distributions fitted to the data by nonlinear least-
squares regression. The horizontal dashed line in each panel indicates the
50% left response rate; the transector location for which the cumulative
Gaussian intersects the dashed line is one measure of perceived line
midpoint (pse). The solid symbols (and vertical dotted lines) in each panel
plot the mean pse (1 SEM) based on the analysis of psychometric
functions from individual observers. The agreement with pse estimates
from the group-averaged fits is excellent, and reveals the systematic left-ward error (pseudoneglect) in perceived line midpoint that typifies the
performance of neurologically normal right-handed observers.
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nents, including P1, N1, and P2 (Fig. 3.). These componentswere maximal over visual cortices.
The earliest robust component, the P1, showed no
amplitude or latency difference between conditions.
However, over right hemisphere occipitoparietal sites,
responses in the Line-Bisection condition showed a
markedly increased negativity relative to those in the
Control Task; this difference began during the peak of the
N1 component and then continued for some 300 ms. At
left hemisphere electrode sites, this negative shift was
seen to onset later during the N1/P2 transition. This
negativity can be seen in Fig. 3, where the right hemi-
sphere electrode site of maximal difference is plottedalong with the equivalently positioned left hemisphere
site.
As expected, the ANOVA for the P1 latency bin (92
100 ms) yielded no significant main effect of condition (P
0.62). The ANOVA for the N1 latency range yielded a
significant interaction of ConditionXHemisphere (F(1,8)
5.19, P 0.05). Follow-up planned comparisons
showed that this was due to a robust main effect of
condition over the right hemisphere electrode sites
(F(1,8) 8.8, P 0.02), whereas no main effect of
condition was seen over the equivalent left hemisphere
sites in this latency range (P 0.44). The ANOVA for
the P2 latency range revealed a significant main effect ofcondition (F(1,8) 31.33, P 0.001) but no interaction
of Condition X Hemisphere (P 0.68). Similarly, the
ANOVA for the N280ot latency range revealed only a
significant main effect of condition (F(1,8) 29.27, P
0.001).
As the ANOVA for N1 showed that the effect of Con-
dition onset earlier over the right hemisphere, we performed
a post hoc analysis to determine rough onset times for the
effect in both hemispheres. We used a series of paired
two-tailed t tests between the Control and Line-Bisection
conditions at the four representative pairs of electrode sites
used in the above analyses. Tests were conducted at laten-cies preceding the P2 peak to mark the earliest timepoint
that conformed to a 0.05 criterion. Onset latencies across the
four left and across the four right hemisphere electrode sites
were averaged to provide a best estimate of onset in a given
hemisphere. A point was only accepted as the earliest di-
vergence if at least 11 subsequent consecutive time points
(20 ms at 500 Hz digitization rate) met the 0.05 criterion
(see also Guthrie and Buchwald, 1991; Foxe and Simpson,
2002). The criterion was met at 172 ms for the right hemi-
sphere and 208 ms for the left, indicating that bisection-
related attentional processes onset in the right hemisphere
some 3040 ms earlier than they do in the left hemisphere.
Fig. 3. Sustained negative modulation during line-bisection task. The 128-channel electrode montage is shown here projected onto a three-dimensional scalp
reconstruction (from structural MRI data) of one of the subjects (B.H.). Group-averaged (n 9) ERP voltage waveforms are plotted for two electrode sites
(darkened in Montage display), showing the marked negative deflection when subjects performed the line-bisection task (solid trace) relative to the control
task (dashed trace). The bottom two panels show the P values (pointwise two-tailed t test comparisons between conditions), illustrating the timecourse of
significant differences at these electrode sites.
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(1) A dipole was fixed in the coordinates of RLOC and
allowed to fit the data in the initial 10-ms period of
the effect (172182 ms). This dipole accounted for
57.9% of the variance in the data for this epoch. Note
that allowing the dipole to freely fit the data in this
period resulted in a dipole that went more medially
and superiorly into the medial occipital cortex (7,
87, 24). This freely fit dipole resulted in no real
improvement in fit with essentially the identical per-
centage of the variance explained (58.1%). The di-
pole was therefore fixed in the RLOC and its orien-
tation was also fixed (see Table 1).
(2) A second dipole was fixed in the RIPP location. The
fitting period was extended for another 10 ms (172
192) and the dipole orientation was allowed to freely
fit the data. This resulted in an explained variance of63.1% over the 20-ms epoch. Again, we allowed this
dipole to freely fit as a test. This caused the dipole to
move to a more inferior and medial location in the
region of the right superior occipital gyrus (32, 84,
29) with an improvement in explained variance to
69.1%. This was a clear improvement in the propor-
tion of explained variance, suggesting that there was
significant activity in the region of the right superior
occipital/occipitotemporal region that needed to be
explained in this early period. We therefore fixed this
second dipole in the freely fit location in the right
superior occipital gyrus (RSOG).
(3) We added a third dipole, again fixing its location in
the RIPP, and allowed it to fit for orientation over the
172 to 192-ms epoch. Explained variance improved
to 73.8%. Allowing this dipole to freely fit for loca-
tion and orientation resulted in only a very slight
change in location (1 cm) and no improvement in
explained variance. We therefore fixed the dipole in
the RIPP cords defined above. We tested the stability
of the fit so far by allowing dipole 2 to freely fit this
epoch again. This resulted in no change in the di-
poles position or orientation, indicative of a rela-
tively stable fit.
(4) We added a fourth freely fitting dipole and extendedthe analysis window out to 202 ms (a 30-ms epoch).
This resulted in a second dipole in the region of the
superior occipital gyrus/middle temporal gyrus (49,
82, 22) that was more lateral than dipole 2 above.
Explained variance for the 30-ms epoch was 71.0%.
(5) The proximity of this dipole to dipole 2 suggested
that these two dipoles might be trying to explain the
same data. We tested this by turning off dipole 2 and
allowing 4 to freely fit over this 30-ms epoch. This
resulted in a dipole that moved slightly more medi-
ally between the two previous dipole locations (45,
85, 25). The explained variance was 71.5%.Clearly, only a single dipole in the RSOG was re-
quired. Therefore, we fixed the location and orienta-
tion of this latter dipole.
(6) We then added a dipole in the RSPP and again
extended the epoch by 10 ms to 212 ms. The reader
will note that during this latter 10-ms period, the
focus over superior parietal scalp has begun to
emerge strongly, as detailed in the topographic anal-
ysis above. Addition of this dipole resulted in an
explained variance of 76.8% over the 40-ms epoch
(172212). It is noteworthy that at the end of the
epoch, explained variance was 83.5% (212 ms). Al-
Fig. 4. Topographic mapping of the line-bisection effect. Group-averaged
(n 9) potential maps are shown for three timepoints during the three
phases of the line-bisection effect. These topographies are derived from the
difference waves, obtained by subtracting the Control condition from the
Line-Bisection condition. Surface negativity is depicted in blue/cyan and
positivity in red/yellow. The first phase of the effect (top maps 176 ms)
shows a highly lateralized negative focus over right temporoparietal scalp.
During the second phase (middle maps 220 ms), at least two distinct
negative foci are evident with the emergence of a strong central parietal
negative focus. During the third phase, the time period of the largest effect(bottom maps 310 ms), the topography is dominated by a right central
parietal negativity.
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lowing this dipole to freely fit resulted in only a very
slight change of location, indicating that its location
in RSPP was a relatively stable fit, so the dipole was
fixed in the coordinates defined above.
(7) We next added both a medial striate and left cere-
bellar dipole and extended the fitting window to 50
ms (172222 ms), allowing the orientations of thesetwo dipoles to fit the data. These six dipoles resulted
in an explained variance of 81.7% over the epoch
with a best fit at the last point (222 ms) of 93.0%. We
fixed these latter two dipoles orientations.
(8) We then opened up the epoch across the duration of
the entire effect (172400 ms) and allowed these six
dipoles to fit the data. This resulted in an average
explained variance of 92.0% over this 228-ms epoch
(see Fig. 6B). It is of note that during the period
when the effect was of largest amplitude, 220320
ms (i.e., highest signal-to-noise in our data), the
averaged explained variance was 97.5%.(9) Finally, it is clear from the topographic data that
there is activity over the left hemisphere that is
unlikely to be simply a result of volume conduction
from right hemisphere activations. Left hemisphere
activations are also seen in the fMRI data although
the exact loci across studies are not consistent (ex-
cept for the left cerebellum). We added a left hemi-
sphere inferior posterior parietal dipole (38, 42,
40) and fit it to a window from 200 to 260 ms. These
coordinates were taken from Fink et al. (2001). It
was during this epoch that the strongest activity over
left scalp was observed electrophysiologically. We
then fixed its orientation and opened the epoch up to
the entire window (172400 ms) and refit the data
with these seven dipoles. Explained variance across
the entire epoch was only marginally improved to
92.4%.
It is noteworthy that Fink and colleagues also find con-
sistent activation of right frontal regions including orbito-
frontal cortex and the right dorsolateral prefrontal cortex. In
our study, we also see foci of electrical activity over frontal
regions but these activations occur relatively late in time,
mostly after the posterior parietooccipital line-bisection ef-
fect is over. These frontal effects are not treated further inthe present data analysis.
It is important to emphasize that the fMRI activations
seen in previous studies, and which we have used to guide
this source analysis, cannot be thought of as discrete acti-
vations of individual brain regions. Rather, as can be seen in
Fig. 7, many of these activation clusters are much larger
than a single cortical area and are likely to represent the
activity of a cluster of functional areas. The same is cer-
tainly the case for the electrophysiological results reported
in the present paper, where a given scalp topography almost
certainly represents coordinated activity within a cluster of
functionally related areas. For instance the lateral occipital
cortex contains a cluster of subregions, and it would be
incorrect to think of the initial phase of the line-bisection
effect as representing activity in just a single one of these.
As such, fitting each cluster or time epoch with a single
equivalent current dipole clearly represents a highly over-
simplified model of activity within a given cluster of areas.
As such, it is important that the reader should consider thesedipoles to represent a center-of gravity rather than a
discrete neural locus or a single neural event.
Discussion
The current findings explicate the brain mechanisms un-
derlying performance of the visual line-bisection task, a
perceptual version of the perceptual-motor task that is fre-
quently employed in the clinic to disclose the presence and
severity of visuospatial neglect syndrome. We define an
electrophysiological correlate of line-bisection judgments,
which manifests as a right parieto-occipital negative poten-
tial that is significantly earlier in latency and larger in
magnitude than that over left scalp. This net negative po-
tential change is seen to persist over a latency window from
approximately 170 to 400 ms poststimulus, and consists of
three distinct phases that can be discerned through topo-
graphic mapping. Dominating the earliest phase of the re-
sponse (165190 ms) is a negative focus concentrated
over the right lateral parieto-occipital scalp. In this early
phase, the effect appears to be largely if not entirely unilat-
eral with no differences seen over the left hemisphere. In the
second phase (190240 ms), a distinct additional negative
focus develops over the more superior right central parietalscalp, during which the right parieto-occipital focus persists.
In the final phase, as the point of maximal amplitude effect
is approached (310 ms), the topographic pattern becomes
dominated by this second focus over the right central pari-
etal scalp. We refer to this net negative difference as the
line-bisection effect.
The intracranial generators of the line-bisection effect
Recent fMRI investigations using very similar experi-
mental paradigms to ours have detailed a network of brain
areas that are activated during line-bisection judgments(see, e.g., Fink et al., 2000a, 2000b, 2001). Fig. 7, which is
adapted from work by Fink and colleagues (Fink et al.,
2001), illustrates the regional distribution of the activated
regions during a line-bisection task, and clearly shows the
involvement of right inferior and superior parietal cortices,
in addition to earlier visual areas, frontal regions, and re-
gions in the cerebellum. Our topographic results (Fig. 3)
appear to be in good agreement with these findings and
provide the critical temporal activation pattern across these
regions. Although topographic mapping of the ERP alone
permits only relatively crude spatial localization in terms of
the intracranial generators responsible for a given scalp
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topography, observation of the correspondence between the
fMRI results of Fink et al. (2001) and the present data
allows for a degree of added confidence in such interpreta-
tions. Clearly, the scalp topography of the earliest phase of
the line-bisection effect suggests that regions in the right
lateral occipital cortex and the right temporoparietal occip-
ital junction (TPJ) are the likeliest generators, rather than
parietal areas. Indeed, previous fMRI results show an ex-tended region of robust activation in these more inferior
right occipital regions (e.g., Fink et al., 2001). In turn, the
second phase of the line-bisection effect, during which a
strong focus develops over the right central parietal scalp, is
likely generated in regions of the right superior parietal
cortex, where again, fMRI shows a very strong regional
activation.
To further assess the putative intracranial sources of the
effect, we conducted a source analysis to augment our
topographic mapping data. A stable fit that accounted for
more than 90% of the variance contained in the data was
found, which included generators in right lateral occipital
cortex, right superior occipital gyrus, right lateral inferior
posterior parietal cortex, and right lateral superior posterior
parietal cortex. Additional generators were modeled in me-
dial occipital cortex, the left cerebellum, and left lateral
inferior parietal cortex. The latter dipole in left inferior
parietal cortex contributed very little to the source solution.
To arrive at this solution, we took advantage of the previousfMRI studies of line bisection to guide our source investi-
gation (e.g., Fink et al., 2001). The model shows that di-
poles seeded in the major activation centers previously
defined provide a very stable fit of our data.
It is noteworthy that the earliest phase of the line-bisec-
tion effect we describe here has a somewhat more inferior
distribution than might be predicted for a generator in the
inferior parietal lobule and may be more consistent with a
primary generator in superior occipitotemporal or lateral
occipital regions of cortex. Source analysis suggested that
two dipoles were needed to account for his early phase, one
in right lateral occipital cortex and a second in right lateral
superior occipital gyrus, directly adjacent to the middle
temporal gyrus. Karnath (2001) has recently suggested that
the rostral portion of the superior temporal cortex, just
inferior to the temporoparietal junction, is the principal
region involved in spatial awareness and the primary site of
injury in cases of neglect (see also Karnath et al., 2001).
Karnath argues that this region represents the interface be-
tween the ventral (object: what) and dorsal (spatial: where)
visual processing streams. Our data appear to accord well
with this interpretation, particularly when the temporal se-
quence of activations is taken into account. First, we find
that there is extensive visual processing prior to the occur-
rence of any spatial attention effects, as indexed by the N1component of the VEP which has been repeatedly impli-
cated as a marker for ventral stream object processing (e.g.,
Allison et al., 1999; Doniger et al., 2000, 2001; Murray et
al., 2002). The onset of the initial phase of the line-bisection
effect is seen to follow the peak of the N1 and appears to be
generated in and around the right lateral occipital region and
the right superior occipital gyrus, which is relatively close to
the TPJ. As Karnath notes, the TPJ is ideally situated at the
junction between the two visual processing streams. Subse-
quently, the effect appears to be transmitted into inferior
parietal and then more superior parietal areas of the dorsal
stream, potentially for further spatial processing. A strongprediction based on the present results is that object recog-
nition processes, as indexed by the N1 component, should
be relatively intact in neglect patients but that the essential
link or relay between initial object-based processing (ac-
complished within the ventral stream) and subsequent
visuospatial processing (subserved by the dorsal stream) is
selectively impaired.
The observation that lesions to the superior temporal
cortices appear to be the primary culprits in spatial neglect
syndrome (Karnath, 2001) raises questions regarding the
functional role played by superior parietal regions during
performance of the line-bisection task. Clearly superior pa-
Fig. 5. Three levels of contrast. Group-averaged (n 9) voltage wave-
forms are shown for the three levels of display contrast used (sameparietooccipital scalp site as shown in Fig. 3). The line-bisection effect can
be seen at each level. As N1 latency increases with decreasing contrast, the
line-bisection effect moves systematically with this N1 latency shift.
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rietal areas are involved in the processing demanded for
line-bisection judgments as represented by the late phase of
the line-bisection effect over superior parietal scalp seen
here, and based on results from the many hemodynamic
imaging studies of line bisection (e.g., Fink et al., 2000a.,
2001). One possible explanation might be that the initial
phase of the line-bisection effect over TPJ represents a
necessarybutinsufficientcomponent of the task whereas the
subsequent processing in more superior parietal regions
represents more refined processes critical to the accuracy of
judgments but not necessaryfor the performance of the task
per se. Such a sequence might help to explain the sometimes
confusing and contradictory findings regarding the multiple
areas of right parietal and/or temporal cortex which appear
to result in neglect when lesioned (e.g., Vallar and Perani,
1986; Vallar, 2001).
Attempts to dissociate the relative contributions of inferior
and superior parietal regions to the processes of visuospatial
Fig. 6. Source analysis of the line-bisection effect. (A) The locations of the seven dipoles used to model the line-bisection effect are shown projected into
a 3-D rendering of an averaged human brain. Each dipole location is color-coded. (B) The amount of variance explained in the data by these seven dipoles
across the 172 to 400-ms epoch of the effect. (C) Waveforms depicting the source strength of each dipole are plotted for the 172 to 400-ms epoch (same color
convention as used in A).
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attention have been made (Corbetta et al., 2000). Using fMRI,
Corbetta and colleagues conducted a typical visuospatial cue-
ing experiment, whereby centrally presented arrows cued
subjects to the likeliest location for a subsequent target stim-
ulus, which was then presented to either the left or the right of
fixation. They found that the right TPJ was primarily activatedwhen visual attention was reoriented to the occurrence of a
target stimulus and that this activation was greater to targets
that occurred in the uncued region of space. Thus, they inter-
preted this finding as evidence that TPJ was involved in the
reorienting of spatial attention when attention was captured
commonly known as exogenous attention in the literature.
Importantly, the TPJ was not found to be active in response to
the cue stimulus that oriented subjects attention to the subse-
quent targets, arguing that TPJ was not involved in the volun-
tary orienting of attention (so-called endogenous attention). On
the other hand, they found that the intraparietal sulcus and
superior parietal regions were involved in the voluntary direct-ing and maintenance of attention, showing sustained activation
to the endogenous cues used to direct attention. This latter
finding accords well with recent electrophysiological studies of
endogenous attention mechanisms, which showed involvement
of right parietal regions in the anticipatory biasing of visuo-
spatial attention in somewhat similar cue-target designs (Foxe
et al., 1998; Worden et al., 2000; Fu et al., 2001).
However, on initial consideration, the present findings do
not appear to accord well with the findings of Corbetta and
colleagues. Here we find that the first and putatively most
critical modulation of the ERP, due to the spatial attention
required for the line-bisection task, appears to occur in the
region of the right lateral occipital cortex and the right TPJ.
Clearly, in this task, attention is not being captured by thestimulus. Rather, this task involves the voluntary/endoge-
nous orienting of spatial attention to an object (the line).
However, the apparent discrepancy between these results
may have to do with the large differences in experimental
paradigm. One clear difference between the current task and
the kinds of tasks that are typically used to investigate
visuospatial attention is that here spatial attention is being
directed to an object, whereas in the Corbetta study, spatial
attention was being directed to a portion of space. Thus,
regions of TPJ may become involved when spatial attention
is voluntarily oriented to an object. In support, Karnath
(2001) argues that the rostral superior temporal gyrus, byvirtue of inputs from both the dorsal and the ventral stream,
may be involved in both spatial orienting and the analysis of
objects in space. By this reasoning, both the findings of
Corbetta et al. (2000) and the present findings are consonant
with activity in the TPJ. Of course, given the spatial uncer-
tainty regarding the exact generator locus of the early line-
bisection effect from ERP topographic mapping alone, an
alternate explanation may be that subregions in and around
Table 1
Results from source analysis
Brain Region Talairach Coordinates Dipole Orientation
X Y Z nx ny nz
R. Lat Occip 34 89 3 0.09 0.75 0.65
R. Lat Sup Post Parietal 23 58 61 0.24 0.72 0.65
R. Lat Inf Post Parietal 41 40 50 0.18 0.83 0.52
R. Sup Occip Gyrus 44 84 25 0.13 0.87 0.48
Med. Striate/Extrastriate 0 80 14 0.20 0.89 0.40
L. Cerebellum 30 60 30 0.33 0.9 0.27
L. Lat Inf Post Parietal 38 42 40 0.57 0.82 0.05
Note. Talairach coordinates and orientations are given for each dipole.
Fig. 7. fMRI activation during the line-bisection task. Comparison of the fMRI activations seen here with the scalp topography of the line-bisection effect
shown in Fig. 4 shows strong correspondence between the electrophysiological and hemodynamic data sets. (adapted with permission from Fink et al.,
NeuroImage 14 (2001) S59.) Areas of significant activation are shown projected into standard stereotactic space in a sagittal, coronal, and axial view (from
left to right).
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the temporoparietal junction serve different roles and that
the region in TPJ that is active during exogenous attention
in the Corbetta et al. (2000) study is not the same region that
is activated in the present study.
Magnocellular and parvocellular involvement in spatial
attention
Arising in the retina, and projecting centrally into tem-
poral and parietal cortical regions, the primate visual system
is composed of two principal neural pathways, the parvo-
cellular (P) and magnocellular (M) processing streams
(Shapley and Perry, 1986). The M stream has a predominant
dorsal projection to areas V2, V3, V4, MT, MST, and 7a in
posterior parietal cortex. The P stream courses ventrally to
areas V2, VP, and V4, and to regions of inferotemporal
cortex (DeYoe and Van Essen, 1988; Webster and Unger-
leider, 1998). Cells in the M pathway possess high-contrast
sensitivity, and demonstrate early response saturation; cells
of the P pathway are far less sensitive, and respond in a
graded fashion even to high-contrast stimuli (Kaplan and
Shapley, 1982, 1986). Neuronal contrast response functions
are well-described by the MichaelisMenten equation,
where C50represents the stimulus contrast producing half-
maximal response (the semisaturation constant). Primate P
and M LGN neurons possess median values of C50of 0.50
and 0.11, respectively; for cells in primary visual cortex
(V1), which receive both M and P input, this value is 0.33,
and for area MT it is 0.07 (Sclar et al., 1990).
It is currently unknown to what extent visuospatial at-
tentional mechanisms depend differentially upon M and P
pathway input. Relevant is evidence that VEPs to luminancecontrast in the neglected hemifield of neglect patients are
delayed compared to the nonneglected hemifield. The delay
is exacerbated at lower luminance contrasts, and no latency
differences are observed for isoluminant chromatic stimuli
(Spinelli et al., 1994, 1996). These findings have been
interpreted to suggest that neglect may, at least in part,
result from a selective impairment of M stream input to
parietal cortical regions. Anatomical evidence also indicates
that the parietal cortex receives rich input from the M
stream, supported by recent functional neuroimaging evi-
dence in humans (Tootell et al., 1995a, 1995b) that neurons
in dorsal V3 are activated at low stimulus contrasts. AreaV3, in turn, receives M stream input from layer 4B of V1
and from the thick stripes of V2, and has reciprocal
connections with other regions innervated by the M stream,
such as parietal cortex and V5 (Shipp et al., 1994; Zeki and
Shipp, 1988; Webster and Ungerleider, 1998).
The present behavioral results, as well as those of a more
extensive study involving 59 observers (McCourt, unpublished
results), reveal that leftward error (pseudoneglect) on the line-
bisection task is remarkably stable as a function of stimulus
contrast over the range of 1.5100%, which readily encom-
passes the transition from M to P stream function. It is clear
from these data that parvocellular inputs do not appear to be a
necessary component for successful performance of the line-
bisection task, as at contrast levels below about 8% there is
little or no parvocellular input (Tootell et al., 1988). The cur-
rent electrophysiological data provide similar evidence against
a major role for parvocellular inputs as at the 3% contrast level,
a robust line-bisection effect is observed and this effect has
highly similar morphology to the effects seen for the 25 and100% contrast conditions. The line-bisection effect at 3% con-
trast persists despite large effects upon P1 amplitude and N1
latency due to the contrast manipulation. Rather, the primary
effect of the contrast manipulation upon the line-bisection
effect was simply to delay its onset, which will be discussed in
the next section.
Confluence of object-based and space-based attentional
processing
While the most prominent omnibus symptom of neglect
syndrome is simply left inattention, the existence of numer-
ous subtypes of neglect has become increasingly clear, dis-
sociated along the dimensions of near (peripersonal) vs far
(extrapersonal) space (Halligan and Marshall, 1991; Vuil-
leumier et al., 1998; Cowey et al., 1994; Tegner and
Levander, 1991b), perceptual vs motor origin (Bisiach et al.,
1990; Tegner and Levander, 1991a), and referenced to ego-
centric (self-centered) vs allocentric (object-based) spatial
coordinate systems (Behrmann, 1999; Driver and Pouget,
2000; Walker, 1995). Similar distinctions have been made
with respect to pseudoneglect, where the degree of leftward
error on line-bisection tasks is influenced by motor activity
(McCourt et al., 2001), viewing distance (McCourt and
Garlinghouse, 2000a), and object geometry (McCourt andJewell, 1999; McCourt and Garlinghouse, 2000b). Our re-
sults are particularly interesting with respect to the distinc-
tion between egocentric and allocentric attention. Note that
spatial judgements of line midpoint in a bisection task
necessarily involve computing the spatial location of a spe-
cific feature (the transector) with respect to features of an
object (the line itself), and thus explicitly invoke object-
based attention. Our electrophysiological results speak to
this issue.
Perhaps the best-known effect of selective visuospatial
attention upon the ERP is the oft-reported modulation of the
P1 component, which is observed when attention is directedto a specific portion of the visual field while other parts of
visual space are ignored (e.g., Van Hoorhis and Hillyard,
1977; Martinez et al., 1999, 2001). This spatial attention
effect occurs relatively early in visual processing (typically
in the 70- to 130-ms latency range) and is an example of the
case where attention is being apportioned in an egocentric
spatial coordinate system. Clearly, the present findings are
wholly different in that the latency of the line-bisection
effect is late (post N1 at 170 ms). Moreover, its latency
tracks with that of N1 (not P1), as a function of decreasing
line contrast. The timecourse of N1 has been shown to be a
marker for ventral stream processing (e.g., Allison et al.,
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1999; Doniger et al., 2001), and as such, the relationship of
N1 peak latency and the onset of the line-bisection effect
suggests that the effect is linked to the timing of object
processing in the visual system. Object-based attention must
have a substrate upon which to act, that is, an object, and the
synthesis of coherent objects from visual primitives such as
blobs, lines, and edges is associated with processing in theventral pathway (e.g., Gross et al., 1972; Sary et al., 1993;
Janssen et al., 2001). Attention that is referenced to partic-
ular objects must therefore receive input from the ventral
stream and this is what is meant by allocentric spatial
attention. Thus, in the present study, the line itself (espe-
cially its endpoints) must first be encoded; spatial attention
is only subsequently engaged upon the object to determine
the location of the transector in relation to the objects
midpoint. Compellingly, several recent studies have re-
vealed that, in the absence of an object upon which to
operate, the severity of both neglect (Vuilleumier and Lan-
dis, 1998) and pseudoneglect are reduced (Post et al., 2001).
Thus, it would appear that neglect might be more severe
during allocentric spatial attention. These findings are con-
sistent with the notion of the superior temporal gyrus as an
interface between object processing and spatial processing
streams, as posited by Karnath (2001).
While our contrast manipulation succeeded in its goal of
systematically lengthening peak N1 latency, especially at
the lowest contrast level used (3%), it is worth emphasizing
that the amplitude of N1 appears to behave quite differently
from that of P1 in response to this manipulation. The P1 gets
consistently smaller as the contrast is dropped, especially
between the 25 and 3% levels, whereas the N1 maintains its
amplitude across the three contrast levels. It is perhapssurprising that the N1 shows such preserved amplitude at
the 3% contrast level given that N1 has been often taken to
predominantly represent processing within ventral visual
stream structures (e.g., Doniger et al., 2000, 2001; Murray
et al., 2002). Clearly, these results show that while the
timing of N1 is affected, the amplitude of N1 is largely
unaffected when stimulation is biased toward magnocellular
input. The relative independence of P1 and N1 amplitudes
suggests that N1 generation does not depend causally on
inputs from the generators of the earlier P1 component and
that the processes represented by the N1 component can be
successfully initiated through magnocellular inputs. Furtherinvestigation is clearly necessary to delineate the relative
contributions of magnocellular and parvocellular inputs to
the componentry of the VEP (see also Foxe et al., 2001;
Butler et al., 2001).
Relation of ERP timecourse to TMS-derived estimates
While fMRI studies have provided strong evidence that
activation of right parietal regions accompanies tasks re-
quiring spatial attention (e.g., line-bisection tasks), and
whereas the present study using high-density topographical
ERP methods has verified the involvement of right parietal
and temporoparietal occipital regions, including the tempo-
ral dynamics of their activation, neither fMRI nor ERP
techniques actually establish a causal relationship between
neural activity and behavioral performance. In this regard a
useful adjunct technique is transcranial magnetic stimula-
tion (TMS), in which briefly generated magnetic fields tran-
siently disrupt neural activity in target cortical regions. Thedeficits in behavior produced by TMS can be causally re-
lated to the disruption of normal activity within the net-
works of stimulated brain regions. Strong evidence that
right parietal regions are essential for spatial attention
comes from the finding that rapid-rate TMS over these
regions attenuates the allocation of attention into left hemi-
space, producing visual extinction similar to that in neglect
patients (Pascual-Leone et al., 1994; Fierro et al., 2000).
More recently a study using single-pulse TMS over right
posterior parietal cortex (electrode site P6) reported signif-
icant rightward deviation of line midpoint judgments in a
line-bisection task (i.e., neglect-like behavior) when pulses
were delivered 150 ms poststimulus (Fierro et al., 2001).
This value agrees very well with the observed timecourse of
initial right parietal activation as revealed in the present
ERP study, and the optimal stimulation location over site P6
is highly similar to the observed center of the initial nega-
tive focus of the line-bisection effect.
One potentially informative direction for a future TMS
study would be to selectively interrupt the early (parietooc-
cipital) and later (superior parietal) phases of the line-bisec-
tion effect to attempt to dissociate and identify the separate
contributions of these two distinct regions to line-bisection
judgments. Mapping both the exact timing and the topog-
raphy of these information processing phases on a subject-by-subject basis could further enhance the specificity of this
method, as some degree of temporal and topographic vari-
ation was observed between subjects in the current sample.
Clinical utility of ERP measures of the line-bisection
effect
One problematic issue surrounding the use of standard
line-bisection tasks to define spatial neglect syndrome is
that motor responses are often required from the patient,
such as asking them to draw bisectors with a pencil through
lines on a page, or to point to the longer/shorter end of adisplayed line. Critically, there are often significant motor
deficits in patients with lesions that involve frontal cortex
and also visual cortices, and this can be a confounding
factor in results of the line-bisection task that involve such
overt responses (see, e.g., Darling et al., 2001). That is,
systematic bisection errors by a motor-compromised patient
could reflect an inability or reluctance to move the drawing
or pointing hand in the necessary direction for correct judg-
ments. The presence of a highly robust ERP effect over right
occipitoparietal areas may provide a simple means for as-
sessing integrity of function in patients without the neces-
sity for overt motor or even verbal responses.
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Summary
In conclusion, the current study reveals a robust negative
potential over the right lateral occipitoparietal and right
central parietal scalp that indexes the neural processing
involved in performance of the line-bisection task.
Through high-density topographic mapping, we detail thespatiotemporal dynamics of this processing on the scalp
surface. The observed topographies are consistent with re-
cent hemodynamic imaging studies that have suggested a
prominent role for regions in and around the right tem-
poroparietal junction and regions of the right superior pari-
etal cortices in this task. Source analysis confirmed the
involvement of these areas as well as early involvement of
right lateral occipital cortices. The present data provide the
critical temporal activation pattern across these regions and
underscore the importance of defining the temporal patterns
of attentional modulation, which will be critical to our
understanding of the mechanisms of attentional control in
the human brain (e.g., Schroeder et al., 2001).
Acknowledgments
We express our sincere appreciation to Deirdre Foxe,
Beth Higgins, and Dr. Micah Murray for their technical help
with this study. We are most grateful to Dr. Gereon Fink
and his colleagues for generously allowing us to reproduce
their fMRI data in the current report. Dr. Antigona Martinez
provided valuable comments on an earlier version of the
manuscript for which we are indebted to her. Our thanksalso go to two anonymous reviewers for their careful and
constructive comments. This work was supported in part by
grants from the National Institute of Mental Health
(MH63434 to J.J.F.; MH49334 to D.C.J.), the National Eye
Institute (EY12267 to M.E.M.), North Dakota EPSCoR
(M.E.M.), the Neuropsychiatric Research Institute, Fargo,
ND (M.E.M.), and the North Dakota State University De-
velopment Foundation (M.E.M.), and by generous support
from the Burroughs Wellcome Fund.
Appendix
Readers are encouraged to visit the online version of this
paper at ScienceDirect to download the animation file which
accompanies the present report.
This animation shows the evolution of the line-bisection
effect at a rate approximately 100 times slower than real
time. It begins with a display of the 128-channel montage as
the 3-D rendered head rotates before settling with a view of
the scalp from directly behind the head. The electrodes then
disappear before the topographic maps begin. The counter at
the bottom right corner of the panel shows time in ms and
the animation of topography begins at 0 ms, which is when
the stimulus appeared. The group-averaged data have been
projected onto a 3-D-rendered scalp surface. Recall that the
line-bisection effect is derived by subtracting the control
condition from the experimental condition and therefore all
activity associated with the common sensory processing of
these stimuli has been eliminated from the data. Therefore,
it can be seen that for the first 150 ms, there is no activitythat exceeds noise.
The following is a description of the evolution of the
effect, which may help to orient the reader as they observe
the animation.
At about 160 ms, a negative focus over right pari-
etooccipital scalp is seen to emerge and build rapidly
in amplitude. By 172 ms (the point at which the effect
reaches statistical significance) the negativity is
clearly discernable. This right parietooccipital nega-
tivity continues to build in amplitude up until 200
210 ms. At approximately 188 ms, the initial right parietooc-
cipital negativity appears to extend dorsally and cen-
trally over midparietal scalp and this extension of the
negativity quickly develops into a second distinct fo-
cus by approximately 206212 ms.
By 210 ms, a third negative limb begins to be apparent
over the left parietooccipital scalp, although this left
scalp negativity never becomes fully distinct from the
two right foci.
At 224 ms, the right parietooccipital focus begins to
decrease in amplitude and the topography becomes
dominated by the central parietal negativity. This con-tinues until approximately 260 ms when the right
parietooccipital focus begins to increase its amplitude
for a second time.
Between 260 and 340 ms, this pattern is essentially
maintained. However, the reader will notice that both
the right parietooccipital and central parietal foci ap-
pear to wax and wane somewhat independently during
this period. While it is impossible to determine at this
stage what these dynamics represent functionally, we
speculate that these modulations are likely to represent
successive rounds of activation within functionally
distinct modules of the spatial attention network, andperhaps communication between two distinct centers
of the network.
By 340 ms, the negativity becomes concentrated over
central parietal scalp and the bilateral parietooccipital
foci are diminishing in amplitude. By 400 ms, the
effectis essentially over.
Note that the reader may find it useful to use the manual
advance option on their media player to control the progres-
sion of the animation in order to get a better look at the
topographic transition points detailed above.
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References
Allison, T., Puce, A., Spencer, D, McCarthy, G., 1999. Electrophysiolog-
ical studies of human face perception. I. Potentials generated in occipi-
totemporal cortex by face and non-face stimuli. Cereb. Cortex 9,
415430.
Behrmann, M., 1999. Spatial reference frames and hemispatial neglect, in:
Gazzaniga, M.S. (Ed.), The New Cognitive Neurosciences, 2nd edition.MIT Press, Cambridge, MA, pp. 651666.
Bentin, S., Mouchetant-Rostaing, Y., Giard, M.H., Echallier, J.F., Pernier,
J., 1999. ERP manifestations of processing printed words at different
psycholinguistic levels: time course and scalp distribution. J. Cogn.
Neurosci. 11, 235260.
Beringer, J., 1995. Experimental Run Time System (Version 3.13).
Berisoft Corp., Frankfurt, Germany.
Bisiach, E., Geminiani, G., Berti, A., Rusconi, M.L., 1990. Perceptual and
premotor factors of unilateral neglect. Neurology 40, 12781281.
Bowers, D., Heilman, K.M., 1980. Pseudoneglect: effects of hemispace on
a tactile line bisection task. Neuropsychologia 18, 491498.
Butler, P.D., Schechter, I., Zemon, V., Schwartz, S.G., Greenstein, V.C.,
Gordon, J., Schroeder, C.E., Javitt, D.C., 2001. Dysfunction of early-
stage visual processing in schizophrenia. Am. J. Psychiatr. 158, 11261133.
Cappa, S.F., Guariglia, C., Messa, C., Pizzamiglio, L., Zoccolotti, P., 1991.
Computed tomography correlates of chronic unilateral neglect. Neuro-
psychology 5, 195204.
Corbetta, M., Kincade, J.M., Ollinger, J.M., McAvoy, M.P., Shulman,
G.L., 2000. Voluntary orienting is dissociated from target detection in
human posterior parietal cortex. Nature Neurosci. 3, 292297.
Coull, J.T., Nobre, A.C., Frith, C.D., 2001. The noradrenergic 2 agonist
clonidine modulates behavioural and neuranotomical correlates of hu-
man attentional orienting and alerting. Cereb. Cortex 11, 7384.
Cowey, A., Small, M., Ellis, S., 1994. Left visuo-spatial neglect can be
worse in far than in near space. Neuropsychologia 32, 10591066.
Darling, W.G., Rizzo, M., Butler, A.J., 2001. Disordered sensorimotor
transformations for reaching following posterior cortical lesions. Neu-
ropsychologia 39, 237254.DeYoe, E.A., Van Essen, D.C., 1988. Concurrent processing streams in
monkey visual cortex. Trends Neurosci. 11, 219226.
Doniger, G.M., Foxe, J.J., Murray, M.M., Higgins, B.A., Schroeder, C.E.,
Javitt, D.C., 2001. Visual perceptual learning in human object recog-
nition areas: a repetition priming study using high-density electrical
mapping. NeuroImage 13, 305313.
Doniger, G.M., Foxe, J.J., Murray, M.M., Higgins, B.A., Snodgrass, J.G.,
Schroeder, C.E., Javitt, D.C., 2000. Activation time-course of ventral
visual stream object-recognition areas: high density electrical mapping
of perceptual closure processes. J. Cogn. Neurosci. 12, 615621.
Driver, J., Pouget, A., 2000. Object centered visual neglect, or relative
egocentric neglect? J. Cogn. Neurosci. 12, 542545.
Fierro, B., Brighina, F., Oliveri, M., Piazza, A., La Bua, V., Buffa, D.,
Bisiach, E., 2000. Contralateral neglect induced by right posteriorparietal rTMS in healthy subjects. Neuroreport 11, 15191521.
Fierro, B., Brighina, F., Piazza, A., Oliveri, M., Bisiach, E., 2001. Timing
of right parietal and frontal cortex activity in visuo-spatial perception:
a TMS study in normal individuals. Neuroreport 12, 26052607.
Fink, G.R., Marshall, J.C., Shah, N.J., Weiss, P.H., Halligan, P.W., Grosse-
Ruyken, M., Ziemons, K., Zilles, K., Freund, H.J., 2000a. Line bisec-
tion judgments implicate right parietal cortex and cerebellum as as-
sessed by fMRI. Neurology 28, 13241331.
Fink, G.R., Marshall, J.C., Weiss, P.H., Shah, N.J., Toni, I., Halligan, P.W.,
Zilles, K., 2000b. Where depends on what: a differential functional
anatomy for position discrimination in one- versus two-dimensions.
Neuropsychologia 38, 17411748.
Fink, G.R., Marshall, J.C., Weiss, P.H., Toni, I., Zilles, K., 2002. Task
instructions influence the cognitive strategies involved in line bisection
judgments: evidence from modulated neural mechanisms revealed by
fMRI. Neuropsychologia 40, 119130.
Fink, G.R., Marshall, J.C., Weiss, P.H., Zilles, K., 2001. The neural basis
of vertical and horizontal line bisection judgments: an fMRI study of
normal volunteers. NeuroImage 14, S59 67.
Foxe, J.J., Doniger, G.M., Javitt, D.C., 2001. Early visual processing
deficits in schizophrenia: impaired P1 generation revealed by high-
density electrical mapping. Neuroreport 12, 38153820.
Foxe, J.J., Simpson, G.V., Ahlfors, S.P., 1998. Parieto-occipital 10 Hz
activity reflects anticipatory state of visual attention mechanisms. Neu-
roreport 9, 39293933.
Foxe, J.J., Simpson, G.V., 2002. Timecourse of activation flow from V1 to
frontal cortex in humans: a framework for defining early visual
processing. Exp. Brain Res. 142, 139150.
Fu, K.G., Foxe, J.J., Murray, M.M., Higgins, B.A., Javitt, D.C., Schroeder,
C.E., 2001. Attention-dependent suppression of distracter visual input
can be cross-modally cued as indexed by anticipatory parieto-occipital
alpha-band oscillations. Cogn. Brain Res. 12, 145512.
Fuchs, M., Drenckhahn, R., Wischmann, H.A., Wagner, M., 1998. An
improved boundary element method for realistic volume-conductor
modeling. IEEE Trans. Biomed. Eng. 45, 980 997.
Galati, G., Lobel, E., Vallar, G., Berthoz, A., Pizzamiglio, L., Le Bihan, D.,
2000. The neural basis of egocentric and allocentric coding of space inhumans: a functional magnetic resonance study. Exp. Brain Res. 133,
156164.
Gross, C.G., Rocha-Miranda, C.E., Bender, D.B., 1972. Visual properties
of neurons in inferotemporal cortex of the Macaque. J. Neurophysiol.
35, 96111.
Guthrie, D., Buchwald, J.S., 1991. Significance testing of difference po-
tentials. Psychophysiology 28, 240244.
Halligan, P.W., Marshall, J.C., 1991. Left neglect for near but not far space
in man. Nature 350, 498500.
He, B., Lian, J., Spencer, K.M., Dien, J., Donchin, E., 2001. A cortical
potential imaging analysis of the P300 and novelty P3 components.
Hum. Brain Mapping 12, 120130.
Heilman, K.M., van den Abell, T., 1980. Right hemisphere dominance for
attention: the mechanism underlying hemispheric asymmetries of inat-
tention (neglect). Neurology 30, 327330.Janssen, P., Vogels, R., Liu, Y., Orban, G.A., 2001. Macaque inferior
temporal neurons are selective for three-dimensional boundaries and
surfaces. J. Neurosci. 21, 94199429.
Jewell, G., McCourt, M.E., 2000. Pseudoneglect: a review and meta-
analysis of performance factors in line bisection tasks. Neuropsycho-
logia 38, 93110.
Kaplan, E., Shapley, R., 1982. X and Y cells in the lateral geniculate
nucleus of macaque monkeys. J. Physiol. 330, 125143.
Kaplan, E., Shapley, R.M., 1986. The primate retina contains two types of
ganglion cells, with high and low contrast sensitivity. Proc. Natl. Acad.
Sci. USA 83, 27552757.
Karnath, H.O., 2001. New insights into the functions of the superior
temporal cortex. Nature Rev. Neurosci. 2, 568576.
Karnath, H.O., Ferber, S., Himmelbach, M., 2001. Spatial awareness is a
function of the temporal not the posterior parietal lobe. Nature 411,950953.
Kerkhoff, G., 2001. Spatial hemineglect in humans. Prog. Neurobiol. 63,
127.
Martinez, A., Anllo-Vento, L., Sereno, M.I., Frank, L.R., Buxton, R.B.,
Dubowitz, D.J., Wong, E.C., Hinrichs, H., Heinze, H.J., Hillyard, S.A.,
1999. Involvement of striate and extrastriate visual cortical areas in
spatial attention. Nature Neurosci. 2, 364369.
Martinez, A., DiRusso, F., Anllo-Vento, L., Sereno, M.I., Buxton, R.B.,
Hillyard, S.A., 2001. Putting spatial attention on the map: timing and
localization of stimulus selection processes in striate and extrastriate
visual areas. Vision Res. 41, 14371457.
McCourt, M.E., 2001. Performance consistency of normal observers in
forced-choice tachistoscopic visual line bisection. Neuropsychologia
39, 10651076.
725J.J. Foxe et al. / NeuroImage 19 (2003) 710726
8/10/2019 Foxe Et Al-Right Hemisphere Control of Visuospatial Attention
17/17
McCourt, M.E., Freeman, P., Tahmahkera-Stevens, C., Chaussee, M.,
2001. The influence of unimanual response on pseudoneglect magni-
tude. Brain Cogn. 45, 5263.
McCourt, M.E., Garlinghouse, M., 2000a. Asymmetries of visuospatial
attention are modulated by viewing distance and visual field elevation:
pseudoneglect in peripersonal and extrapersonal space. Cortex 36,
715732.
McCourt, M.E., Garlinghouse, M., 2000b. Stimulus modulation of pseudo-neglect: effect of line geometry. Neuropsychologia 38, 520524.
McCourt, M.E., Garlinghouse, M., Slater, J., 2000. Centripetal versus
centrifugal bias in visual line bisection: focusing attention on two
hypotheses. Front. Biosci. 5, D5871.
McCourt, M.E., Jewell, G., 1999. Visuospatial attention in line bisection:
stimulus modulation of pseudoneglect. Neuropsychologia 37, 843
855.
McCourt, M.E., Olafson, C., 1997. Cognitive and perceptual influences on
visual line bisection: psychophysical and chronometric analyses of
pseudoneglect. Neuropsychologia 35, 369380.
Mesulam, M.M., 2000. Attentional networks, confusional states, and ne-
glect syndromes, in: Mesulam, M.M. (Ed.), Principles of Behavioral
and Cognitive Neurology, 2nd edition. Oxford University Press, Ox-
ford, pp. 174256.
Michel, C.M., Thut, G., Morand, S., Khateb, A., Pegna, A.J., Grave dePeralta, R., Gonzalez, S., Seeck, M., Landis, T., 2001. Electric source
imaging of human brain functions. Brain Res. Brain Res. Rev. 36,
108118.
Murray, M.M., Wylie, G.R., Higgins, B.A., Javitt, D.C., Schroeder, C.E.,
Foxe, J.J., 2002. The spatio-temporal dynamics of illusory contour
processing: combined high-density electrical mapping, source analysis,
and functional magnetic resonance imaging. J. Neurosci. 22, 5055
5073.
Na, D.L., Adair, J.C., Hye Choi, S., Won Seo, D., Kang, Y., Heilman,
K.M., 2000. Ipsilesional versus contralesional neglect depends on at-
tentional demands. Cortex 36, 455467.
Oldfield, R.C., 1971. The assessment and analysis of handedness: the
Edinburgh Inventory. Neuropsychologia 9, 97113.
Pascual-Leone, A., Gomez-Tortosa, E., Grafman, J., Alway, D., Nichelli,
P., Hallett, M.., 1994. Induction of visual extinction by rapid-rate
transcranial magnetic stimulation of parietal lobe. Neurology 44, 494
498.
Post, R.B., Caufield, K.J., Welch, R.B., 2001. Contributions of object- and
space-based mechanisms to line bisection errors. Neuropsychologia 39,
856864.
Ritter, W., Vaughan Jr., H.G., 1969. Averaged evoked responses in vigi-
lance and discrimination: a reassessment. Science 164, 326328.
Robertson I.H., Halligan P.W., 1999. Spatial Neglect: A Clinical Hand-
book for Diagnosis and Treatment. Psychology Press.
Sary, G., Vogels, R., Orban, G.A., 1993. Cue-invariant shape selectivity of
macaque inferior temporal neurons. Science 260, 995997.
Scherg, M., Picton, T.W., 1991. Separation and identification of event-
related potential components by brain electric source analysis. Electro-
encephalogr. Clin. Neurophysiol. Suppl. 42, 2437.Scherg, M., Berg, P., 1996. New concepts of brain source imaging and
localization. Electroencephalogr. Clin. Neurophysiol. Suppl. 46, 127
137.
Schroeder, C.E., Mehta, A.D., Foxe, J.J., 2001. Determinants and mecha-
nisms of attentional modulation of neural processing. Front. Biosci. 6,
D672684.
Sclar, G., Maunsell, J.H.R., Lennie, P., 1990. Coding of image contrast in
central visual pathways of the macaque monkey. Vision Res. 30, 110.
Shapley, R., Perry, V.H., 1986. Cat and monkey retinal ganglion cells and
their functional roles. Trends Neurosci. 9, 229235.
Shipp, S., de Jong, B.M., Zihl, J., Frackowiak, R.S.J., Zeki, S., 1994. The
brain activity related to residual motion vision in a patient with a
bilateral lesion of V5. Brain 117, 10231038.
Simpson, G.V., Foxe, J.J., Vaughan Jr., H.G., Mehta, A.D., Schroeder,
C.E., 1995b. Integration of electrophysiological source analyses, MRI
and animal models in the study of visual processing and attention.
Electroencephalogr. Clin. Neurophysiol. Suppl. 44, 7692.
Simpson, G.V., Pfleiger, M.E., Foxe, J.J., Ahlfors, S.P., Vaughan, Jr., H.G.,
Hrabe, J., Ilmoniemi, R.J., Lantos, G., 1995a. Dynamic neuroimaging
of brain function. J. Clin. Neurophysiol. 12, 118.
Spinelli, D., Angelina, P., De Luca, M., Burr, D.C., 1996. VEP in neglect
patients have longer latencies for luminance but not for chromatic
patterns. Neuroreport 7, 815819.
Spinelli, D., Burr, D.C., Morrone, M.C., 1994. Spatial neglect is associated
with increased latencies of visual evoked potentials. Vis. Neurosci. 11,
909918.
Talairach, J., Tournoux, P., 1988. Co-Planar Stereotaxic Atlas of the
Human Brain. Thieme, New York.
Tegner, R., Levander, M., 1991a. Through a looking glass. A new tech-
nique to demonstrate directional hypokinesia in unilateral neglect.
Brain 114, 19431951.
Tegner, R., Levander, M., 1991b. The influence of stimulus properties on
visual neglect. J. Neurol. Neurosurg. Psychiatry. 54, 882887.
Tootell, R.B., Hamilton, S.L., Switkes, E., 1988. Functional anatomy of
macaque striate cortex. IV. Contrast and magno-parvo streams. J. Neu-
rosci. 8, 1594 1609.Tootell, R.B., Reppas, J.B., Dale, A.M., Look, R.B., Sereno, M.I., Malach,
R., Brady, T.J., Rosen, B.R., 1995a. Visual motion aftereffect in human
cortical area MT revealed by functional magnetic resonance imaging.
Nature 375, 139141.
Tootell, R.B.H., Reppas, J.B., Kwong, K.K., Malach, R., Born, R.T.,
Brady, T.J., Rosen, B.R., Belliveau, J.W., 1995b. Functional analysis of
human MT and related visual cortical areas using functional magnetic
resonance imaging. J. Neurosci. 15, 32153230.
Vallar, G., 2001. Extrapersonal visual unilateral spatial neglect and its
neuroanatomy. NeuroImage 14, S5258.
Vallar, G., Lobel, E., Galati, G., Berthoz, A., Pizzamiglio, L., Le Bihan, D.,
1999. A fronto-parietal system for computing the egocentric spatial
frame of reference in humans. Exp. Brain Res. 124, 281286.
Vallar, G., Perani, D., 1986. The anatomy of unilateral neglect after
right-hemisphere stroke lesions. A clinical/CT-scan correlation study inman. Neuropsychologia 24, 609 622.
Vallar G., Perani D., 1987. The anatomy of spatial neglect in humans, in:
Jennerod M. (Ed.), Neurophysiological and Neuropsychological As-
pects of Spatial Neglect, North Holland, Amsterdam, pp. 235258.
Van Hoorhis, S.T., Hillyard, S.A., 1977. Visual evoked potentials and
selective attention to points in space. Percept. Psychophys. 22, 5462.
Vuilleumier, P., Landis, T., 1998. Illusory contours and spatial neglect.
Neuroreport 9, 24812484.
Vuilleumier, P., Valenza, N., Mayer, E., Reverdin, A., Landis, T., 1998.
Near and far visual space in unilateral neglect. Ann. Neurol. 43,
406410.
Walker, R., 1995. Spatial and object-based neglect. Neurocase 1, 371383.
Webster, M.J., Ungerleider, L.G., 1998. Neuroanatomy of visual attention,
in: Parasuraman, R. (Ed.), The Attentive Brain. MIT Press, Cambridge,
MA, pp. 1934.Weintraub, S., Mesulam, M.M., 1987. Right cerebral dominance in spatial
attention. Further evidence based on ipsilateral neglect. Arch. Neurol.
44, 621625.
Weiss, P.H., Marshall, J.C., Wunderlich, G., Tellmann, L., Halligan, P.W.,
Freund, H.J., Zilles, K., Fink, G.R., 2000. Neural consequences of
acting in near versus far sp
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