Foxe Et Al-Right Hemisphere Control of Visuospatial Attention

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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.

712 J.J. Foxe et al. / NeuroImage 19 (2003) 710726


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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



10/17

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.



11/17

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).



12/17

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).



13/17

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.,



14/17

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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