Neural Sources of Focused Attention in Visual Search

Abstract

Previous studies of visual search in humans using event-related potentials (ERPs) have revealed an ERP component called ‘N2pc’ (180–280 ms) that reflects the focusing of attention onto potential target items in the search array. The present study was designed to localize the neuroanatomical sources of this component by means of magnetoencephalographic (MEG) recordings, which provide greater spatial precision than ERP recordings. MEG recordings were obtained with an array of 148 magnetometers from six normal adult subjects, one of whom was tested in multiple sessions so that both single-subject and group analyses could be performed. Source localization procedures revealed that the N2pc is composed of two distinct neural responses, an early parietal source (180–200 ms) and a later occipito-temporal source (220–240 ms). These findings are consistent with the proposal that parietal areas are used to initiate a shift of attention within a visual search array and that the focusing of attention is implemented by extrastriate areas of the occipital and inferior temporal cortex.

Full text

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Previous studies of visual search in humans using event-related
potentials (ERPs) have revealed an ERP component called ‘N2pc’
(180–280 ms) that reflects the focusing of attention onto potential
target items in the search array. The present study was designed to
localize the neuroanatomical sources of this component by means of
magnetoencephalographic (MEG) recordings, which provide greater
spatial precision than ERP recordings. MEG recordings were obtained
with an array of 148 magnetometers from six normal adult subjects,
one of whom was tested in multiple sessions so that both single-
subject and group analyses could be performed. Source localization
procedures revealed that the N2pc is composed of two distinct
neural responses, an early parietal source (180–200 ms) and a later
occipito-temporal source (220–240 ms). These findings are consist-
ent with the proposal that parietal areas are used to initiate a shift
of attention within a visual search array and that the focusing of
attention is implemented by extrastriate areas of the occipital and
inferior temporal cortex.
Introduction
Searching a cluttered visual scene for objects of interest is a
fundamental task for the primate visual system. To study this
aspect of visual perception, many researchers have used visual
search tasks in which subjects search for a predefined target in
an array containing multiple items (Treisman and Gelade, 1980;
Luck and Hillyard, 1995; Braun and Julesz, 1998; Luck and
Ford, 1998). The present study examines the neural substrates
of visual search by localizing the source of an event-related
potential (ERP) component that reflects the focusing of attention
onto a visual search target (Luck et al., 1990, 1997b; Luck and
Hillyard, 1994a,b). This ERP component is called the ‘N2pc’
wave to indicate its latency range (180–280 ms) and its unique
posterior contralateral scalp distribution. Specifically, the N2pc
is observed as a negative-going voltage deflection at scalp sites
contralateral to the position of a target object within a bilateral
stimulus array. That is, the N2pc is more negative at left hemi-
sphere electrode sites for right visual field (RVF) targets than
for left visual field (LVF) targets, and it is more negative at right
hemisphere electrode sites for LVF targets than for RVF targets
(Luck and Hillyard, 1994a,b).
Several experiments have linked the N2pc wave with the
focusing of attention. First, the N2pc component is present
for targets and also for nontarget stimuli that require careful
scrutiny to be distinguished from the distracters, but it is absent
for nontargets that can be rejected on the basis of a salient feat-
ure (Luck and Hillyard, 1994b). Second, the N2pc is substantially
reduced in amplitude when attentional demands are reduced
by eliminating the distracters (Luck and Hillyard, 1994b) or by
reducing the number of nearby distracters (Luck et al., 1997b).
Third, the N2pc component is larger for conjunction targets than
for feature targets (Luck and Hillyard, 1995; Luck et al., 1997b),
which corresponds with the greater attentional requirements
of conjunction targets. Fourth, the N2pc switches rapidly from
hemisphere to hemisphere during difficult visual search tasks,
mirroring the shifts of attention predicted by serial models of
visual search performance (Woodman and Luck, 1999). Moreover,
the N2pc component appears to be related to attention-related
modulations of single-unit activity that have been observed in
monkeys in the infero-temporal cortex and area V4 (Chelazzi
et al., 1993, 1998; Chelazzi and Desimone, 1994; Luck et al.,
1997b). Specifically, both the N2pc and these single-unit atten-
tion effects are larger for difficult discrimination tasks than
for simple detection tasks, are larger when distracters are near to
the target and are larger for tasks that require target localization.
Thus, localization of the N2pc component will be very useful for
understanding the neural and cognitive mechanisms of attention
in visual search.
Previous ERP experiments have shown that the N2pc com-
ponent has a largely posterior scalp distribution, focused over
the lateral occipital cortex (Luck and Hillyard, 1994a). However,
ERP localization is greatly limited by the spatial blurring of the
voltage distribution produced by the high resistance of the skull.
Alternatively, recording the event-related magnetic response
of the brain [for a comprehensive outline of this method see
(Williamson and Kaufman, 1987; Hari and Lounasmaa, 1989;
Hämäläinen et al., 1993)] is much less compromised by limita-
tions due to skull and scalp conductivities (Hämäläinen et al.,
1993) and allows a fairly high localization accuracy (Yamamoto
et al., 1988). With the recent availability of whole-head record-
ing systems, magnetoencephalography (MEG) has provided
valuable knowledge about cortical regions involved in visual
processing (Aine et al., 1995; Portin et al., 1998; Ahlfors et al.,
1999; Halgren et al., 2000; Vanni and Uutela, 2000). The present
study aimed to provide a more detailed localization of the N2pc
component by recording the magnetic analog of the N2pc com-
ponent. We expected that the majority of N2pc activity would
occur along the ventral pathway, potentially including a number
of visual areas (e.g. the human homologues of area V4, TEO, TE).
This prediction is based on the N2pc scalp distribution observed
in previous electrical recordings (Luck and Hillyard, 1994a)
and the similarity between the N2pc and single-unit effects in
area V4 and in infero-temporal cortex (Luck et al., 1997b). In
addition, because of the important role of the parietal lobes in
controlling the direction of attention (Mesulam, 1981; Corbetta
et al., 1993; Rafal and Robertson, 1995), we anticipated that
parietal activity might precede the predicted ventral-stream
activity.
Neural Sources of Focused Attention in
Visual Search
Jens-Max Hopf, Steven J. Luck1,Massimo Girelli2,Tilman
Hagner, George R. Mangun3, Henning Scheich4and
Hans-Jochen Heinze
Department of Neurology II, Otto-von-Guericke-University,
D-39120 Magdeburg, Germany,1Department of Psychology,
University of Iowa, Iowa City, IA 52242-1407, USA,
2Department of Neurological and Visual Sciences, University of
Verona, I-37134 Verona, Italy,3Center for Cognitive
Neuroscience, Duke University, Durham, NC 27708-0999, USA
and4Institute for Neurobiology, Otto-von-Guericke-University,
D-39120 Magdeburg, Germany
Cerebral Cortex Dec 2000;10:1233–1241; 1047–3211/00/$4.00
© Oxford University Press 2000. All rights reserved.

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Materials and Methods
Subjects
Twelve subjects were initially screened in a conventional ERP experiment
(data not presented here). From this group, six subjects with good
fixation, low noise levels (<20% rejected trials due to eye movement
artifacts) and a substantial N2pc effect (>1.5 µV) were selected for the
main MEG experiment (three male and three female subjects, mean age
26.5 years, all right handed). Each subject had normal visual acuity and
was paid for participation. One of the subjects, labeled AA01, was run in
three additional sessions on separate days, resulting in almost 5000 trials
per condition for that subject. All experiments were undertaken with the
understanding and written consent of each subject.
Stimuli and Procedure
The stimuli used in this experiment are illustrated in Figure 1. Subjects
were instructed to attend either to a red bar or to a green bar, and to
discriminate whether the attended-color bar was horizontal or vertical
in each stimulus array. The stimuli were presented via microcomputer-
controlled back projection at a viewing distance of 120 cm. Each stimulus
array contained 24 randomly distributed bars, including 22 blue distracter
bars (11 on each side) plus one red bar and one green bar that served
as potential targets. The red and green bars were always presented
on opposite sides of the vertical meridian, either both above or both
below the horizontal meridian. Each bar was 0.14 × 1.0° of visual angle
and was either horizontally or vertically oriented, selected at random. The
minimum distance between bars was 1.4°. Each array was presented for
750 ms, with a variable-duration blank interval of 600–900 ms between
arrays. A fixation point was continuously present in the center of the
screen, and subjects were instructed to fixate this point and to minimize
blinking. Fixation was monitored by an infrared video camera with a
zoom lens. Subjects were informed about the quality of fixation between
runs. The obtained data were not used during subsequent data analysis.
At the beginning of each trial block, the to-be-attended color for that
block (red or green) was designated. For each stimulus array, the subject
was instructed to press a left-hand button if the bar of the attended color
was vertical or a right-hand button if the bar was horizontal. Speed and
accuracy were stressed equally. Subjects performed 12 trial blocks,
six with each of the two attended colors. Each block consisted of 200
stimulus presentations, yielding 1200 trials with an LVF target and 1200
trials with an RVF target for each subject. It should be noted that, because
both red and green items were present in each array and only the atten-
tional instructions varied, the same physical stimuli were used for LVF
and RVF targets, thus avoiding any physical stimulus confounds (Luck et
al., 1997b).
Recording and Analysis
The MEG and electroencephalogram (EEG) signals were recorded simul-
taneously using a BTi Magnes 2500 whole-head MEG system with 148
magnetometers (Biomagnetic Technologies, Inc., San Diego, CA) for the
MEG and an electrode cap (Electrocap International, Eaton, OH) in
conjunction with a 32-channel Synamps amplifier (NeuroScan, Inc.,
Herndon, VA) for the EEG. The EEG was recorded with reference to the
right mastoid. The MEG and EEG signals were filtered with a bandpass of
DC-50 Hz and digitized with a sampling rate of 254 Hz. The MEG was also
subjected to an online noise reduction process that removes a weighted
sum of environmentally induced magnetic noise (first-order spatial
gradients of the field) recorded by eight remote reference channels that
do not pick up brain activity (Robinson, 1989). Artifact rejection was
performed offline by removing epochs with peak-to-peak amplitudes ex-
ceeding a threshold of 3.0 × 10–12T for the MEG and 100 µV for the EEG.
Individual head shapes and the sensor frame coordinate system were
spatially co-registered by using a Polhemus 3Space Fastrak system to
digitize individual skull/scalp landmarks (nasion and left and right
preauricular points). The locations of these landmarks in relation to
sensor positions were derived on the basis of precise localization signals
provided by five spatially distributed coils attached to the head with a
fixed spatial relation to the landmarks. These landmarks, in turn, were
matched with the individual subjects’ anatomical magnetic resonance
(MR) scans. The 32 EEG electrode locations were also digitized to permit
co-registration with the coordinate system. To compute the grand aver-
age activity over all subjects, the coordinate system for each subject
was readjusted to the coordinate system of one subject whose anatomical
MR scan was used for the grand-average source analyses. This MR scan
was, in turn, spatially normalized into the standardized reference frame
of Talairach and Tournoux for the purpose of reporting MEG source
localization coordinates (Talairach and Tournoux, 1988).
Separate averages for both MEG and EEG were computed for targets
occurring in the LVF and the RVF, and the data were collapsed over
the two target colors (red and green). The N2pc was then isolated
by computing LVF-minus-RVF difference waves (i.e. difference waves
constructed by subtracting the RVF-target waveforms from the LVF-target
waveforms). These difference waves eliminated activity due to purely
sensory responses, because all arrays contained a red pop-out item in one
visual field and a green pop-out item in the other field, with red being
attended in some trial blocks and green being attended in others. Any
higher-level cognitive activity that was equal for ipsilateral and contra-
lateral targets was also eliminated in these difference waves, leaving only
lateralized cognitive responses (primarily the N2pc wave). Statistical
analyses of the N2pc were conducted using within-subjects analyses of
variance (ANOVAs) with the Greenhouse–Geisser epsilon adjustment for
nonsphericity.
MEG source analysis was performed on the LVF-minus-RVF difference
wavesusingthemultimodalneuroimaging
(NeuroScan, Inc., Sterling, VA). First, a 3-D reconstruction of the head and
cortical surface was created using the boundary element method
(Hämäläinen and Sarvas, 1989). Second, this model of the head and brain
was used in conjunction with the observed MEG fields to compute a
model of the distribution of current over the cortical surface (henceforth
called a distributed-source model) using the minimum norm least squares
method (Fuchs et al., 1999).
The distributed-source model had the following characteristics. (i)
The BEM surface grid served as a predefined source compartment.
(ii) Due to the large number of parameters, the minimum norm least
square model minimized the data misfit by taking into account an
additional model term that used a diagonal location weighting matrix to
remove the natural bias towards high gain source locations that would
overemphasize superficial source locations. The regularization parameter
necessary to link the model term to the data was determined by the
chi-square criterion relying on the assumption that the data misfit is in the
order of the amount of noise in the data (see Philips Electronics N.V., User
Guide Curry, version 3.0, 1998, pp. 151–154).
Finally, to provide discrete estimates of the current sources, equi-
valent current dipoles were also fitted to the MEG data. These procedures
were applied to the MR and MEG data from the single-session data from
each subject, to the multi-session data from subject AA01 and to the
grand-average data. This multipronged approach was used to gain the
advantages of both grand-average analyses (e.g. improved signal-to-noise
ratios) and single-subject analyses (e.g. direct linkage between anatomical
and electromagnetic data).
softwareCurry3.0
Figure 1. Example stimulus array. While fixating the central cross, subjects were
requiredtoattendtoredortogreenandtopressoneoftwobuttonstoindicatewhether
the bar of the attended color was horizontal or vertical.
1234 Neural Sources of Focused Attention in Visual Search • Hopf et al.

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Results
Electrical Waveforms and Distributions
The ERP waveforms and scalp distributions for the grand-average
data are shown in Figure 2A, with the multisession data from
one subject (AA01) shown in Figure 2B. These waveforms were
recorded at lateral occipital scalp sites TO1 (halfway between
O1 and T5) and TO2 (halfway between O2 and T6). The corres-
ponding difference waves (LVF-minus-RVF) are superimposed
as thick lines. In both the grand-average and single-subject data,
the N2pc wave can be seen as a more negative response for
the contralateral waveform relative to the ipsilateral waveform
between ∼180 and 280 ms post-stimulus. To test the statistical
significance of this effect in the group data, the N2pc was mea-
sured as the mean amplitude from 180 to 280 ms, relative to a
100 ms pre-stimulus baseline, and these data were entered into a
two-way within-subjects ANOVA with factors of electrode site
(O1, TO1, PO1, T5, P3 and their right hemisphere equivalents)
and target lateralization (ipsilateral or contralateral relative to
the electrode site). In this analysis, the more negative voltage
for contralateral targets relative to ipsilateral targets led to a
significant main effect of target lateralization [F(1,5) = 14.14, P <
0.02].
An analogous ANOVA was conducted for subject AA01.
Separate measurements were obtained from the subject for each
of the four sessions, and these data were entered into the ANOVA
with session as the random factor. This analysis also yielded a
significant main effect of target lateralization [F(1,3) = 12.24,
P < 0.05]. Thus, a significant electrical N2pc component was
present both in the group and in subject AA01.
The scalp distributions of the LVF-minus-RVF difference waves
are shown in Figure 2 for two different time intervals, 180–200
ms and 220–240 ms. These distributions show peak voltages at
lateral and superior occipital sites, with similar but subtly differ-
ent distributions in the two time ranges.
Magnetic Waveforms and Distributions
The magnetic field distributions of the LVF-minus-RVF difference
waves are shown for the group and for subject AA01 in Figure 3.
Two different time intervals, 180–200 ms and 220–240 ms, are
shown. These field distributions are quite complex, so we will
first discuss in detail the data for 220–240 ms in subject AA01
(Fig. 3C). Peaks of magnetic flux leaving the cortex (shown in
red lines) can be observed over both the left and right occipital
lobes, with a transition to magnetic flux entering the cortex
(shown in blue lines) near the occipito-temporal border. The
polarity-reversal regions, which are typically associated with
the location of the underlying current dipole, are marked with
large black circles. The polarity of the magnetic field for this
LVF-minus-RVF difference wave appears to be the same over
the left and right hemispheres, whereas opposite polarities were
observed over the left and right hemispheres in the ERP distri-
butions (see Fig. 2), but this superficial difference between the
potential distribution and the magnetic fields was expected.
Specifically, the magnetic field flowing out of the left occipital
lobe and into the left occipito-temporal region is consistent
with a downward-pointing current dipole (due to the right-hand
rule), whereas the magnetic field flowing out of the right
occipital lobe and into the right occipito-temporal region is
consistent with an upward-pointing current dipole. Thus, the
magnetic field distribution shown in Figure 3C indicates the
presence of opposite-polarity current sources in the left and
right hemispheres, consistent with the corresponding potential
distribution shown in Figure 2B.
The grand-average magnetic field distribution of the LVF-
minus-RVF difference wave for the 220–240 ms interval is shown
in Figure 3D. This field is qualitatively similar to the single-
subject field but is somewhat more complex, presumably due
to differences in the distributions across the subjects who were
Figure 2. ERP waveforms and voltage maps for difference waves constructed by
subtracting RVF-target trials from LVF-target trials. The grand-average data are at the
top (A) and the single-subject multisession data are at the bottom (B). The waveforms
were recorded from lateral occipital electrode sites (TO1, TO2). The broken line reflects
the response to a target that was in the RVF, the solid line reflects the response to a
target in the LVF. Thick lines reflect difference waves (LVF-minus-RVF target). Negative
is plotted upward.
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averaged together to create the grand average. In particular,
there is evidence of a midline parietal source in the grand-
average data that is weak or absent in the data from subject
AA01.
Magnetic field distributions (LVF-minus-RVF difference) from
subject AA01 and from the grand average are shown for the
180–200 ms interval in Figure 3A and B, respectively. The
polarity reversals — again indicated with black circles — were
more medial and dorsal than the distributions observed at
220–240 ms, and the response appeared to be much stronger
over the left hemisphere than over the right hemisphere. The
different field distributions in the 180–200 and 220–240 ms time
ranges suggest that the N2pc component is composed of at least
two separate sources, an early source that is relatively dorsal,
medial and left-lateralized, and a later source that is relatively
ventral, lateral and bilaterally symmetrical.
Grand-average MEG waveforms, as well as difference waves
(LVF-minus-RVF) from selected sites, are shown in Figure 4. As
discussed above, the opposite voltage polarities observed in the
left and right hemispheres are not accompanied by opposite
magnetic field polarities, and it is therefore not useful to create
the sort of ipsilateral and contralateral waveforms that were
used to display the ERPs in Figure 2. The waveforms in Figure 4
instead show the responses for LVF and RVF targets at pairs of
sensors located on opposite sides of magnetic field polarity
inversions. For example, the sites labeled A1 and A2 are located
on opposite sides of the magnetic field inversion shown in the
circled area in Figure 3B, and these sites show opposite-polarity
effects in the 180–200 ms time range. Similarly, the sites labeled
B1 and B2 are located on opposite sides of the magnetic field
inversion shown in the circled area in Figure 3D, and these sites
show opposite-polarity effects in the 220–240 ms time range.
These effects are the magnetic analog of the electrical N2pc
component.
Distributed Source Estimates
To estimate the internal distribution of electrical current
responsible for the observed magnetic field distributions, the
data were subjected to distributed source analyses of the MEG
LVF-minus-RVF difference waves. A separate analysis was con-
ducted for each subject using a 3-D reconstruction of his or her
cortical surface. An additional source analysis was performed for
the grand-average data using a 3-D reconstruction of the cortical
surface of a single subject; this leads to less anatomical precision,
but this is partially compensated for by the increased signal-
to-noise ratio and greater generality of the grand-average data.
The MEG field distributions shown in Figure 3 indicated that
the N2pc component is accompanied by different patterns of
activity at different latencies, and separate source analyses were
therefore conducted using two different latency ranges. For the
grand-average data, these ranges were 180–200 and 220–240 ms;
for the individual subjects, these ranges were widened to
Figure 3. Magnetic field distributions over two different time ranges from the
single-subject multisession data (A and C) and the grand-average data (B and D),
measured from LVF-minus-RVF target difference waves. The black circles indicate
transition points between locations where the magnetic flux leaves (red) and enters
(blue) the skull, which are usually associated with an underlying dipole.
Figure 4. MEG waveforms at selected sensor sites that show N2pc-like effects of the
position of thevisual search target. Thick lines reflect difference waves(LVF-minus-RVF
target).
1236 Neural Sources of Focused Attention in Visual Search • Hopf et al.

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180–220 ms and 220–280 ms to offset the smaller signal-to-noise
ratios of the single-subject data.
Figure 5A shows the estimated pattern of activity in a poster-
ior view of the cortex for each of the six subjects. To maintain
comparability across subjects, the activity for subject AA01
(subject 1) in this figure was derived from his first session, not
averaged across all four of his sessions. For all six subjects, the
estimated activity was relatively dorsal and medial in the early
interval and relatively ventral and lateral in the late interval. For
both intervals, the relative strength of the activity in the left
and right hemispheres varied across subjects, and there was no
obvious and consistent dominance of one hemisphere in either
interval. This is also evident in a within-subject comparison.
Figure 5D illustrates the time course of source activity for
the second session of subject AA01. While the pattern unfolding
over time consistently shows an early parietal source followed by
later lateral ventral sources, the early right parietal source was
more prominent in this session.
It is important to note that the single-subject data were
relatively noisy, and the differences in the estimated patterns of
activity may reflect noise in the recordings rather than variabil-
ity in the actual generator sites of the neural activity. However,
despite the noise, there was quite a bit of consistency across
subjects in the general pattern of an early parietal focus and a
later occipito-temporal focus.
Figure 5B shows the estimated patterns of activity in the multi-
session single-subject data and Figure 5C the grand-average data.
In the 180–200 ms interval, the estimated activity was primarily
present in the left parietal lobe, although some activity was also
present in the right parietal lobe (mostly below the plotting
threshold). In the 220–240 ms interval, the estimated activity
was focused in the region of the occipital–temporal border, with
approximately equal levels of activity in the two hemispheres.
These patterns were quite similar for the grand-average and
single-subject data, although the parietal effect was more dorsal
in the single-subject data. These estimates are also similar to the
estimates that were obtained for the single-session data from the
six individual subjects (see Fig. 5A), indicating that the estimates
from the grand-average data are not significantly distorted by
averaging over subjects and that the estimates from the multi-
session single-subject data are reasonably representative of the
larger sample.
Dipole Source Estimates
To provide discrete estimates of these areas of activity, dipoles
were fitted to the MEG distributions from 180 to 280 ms for the
group and also for subject AA01. Because there were three
obvious sources in the minimum norm estimates, one in the left
parietal lobe and one in each inferior temporal lobe, three di-
poles were used in this analysis. Each dipole was initially located
near the center of the region of activity as determined by the
minimum norm analyses, but the locations and orientations were
then allowed to vary to maximize the fit between the dipole
model and the observed magnetic field distributions.
The best-fit dipole solutions for the group data are illustrated
in Figure 6A. A single dipole was first fit from 180 to 200 ms.
This best-fit dipole accounted for 70% of the variance, and was
located at the magnetic polarity reversal circled in Figure 3B,
near the center of the activity in the minimum norm solution.
[Due to temporal overlap of the parietal source with the subse-
quent temporal sources, estimates using three dipoles were
computed for the early time window (180–200 ms) and a later
time window (200–220 ms). The explained variance (70%) was
not significantly altered between 180 and 200 ms, but was
increased to 81% between 200 and 220 ms, indicating an
initial contribution of the ventral sources in this later time
window.] According to reference coordinates in the Talairach
and Tournoux system (x, y, z: –5, –77, 55), this dipole is located
in the posterior part of the parietal lobe (Brodmann’s area 7).
Two dipoles, one in each hemisphere, were used to fit the
group data from 220 to 240 ms. The best-fit dipoles in this time
range explained >90% of the variance in the field distributions,
and they were located in the posterior half of the left and right
inferior temporal lobes, with a somewhat more posterior loca-
tion in the left hemisphere than in the right. The Talairach
and Tournoux coordinates of the dipole in the left hemisphere
indicated a region in the posterior part of the fusiform gyrus
(x, y, z: –33, –65, –7), whereas the coordinates of the dipole in
the right hemisphere dipole indicated a region in the posterior
middle temporal gyrus close to the border to the inferior tem-
poral gyrus (x, y, z: 53, –45, 1).
A similar pattern was obtained in the analysis of subject AA01,
as shown in Figure 6B. In this analysis, a dipole in the parietal
lobe — somewhat more superior and medial to that estimated
from the group data — explained 70% of the variance from 180 to
200 ms. In the 220–240 ms time range, one of the two dipoles
was located in the left inferior temporal lobe, quite close to the
dipole estimated from the group dipole. The second dipole, how-
ever, was substantially more dorsal than the corresponding
dipole from the group analysis. Together, these two dipoles
explained 92% of the variance from 220 to 240 ms.
It should be noted that the purpose of estimating these dipole
locations was simply to provide a discrete location in the
standard Talairach reference frame, and these estimates are not
intended as strong claims about the center of mass of the activity.
The distributed source estimates shown in Figure 5 provide a
more reasonable estimate of the actual pattern of activity, but are
not easily reduced to a simple set of coordinates.
Forward Solution
In contrast to the magnetic field distribution, there was no clear
indication of separate parietal and temporal sources for the early
and late N2pc intervals in the ERP scalp distribution. To gain
clarification about this discrepancy, a more direct estimation of
the relation between the magnetic source configuration and the
ERP scalp distribution is of interest. To this end the grand-
average potential distribution was simulated by forward compu-
tation using the set of dipole parameters (location, orientation)
as revealed by the magnetic source analysis. As illustrated by
traces in Figure 7, the time course of relative contribution
(dipole strength) of the early parietal and the later two temporal
dipoles was modeled by separate but overlapping sine waves
(2sin2x). The first map (A) shows the potential distribution that
would result from a predominant contribution of the parietal
dipole. This distribution is not visible in the original grand aver-
age (Fig. 2A). Map C shows the resulting potential distribution
when the two temporal dipoles alone contribute to the forward
solution. Here the forward solution resembles the original ERP
distribution, particularly in the 220–240 ms time range. Interest-
ingly, during the time range of overlap (map B) the potential
distribution resembles the distribution in map C but shows a
slight deformation over the left hemisphere that can also be seen
in the original average ERP distribution between 180 and 200
ms. Taken together, the forward solution gives some indication
that the early parietal source may not have been causing signif-
icant ERP effects due to the particular orientation, relatively low
Cerebral Cortex Dec 2000, V 10 N 12 1237