The spatiotemporal dynamics of illusory contour processing: combined high-density electrical mapping, source analysis, and functional magnetic resonance imaging.

Abstract

Because environmental information is often suboptimal, visual perception must frequently rely on the brain's reconstruction of contours absent from retinal images. Illusory contour (IC) stimuli have been used to investigate these "filling-in" processes. Intracranial recordings and neuroimaging studies show IC sensitivity in lower-tier area V2, and to a lesser extent V1. Some interpret these data as evidence for feedforward processing of IC stimuli, beginning at lower-tier visual areas. On the basis of lesion, visual evoked potentials (VEP), and neuroimaging evidence, others contend that IC sensitivity is a later, higher-order process. Whether IC sensitivity seen in lower-tier areas indexes feedforward or feedback processing remains unresolved. In a series of experiments, we addressed the spatiotemporal dynamics of IC processing. Centrally presented IC stimuli resulted in early VEP modulation (88-100 msec) over lateral-occipital (LOC) scalp--the IC effect. The IC effect followed visual response onset by 40 msec. Scalp current density topographic mapping, source analysis, and functional magnetic resonance imaging results all localized the IC effect to bilateral LOC areas. We propose that IC sensitivity described in V2 and V1 may reflect predominantly feedback modulation from higher-tier LOC areas, where IC sensitivity first occurs. Two additional observations further support this proposal. The latency of the IC effect shifted dramatically later (approximately 120 msec) when stimuli were laterally presented, indicating that retinotopic position alters IC processing. Immediately preceding the IC effect, the VEP modulated with inducer eccentricity--the configuration effect. We interpret this to represent contributions from global stimulus parameters to scene analysis. In contrast to the IC effect, the topography of the configuration effect was restricted to central parieto-occipital scalp.

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The Spatiotemporal Dynamics of Illusory Contour Processing:
Combined High-Density Electrical Mapping, Source Analysis, and
Functional Magnetic Resonance Imaging
Micah M. Murray,1,2Glenn R. Wylie,1Beth A. Higgins,1Daniel C. Javitt,1,4Charles E. Schroeder,1,2and
John J. Foxe1,2,3
1The Cognitive Neurophysiology Laboratory, Program in Cognitive Neuroscience and Schizophrenia, Nathan S. Kline
Institute for Psychiatric Research, Orangeburg, New York 10962, Departments of2Neuroscience and3Psychiatry and
Behavioral Science, Albert Einstein College of Medicine, Bronx, New York 10461, and4Department of Psychiatry, New
York University School of Medicine, New York, New York 10016
Because environmental information is often suboptimal, visual
perception must frequently rely on the brain’s reconstruction of
contours absent from retinal images. Illusory contour (IC) stimuli
have been used to investigate these “filling-in” processes. In-
tracranial recordings and neuroimaging studies show IC sensi-
tivity in lower-tier area V2, and to a lesser extent V1. Some
interpret these data as evidence for feedforward processing of
IC stimuli, beginning at lower-tier visual areas. On the basis of
lesion, visual evoked potentials (VEP), and neuroimaging evi-
dence, others contend that IC sensitivity is a later, higher-order
process. Whether IC sensitivity seen in lower-tier areas indexes
feedforward or feedback processing remains unresolved. In a
series of experiments, we addressed the spatiotemporal dy-
namics of IC processing. Centrally presented IC stimuli resulted
in early VEP modulation (88–100 msec) over lateral–occipital
(LOC) scalp—the IC effect. The IC effect followed visual re-
sponse onset by 40 msec. Scalp current density topographic
mapping, source analysis, and functional magnetic resonance
imaging results all localized the IC effect to bilateral LOC areas.
We propose that IC sensitivity described in V2 and V1 may
reflect predominantly feedback modulation from higher-tier
LOC areas, where IC sensitivity first occurs. Two additional
observations further support this proposal. The latency of the
IC effect shifted dramatically later (?120 msec) when stimuli
were laterally presented, indicating that retinotopic position
alters IC processing. Immediately preceding the IC effect, the
VEP modulated with inducer eccentricity—the configuration
effect. We interpret this to represent contributions from global
stimulus parameters to scene analysis. In contrast to the IC
effect, the topography of the configuration effect was restricted
to central parieto–occipital scalp.
Key words: event-related potentials; ERP; VEP; dipoles; fMRI;
Kanizsa; lateral–occipital cortex; LOC
Object recognition occurs despite ambiguous or incomplete in-
formation in the retinal image, as in situations of partial occlusion
and poor lighting. The brain can therefore reconstruct contours
absent from visual images (Petry and Meyer, 1987; Doniger et al.,
2000, 2001). One method of investigating these processes involves
illusory contour (IC) stimuli (Kanizsa, 1976), wherein subjects
perceive an object’s borders in the absence of luminance discon-
tinuities. At present, the locus and timing of IC processing are
unresolved.
Some report IC sensitivity at the lowest cortical processing
stages. Animal intracranial studies have observed effects in area
V2 (von der Heydt et al., 1984; Leventhal et al., 1998; Nieder and
Wagner, 1999; Bakin et al., 2000) and sometimes also in area V1
(Redies et al., 1986; Grosof et al., 1993; Sheth et al., 1996; Lee and
Nguyen, 2001; Ramsden et al., 2001). Human neuroimaging stud-
ies report similar effects (Hirsch et al., 1995; ffytche and Zeki,
1996; Larsson et al., 1999; Seghier et al., 2000). From these
findings, some concluded that IC sensitivity is a feedforward
bottom-up process (Grosof et al., 1993; ffytche and Zeki, 1996;
Sheth et al., 1996; Leventhal et al., 1998; Albert and Hoffman,
2000).
Others propose that IC sensitivity occurs at higher processing
stages. Behavioral investigations suggest critical roles for ma-
caque V4 (De Weerd et al., 1996; Merigan, 1996) and inferotem-
poral (IT) areas (Huxlin and Merigan, 1998; Huxlin et al., 2000).
In humans, visual evoked potentials (VEP) modulate relatively
late (?150 msec after stimulus) to IC stimuli (Brandeis and
Lehmann, 1989; Sugawara and Morotomi, 1991; Tallon-Baudry et
al., 1996, 1997; Hermann et al., 1999; Korshunova, 1999; Csibra et
al., 2000; Hermann and Bosch, 2001). Most VEP studies exam-
ined ? modulations (30–60 Hz) as an index of perceptual binding
(Tallon-Baudry et al., 1996, 1997; Csibra et al., 2000) or target
selection (Hermann et al., 1999; Hermann and Bosch, 2001), and
some observed no wide-band effects before 200 msec (Tallon-
Received Oct. 9, 2001; revised March 6, 2002; accepted March 7, 2002.
This work was supported by National Institutes of Health Grants MH63434
(J.J.F.) and MH49334 (D.C.J.) and the Burroughs Wellcome Fund. Data from this
study are from a thesis submitted in partial fulfillment of the requirements for the
degree of Doctor of Philosophy in the Sue Golding Graduate Division of Medical
Sciences, Albert Einstein College of Medicine, Yeshiva University. We thank Dr.
Glen Doniger for comments on previous versions of this manuscript and Deirdre
Foxe for technical assistance. Special thanks are extended to Dr. Kevin Knuth, Dr.
David Guilfoyle, and Raj Sangoi of the Center for Advanced Brain Imaging at The
Nathan S. Kline Institute for Psychiatric Research for technical expertise in fMRI
data acquisition. Sincere appreciation also goes to two anonymous reviewers for
their detailed constructive comments.
Correspondence should be addressed to Dr. John J. Foxe, The Cognitive Neuro-
physiology Laboratory, Program in Cognitive Neuroscience and Schizophrenia,
Nathan S. Kline Institute for Psychiatry Research, 140 Old Orangeburg Road,
Orangeburg, NY 10962. E-mail: foxe@nki.rfmh.org.
M. M. Murray’s present address: Functional Brain Mapping Laboratory, Depart-
ment of Neurology, University Hospital of Geneva, 24 rue Micheli-du-Crest, CH-
1211 Geneva, Switzerland.
Copyright © 2002 Society for Neuroscience 0270-6474/02/225055-19$15.00/0
The Journal of Neuroscience, June 15, 2002, 22(12):5055–5073

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Baudry et al., 1996, 1997). Nevertheless, the dynamics of IC
processing (relative to cortical response onset) as well as its locus
remain undetermined. Regarding the latter, one detailed func-
tional magnetic resonance imaging (fMRI) study found differen-
tial activation to IC shapes of varying types, sizes, and gap-widths
in higher-tier lateral–occipital (LOC) areas but not in lower-tier
areas including V1 and V2 (Mendola et al., 1999).
Although all human neuroimaging studies observed effects in
higher-tier cortical areas, studies in animals have neither investi-
gated IC sensitivity in such areas nor addressed the timing of IC
sensitivity relative to sensory response onset. We investigated
these issues in a series of experiments, in which we combined
high-density electrical mapping, source analysis, and fMRI to
elucidate the mechanisms underlying IC processing in humans.
The high temporal resolution of our electrophysiological tech-
nique permitted us to assess the relative timing of IC processes
while recording simultaneously over the entire scalp. In experi-
ment 1, stimuli were centrally presented while high-density event-
related potential (ERP) recordings were made, to determine the
timing and likely sources of VEP modulations specific to IC
presence. In experiment 2, fMRI data were obtained from a
second cohort of subjects, and their hemodynamic activations
were used to constrain VEP source analyses of the electrophysi-
ological data recorded from these same subjects. Experiment 3
examined whether the timing and locus of IC processing was
affected by the contrast polarity of the display. Experiment 4 (a
reanalysis of data from experiments 1 and 3) examined the effects
of variations in the low-level features of the stimuli on the earliest
VEP componency, allowing us to determine whether our ap-
proach was sensitive to modulations in lower-tier visual areas.
Experiment 5 tested the effect of varying the retinotopic position
of IC stimulus presentation on the timing and locus of IC pro-
cessing. Collectively, our data support a model of IC processing
wherein dorsal stream regions, which are initially insensitive to IC
presence, coarsely demarcate the spatial extent of a given stimu-
lus array and then input to ventral stream structures (e.g., the
LOC) where IC sensitivity first occurs. Our data support the
notion that IC effects observed previously in lower-tier areas are
likely to be driven by feedback inputs from higher-tier areas.
Portions of this study have been published previously in ab-
stract form (Murray et al., 2000).
MATERIALS AND METHODS
This study is composed of five separate experiments, the details of which
are described in turn.
Experiment 1: central presentations of illusory contours
Subjects. Twenty-eight (10 female) neurologically normal, paid volun-
teers, aged 20–57 (mean ? SD ? 33.4 ? 12.4) participated. All had
normal or corrected-to-normal vision, and all but two of the subjects
were right-handed (Oldfield, 1971). All subjects provided written in-
formed consent, and the Institutional Review Board of the Nathan Kline
Institute for Psychiatric Research approved all procedures.
Stimuli and task. Kanizsa-type (Kanizsa, 1976) illusory contour stimuli
were presented to subjects on a computer monitor located 114 cm away
while subjects fixated a central cross. These stimuli were constructed
from “pacmen” inducers (Fig. 1) that were oriented to either form or not
form an illusory shape (“IC present” and “IC absent,” respectively). Five
shapes were used: square, circle, triangle, pentagon, and five-pointed
star. Inducers were circular, subtended 3° of visual angle in diameter, and
appeared black on a gray background. To produce illusory shapes of the
same maximal width and height (6° in either plane), the eccentricity (?)
of inducers varied across shapes. In the case of the square, inducers were
centered at 4.25° eccentricity along 45° radii from central fixation. For the
circle, they were located at 3° eccentricity along the horizontal and
vertical meridians. For the triangle, the two lower field inducers were
centered at 4.25° eccentricity, and the upper field inducer was centered
at 3°. For both the pentagon and star, the two lower field inducers were
centered at 3.5° eccentricity, the two lateral upper field inducers were
centered at 3.1°, and the top inducer was centered at 3° along the vertical
meridian. Likewise, different numbers of inducers were used to define
these shapes (Table 1). The support ratio (?), defined as the percentage
of the perimeter of the illusory shape revealed by the inducers (Ringach
and Shapley, 1996), varied similarly, as did the surface area (?) of the IC
shapes (Table 1).
After the presentation of each stimulus a “Y/N” cue appeared prompt-
ing a forced-choice response. Subjects pressed one button for a “No”
response, indicating that they did not perceive an illusory shape, or a
second button for a “Yes” response, indicating that they perceived such
a shape. The entire experiment consisted of at least nine blocks (mean ?
SD ? 15.5 ? 6), each block containing 66 stimuli. IC present and IC
absent inducer configurations were presented randomly and were equally
probable. Subjects were encouraged to take breaks between blocks to
maintain high concentration and prevent fatigue. The timing of presen-
tations was such that each stimulus appeared for 500 msec, followed by a
blank screen for 1000 msec. Then the Y/N response prompt appeared
and remained on the screen until a response was made, allowing subjects
to control stimulus delivery. A blank screen (1000 msec duration) fol-
lowed responses. Use of the response prompt was motivated by the desire
to diminish the impact of motor responses on the sensory VEP.
EEG data acquisition. Continuous EEG was acquired through Neuro-
scan Synamps from 64 scalp electrodes (impedances ?5 k?), referenced
to nose, bandpass filtered from 0.05 to 100 Hz, and digitized at 500 Hz
(see Fig. 2, inset). Epochs of continuous EEG (?100 msec before stim-
ulus onset to 500 msec after stimulus onset) were averaged from each
subject separately for both the IC present and IC absent stimulus con-
figurations to compute the VEP. Baseline was defined as the epoch from
?100 msec to stimulus onset. Trials with blinks and eye movements were
rejected off-line on the basis of horizontal and vertical electro-
oculograms (resolution of ?0.5°) (Murray et al., 2001). An artifact
criterion of ?60 ?V was used at all other scalp sites to reject trials with
excessive EMG or other noise transients. The average (?SD) EEG
Figure 1.
ers were used in all experiments of the present study to define various
geometric shapes (see Materials and Methods and Table 1 for details).
Inducers were oriented to either form or not form an illusory shape. Right,
The timing of stimulus presentations was such that inducers were pre-
sented for 500 msec duration (400 msec in experiment 5), followed by a
blank screen for 1000 msec (450 msec in experiment 5). This was followed
in turn by a Y/N prompt that remained on the screen until subjects made
a forced-choice button-press response indicating whether an illusory
shape was presented. A blank screen (1000 msec duration in experiments
1, 3, and 4; 700 msec duration in experiment 5) then preceded the next
trial. Paradigmatic differences in experiment 2 are described in Materials
and Methods.
Stimuli and experimental paradigm. Left, Kanisza-type induc-
Table 1. IC stimulus characteristics (experiments 1–3)
Shape
Number of
inducers
Support
ratio (?)
Induced
surface
area (?)
Eccentricity
of farthest
inducer (?)
Triangle
Square
Circle
Pentagon
Star
3
4
4
5
5
37.5%
50%
?64%
?80%
?62.5%
18°2
36°2
28.3°2
24.4°2
12°2
4.25°
4.25°
3°
3.5°
3.5°
5056 J. Neurosci., June 15, 2002, 22(12):5055–5073 Murray et al. • Dynamics of Illusory Contour Processing

Page 3

epoch acceptance rate was 88% (?9). The average number of accepted
sweeps per condition was 462 (?196), with the lowest for any subject
being 248 and the highest being 975. Statistical and dipole source anal-
yses as well as topographic mapping for all experiments in this study were
performed on broad-band data, although filtered waveform data (40 Hz
low-pass; 24 dB/octave) are displayed in the Figures.
Experiment 2: illusory contour processing assessed with
combined EEG and fMRI
Subjects. Five subjects (three female; aged 24–33; mean ? SD ? 29.0 ?
3.9) participated. Two of these subjects had also participated in experi-
ment 1. All had normal or corrected-to-normal vision, and four of the five
were right-handed. All subjects provided written informed consent, and
the Institutional Review Board of the Nathan Kline Institute for Psychi-
atric Research approved all procedures.
Stimuli and task. Stimuli were physically identical to those of experi-
ment 1; however, the task requirements and the timing parameters
differed slightly in this experiment. Both the EEG and fMRI components
required only passive viewing of the stimuli. In the case of the EEG
portion, stimuli were presented for 300 msec duration [1–2 sec random-
ized interstimulus interval (ISI)], and both IC present and IC absent
configurations appeared with equal probability from one trial to the next
(randomized across trials). The entire EEG portion consisted of at least
10 stimulus blocks (mean ? SD ? 10.8 ? 1.3), each containing 100 trials.
Stimulation for the fMRI portion followed a “box-car” block design.
Subjects viewed the following sequence: a blank gray screen (20 sec
duration; hereafter, “rest”), a series of 20 IC shapes (300 msec duration;
700 msec ISI; randomized across the five shapes of experiment 1), rest for
20 sec, and a series of 20 IC absent configuration stimuli (300 msec
duration; 700 msec ISI). In each set of functional images, this sequence
was repeated three times. Likewise, the order of stimulus types was
counterbalanced across subjects.
EEG data acquisition. Continuous EEG was acquired and the VEP
(?100 msec before stimulus onset to 300 msec after stimulus onset) was
calculated in an identical manner to experiment 1, except that data were
collected from a 128-channel montage (?2.4 cm inter-electrode spacing).
The average EEG epoch acceptance rate was 84% (?15). The average
number of accepted sweeps per condition was 464 (?107), with the
lowest for any subject being 307 and the highest, 622. Electrophysiolog-
ical results of this experiment are displayed on the three-dimensional
(3-D) reconstruction of one subject’s (B.H.) anatomical MRI, using the
boundary element method (BEM) (Fuchs et al., 1998) as implemented in
CURRY (Philips Research, Hamburg, Germany). Topographic mapping
on this BEM reconstruction was based on the digitization of this same
subject’s fiduciary landmarks and electrode locations (Polhemus Fastrak
and 3DspaceDX software, Neuroscan, Inc.). In addition to preserving the
exact geometric relationships between electrodes, this technique facili-
tates the spatial co-registration of EEG and fMRI data by overlapping
these fiduciary landmarks.
fMRI data acquisition and analysis. A 3 tesla SMIS (Marconi) system
equipped with a head volume coil at the Center for Advanced Brain
Imaging at the Nathan Kline Institute was used to acquire T2*-weighted
echo-planar images (EPIs) (repetition time/echo time/flip angle ? 3
sec/40 msec/90°; voxel size ? 3.5 mm3; matrix size ? 64?64). This
sequence emphasizes the blood oxygenation level dependent (BOLD)
response, which is an indirect index of local neural activity (Kwong et al.,
1992; Ogawa et al., 1992; Logothetis et al., 2001). In each block of EPI
scans, 90 volumes were acquired, each of which consisted of 31 contig-
uous slices, which covered the entire brain. The first five volumes were
discarded to allow for stabilization of the BOLD signal. Visual stimula-
tion was delivered through MR-compatible liquid crystal display goggles
(Resonance Technology Inc., Northridge, CA). Head movement was
minimized with the use of a custom-made head holder. In all five
subjects, motion never exceeded 0.75 mm along any axis. All fMRI data
analyses were conducted with SPM99 software (Welcome Department of
Cognitive Neurology, London, UK). Images were realigned to the first
included volume, normalized into Talairach coordinate space, and
smoothed (7 mm3full width at half maximum Gaussian kernel). We
convolved the timing of the stimulation epochs (delayed box-car) with a
canonical hemodynamic response function, and cross-correlated the re-
sulting design matrix with the signal intensity changes observed in the
EPI images. A t test parametric map was generated using a height
threshold of t ? 4.70 ( p ? 0.05, corrected for multiple comparisons).
First, we rendered this activation map on a brain provided by SPM99
software. Then, we spatially co-registered this activation map with one
subject’s (B.H.) anatomical MRI data as well as with the group-averaged
VEP data using CURRY software.
Experiment 3: the IC effect and contrast polarity
Subjects. Seventeen subjects (five female; aged 20–42; mean ? 25.6 ?
6.5) from experiment 1 participated. All subjects, except one, were
right-handed (Oldfield, 1971).
Stimuli and task. All experimental parameters were identical to exper-
iment 1, except that the contrast polarity was inverted, such that inducers
appeared gray on a black background. The entire experiment consisted
of at least nine blocks (mean ? SD ? 13.9 ? 4.3), each block containing
66 stimuli.
Data acquisition. Continuous EEG was acquired and the VEP was
calculated in an identical manner to experiment 1. EEG epoch accep-
tance rate was 89.9% (?7.6). The average number of accepted sweeps
per condition was 416 (?132), with the lowest for any subject being 251
and the highest, 750.
Experiment 4: stimulus features affecting scene analysis
Subjects. Data from the 17 subjects (5 female; aged 20–42; mean ?
25.6 ? 6.5) who participated in both experiments 1 and 3 were included.
Stimuli and task. The stimuli were those of experiments 1 and 3. As
described above, five IC shapes were defined by varying numbers of
Kanizsa-type inducers. Additional stimulus parameters are listed in
Table 1.
Data acquisition. For each subject, the EEG epochs from experiments
1 and 3 were pooled and averaged according to shape as well as IC
presence versus absence to compute the VEP. This resulted in 10 VEPs
for each subject (five shapes ? two stimulus configurations). The average
number of accepted sweeps per condition was 147 ? 44, with the lowest
for any subject being 78 and the highest, 280.
Experiment 5: the IC effect and lateral presentations
Subjects. Twelve (seven female) neurologically normal, paid volunteers,
aged 19–47 (mean ? SD ? 26.3 ? 7.2) participated. Eleven of the 12
subjects were right-handed (Oldfield, 1971). Eight of these twelve sub-
jects had also participated in experiment 1.
Stimuli and task. Stimulation was similar to experiments 1 and 3, except
that the inducer array was presented entirely to the left or right of central
fixation. As before, inducer stimuli were oriented either to form or not
form an illusory shape and were presented to subjects on a computer
monitor located 114 cm away as they centrally fixated. Contrast polarity
was identical to experiment 1. All stimulus presentations were composed
of five inducers, centered at identical locations for all shapes. Three of
the five shapes from experiment 1 were used: the circle, pentagon, and
five-pointed star. For each illusory shape there were three versions in
which the inducers were not aligned, to make it less likely that subjects
completed the task by remembering the orientation of a particular
inducer. Inducers were circular and subtended 1.75° of visual angle in
diameter. When present, illusory shapes maximally subtended 4° in both
the horizontal and vertical planes. Stimuli were presented to either the
left or right of a central fixation cross that remained on the screen (2°
from the vertical meridian to the nearest edge of the stimulus). The
entire stimulus subtended 5.5° in both horizontal and vertical planes.
The experiment consisted of at least 20 blocks (mean ? SD ? 30 ?
9.7), each block containing 72 stimulus presentations. IC present and IC
absent inducer configurations were equally probable, as was hemifield of
presentation. The timing of stimulus presentations was such that each
stimulus appeared for 400 msec, followed by a blank screen for 450 msec.
After this, a Y/N cue appeared centrally, prompting a forced-choice
response identical to that in experiment 1. The Y/N cue remained on the
screen until a response was made, allowing subjects to control stimulus
delivery. A blank screen (700 msec duration) followed responses.
Data acquisition. Continuous EEG was acquired in an identical man-
ner to experiment 1. The average EEG epoch acceptance rate was 93.8%
(?4.2). Epochs of continuous EEG (?100 msec before stimulus onset to
500 msec after stimulus onset) were averaged from each subject sepa-
rately for both the IC present and IC absent stimulus configurations and
each hemifield of presentation to compute the VEP. This resulted in four
VEPs for each subject. Baseline was defined as the epoch from ?100
msec to stimulus onset. The average number of accepted sweeps per
condition was 336 (?75), with the lowest for any subject being 171 and
the highest, 529.
Murray et al. • Dynamics of Illusory Contour Processing J. Neurosci., June 15, 2002, 22(12):5055–5073 5057

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Scalp current density and dipole source analyses
Information about the intracranial generators contributing to IC sensi-
tivity was obtained using two methods. The first was topographic map-
ping based on scalp current density (SCD) analysis according to the
methods of Huiskamp (1991) and as implemented in CURRY. SCD
analysis takes advantage of the relationship between local current density
and field potential defined by Laplace’s equation, by calculating the
second spatial derivative of the field potential, which is directly propor-
tional to the current density. The SCD technique eliminates the contri-
bution of the reference electrode and mathematically eliminates effects
of volume conduction on the surface potential caused by tangential
current flow within the scalp. The major strength of the SCD analysis is
the possibility for improved visualization of approximate locations of
intracranial generators through the reduced spatial superposition of
gradients originating from multiple intracranial sources (Urbano et al.,
1996).
The second method was dipole source analysis using electromagnetic
source estimation as applied through CURRY software. This method
assumes that there are a limited and distinct number of active brain
regions over the VEP epoch, each of which can be approximated by an
equivalent dipole. Dipole generators are placed within a three-shell
spherical volume conductor model and overlaid on and adjusted to one of
our cohort’s (B.H.) BEM-segmented structural MRI. The forward solu-
tion to this dipole configuration is tested against the observed experi-
mental 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, 1990). If the number of modeled sources, m, is adequate, then
addition of another source (test dipole) and solving for m ? 1 sources
would not be expected to further reduce the residual variance, above that
attributable to noise. For both SCD and dipole analyses, group-averaged
VEP data were used to maintain the highest possible signal-to-noise ratio
as well as to generalize our results across individuals.
One further note about the SCD topographic mapping method is
warranted. An 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 both the stability of a given topography
over time and the extent of contribution of background noise to the
maps. In determining the display gain to be used for the maps in the
current study, we followed the topography over its entire time course for
each subject (through observing animated time-series). Observation of
these maps in the baseline period (from ?100 msec until the onset of
significant activity at ?50 msec) allowed us to determine the relative
contribution of noise to the SCD topographic maps. The gain was then
set so that background noise during this baseline period accounts for
only one to two topographic lines of current density in a given SCD map.
In all cases in which a topography is plotted for a single time point in this
manuscript, we observed that topography to be stable over time (see
Fig. 3C).
Statistical cluster plots
Because our primary interest was in determining the timing of IC
sensitivity, we calculated point-wise paired t tests between VEP re-
sponses. By this method, we can identify the onset of differential re-
sponses between both the IC present versus IC absent conditions (here-
after, the IC effect) as well as VEP response onset versus the 0 ?V
baseline. For each electrode, the first time point where the t test exceeded
the 0.05 ? criterion for at least 11 consecutive data points (?20 msec at
a 500 Hz digitization rate) was labeled as onset of either the IC effect or
the VEP response (Guthrie and Buchwald, 1991).
The results of the point-wise t tests from 55 of the 64 electrodes are
displayed as an intensity plot (see Fig. 11) to efficiently summarize and
facilitate the comparison of the multiple data sets comprising this study.
The x-, y-, and z-axes, respectively, represent time (post-stimulus onset),
electrode location, and the t test result (indicated by a color value) at each
data point. Posterior electrodes (from right to midline to left) include O2,
PO8, P8, P6, PO6, PO4, P4, P2, Pz, POz, Oz, P1, P3, PO3, PO5, P5, P7,
PO7, and O1. Central electrodes (from right to midline to left) include
TP8, T8, C6, CP6, CP4, C4, C2, CP2, CPZ, CZ, C1, CP1, CP3, C3, C5,
CP5, TP7, and T7. Frontal electrodes (from right to midline to left)
include FT8, F8, F6, FC6, FC4, F4, F2, FC2, FCZ, FZ, F1, FC1, FC3, F3,
F5, FC5, FT7, and F7. The nomenclature of these electrode sites is
modified from the “International extended 10–20 system” (American
Electroencephalographic Society, 1991).
RESULTS
Experiment 1: central presentations of
illusory contours
Behavioral results
Subjects correctly indicated the presence or absence of IC shapes
94.6% (?8.3%) of the time.
Electrophysiological results
Inspection of group-averaged VEPs for the IC present and IC
absent conditions revealed the traditional series of VEP compo-
nents, including the P1, N1, and P2. These components were
maximal over posterior scalp sites. Direct comparison of these
conditions revealed a differential response to centrally presented
IC shapes over posterior scalp sites (Fig. 2). The rising phase of
the P1 component showed neither amplitude nor latency modu-
lation with the presence versus absence of IC shapes. In contrast,
a large difference in waveform morphology was apparent begin-
ning at the falling phase of the P1 component.
We were interested in determining the onset latency of the
differential response to the presence versus absence of IC shapes.
The point-wise paired t tests indicate that the earliest IC effect
onset is at 88 msec after stimulus at scalp site P2. IC effect onset
latencies at the remaining posterior scalp sites ranged between 88
and 102 msec, with near simultaneous onsets over both hemi-
spheres (88 msec at site P2 vs 90 msec at site CP3; see Fig. 11, top
left). The initial IC effect was largest at sites PO5 and PO6 (Fig.
2), peaked at ?146 msec after stimulus onset, and persisted until
?215 msec after stimulus onset (see Fig. 11). A second phase of
the IC effect began at ?230 msec and persisted until ?320 msec.
No IC effect was observed over frontal sites until ?250 msec,
persisting until ?320 msec after stimulus. Figure 2 displays the
VEP waveforms as well as the results of point-wise t tests from
sites PO5 and PO6 in addition to two representative frontal sites.
For each subject and stimulus condition, we then calculated the
area (vs the 0 ?V baseline) for the 136–156 msec after stimulus
window at sites PO5, PO6, P5, and P6 (sites of maximal IC effect)
and submitted these area measures to a 2 ? 2 ? 2 repeated
measures ANOVA. The within subjects factors were stimulus
condition (IC absent vs IC present), hemisphere (left vs right),
and electrode (two over each hemisphere). There was a main
effect of stimulus condition (F(1,27)? 122.13; p ? 0.001), which
confirmed the results of the point-wise t tests. There was also a
main effect of hemisphere (F(1,27)? 5.73; p ? 0.03). Furthermore,
there was a significant interaction between the factors of stimulus
condition and hemisphere (F(1,27)? 4.52; p ? 0.05), indicating a
larger IC effect over the right versus left hemisphere.
A further emphasis of this study was the registration of the
onset of the IC effect (as well as its time course) relative to onset
of the initial visual cortical activation. That is, we sought to
determine when the brain responded differentially to IC presence
versus absence relative to when the brain responded to any visual
stimulus (regardless of IC presence versus absence). We therefore
determined the latency of the earliest deviation from baseline
activity of the VEP response at each of the 64 scalp sites by
calculating point-wise paired t tests (two-tailed) between the 0
?V baseline and the collapsed data from the two stimulus condi-
tions. The first time point forward from 30 msec after stimulus
onset where the response exceeded the 0.05 ? criterion for at least
11 consecutive data points was labeled as the VEP response onset
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at that site. The earliest response onset was 48 msec (e.g., sites P3,
P4), consistent with other observations of what has been termed
the C1 component (Clark et al., 1995; Murray et al., 2001; Foxe
and Simpson, 2002; Molholm et al., 2002). From this average
VEP response onset latency, we calculated a minimal lag of 40
msec until the onset of the IC effect.
SCD topographic maps of the group-averaged IC effect are
shown in Figure 3, A and C, and illustrate the scalp topography of
the IC effect at its peak (146 msec) (Fig. 3A), as well as the
stability of the lateral–occipital distribution of the IC effect over
time (Fig. 3C). Although the point-wise t test analysis described
above provides the latency of the IC effect, the scalp topography
indicates that this effect occurs first over lateral–occipital areas
and is larger over the right hemisphere. Moreover, no current
density foci are observed over frontal scalp regions. In addition to
the SCD maps, Figure 11 (top left panel) displays a statistical
cluster plot of the results of point-wise t tests for the IC effect at
55 of the 64 electrodes over the 500 msec post-stimulus epoch.
Two phases of the IC effect over lateral–posterior scalp sites, as
well as the absence of a robust frontal effect, are readily apparent
(?100–300 msec and ?325–425 msec).
Source analyses were performed on the difference between the
group-averaged VEP responses from the IC present minus IC
absent conditions. We seeded the locations of two dipoles based
on the average Talairach coordinates of the LOC areas described
by Mendola et al. (1999) as responsive to the presence of
Kanizsa-type IC shapes. The right hemisphere LOC dipole was
Figure 2.
dB/octave roll-off). Data from a pair of frontal (F3/F4) and parieto–
occipital (PO5/PO6) electrode sites are shown. Their locations on the
scalp are indicated by large white discs on the 3-D reconstruction (BEM as
implemented in CURRY) of one subject’s (B.H.) anatomical MRI. Black
traces indicate the VEP response to the presence of an illusory contour
(IC present), whereas light gray traces indicate the corresponding VEP
response to the non-inducing configurations (IC absent). Dark gray dashed
traces represent the IC present minus IC absent difference. Black traces in
the insets illustrate the p value of point-wise t tests between the IC present
and IC absent conditions across the VEP epoch.
Experiment 1 VEP waveforms (40 Hz low-pass filter; 24
Figure 3.
stimuli. Maps in this and similar figures are displayed on the 3-D recon-
struction (BEM as implemented in CURRY) of one subject’s (B.H.)
anatomical MRI data. These SCD foci are consistent with bilateral
lateral–occipital generators, although they are more pronounced over the
right hemisphere. Polarity of these maps is arbitrary, depending on the
direction of the subtraction, and scales are shown. A, SCD topographic
map (left-sided, back, and right-sided views) at 146 msec after stimulus
onset depicting the IC effect in experiment 1 (inducers appeared black on
a gray background). B, SCD topographic map at 156 msec after stimulus
onset depicting the IC effect in experiment 3 (inducers appeared gray on
a black background; identical scale as in A). C, A series of maps (back
view) depicting the stability of the SCD topography over the 88–168 msec
post-stimulus epoch for the IC effect shown in A.
SCD topographic maps of the IC effect for centrally presented
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located at 35, 85.4, 8 mm, and the left hemisphere LOC dipole
was located at ?35, 85.4, 8 mm. Initially, these two dipoles were
fit to a single time point (146 msec) at the peak difference
between the IC present and IC absent conditions. Only their
locations were fixed, allowing free and independent dipole rota-
tion until a minimal residual variance was achieved. We then fixed
the orientation of both dipoles and widened the epoch to 116–156
msec, so as to encompass both the peak as well as the immediately
preceding period. These two fixed dipoles explained on average
95.1% of the variance over this epoch (Fig. 4). The strength of
each source over the 0–500 msec epoch indicates that both
generators are activated simultaneously.
Experiment 2: illusory contour processing assessed
with combined EEG and fMRI
The results of experiment 1 provide compelling evidence that IC
sensitivity follows the onset of the VEP response by a consider-
able lag (40 msec) and occurs first in higher-tier lateral–occipital
brain areas. To interpret the IC effect observed in the VEP
response and dipole source analysis in relation to previous hemo-
dynamic imaging results more directly, we combined 128-channel
EEG and fMRI data sets from a cohort (n ? 5) of subjects (see
Materials and Methods).
Electrophysiological results
VEP morphology was similar to that seen in experiment 1. The
comparison of the IC present and IC absent conditions revealed
a similar IC effect, beginning during the falling phase of the P1
and persisting through the peak of the N1 component (Fig. 5A).
Visual inspection of the difference waveforms between IC present
and IC absent stimulus conditions indicated that in these subjects
the IC effect peaked at ?126 msec. We tested for an IC effect with
a 2 ? 2 ? 3 repeated measures ANOVA using area measures (vs
the 0 ?V baseline) from each subject and stimulus condition over
the 106–146 msec post-stimulus epoch at six parieto–occipital
sites. The within subjects factors were stimulus condition (IC
Figure 4.
are rendered in the 3-D reconstruction of one subject’s (B.H.) anatomical
MRI (BEM as implemented in CURRY; back and side views shown) at
the peak of the IC effect (146 msec). B, On average, this pair of dipoles
accounts for 95.1% of the variance between the observed data and the
forward solution to these dipoles over the 116–156 msec post-stimulus
epoch. C, The strength of these dipoles over the post-stimulus epoch
indicates synchronous bilateral IC processing in the lateral–occipital
areas.
A, The positions and orientations of two fixed dipoles (cyan)
Figure 5.
(40 Hz low-pass filter; 24 dB/octave roll-off; identical color scheme as in
Fig. 2) from two representative scalp sites illustrate the IC effect under
passive viewing conditions. Electrode locations are indicated with large
white discs on the 3-D scalp reconstruction of one subject’s anatomical
MRI shown in the insets. B, The locations of fMRI results are shown on
axial slices of a standard brain supplied with SPM99 software. White pixels
indicate areas of significant BOLD signal increase for the IC present
versus IC absent conditions ( p ? 0.05; corrected for multiple comparisons
across the entire image volume).
VEP and fMRI results from experiment 2. A, VEP waveforms
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absent vs IC present), hemisphere (left vs right), and electrode
(three electrodes over each hemisphere). There was a main effect
of stimulus condition (F(1,4)? 233.66; p ? 0.0001), confirming the
IC effect. No other effects or interactions reached the 0.05 signif-
icance threshold.
fMRI results
Activation maps of the IC effect (IC present vs IC absent) localize
IC processing to the LOC areas bilaterally. This localization was
independent of any a priori hypotheses in our analysis, because
no regions of interest were specified, and we corrected for mul-
tiple comparisons across the entire image volume. Figure 5B
displays the BOLD response activation maps for the IC present
versus IC absent comparison as rendered on axial slices of the
standard brain supplied by SPM99 software. The locations (Ta-
lairach coordinates) and extent of these activation clusters as well
as the results of the statistical tests are listed in Table 2. On the
basis of the coordinates reported in previous studies of IC pro-
cessing (Mendola et al., 1999) and object recognition processes
(Malach et al., 1995; Grill-Spector et al., 1998a,b), we interpret
the location of the two posterior clusters as within the LOC. It
should be noted that the LOC region is considered to be com-
posed of several areas that branch off dorsally and ventrally from
a posterior–lateral vertex (Malach et al., 1995; Grill-Spector et al.,
1998a,b). Consequently, there is often variation in coordinates
ascribed to LOC area activation both within and across studies.
Moreover, additional analyses using a region of interest (uncor-
rected threshold of p ? 0.001 within a 10 mm sphere) approach
failed to reveal significant clusters of activation in lower-tier areas
V1 or V2 (data not shown), using the average Talairach coordi-
nates for these areas described by Mendola et al. (1999). Activa-
tions in these lower-tier areas were observed, however, in the
comparison of either stimulus condition versus rest. These col-
lective fMRI results provide strong support for the proposal that
the strongest IC processing occurs in higher-tier visual areas.
In addition to the LOC clusters, we also observed significant
activation in the right parietal cortex. On the basis of previous
findings (Grill-Spector et al., 1998a,b; Mendola et al., 1999), this
cluster is likely situated in and just superior to V3A. Interestingly,
in the across subjects analysis of Mendola et al. (1999), this area
showed a small but statistically significant BOLD signal modula-
tion to IC presence versus absence. Responses in V3A have
likewise been reported in a recent study of cue-invariant object
recognition (Grill-Spector et al., 1998a), implicating a role for
dorsal visual areas in object processing (Sugio et al., 1999) (results
of experiment 4, below). The present results may be detecting a
similar modulation in parietal areas. However, the SCD topogra-
phy and source analysis results of experiment 1 would argue
against a prominent role of parietal areas in the earliest phase of
the IC effect.
EEG/fMRI co-registration and source analysis
To better interpret the IC effect observed in the VEP, we co-
registered both the EEG and fMRI data sets from the same five
subjects into the 3-D reconstruction of one subject’s (B.H.) ana-
tomical MRI and constrained the location of dipole sources to the
fMRI activation clusters. By this approach we were able to visu-
alize the results from both neuroimaging techniques concurrently
in the same coordinate space. This representation of the data
emphasizes both the high temporal resolution of the electrophys-
iological technique as well as the high spatial resolution of fMRI.
As in experiment 1, source analyses were performed on the
difference between the group-averaged (n ? 5) VEP responses
from the IC present minus IC absent conditions. We fixed the
locations and orientations of three dipoles to the Talairach coor-
dinates of the fMRI clusters listed in Table 2 (Fig. 6A). Figure 6B
displays the percentage of the variance explained (across all
recording channels) by these three dipoles over a ?20 msec
window surrounding the peak (?120 msec) of the earliest IC
sensitivity. On average, these dipoles explained 87.7% of the
variance over the 106–146 msec epoch. Figure 6C plots the
strength of each source over the epoch after onset of IC sensitiv-
ity (80–300 msec). It is important to emphasize that the fMRI
clusters seen in this experiment almost certainly represent the
activity of more than a single functional area within the LOC,
just as the electrophysiological effect represents co-coordinated
activity within a cluster of LO regions rather than a discrete
activation of a single region. Thus, fitting each cluster with a
single equivalent current dipole clearly represents an overly sim-
plified modeling of the activity within this complex. As such,
these dipoles should be considered to represent a “center of
gravity” rather than a discrete neural locus.
The co-registration of electrophysiological data (ERP) and
hemodynamic data (fMRI - BOLD) from the same subjects
(so-called multimodal imaging) has been widely acclaimed as a
major advance in our abilities to detail the spatiotemporal dy-
namics of brain function, and such methods have been applied
extensively in recent research (Simpson et al., 1995a,b; Martinez
et al., 1999; Bonmassar et al., 2001; Dale and Halgren, 2001).
However, the co-registration of these separate data sets is based
on the premise that the same neural events are represented in
both, and therefore it is critical to consider the relationship or
coupling between the physiological phenomena underlying the
different signal modalities. Within a single neuron, the current
flow generated when postsynaptic receptors are bound by their
respective neurotransmitter(s) results in both EPSPs and IPSPs,
the balance and spatial distribution of which determine whether
the neuron will generate an action potential. Synchronous trans-
membrane current flow in a population of neurons produces a
macroscopic version of the PSP, the local field potential (LFP).
The LFP distribution in the extracellular medium has a lawful
Table 2. Volume summary of fMRI results with p values corrected for multiple comparisons within the entire volume
Location
Cluster-level Voxel-level
Talairach coordinates
x, y, z (mm)pcorrected
kE
pcorrected
T; (Z?)
LOC(right)
LOC(left)
Parietal
0.000
0.000
0.000
0.003
190
69
48
20
0.000
0.002
0.001
0.007
6.87; (6.67)
5.41; (5.31)
5.59; (5.48)
5.17; (5.09)
32, ?96, 4
?46, ?76, ?8
20, ?84, 36
30, ?74, 46
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spatial relationship to the net direction (i.e., inward or outward)
and macroscopic anatomical profile of the current flow pattern
across the active cellular elements (Freeman and Nicholson, 1975;
Schroeder et al., 1995). A portion of these LFPs volume conduct
to the scalp surface, and it is these and not the action potentials
that are recorded during ERP studies (Mitzdorf, 1991; Schroeder
et al., 1995). In fact, in most cases, transmembrane current flow
does not lead to action potentials because the net combination of
EPSP and IPSP results in either subthreshold activation or inhi-
bition. In such cases, action potential measures might suggest low
activity in a region, although a significant LFP is being generated.
Clearly, the LFP would be a likelier index of metabolic activity in
these cases and would be a likelier correlate of the BOLD re-
sponse. Furthermore, transmembrane current flow has a clearer a
priori relationship to hemodynamic responses than do action
potentials. The reason is that most of the cortical energy produc-
tion of a neuron, and therefore its metabolic load, supports
functional (synaptic) glutamatergic neuronal activity (Sibson et
al., 1998). A measure of support for these contentions is found in
a recent study in which intracranial electrophysiologic recordings
were compared with fMRI data concurrently recorded from the
same monkeys (Logothetis et al., 2001). These authors found that
the LFP (measured as activity between 10 and 130 Hz) was a
better estimate of the BOLD response than multiunit activity
(300–3000 Hz). They interpreted their data, which showed a
strong correlation between the LFP and the BOLD response, as
strong evidence that both were indexing the same physiological
events in the brain.
Experiment 3: the IC effect and contrast polarity
Several sources of evidence indicate that the processing of illu-
sory contours and figure-ground segregation are independent of
the contrast polarity of the inducers. Intracranial studies of ma-
caques reveal that figure-ground responses (Baumann et al., 1997;
Zhou et al., 2000; Peterhans and Heitger, 2001) and illusory
contour presence versus absence (Heider et al., 2000) are signaled
in lower-tier areas independent of contrast polarity. Psychophys-
ical studies demonstrate that illusory contour strength persists
during variation in the contrast polarity across inducers (Prazdny,
1983, 1986; Shapley and Gordon, 1985; Dresp and Grossberg,
1997; Victor and Conte, 1998; Spehar, 2000) and subjective
brightness (Ware, 1981) of the induced shape. Similarly, recent
fMRI results from humans demonstrate contrast polarity inde-
pendence for the differential activation to IC presence versus
absence in LOC regions (Mendola et al., 1999). We therefore
sought to determine whether the timing and locus of the IC effect
was independent of the contrast polarity between inducers and
the background.
Behavioral results
Subjects correctly indicated the presence or absence of IC shapes
96.3% (?3.2%) of the time. There was no difference in perfor-
mance across contrast polarities (t16? 1.48; p ? 0.16) from the
data of those subjects who participated in both experiments 1
and 3.
Electrophysiological results
VEP morphology was similar to that seen in experiment 1, and
direct comparison of the IC present and IC absent conditions
revealed a similar IC effect (Figs. 3B, 7). As before, the rising
phase of the P1 component showed neither amplitude nor latency
modulation with the presence versus absence of IC shapes. In
agreement with the results of experiment 1, a large difference in
waveform morphology was apparent during the falling phase of
the P1 component.
Point-wise paired t tests between the VEP for the IC present
versus IC absent condition were calculated to determine the onset
latency of the IC effect (identical criterion as in experiment 1).
The earliest IC effect onset was at 104 msec after stimulus at scalp
site P3. IC effect onset latencies at the remaining posterior scalp
sites ranged from 106 to 112 msec, with near simultaneous onset
over both hemispheres (110 msec at site PO4). The IC effect was
largest at sites PO5 and PO6, peaking at ?162 msec after stimulus
onset. No IC effect was observed over frontal sites. Figure 7
displays the VEP waveforms as well as the results of point-wise t
tests from sites PO5 and PO6.
For each subject and stimulus condition, we then calculated the
area (vs the 0 ?V baseline) for the 152–172 msec post-stimulus
window at sites PO5, PO6, P5, and P6 and submitted these area
measures to a 2 ? 2 ? 2 repeated measures ANOVA. The within
subjects factors were stimulus condition (IC absent vs IC present),
hemisphere (left vs right), and electrode (two over each hemi-
Figure 6.
tions of the fMRI activation clusters ( yellow) and VEP dipoles (blue),
both from analyses of the group (n ? 5) data of experiment 2, are shown
spatially co-registered on the 3-D reconstruction of one subject’s anatom-
ical MRI. B, On average, these dipoles account for 87.7% of the variance
between the observed data and the forward solution to these dipoles over
the 106–146 msec post-stimulus epoch. C, The strength of these dipoles
over the 80–300 msec post-stimulus epoch reveals the relative contribu-
tion of each source over time.
VEP/fMRI co-registration and source analysis. A, The loca-
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sphere). There was a main effect of stimulus condition (F(1,16)?
37.51; p ? 0.001), which confirmed the results of the point-wise t
tests. There was no main effect of hemisphere (F(1,16)? 2.30; p ?
0.15). However, there was a significant interaction between the
factors of stimulus condition and hemisphere (F(1,16)? 4.45; p ?
0.05), indicating a larger IC effect over the right versus left
hemisphere.
As before, we were interested in registering the latency of the
IC effect within the framework of visual cortical response onset.
We determined the onset latency of the earliest activity at each of
the 64 scalp sites by calculating point-wise paired t tests (two-
tailed) between the 0 ?V baseline and the VEP response col-
lapsed across IC presence and absence. The first time point
forward from 30 msec post-stimulus onset where the response
exceeded the 0.05 ? criterion for at least 11 consecutive data
points was labeled as the VEP response onset at that site. The
earliest VEP response onset latency was 52 msec (sites P3 and
P4), resulting in a lag of 52 msec until the onset of the IC effect.
SCD topographic maps of the group-averaged IC present mi-
nus IC absent difference are shown in Figure 3B. As in experi-
ment 1, scalp topography of the IC effect is consistent with
lateral–occipital areas and is larger over the right hemisphere.
Moreover, no current density foci are observed over frontal scalp
regions. In addition to the SCD maps, Figure 11 (top right panel)
displays a statistical cluster plot of the results of point-wise t tests
for the IC effect at 55 of the 64 electrodes over the 500 msec
post-stimulus epoch. Two phases of the IC effect over lateral–
posterior scalp sites, as well as the absence of a robust frontal
effect, are again readily apparent (?100–250 and ?325–425
msec).
Experiment 4: stimulus features affecting
scene analysis
Because a potential criticism of the findings of experiments 1–3
might be that the present VEP measurements are insensitive to
earlier modulations, originating in lower-tier visual areas, we
performed a further analysis of these data. Variation of low-level
stimulus features (such as those listed in Table 1) is one means of
optimizing the ability to detect modulations in lower-tier visual
areas. For example, early modulation of the VEP (?60 msec after
stimulus onset) was observed for Kanizsa-type stimulus configu-
rations consisting of four versus three inducers, independent of
IC presence versus absence (Hermann et al., 1999). This modu-
lation of the P1 component was interpreted to reflect an overall
brightness difference between stimulus configurations with more
versus fewer inducers and demonstrates the sensitivity of the
VEP to changes in low-level stimulus features. Our question then
is whether the low-level features that varied across the stimulus
set used in experiments 1 and 3 produce modulations of the VEP
responses before the IC effect. Such a result would indicate that
our methodological approach is sensitive to modulation of activ-
ity in lower-tier visual areas. Such a finding would bolster our
contention that the IC effect described in experiments 1–3 does
indeed index the earliest IC sensitivity in cortex.
We used the stimuli from experiments 1 and 3 to address this
question of scene analysis as well as to optimize the likelihood of
observing modulations in lower-tier visual areas. We separately
averaged the VEP responses to each of the IC present and
corresponding IC absent inducer configurations for each of the
five shapes (circle, pentagon, star, square, and triangle) used in
experiments 1 and 3. Because the results of experiment 3 indi-
cated that the IC effect is insensitive to contrast polarity, we
collapsed the data sets from the 17 subjects who participated in
both experiments 1 and 3. Recall that across these five shapes,
several stimulus parameters varied: (1) the number of inducers,
(2) support ratio, (3) surface area of the induced IC shapes, and
(4) farthest eccentricity of any inducer (Table 1).
Electrophysiological results
Consistent with our description of the timing of IC sensitivity, we
find no evidence for an IC effect before 88 msec after stimulus
onset for any individual shape (data not shown). Rather, there
was early (?66 msec) modulation of the VEP responses to the
square and triangle (both IC present and absent configurations)
versus each of the other three stimulus shapes. In further contrast
to the IC effect, this modulation began over posterior midline
scalp and remained restricted primarily to central parieto–occi-
pital scalp with some later but weak modulation over lateral
parieto–occipital scalp (Fig. 8). The mainly central occipital
distribution of this effect is consistent with modulations of lower-
tier areas. To assess which stimulus parameters influence this
modulation, we calculated the area (vs the 0 ?V baseline) over the
66–86 msec post-stimulus epoch for each shape and stimulus
configuration and submitted these values to a 5 ? 2 ? 3 repeated
measures ANOVA. The within subjects factors were shape (cir-
cle, pentagon, star, square, and triangle), stimulus configuration
(IC present and IC absent), and scalp site (P7, Pz, and P8). There
was a main effect of shape (F(4,13)? 8.69; p ? 0.001), but no main
effect of either configuration (F(1,16)? 0.32; p ? 0.58) or scalp site
(F(2,15)? 0.56; p ? 0.58). Only the interaction between the factors
of shape and scalp site was significant (F(8,9)? 9.85; p ? 0.001),
indicating that the effect of shape varied across scalp sites. A
series of post hoc t tests (Table 3) using these area measures
collapsed across IC present and IC absent stimulus configurations
reveal a graded amplitude decrease across shapes over posterior
midline scalp (Fig. 8A). An overlay of the shapes used in this
experiment reveals that this amplitude decrease corresponds with
Figure 7.
dB/octave roll-off). Data are shown in an identical manner as in Figure 2
from two representative electrode sites (PO5 and PO6).
Experiment 3 VEP waveforms (40 Hz low-pass filter; 24
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the maximal eccentricity of inducer elements (Fig. 8B). Next, we
collapsed the VEP responses (both IC present and IC absent)
from the triangle and square and contrasted them against the
corresponding collapsed data from all other shapes and stimulus
configurations (hereafter “wide” and “narrow” extent, respec-
tively) (Fig. 8C). The SCD topography of this wide minus narrow
difference is displayed in Figure 8D at 70 msec after stimulus
onset and illustrates its central parieto–occipital distribution.
Hereafter, we refer to this wide minus narrow difference as the
configuration effect. In addition to its earlier timing, this config-
uration effect contrasts with the lateral–occipital distribution that
characterizes the IC effect (Fig. 8C, compare with Fig. 3).
The configuration effect reveals stages of scene analysis pre-
ceding IC sensitivity that appear to modulate according to the
spatial distribution of inducers rather than to several other low-
level stimulus parameters (Table 1). We posit that the maximal
eccentricity of inducer elements (Table 1, ?) demarcates a coarse
extent of visual feature space and is the determining parameter in
this early modulation. Several findings from this analysis support
this hypothesis. In contrast to Hermann et al. (1999), we observe
no difference in the VEP responses to the triangle versus square
(Table 3), arguing against a difference in overall brightness
(caused by the number of inducers) as an explanation of this early
modulation. Rather, there was a significant amplitude difference
between the responses to the square and circle, both of which are
defined by four inducers. Likewise, there were smaller amplitude
responses from stimuli defined by five inducers, relative to those
defined by fewer inducers. This pattern also is in contrast to the
prediction based on a strict interpretation of a “spotlight” of
spatial attention (Eriksen and Yeh, 1985; Sto ¨ffer, 1994), because
stimulus displays containing more local elements (inducers)
should have yielded a larger response. The induced surface area
(?) of IC shapes is highly unlikely to be the mediating parameter,
because the configuration effect occurs when IC shapes are both
present and absent. Likewise, similar responses are observed
across shapes with large surface area differences (Table 3, pen-
tagon vs star). Variation in support ratio (?) likewise cannot
account for this modulation. The pattern of VEP response am-
plitudes across shapes does not follow the prediction of a linearly
increasing variation across support ratios (Gegenfurtner et al.,
Figure 8.
response (sites P7, Pz, and P8) to each shape independent of IC presence versus absence. A step function in mean area is observed over midline but not
lateral scalp sites. A single asterisk indicates a significant difference ( p ? 0.01; paired t test) in mean area between both the response to the square and
triangle versus a particular shape. The double asterisk indicates a significant difference ( p ? 0.01; paired t test) in mean area between the circle and each
of the other shapes. Mean area would appear to follow the eccentricity of inducers. B, A diagram of each of the IC shapes (thick black lines) indicates
the variation in the farthest eccentricity of any inducer across shapes. C, VEP waveforms from sites P7, Pz, and P8. These data have been collapsed across
wide shapes (square and triangle) and narrow shapes (star, pentagon, and circle). D, SCD topographic maps of the wide ? narrow difference at 76 msec
after stimulus onset.
Shape-wise analyses demonstrating the configuration effect. A, Bar graphs display the mean area (66–86 msec after stimulus) of the VEP
Table 3. Shape-wise t tests (experiment 4)
Triangle SquareStarPentagonCircle
Triangle
Square
Star
Pentagon
t16? 0.47; p ? 0.65
t16? 2.81; p ? 0.01
t16? 3.22; p ? 0.005
t16? 4.07; p ? 0.001
t16? 5.11; p ? 0.001
t16? 0.19; p ? 0.85
t16? 6.06; p ? 0.001
t16? 5.57; p ? 0.001
t16? 3.48; p ? 0.003
t16? 4.10; p ? 0.001
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1997), because by this reasoning the pentagon would be expected
to yield the largest early VEP response.
Experiment 5: the IC effect and lateral presentations
It is also possible that our results from experiments 1–3 were
biased toward the involvement of higher-tier visual areas, because
centrally presented IC shapes were relatively large (maximally 6°)
and the inducers were quite eccentric. Reducing the gap between
inducing elements has been used experimentally to bias IC pro-
cessing toward lower-tier visual areas. However, IC processing
appears to be size invariant and did not produce modulation in
lower-tier areas despite robust modulation in the LOC area
(Mendola et al., 1999). It is important to note, however, that
changes in gap width when inducers still straddle the vertical
meridian limit the possible biasing of IC processes into lower-tier
areas because of the anatomical and physiological properties of
lower-tier visual areas. That is, when inducers straddle the verti-
cal and horizontal meridians, IC borders must be established
between anatomically separated cortical representations.
One physiological implication is that the formation of IC bor-
ders, even for IC shapes induced over small gaps, would be
preceded by interhemispheric and intrahemispheric neural re-
sponse interactions when stimuli are spatially dispersed across the
horizontal and vertical meridians (Murray et al., 2001). Such
interactions are mediated by horizontal and callosal connections.
Candidate areas mediating the earliest IC sensitivity may there-
fore be restricted to those with sufficient callosal and horizontal
connectivity (Van Essen et al., 1982; Tootell et al., 1988, 1998;
Clarke and Miklossy, 1990). In agreement with this possibility, a
recent VEP study from our laboratory has shown that these
interactions are delayed relative to the initial cortical response
onset by ?25 msec (Murray et al., 2001).
A further limitation is based on the observation that the in-
duction of IC sensitivity requires a minimal stimulus extent that
is larger than the classical receptive fields of neurons in area V1
or V2 (von der Heydt et al., 1984; Peterhans and von der Heydt,
1989; von der Heydt and Peterhans, 1989). One implication of this
observation is that IC sensitivity relies on contextual modulations
(Lamme and Spekreijse, 2000). Thus, at least two possible mech-
anisms of IC sensitivity present themselves. Either long-range
horizontal connections within a visual area (intrinsic and callosal)
or feedback projections from higher-tier visual areas with suffi-
ciently large receptive fields to span the inducing elements would
be required to process IC stimuli (Rockland and Pandya, 1979;
Gilbert, 1983; Maunsell and Van Essen, 1983; Van Essen et al.,
1994; Gilbert et al., 1996; Spillmann and Werner, 1996). Evidence
for top-down (i.e., feedback) modulation of activity in lower-tier
visual areas has been shown in macaque V1 during figure-ground
segregation (Kapadia et al., 1995, 1999; Lamme, 1995; Zipser et
al., 1996; Hupe ´ et al., 1998; Lamme et al., 1998a,b, 1999; Lamme
and Spekreijse, 2000). These modulations occurred ?80–100
msec after stimulus onset, considerably later than the initial onset
of activity in the same V1 neurons (Zipser et al., 1996; Lamme
and Spekreijse, 2000), and are suppressed by anesthesia (Lamme
et al., 1998b), consistent with reentrant, feedback modulations
from higher-tier areas (Lamme and Roelfsema, 2000). This time
frame for contextual modulations in area V1 corresponds well to
the results of a recent intracranial investigation that focused on
the latency of IC sensitivity across the cortical layers of areas V2
and V1 (Lee and Nguyen, 2001). In this study, the earliest
differential response to Kanizsa-type IC stimuli onset at 70 msec
was in superficial layers of V2, with an effect in both deep layers
of V2 as well as superficial layers of V1 lagging by ?25–30 msec
(Lee and Nguyen, 2001).
An alternative means of biasing illusory contour processing
may be to vary the retinotopic position of inducers such that
interhemispheric interactions (and potentially the involvement of
higher-tier areas with large bilateral receptive fields) would not be
required for IC processing. Lateral presentations should distin-
guish between two possible hypotheses of IC processing. The first
contends that if IC sensitivity is a low-level and bottom-up pro-
cess, then confining inducers to a single visual field should result
in earlier modulation of the VEP than when centrally presented,
because this would potentially be mediated by long-range local
connectivity. Likewise a change in the scalp topography of the IC
effect would be expected (relative to central presentations) that
should be consistent with lower-tier visual areas. A second hy-
pothesis contends that object recognition of centrally presented
stimuli would be faster than when stimuli are laterally presented,
in part because of the overrepresentation of central vision in
cortex (Popovic and Sjostrand, 2001). Several additional lines of
evidence support this hypothesis. Previous electrophysiological
studies of laterally presented IC shapes did not observe differen-
tial modulation of the VEP response until ?224 msec after
stimulus onset (Brandeis and Lehmann, 1989), which is consid-
erably later than our IC effect with centrally presented stimuli.
Psychophysical studies involving the discrimination of illusory
shapes of contours reveal that performance is best when stimuli
appear centrally rather than laterally or either above or below
fixation (Rubin et al., 1996) [see also Juttner and Rentschler
(2000) for demonstrations with object categorization]. Still others
demonstrate that performance on shape discrimination tasks is
improved when stimuli are distributed between the left and right
visual hemifields (Banich and Belger, 1990; Mohr et al., 1994).
The prediction from these collective data would therefore be that
the IC effect for laterally presented stimuli would shift later
relative to central presentations. In experiment 5, we presented
IC stimuli laterally to one or the other visual hemifield to distin-
guish between these two hypotheses.
Behavioral results
Subjects correctly indicated the presence or absence of IC shapes
in the left visual field 97.8% (?2.3%) of the time and in the right
visual field 97.5% (?2.2%) of the time. There was no difference
in performance across visual fields (t11? 0.96; p ? 0.36). For the
eight subjects who also participated in experiment 1, there was no
difference in the accuracy of performance when stimuli were
centrally presented versus when they were presented to the left
(t7? 1.06; p ? 0.32) or right (t7? 0.72; p ? 0.49) visual field.
However, it is important to recall that subjects were able to
self-pace stimulus delivery and that accuracy was emphasized
over speed. Subjective reports on debriefing of subjects who had
participated in experiment 1 indicated that the IC shapes did not
“pop out” with the same saliency as when they were presented
centrally.
Electrophysiological results
As in experiments 1 and 3, inspection of group-averaged VEPs
for the IC present and IC absent conditions revealed the tradi-
tional series of VEP components, including P1, N1, and P2. These
components were maximal over posterior scalp sites. Direct com-
parison of these conditions revealed a differential response to
laterally presented IC shapes over posterior scalp sites (Fig. 9).
The P1 and N1 components showed neither amplitude nor latency
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modulation with the presence versus absence of IC shapes. In
contrast, two large, successive differences in waveform morphol-
ogy were apparent after the P2 component.
As before, to determine the onset latency of the IC effect with
lateralized stimuli, we calculated point-wise paired t tests be-
tween the VEP for the IC present versus IC absent condition. For
each electrode, the first time point where this comparison ex-
ceeded the 0.05 ? criterion for at least 11 consecutive data points
was labeled as the onset of the IC effect. For left visual field
presentations, the earliest IC effect onset at 218 msec after stim-
ulus was at scalp site PO4, and at 220 msec it was at site PO3. For
right visual field presentations, the earliest IC effect onset at 202
msec after stimulus was at scalp site PO3, and at 214 msec after
stimulus onset it was at site PO4. For both left and right visual
field stimuli, the IC effect was largest at sites PO5 and PO6,
peaking at 242 msec after stimulus onset over the hemisphere
contralateral to the stimulated visual field and at 252 msec over
the ipsilateral hemisphere (Fig. 9). No IC effect was observed over
frontal sites in response to either visual field presentation. Fig-
ure 9 displays the VEP waveforms, as well as the results of
point-wise t tests, for both left (PO6) and right (PO5) visual field
presentations.
For each subject and stimulus condition, we then calculated the
area (vs 0 ?V baseline) for the 236–256 msec window at sites
PO5, PO6, P5, and P6. For each visual field of presentation, we
then submitted these area measures to a 2 ? 2 ? 2 repeated
measures ANOVA. For each of the two ANOVAs, the within
subjects factors were stimulus condition (IC absent vs IC present),
hemisphere (left vs right), and electrode (two over each hemi-
sphere). For left visual field presentations, there was a main effect
of stimulus condition (F(1,11)? 13.82; p ? 0.003), which con-
firmed the results of the point-wise t tests. There was no main
effect of hemisphere (F(1,11)? 0.33; p ? 0.58). Furthermore, there
was no interaction between the factors of stimulus condition and
hemisphere (F(1,11)? 1.88; p ? 0.20), indicating an equivalent IC
effect over both hemispheres. None of the other tests reached
significance. For right visual field presentations, there was a main
effect of stimulus condition (F(1,11)? 22.12; p ? 0.001), which
confirmed the results of the point-wise t tests. There was no main
effect of hemisphere (F(1,11)? 1.49; p ? 0.25); however, the
interaction between the factors of stimulus condition and hemi-
sphere approached the significance criterion (F(1,11)? 4.53; p ?
0.06), suggestive of a larger IC effect over the left (contralateral)
hemisphere.
It may be contended that the later IC effect latency in this
experiment reflects a more general lag in visual cortical process-
ing for lateral versus central presentations. We assessed this
possibility by registering this IC effect within the framework of
the visual cortical response by determining the onset latency of
the earliest activity at each scalp site. For each of the 64 scalp
sites, point-wise paired t tests (two-tailed) were conducted be-
tween the 0 ?V baseline and the collapsed response across stim-
ulus conditions. The first time point forward from 30 msec post-
stimulus onset where the response exceeded the 0.05 ? criterion
for at least 11 consecutive data points was labeled as the VEP
response onset at that site. This latency was 62 msec for left and
64 msec for right visual field stimuli, ?10 msec later than when
stimuli were centrally presented. From this VEP response onset
latency, we calculated a lag of ?120 msec until the onset of the IC
effect for laterally presented stimuli.
It is noteworthy that despite the dramatic latency shift in the IC
effect, we observed no drop in performance accuracy between
central and lateral presentations. The most parsimonious expla-
nation is that we are observing a ceiling effect, because the
determination of IC presence versus absence is a relatively trivial
task as evidenced by ?94% accuracy in all conditions. One
possibility is that performance differences might have been ob-
served had we asked our subjects to make a more fine-grained
discrimination (e.g., discriminate between shapes). Likewise, this
pattern of results would predict that reaction times, in correspon-
dence with the latency of the IC effect, would be slower for lateral
versus central presentations. The design of the present study,
however, did not include collection of reaction times, because
subjects responded after a response prompt to minimize the
overlap of motor potentials with the early VEP components.
Therefore, this prediction will be tested in future studies.
SCD topographic maps of the group-averaged IC present mi-
nus IC absent difference are shown in Figure 10. As with centrally
presented stimuli, the scalp topography of the IC effect indicates
that IC sensitivity occurs first over lateral–occipital areas of the
hemisphere contralateral to stimulus presentation (although also
involving the ipsilateral hemisphere). No current density foci are
observed frontally. This is visualized more extensively in Figure
11 (bottom two panels), which displays statistical cluster plots of
the results of point-wise t tests for the IC effect at 55 of the 64
electrodes over the 500 msec post-stimulus epoch. In sharp con-
trast to the results of experiments 1–3, the IC effect for laterally
presented stimuli onsets is much later and is composed of a single
phase (?200–275 msec).
The IC effect for laterally presented stimuli is shifted ?120
msec later relative to when stimuli are presented centrally. Im-
portantly, for both central and lateral presentations, the latency of
VEP response onset is indistinguishable, and the scalp topogra-
phy of the IC effect is highly similar. A comparison of Figures 3
Figure 9.
visual field presentations (40 Hz low-pass filter; 24 dB/octave roll-off).
Data are shown in an identical manner as in Figure 2 from one contralat-
eral representative electrode site for left visual field (PO6) and right
visual field (PO5) stimulus presentations.
Experiment 5 VEP waveforms in response to left and right
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and 10 shows this general topographic similarity, suggesting the
activation of a similar complex of LOC generators in both cases;
however, some clear variations are apparent. Given the relative
crudeness of the ERP topographic technique for spatially local-
izing intracranial generators, it would be premature to interpret
these differences to any great extent. Nonetheless, it is reasonable
to predict that although central and lateral stimuli both result in
LOC activation, there is likely to be some variation in the relative
contributions to this effect from the sub-areas that comprise the
LOC region. Clearly, further investigation will be required to test
this prediction.
More importantly, this pattern of results is not consistent with
the prediction that IC processing is a strictly feedforward,
bottom-up phenomenon. Likewise, this shift in the latency of the
IC effect contrasts with previous VEP studies in which similar
latencies (?200 msec) for the IC effect have been observed for
both central (Sugawara and Morotomi, 1991; Tallon-Baudry et
al., 1996, 1997) and lateral presentations (Brandeis and Lehmann,
1989). That is, the onset of our IC effect for central presentations
is earlier than in previous studies, whereas the timing of our IC
effect for lateral presentations is in agreement with the results of
Brandeis and Lehmann (1989).
DISCUSSION
High-density electrical mapping, source analysis, and fMRI were
combined to elucidate mechanisms underlying IC processing in
humans. We present four main findings. (1) The VEP modulated
with IC presence versus absence—the IC effect. This effect onset
was at 88 msec, it followed cortical response onset by 40 msec, and
it was robust across contrast polarities. (2) Lateral presentations
shifted IC effect onset 120 msec later. From these latencies, we
propose that the earliest IC sensitivity is unlikely to occur during
the initial phase of activation in lower-tier hierarchical areas. (3)
Strongly supporting this hypothesis are combined VEP and fMRI
results localizing the earliest IC effect to the LOC, regardless of
the shape or the retinotopic position of the IC. (4) Visual feature
analyses preceded IC sensitivity over central parieto–occipital
scalp—the configuration effect. Collectively, our data support a
model of IC processing wherein dorsal stream regions, which are
initially insensitive to IC presence, coarsely demarcate the spatial
extent of a given stimulus array and then input to ventral stream
structures (e.g., the LOC area), where IC sensitivity first occurs.
Our data support the notion that IC effects previously observed in
lower-tier areas are likely to be driven by feedback (perhaps in
combination with horizontal) inputs from higher-tier areas.
Timing of IC processing
The earliest VEP modulation to IC shape presence versus ab-
sence was onset at 88 msec. In contrast, the initial visual cortical
response was onset at 48 msec. Thus, there is a 40 msec lag
between cortical response onset and IC effect onset, which is
sufficient for signal transmission to the LOC area. Response onset
in human LOC areas can be predicted from intracranial record-
ings detailing the timing of activation across cortical regions
(Schroeder et al., 1998) in awake monkeys. Onset latencies are
?25 msec in V1 (Maunsell and Gibson, 1992; Givre et al., 1994;
Schroeder et al., 1998, 2001) and ?50 msec in IT (Schroeder et
al., 1998, 2001; Mehta et al., 2000a,b). By applying a 3:5 ratio to
draw correspondence between monkeys and humans (Schroeder
et al., 1995, 1998, 2001), a conservative estimate of human V1
onset is 40–50 msec (consistent with the current findings) and
80–100 msec in LOC areas. From these estimates, the IC effect
after central presentations is likely to commence with or shortly
after initial activity in higher-tier ventral stream areas.
These timing data can be applied similarly to the interpretation
of a recent investigation of IC processing dynamics in monkeys
(Lee and Nguyen, 2001). The earliest IC response occurred in
superficial V2 (70 msec), followed by deep V2 (95 msec) and
superficial V1 (100 msec). Unfortunately, activation onset laten-
cies were not reported. However, on the basis of known anatom-
ical connections (Felleman and Van Essen, 1991) and previous
studies in monkeys (Givre et al., 1994; Schroeder et al., 1998,
2001; Mehta et al., 2000a,b), this laminar activation profile is
characteristic of a feedback input pattern. Likewise, mean laten-
cies in V1 and V2 of the awake monkey are 24 and 32 msec,
respectively (Schroeder et al., 1998). Therefore, the findings of
Lee and Nguyen (2001) are unlikely to reflect modulation of the
initial afferent volley to V1 or V2. This hypothesis is further
supported by the impaired discrimination of IC shapes after
lesions in V4 (De Weerd et al., 1996) or IT cortex (Huxlin and
Merigan, 1998; Huxlin et al., 2000) and studies of contextual
modulation (Hupe ´ et al., 1998; Lamme and Spekreijse, 2000), as
well as by the present SCD topography, dipole analysis, and fMRI
results that all localized the IC effect to the LOC area.
Lateral versus central presentations reveal modes of
object processing
Several lines of evidence support the interpretation that the
temporal disparity in the IC effect for central versus lateral
Figure 10.
views) of the IC effect for laterally presented stimuli of experiment 5. Top,
SCD topographic maps at 236 msec after stimulus onset depicting the IC
effect in response to left visual field stimuli. Bottom, SCD topographic
maps at 236 msec after stimulus onset depicting the IC effect in response
to right visual field stimuli. These SCD foci are consistent with bilateral
lateral–occipital generators, although more pronounced over the con-
tralateral hemisphere. Polarity of these maps is arbitrary, depending on
the direction of the subtraction, and scales are shown.
SCD topographic maps (left-sided, back, and right-sided
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presentations likely represents successive stages of differential
activation in the LOC area. Previous studies of object recognition
from our laboratory have revealed a series of VEP modulations
that were interpreted to reflect a change in the mode of object
recognition (Doniger et al., 2000, 2001). We observed a late
modulation, termed Ncl(?290 msec peak) associated with per-
ceptual closure processes leading to object identification of frag-
mented line drawings (Doniger et al., 2000, 2001). Identification
under these conditions was considered to be effortful and to rely
on semantic/episodic memory representations. An earlier modu-
lation of the N1 component was observed for object recognition
as a result of repetition priming, which was interpreted as an
instance of automatic recognition based on a sensory trace (Doni-
ger et al., 2001). Similar to the N1 and Nclmodulations of Doniger
et al. (2000, 2001), the latency shift in the present IC effects for
central and lateral presentations may be reflecting a shift in object
recognition mode from automatic and “perceptual” to effortful
and “conceptual” (Ritter et al., 1982; Tulving and Schacter, 1990;
Humphreys et al., 2000). The implication is that these lateral
presentations rely more on perceptual closure processes than on
Figure 11.
electrode positions ( y-axis) for 55 of the 64 electrodes (see Materials and Methods for details of electrode locations) used in experiments 1, 3, and 5.
For clarity, only p values ? 0.01 are color encoded. Top, Results from experiments 1 and 3 using centrally presented inducers (left, gray background; right,
black background) indicate a biphasic IC effect over posterior scalp sites. No robust IC effect is observed over frontal sites. Bottom, Results from
experiment 5 using laterally presented inducers indicate that the IC effect observed with centrally presented inducers shifts ?120 msec later. As with
centrally presented stimuli, no IC effect is observed over frontal sites. The lag in onset of the IC effect over the direct (contralateral) and indirect
(ipsilateral) hemispheres can be readily seen for both left and right visual field presentations.
Statistical cluster plots. Color values indicate the result of point-wise t tests evaluating the IC effect across post-stimulus time (x-axis) and
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rapid object recognition. That is, the IC effect after lateral pre-
sentations resulted in modulation of the VEP very similar to the
Nclobserved during perceptual closure paradigms, whereas the
corresponding IC effect after presentations of stimuli centrally
yielded modulation of the N1 as well as the Nclcomponency.
These early (in the case of central presentations) and late mod-
ulations (in the cases of both central and lateral presentations)
can be readily visualized in Figure 11.
Locus of IC processing
Our combined VEP and fMRI results localized the IC effect to
the LOC area, which has been implicated repeatedly in object
recognition (Malach et al., 1995; Moscovitch et al., 1995; Vanni et
al., 1996; Grill-Spector et al., 1998a,b; Ishai et al., 1999; Kourtzi
and Kanwisher, 2000), perceptual closure (Doniger et al., 2000,
2001), and IC sensitivity (Hirsch et al., 1995; ffytche and Zeki,
1996; Larsson et al., 1999; Mendola et al., 1999; Seghier et al.,
2000). The LOC area is considered the homolog of macaque IT
cortex (Gross et al., 1972; Sary et al., 1993; Kobatake and Tanaka,
1994) and has large, bilateral receptive fields (Kobatake and
Tanaka, 1994; Tootell et al., 1998). Critically, areas V1 and V2
have small receptive fields and lack substantial callosal inputs,
making it unlikely for these areas to mediate IC effects with large,
centrally presented stimuli used in the present study (although
these areas are likely to be involved in IC processing using
feedback from areas like the LOC area). Object recognition in
higher-tier areas has further been shown to be size, position, and
cue invariant (Sary et al., 1993; Lueschow et al., 1994; Ito et al.,
1995; Grill-Spector et al., 1998a). Some of these properties have
recently been generalized to include IC processing (Mendola et
al., 1999). Here, we replicate the localization of IC processing to
the LOC area of Mendola et al. (1999) and further extend this
finding to include both centrally and laterally presented IC
shapes, demonstrating the position invariance of IC sensitivity in
the LOC area. Although the lateral displacement of the inducer
stimuli appears not to alter the locus of IC sensitivity, it would
appear to shift its mode from automatic to effortful. Although it
may be contended that Kanizsa-type stimuli biased IC processing
toward higher-tier regions and that other IC varieties would have
better detected IC processing in lower-tier areas, recent fMRI
results suggest a central role for the LOC area in the processing
of multiple IC varieties (Mendola et al., 1999). Nonetheless, one
question for future research is the extent to which the dynamics
of the present results generalize across IC varieties.
Hemispheric asymmetry of IC processing
This study found that although both hemispheres are involved in
IC processing, the IC effect is larger over the right versus left
hemisphere after central presentations, but not after lateral pre-
sentations. In the case of lateral presentations, the onset of the IC
effect occurs first over the contralateral hemisphere with activa-
tion over the ipsilateral hemisphere lagging by ?10 msec (con-
sistent with estimates of interhemispheric transfer time) (Murray
et al., 2001). However, the amplitude (measured from a 20 msec
epoch centered on the peak of the IC effect over the contralateral
hemisphere) did not differ over the two hemispheres in the
case of lateral presentations. Further experiments will be re-
quired to resolve any hemispheric asymmetry of illusory contour
processing.
Nonetheless, one hypothesis would maintain that the right
hemisphere is specialized for global form processes, whereas the
left hemisphere is specialized for analytic local processes (Rob-
ertson et al., 1988; Atchley and Atchley, 1998; Corballis et al.,
1999; Han et al., 2000a,b). Psychophysical investigations of IC
perception, however, provide no clear consensus regarding hemi-
spheric dominance (Mattingley et al., 1997; Rasmjou et al., 1999),
whereas a previous VEP study of attention and IC processes
revealed simultaneous and symmetric effects for both left and
right visual field presentations (Brandeis and Lehmann, 1989).
The asymmetric IC effect after central presentations may reflect
a relatively greater contribution of global processing in perceiv-
ing Kanizsa-type IC shapes. Recent reports describe a similar
asymmetry during perceptual closure of fragmented images
(Wasserstein et al., 1984; Doniger et al., 2001) as well as when
selective attention to global versus local features of a stimulus
array was manipulated directly (Fink et al., 1997; Heinze et al.,
1998; Han et al., 2000a,b). It may instead be the case that global
processes predominate in early perceptual processing preceding
IC sensitivity (Navon, 1977; Sekuler, 1994). The configuration
effect described in experiment 4 supports this postulation in so far
as this early VEP modulation is sensitive to the overall, global
distribution of inducer elements in the visual field, rather than to
their number or IC presence (Fig. 8, Table 1). It is critical to note
that despite its magnitude asymmetry, the onset of the IC effect
occurs simultaneously over left and right hemispheres (when
stimuli appear centrally), suggesting that both hemispheres per-
form IC completion processes in parallel, even if potentially
performing different types of scene analysis.
Models of IC processing
Although an extensive review of existing models is beyond the
scope of this study (Mendola, 2002), the present study endorses
some modifications and highlights the importance of temporal
information in modeling brain processes (Schroeder et al., 1998,
2001; Martinez et al., 1999). This study temporally and spatially
dissociated several VEP modulations in response to Kanizsa-type
IC shapes. At 66–86 msec the configuration effect appears to
track the extent of inducers in the visual field. This modulation,
focused over central parieto–occipital scalp, is insensitive to IC
presence. In contrast, the IC effect (88–100 msec), focused over
lateral–occipital scalp sites, is sensitive to IC presence indepen-
dently of inducer extent. However, this later modulation is de-
pendent on the retinotopic position of IC shapes, and its onset
shifts 120 msec later with inducers confined within a hemifield.
The timing of these modulations with respect to the onset of the
visual cortical response supports the view that IC sensitivity as
described previously in areas V2 and V1 may reflect modulation
caused by feedback inputs from higher-tier visual areas. The
present data thus support a three-phase model of IC processing
wherein dorsal stream regions, mediating the configuration effect,
establish a coarse global representation of object space that
guides the subsequent IC effect in LOC areas of the ventral
stream (Schroeder et al., 1998; Vidyasagar, 1999), in turn provok-
ing feedback to lower-tier areas (e.g., V2 and V1).
Although this study highlights the importance of feedback
modulations for IC sensitivity in lower-tier areas, the role of
horizontal inputs should not be disregarded (Das and Gilbert,
1999; Mendola, 2002). In fact, our proposition of dorsal stream
guidance of subsequent ventral stream IC processing is supported
by the finding that the initial activity in ventral stream areas is
consistent with lateral (vs feedforward) inputs, perhaps from
dorsal stream areas (Schroeder et al., 1998). Likewise, several
laboratories have reported dorsal versus ventral stream latency
advantages in both monkeys (Nowak and Bullier, 1997;
Murray et al. • Dynamics of Illusory Contour ProcessingJ. Neurosci., June 15, 2002, 22(12):5055–5073 5069