© European Neuroscience Association
European Journal of Neuroscience, Vol. 10, pp. 3689–3699, 1998
Regions in the human brain activated by simultaneous
orientation discrimination: a study with positron emission
tomography
Patrick Dupont, Rufin Vogels,1 Rik Vandenberghe,1 Annemie Rosier,1 Luc Cornette,1 Guy Bormans, Luc Mortelmans
and Guy A. Orban1
Centrum voor Positron Emissie Tomografie, Departement Nucleaire Geneeskunde, Universitair Ziekenhuis Gasthuisberg,
Herestraat 49, B-3000 Leuven, Belgium
1Laboratorium voor Neuro- en Psychofysiologie, Katholieke Universiteit Leuven, Campus Gasthuisberg, Herestraat 49,
B-3000 Leuven, Belgium
Keywords: cerebellum, functional imaging, regional cerebral blood flow, spatial comparison, visual cortex, task
Abstract
In order to compare regional cerebral activity involved in simultaneous as opposed to successive orientation
discrimination, we used positron emission tomography to measure regional cerebral blood flow, in two threefold
sets of conditions, in a large number of subjects. The first such triad involved simultaneous orientation
discrimination, orientation identification and detection, with all tasks using the same pair of gratings. The second
triad consisted of successive orientation discrimination with its corresponding identification and detection tasks.
Comparisons between tasks within each triad isolate attention to orientation and, respectively, spatial or temporal
comparison. The subtraction of detection from simultaneous discrimination revealed activation of right fusiform,
right lingual, left precentral, left cingulate and left temporal cortex, in addition to right insula, cerebellum and left
thalamus. Only the fusiform, insular and precentral activations remained when the corresponding identification
was subtracted from simultaneous discrimination. In contrast, most of the non-visual activation sites remained
when simultaneous discrimination was compared with successive discrimination, which also revealed a left
lingual activation. These experiments provide further evidence for task-dependent processing in the human
visual system and suggest that the right fusiform cortex is involved in spatial as much as temporal comparisons.
Introduction
In a previous experiment (Orban et al., 1997), we studied the human
brain activity related to orientation discrimination tasks. The stimulus
used was a single grating presented in the central visual field. The
subjects were instructed to identify the orientation of the stimulus
(orientation identification) or to compare the orientation of two
successive gratings (successive orientation discrimination). As control
tasks, we used detection and passive viewing of the stimulus. The
subtraction of detection from successive discrimination revealed
activation sites in the right posterior and middle fusiform gyrus,
among other regions. The right middle fusiform region remained
differentially active in successive discrimination when compared with
identification, suggesting that activity in this region is related to the
temporal comparison of orientation. This result shows – as the stimuli
were always the same – that the flow of information in the human
visual cortex depends not only on the attribute, as shown by Zeki
et al. (1991) and Corbetta et al. (1991), but also on the nature of the
task. We call this principle the task-dependency of visual processing.
In a companion paper (Vogels et al., 1997) we demonstrated,
using the lesion paradigm, a similar dissociation between successive
discrimination and identification in the visual system of the monkey.
An analogous dissociation has also been shown in the monkey
auditory system (Colombo et al., 1990). Finally, we have provided
evidence that task dependency in humans also applies to visual
attributes other than orientation, particularly direction and speed of
motion, again by comparing successive discrimination and identification (Cornette et al., 1998a; Orban et al., 1998). Hence, all currently
available evidence supporting the task-dependency of sensory system
processing, in both monkey and humans, derives from using successive
discrimination tasks. Therefore, we wondered whether we could
extend the principle to other discrimination tasks, notably simultaneous discrimination, rather than successive discrimination. This was
the primary aim of the experiments reported here.
In the present study we used pairs of gratings as stimuli and
subjects were required to compare the orientations of the two gratings
across space, to identify the orientation of one of the gratings or
to detect the stimulus, corresponding to simultaneous orientation
discrimination, orientation identification, and detection, respectively.
In addition we replicated the three tasks, successive orientation
discrimination, orientation identification, and detection using a single
grating presentation like that of Orban et al. (1997). With this
Correspondence: Dr Guy A. Orban, as above. E-mail: guy.orban@med.kuleuven.ac.be
Received 19 February 1998, revised 24 June 1998, accepted 10 July 1998
3690 P. Dupont et al.
approach, we hoped to answer three questions. Firstly, can task
dependency also be demonstrated in the simultaneous orientation
discrimination? Secondly, which areas are involved in simultaneous
orientation discrimination and how are they related to the areas
involved in successive discrimination of grating orientation (Dupont
et al., 1993; Orban et al., 1997), and in particular, which areas are
involved in spatial comparison and how do they relate to those
involved in temporal comparison? The third issue concerned the
replication of the results of Orban et al. (1997) in a larger sample of
subjects of both genders.
All stimuli used in the present study consisted of gratings. The
presentation of such stimuli can be controlled very carefully, so that
subjects can make use of only a single cue, orientation, to make their
discrimination. The use of simple stimuli and simple tasks has
advantages in the interpretation of human PET activation studies
since complex visual stimuli may elicit cognitive processes beyond
the explicit requirements of the task performed (Sergent, 1994).
Materials and methods
Subjects
Twenty-four (13 male, 11 female) volunteers, aged between 20 and
28 years, participated in this experiment. All subjects were strictly
right-handed as judged by the Edinburgh inventory, drug-free, had
no neurological or psychiatric history, had normal or corrected-tonormal vision and a normal brain structure as visualized with MRI.
All scanning procedures were undertaken with the understanding and
written consent of each participating subject, in accordance with the
Declaration of Human Rights, Helsinki 1975. The study has been
approved by the Ethical Committee of the medical school, Katholieke
Universiteit Leuven.
Stimulus characteristics
The stimulus consisted of square wave gratings presented for 300 ms
on an Atris monitor (70 Hz) at 114 cm. The subjects viewed the
display binocularly in a dimly lit room (0.075 cd/m2). The grating’s
mean luminance was 23.1 cd/m2, its contrast 90%, and its cycle width
was 0.5° (2 cycles/deg). The grating’s phase was randomized between
trials and noise was superimposed on the edges of the bars to eliminate
cues other than orientation. Two different types of stimulus displays
were used: (i) a pair of gratings (diameter 2.8°) with their centres
vertically aligned at a distance of 3.75° and (ii) a single grating
(diameter 4°). The size of the two gratings presented in tandem was
adjusted so that the sum of their areas matched the area of the
single grating.
Subjects were instructed to fixate on a virtual point halfway
between the two gratings for the first stimulus display or to fixate on
the centre of the single grating for the second. During the training
sessions (see below) the fixation point was present. It was omitted
during scanning because it has been claimed that the presence of a
fixation point decreases responses of visual neurons to a visual
stimulus (Richmond & Sato, 1987). Fixation during the scanning was
controlled by electro-oculography (EOG). There were no saccades
over 2° nor were there detectable slow eye movements, despite the
absence of a fixation point during PET scanning.
The stimulus rate was 50 stimuli/min. This is lower than in the
Orban et al. (1997) study, but subjects experienced great difficulty in
achieving acceptable performance in the simultaneous discrimination
at faster rates.
Visual tasks
There were three separate tasks: (i) a same–different task involving
a comparison, (ii) an identification task and (iii) a detection task.
Responses were given by pushing response keys followed by auditory
feedback in all tasks. in the detection task, the subject was instructed
to push one of the two keys at random. In the discrimination tasks
the response given was conditional on the stimulus displayed. Each
subject had to perform all three tasks using both types of stimulus
displays, resulting in a total of six different conditions (see Fig. 1).
Only two orientations were presented, the vertical and a near-vertical
(oblique) orientation, in each condition.
Using stimulus display 1 (paired gratings), the subjects performed
the tasks listed below.
1 A simultaneous orientation discrimination or spatial same–different
task (SSD): the subject had to indicate whether or not the two gratings
had the same orientation.
2 A spatial identification task (SID): the subject had to identify the
grating which had an oblique orientation (in every trial one and only
one grating had an oblique orientation). This task is similar to that
used earlier in cat and monkey behavioural studies (Orban et al.,
1990; Vogels et al., 1997) and can be accomplished by attending to
the orientation of one of the two gratings.
3 A detection task (SDET): the subject had simply to detect the
presence of the stimulus.*
Furthermore, using stimulus display 2 (single grating), subjects
performed three similar tasks described below.
4 A successive orientation discrimination or temporal same–different
task (TSD): the subject had to indicate whether or not the two
successive gratings had the same orientation.
5 An identification task (TID): the subject had to identify the
orientation of the grating as vertical or oblique. To distinguish this
task from SID we designated it TID.
6 A detection task (TDET): the subject had merely to detect the
presence of the stimulus. To distinguish this task from SDET, it was
labelled TDET.*
In all tasks the stimulus was present for 300 ms during each trial.
The 600 ms response window started from the onset of the stimulus
for most discrimination tasks (SSD, SID and TID), but from the onset
of the second stimulus for the successive same–different task (TSD).
For the detection tasks the 400-ms window started from the onset of
the stimulus for the detection tasks. In the detection tasks, we used
a random intertrial interval (temporal jitter of 350 ms). In the TSD
and SSD tasks, stimuli were randomized in such a way that subjects
could solve the task only by making a comparison. Tasks 4–6 are
exactly the same as those used in Orban et al. (1997), except that in
TDET subjects choose between two response keys rather than using
the right key exclusively as in Orban et al. (1997). The orientation
differences ranged from 1° to 3° and were adjusted to yield similar
levels of performance in all four discrimination tasks.
Data acquisition
Subjects were trained in two sessions on different days. During the
first session subjects practised until they could perform each of the
orientation discrimination tasks at the level of at least 75% correct.
In this session the orientation difference was set at 4°. During the
second session, the subjects were trained to perform the tasks at
*These tasks are not genuine detection tasks in the sense that subjects are not
required to distinguish between presence or absence of a stimulus. The
terminology is used for consistency with previous publications (Orban et al.,
1997; Cornette et al., 1998a).
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
Cerebral activation by orientation discrimination 3691
FIG. 1. The six different tasks: stimulus and response window timing in (A) detection of a single central grating (TDET), (B) identification of grating orientation
(TID), (C) successive orientation discrimination or temporal same-different (TSD), (D) detection of a pair of paracentral gratings (SDET), (E) spatial identification
of orientation (SID), (F) simultaneous orientation discrimination or spatial same-different (SSD).
smaller orientation differences. The difference was decreased in steps
of 1° until performance dropped below 75% on that task. The final
orientation difference ranged between 1° and 3°, depending on the
subject and on the task. The orientation differences selected for the
PET session were chosen to be close to the just noticeable difference
level to increase the level of attention, hence maximizing the demands
on the visual system, and to equate performance across tasks.
In a third session, the subject had to perform the tasks while lying
in the PET scanner (Siemens-CTI 931/8/12). The subjects viewed the
stimuli in a dimly lit room at a distance of 114 cm. The order of
tasks was randomized according to a Latin square design. The head
of the subject was immobilized with a foam headholder (Smither
medical products, Akron OH, USA). The start of the task coincided
with the start of the injection of 50 mCi (1.85 GBq) H215O. The
injection lasted 12 s and the scan began when activity reached the
brain as evidenced by the sharp rise in measured counts (around 30 s
post-injection). Duration of the scan was 40 s. Each subject underwent
six emission scans separated by a 15-min time interval to allow the
tracer to decay.
The PET scanner measured 15 planes (slice thickness 6.75 mm)
parallel to the inferior orbito-meatal plane. Prior to the emission
scans, a transmission scan was acquired which was used to correct
for attenuation. The corrected images were reconstructed using filtered
back projection with a Hanning filter with a cut-off frequency of
0.5 cycles/pixel. Each reconstructed image represents the radioactivity
distribution during the measurement which is related to the regional
cerebral blood flow (rCBF).
After the PET scanning, each subject underwent an MRI (magnetic
resonance imaging) scan. The MRI images were acquired using
a three-dimensional Magnetization-Prepared RApid Gradient Echo
(MPRAGE) sequence. Acquisition parameters were: repetition time
10 ms, echo time 4 ms, flip angle 8°, field of view 256 mm, acquisition
matrix 256 3 256. The 3D volume had a thickness of 160 mm,
partitioned into 128 sagittal slices.
Data analysis
The data were analysed with statistical parametric mapping (using
the SPM software, version SPM95, from the Wellcome Department
of Cognitive Neurology, London, UK) implemented in MATLAB
(Mathworks Inc. Sherborn MA, USA). Statistical parametric maps
are spatially extended statistical processes that are used to characterize
regionally specific effects in imaging data. Statistical parametric
mapping combines the general linear model (to create the statistical
map or SPM) and the theory of Gaussian fields to make statistical
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
3692 P. Dupont et al.
TABLE 1. Processes involved in the different tasks
Process
SSD
SID
SDET
TSD
TID
TDET
Fixation
Pre-attentive processing auditory stimulus
Pre-attentive processing visual stimulus
Attention to visual stimulus
Decision and motor selection
Motor response
Attention to orientation
Spatial attention
Comparison with internal standard
Temporal comparison
Spatial comparison
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1*
1*
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
*Half the rate of the other tasks.
TABLE 2. Performance of the subjects
Mean
SD
SSD
SID
SDET
TSD
TID
TDET
77.1
9.7
86.1
7.2
94.3
5.3
85.5
4.8
85.5
8.6
94.2
8.8
inferences about regional effects (Friston et al., 1991 1994; Worsley
et al., 1992).
Spatial realignment and normalization
The scans from each subject were realigned using the first scan as a
reference. The six parameters of this rigid body transformation were
estimated using a least-square approach (Friston et al., 1995a). This
approach is based on an approximate linear relationship between the
images and their partial derivatives with respect to parameters of the
transformation. Following realignment, all images were transformed
into a standard space (Talairach & Tournoux, 1988). This normalizing
spatial transformation matches each scan (in a least square sense) to
a reference or template image that already conforms to the standard
space. The procedure involves a 12 parameter affine (linear) and
quadratic (non-linear) 3-dimensional transformations. This is followed
by a 2-dimensional piece-wise (transverse slices) non-linear matching,
using a set of smooth basis functions that allow for normalization at
a finer anatomical scale (Friston et al., 1995a).
Again the parameters were estimated using standard least squares
after linearizing the problem. As a final preprocessing step the images
were smoothed using a Gaussian kernel [20 3 20 3 12 mm3 at full
width at half maximum (FWHM)].
Statistical analysis
After specifying the appropriate design matrix, the condition, subject,
and covariate effects were estimated according to the general linear
model at each and every voxel (Friston et al., 1995b). The design
matrix included global activity as a confounding covariate and this
analysis can therefore be regarded as an ANCOVA (Friston et al., 1990).
To test hypotheses about regionally specific condition effects, the
estimates were compared using linear contrasts. The resulting set of
voxel values for each contrast constitute a statistical parametric map
of the t statistic SPM{t}.The SPM{t} were transformed to the unit
normal distribution (SPM{Z}) and thresholded at 4.05 (P , 0.05 after
the correction for multiple testing as used in SPM95).
The final image resolution was estimated at 20.2 3 25.2 3 18 mm3
full width at half maximum. The analysed brain volume extended
from z 5 –24 mm to z 5 156 mm with respect to the AC–PC plane.
To test the a priori hypothesis generated by our previous experiment
(Orban et al., 1997), the maximum Z-values in anatomically constrained regions (a sphere with radius 12 mm around the local maxima
found in our previous experiment) were used and P-values were
corrected by dividing the uncorrected value by the number of regions
examined (Bonferroni correction).
Planned linear contrasts
In the six behavioural tasks, a number of processes can be distinguished
(Table 1). The four discrimination tasks (SSD, SID, TSD, TID) share
featural attention to orientation as a component. Both the SSD and
SID tasks involve spatial attention as a component. The TSD task
involves a temporal comparison while the optimal strategy for TID
is the comparison with an internal standard (Vogels & Orban, 1986).
The SSD on the other hand involves a spatial comparison, while the
SID can be solved by identifying the orientation of one of the gratings.
All conditions share the fixation and pre-attentive processing of
auditory and visual inputs. Furthermore, they share attention to a visual
stimulus, movement selection, and motor response. The movement
selection in the detection tasks is made randomly, in contrast to the
discrimination tasks, in which it is conditional on the visual stimulus.
In all tasks except TSD, the rate of movement selection and the
motor response were equal (50 responses per minute). In the TSD
task, both were half the response rate of the other tasks, because
subsequent stimuli have to be compared before a response can be made.
The following subtractions were planned: (1) TSD–TDET and (2)
TSD-TID to replicate the results of our previous experiment (Orban
et al., 1997). (3) SSD–SDET and (4) SID–SDET to study the areas
active in simultaneous orientation discrimination and identification
tasks.
To test the hypothesis of task dependency, we studied the contrasts
(5) SID–SSD and (6) SSD–SID, the latter isolating the spatial
comparison component.
Finally, to compare the simultaneous discrimination task and the
successive discrimination task, we used the contrasts (7) (SSD–
SDET)–(TSD–TDET) and (8) (TSD–TDET)–(SSD–SDET). Notice
that SSD and TSD could not be compared directly since they differ
in visual input. This factor is controlled in subtractions (7) and (8)
by referring each discrimination to its detection task. These last two
subtractions also provide an additional test of task dependency. Since
testing for differences of differences is always less sensitive, we
performed these subtractions only in voxels in which the same–
different tasks yielded more activity than their detection counterparts,
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
Cerebral activation by orientation discrimination 3693
TABLE 3. Temporal same-different vs. detection (TSD–TDET)
Brain region
Occipital
right fusiform gyrus BA19/37
right fusiform gyrus BA19
Frontal
left middle frontal gyrus BA9
left inferior frontal gyrus BA44
Subcortical
right lateral cerebellum/fusiform gyrus BA37
Occipital
1* right fusiform gyrus BA19/37
2 right lingual gyrus BA19
Frontal
3 left precentral gyrus BA6
4 right insula
5 right insula
6 left cingulate gyrus BA32
Temporal
7 left temporal gyrus BA22
Subcortical
8 vermis
9 left thalamus
Present experiment
Coordinates
(x, y, z)
Coordinates
(x, y, z)
Z-score
40, –62, –12
20, –78, –12
40, –66, –12
30, –72, –12
4.77
2.59
–42, 32, 32
–54, 12, 24
–46, 24, 24
–48, 22, 24
3.18
3.39
42, –48, –24
40, –56, –16
3.75
TABLE 5. Spatial identification vs. detection (SID–SDET)
TABLE 4. Spatial same-different vs. detection (SSD–SDET)
Brain region
Orban et al. (1997)
Coordinates (x, y, z)
Z-score
Brain region
40, –62, –12
16, –80, –8
5.07
5.14
Parietal
10 left precuneus BA7
Subcortical
9 left thalamus
–56, 2, 24
34, 14, 20
28, 18, 8
–8, 6, 40
5.27
5.10
5.21
4.87
–58, –6, 8
4.78
8, –70, –16
–14, –16, 0
5.98
4.41
*Numbers refer to the different significant activation sites identified in the
present experiments.
i.e. the voxels significant in the subtraction [(SSD 1 TSD)–
(SDET 1 TDET)].
Results
Performance of the subjects (Table 2)
During the experiment, we adapted the orientation difference in each
orientation discrimination task to try to equalize the perceptual
demands as much as possible. The mean orientation differences were
1.9 and 2.5° for the TID and SID tasks, respectively (range for both:
1°–3°) and 2.4 and 2.7° in the TSD and SSD tasks, respectively
(range for both 2–3°). Performance in the detection tasks was clearly
better than in the discrimination tasks, as could be expected. The
performance during the simultaneous orientation discrimination task
was not as high as in the spatial identification task, probably due to
the higher computational demands in SSD, which were incompletely
compensated by the increase in orientation difference.
Comparison of the successive discrimination task with the
detection task using the single grating (Table 3)
In our previous experiment (Orban et al., 1997) we found five regions
significantly activated by the temporal same–different task when
compared with the detection task. One of these regions, right fusiform
gyrus, remained active when TSD was compared with TID. In the
present experiment, we tested the replicability of these results. In
Table 3, the different regions are listed for the subtraction TSD–
TDET and the maximum Z-score in a sphere with radius 12 mm
Coordinates (x, y, z)
Z-score
–12, –66, 40
4.21
–14, –16, 0
4.67
around these points are given. We applied a Bonferroni correction to
the P-value by dividing the uncorrected P-value (P 5 0.05) by the
number of regions (5 1 1). This corresponds to a threshold in our
case of Z 5 2.39. As a result, we find differential activation in two
visual regions, both located in the right fusiform gyrus, one posteriorly
in BA19 and one in the middle at the occipitotemporal junction in
BA19/37. Also, the activation of two frontal regions was replicated.
Finally, we observed differential activation for the subtraction TSD–
TDET in a region within 12 mm of the right lateral cerebellum site
of the previous experiment. Its coordinates however, suggest that this
activation lies within the right fusiform gyrus. This was confirmed
by comparison of these coordinates with the anatomical MRIs of the
single subjects. In almost every subject (20/21 studied) this voxel
was indeed located within the right fusiform gyrus. While most, if
not all, the findings of Orban et al. (1997) were replicated for the
subtraction TSD–TDET, the difference in activity in right fusiform
cortex in TSD and TID did not reach significance in the present
experiment (38, –50, –12, Z 5 1.47). This suggests that orientation
identification activated fusiform cortex more in the present study than
in the previous.
Comparison of the simultaneous discrimination task with the
corresponding detection task (Fig. 2, Table 4)
Comparing the spatial same–different task (SSD) with the corresponding detection task (SDET) revealed a number of significant activation
sites. Two such sites were located in occipito-temporal cortex: one
in the right fusiform gyrus and another in the right lingual gyrus. In
the frontal cortex, activation sites were located in the left precentral
gyrus, the right insula, and the left cingulate gyrus. Auditory association cortex was also activated, perhaps because of increased attention
to auditory feedback in the most demanding task. Finally, two
subcortical structures, the posterior vermis and the left thalamus were
significantly activated. The activation site in the right fusiform gyrus
is the same as in the subtraction TSD–TDET (Table 3). This region
is activated by all discrimination tasks compared with their detection
task, except for the SID, as can be seen in Fig. 4 showing the
functional profiles of visual activation sites. This profile clarifies the
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
3694 P. Dupont et al.
FIG. 2. Statistical parametric maps showing regions differing between spatial same-different as the experimental condition and detection as control condition
(A) and between spatial identification and detection (B). Yellow and black pixels indicate increased rCBF in the experimental condition compared with the
control condition which was significant at the P , 0.001 uncorrected and the P , 0.05 corrected level, respectively. These regions are superimposed on
horizontal sections, from –20 mm below to 48 mm above the anterior commissure–posterior commissure (AC–PC) line, through standard average magnetic
resonance imaging scans to show the anatomical brain features. L in the lowest section indicates the left side of the brain.
failure to observe any activation in right fusiform gyrus in the
subtraction TSD–TID: it is not due to a reduced activity in TSD, but
to an increase in activity in TID which was not observed in the study
of Orban et al. (1997). The activation of precentral, cingulate and
thalamic regions might be related to the spatial attention required by
the simultaneous discrimination tasks. Activation of these regions has
been observed in other studies of spatial attention (Nobre et al., 1997;
Vandenberghe et al., 1996, 1997). Their profiles (Fig. 5) indeed
confirm that they are active in both of the discrimination tasks using
two gratings, with the exception of the precentral focus.
Comparison of the spatial identification task with the
corresponding detection task (Fig. 2, Table 5)
Two activation sites were observed in this comparison, both located
in the left hemisphere: the precuneus and the thalamus. The latter
activation site was identical to that in the comparison between the
simultaneous discrimination task and the detection task. Activation
in both regions most likely reflects the spatial attention required in
solving the task, as they are close to regions previously shown to be
involved in spatial attention (Vandenberghe et al., 1996, 1997). As
can be seen on the functional profile of the precuneus (Fig. 5), this
region was also weakly active in the simultaneous discrimination task.
Comparison between the simultaneous discrimination task
and the spatial identification task using the same stimulus
(Fig. 3, Table 6)
When we compare the two orientation discrimination tasks using the
paired gratings, we observe that right fusiform gyrus, left precentral
gyrus and right insula were more active in the same–different task
than in the identification task. On the other hand, the right frontal
gyrus and the left insula were more active in the identification task
with respect to the same–different task. The activity profile of the
left insula (Fig. 6) indicates that the activation observed in the
subtraction SID—SSD is in fact due to a deactivation in SSD. This
was also the case for the right frontal gyrus (not shown). Because
the stimuli were the same in the two conditions but the nature of the
task was different, we have yet another example of the taskdependency of visual processing, one which again involved the right
fusiform gyrus, as in Orban et al. (1997) and Cornette et al. (1998a).
Comparison between the same–different tasks compared with
their corresponding detection task (Fig. 3, Table 7)
We are interested only in the voxels which were significantly
activated in the main effect of same-different with respect to detection
[(SSD 1 TSD)-(SDET 1 TDET)[ and which show a significant inter-
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
Cerebral activation by orientation discrimination 3695
FIG. 3. Statistical parametric maps showing regions differing between spatial same-different and spatial identification (SSD–SID) (A) and between the same–
different tasks compared with their corresponding detection task (SSD – SDET) – (TSD – TDET) (B). Blue and black pixels indicate decreased rCBF in the first
condition with respect to the second which was significant at the P , 0.001 uncorrected and the P , 0.05 corrected level, respectively. Same conventions as in Fig. 2.
action effect between task and stimulus [(SSD-SDET)-(TSD-TDET)].
If two contrasts are orthogonal, the probability used to reject the null
hypothesis in a combined set of two contrasts is approximately equal
to the product of the P-values obtained in each of the contrasts
(Fletcher et al., 1995). We selected those voxels which showed a
main effect of task (uncorrected P , 0.01) and an interaction effect
between task and stimulus (uncorrected P , 0.01), which therefore
have an overall significance of an uncorrected P , 0.0001. With this
analysis, we found activity in the left lingual gyrus, the left precentral
gyrus, left cingulate gyrus, left inferior frontal gyrus, right insula,
left temporal gyrus, vermis and left thalamus to be stronger in the
simultaneous same–different task compared with the temporal same–
different task. This was also the case in the right lingual gyrus
(Table 4) which reached Z 5 2.68. This site is not listed in Table 7,
however, because it did not appear as a local maximum, due to the
close proximity of the strong vermis activation (Fig. 3). Because most
regions were also differentially active in the subtraction SSD–SDET,
this reflects increased activity in the simultaneous discrimination task.
In the left lingual gyrus however, the differential activity largely
reflects a deactivation in the successive discrimination (Fig. 4) Only
in the left middle occipital gyrus was activity stronger in the temporal
same–different task than in its spatial counterpart. Again this seems
to result from a deactivation in the simultaneous discrimination, as
much as an activation in the successive discrimination (Fig. 4). These
TABLE 6. Spatial same-different vs. spatial identification (SSD–SID)
Brain region
Occipital
1 right fusiform gyrus BA19/371
Frontal
3 left precentral gyrus BA6
4 right insula
11 right middle frontal gyrus BA46
12 left insula
Coordinates (x, y, z)
Z-score
46, –66, –12
4.08
–56, 2, 24
34, 16, 16
12, 30, 24
–30, 24, 8
4.50
4.92
–5.72
–4.13
*Normal typeface indicates regions activated in SSD compared with SID.
Italics: regions deactivated in the SSD relative to SID.
differences in the activities of right fusiform cortex, left and right
lingual and middle occipital regions are further evidence of task
dependent processing in the human visual system. These last two
comparisons also show that many more brain regions were engaged
in the simultaneous than in the successive discrimination task, in line
with the greater computational demands of the former task compared
with the latter. This again extends our previous findings that the
computationally most demanding task engages the largest network
(Orban et al., 1997).
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
3696 P. Dupont et al.
TABLE 7. Comparison between the same–different tasks compared with their
corresponding detection task (SSD – SDET) – (TSD – TDET) masked with
(SSD 1 TSD) – (SDET 1 TDET) using P , 0.01 as threshold
Brain region
Coordinates (x, y, z)
Occipital
13 left lingual gyrus BA19*
–26, –80, –12
14 left middle occipital gyrus BA19 –50, –70, –4
Frontal
3 left precentral gyrus BA6
–56, –4, 24
6 left cingulate gyrus BA32
–8, 6, 40
15 left inferior frontal gyrus BA45/46–52, 26, 12
4 right insula
32, 16, 24
Temporal
7 left temporal gyrus BA22
–58, –8, 8
Subcortical
8 vermis
6, –66, –16
9 left thalamus
–10, –14, –4
Z-score
2.35
–2.45
4.69
4.49
2.65
2.47
4.07
4.68
3.24
*Normal typeface indicates regions in which the difference of activity in
SSD–SDET exceeds that in TSD–TDET, italics indicate region in which the
opposite is true.
FIG. 5. Functional activity profiles of four regions tentatively identified as
involved in visuo-spatial attention. Same conventions as in Fig. 4.
FIG. 4. Functional activity profiles of four visual activation sites. The adjusted
rCBF (in arbitrary units) is plotted for the six conditions (from left to right:
SSD, SID, SDET, TSD, TID, TDET). Vertical lines indicate the standard error
on the mean (SEM).
Discussion
Our results provide a definitive answer to our first question: taskdependent processing in the human visual system can indeed be
demonstrated with simultaneous discrimination tasks (Tables 6 and
7). Secondly, our results show that although the network engaged in
simultaneous orientation discrimination is partially distinct from that
engaged in successive discrimination (Table 7), the cortical region
concerned with spatial comparison, the right middle fusiform gyrus
(Table 6), is also involved in temporal comparison (Orban et al.,
1997). Finally the answer to the third question, that concerning the
replicability of the results of Orban et al. (1997), is a qualified
affirmative, due to an unanticipated increase in fusiform activity
during the identification of the orientation of single gratings.
Visual activation sites
Little or no functional imaging work has been carried out on
orientation discrimination outside our group, so the most closely
related studies from other groups are those investigating shape
discrimination and analysis.
As mentioned above, the right middle fusiform cortex (46, –66,
–12) involved in simultaneous orientation discrimination, and in
particular its spatial comparison component, is exactly the same
region as that involved in the temporal comparison of orientation
(40, –62, –12, Orban et al., 1997), and, in all likelihood, of motion
direction (34, –58, –8, Cornette et al., 1998a), and speed of motion
(48, –62, –12, Orban et al., 1998).
One could argue that the spatial comparison in the simultaneous
discrimination can be converted into a temporal comparison by shifts
of the attention focus. Hence the activation of right middle fusiform
cortex could be due to the subjects somehow performing a temporal
comparison rather than a spatial one. This is highly unlikely. Indeed
the duration of stimulus presentation is barely sufficient for the
subjects to shift their attention from one peripheral grating to the
other (Weichselgartner & Sperling, 1987). Moreover, the many
significant differences in activation outside the fusiform gyrus between
successive and simultaneous discrimination (Table 7) indicate that
different cerebral networks are engaged in the simultaneous and
successive tasks.
The failure of the present experiments to replicate the involvement
of fusiform cortex in temporal comparison is due to a high level of
activity of this region during TID. Although the tasks were identical
in the two studies, there were minor differences in parameter settings.
Presentation rate was lower in the present experiments than in that
of Orban et al. (1997), which should, however, have little effect.
There was no explicit fixation point, making the concomitant fixation
task more difficult. In other experiments we have also seen recruitment
of fusiform cortex when the subject had difficulty in performing an
identification task (Cornette et al., 1998a). In this respect it is worth
noting that we (Cornette et al., 1998b) recently have replicated the
differential activation of the right fusiform cortex in the subtraction
TSD–TID, using parameter settings more similar to those used by
Orban et al. (1997).
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
Cerebral activation by orientation discrimination 3697
FIG. 6. Functional activity profiles of insular and cerebellar activation sites.
Same conventions as in Fig. 4.
The right middle fusiform region is posterior to a region (35,
–48, –11) described by Corbetta et al. (1991) as involved in successive
shape discrimination. It is also posterior to the region (40, –48, –12)
reported by Schacter et al. (1995) to be involved in computation of
representations of 3D objects. It corresponds rather well, however, to
a region (52, –58, –8) implicated in object recognition by Kosslyn
et al., 1994) and to a region (43, –61, –16) involved in object analysis
according to Kanwisher et al. (1997). It also closely matches the
right fusiform region (48, –60, –18), whose activity correlated with
presentation rate of ellipses irrespective of the categorization task
performed, according to Rees et al. (1997). Furthermore, it is close
to a region (50, –56, –10) described very recently by Faillenot, I.,
Decety, J. & Jeannerod, M. (unpublished) as involved in simultaneous
discrimination of 2D and 3D spatial object features. Finally, it is
worth mentioning that in inferotemporal cortex of the monkey,
neurons have been shown to give significantly different responses to
pairs of identical stimuli as compared with pairs of different stimuli
(Sato, 1995).
The other visual region engaged by the simultaneous discrimination
task, the right lingual gyrus (16, –80, –8), corresponds to the region
activated by an increased rate of presentation of single gratings (24,
–82, –16, Orban et al., 1997) and by presentation of a central grating
as compared with a peripheral one (24, –84, –16, Vandenberghe et al.,
1996). It also corresponds closely the right lingual region involved
in successive discriminations of direction (22, –86, –4, Cornette et al.,
1998a) and speed (16, –84, –8, Orban et al., 1998), as well as that
involved (weakly) in an object matching task (28, –86, –16, Köhler
et al., 1995). A symmetrically placed region in the left hemisphere
(–26, –80, –12) was revealed by comparing simultaneous and successive orientation discrimination (Table 7). Weak lingual activation in
the left hemisphere has also been observed in speed discrimination
tasks (–16, –84, – 12 and –14, –88, –16, Orban et al., 1998). The
right and left lingual activation also correspond closely to the right
and left posterior fusiform region (22, –82, –12 and –24, –86, –12)
shown by Price et al. (1996) to increase activity with increasing word
rate. Finally the left middle occipital region, which was more active
in successive discrimination than in simultaneous discrimination
(–50, –70, –4), is located symmetrically with respect to another region
found to be dependent on grating presentation rate (42, –72, –12) by
Orban et al. (1997).
This convergence among studies suggests that these activation sites
may well represent distinct extrastriate regions. For example, Cornette
et al. (1998a) have argued that the visual lingual region corresponds
to area V4v mapped in retinotopic experiments (Sereno et al., 1995;
DeYoe et al., 1996; Engel et al., 1997). More recent evidence suggests
that, since it represents central vision, the visual lingual region instead
corresponds to the recently mapped colour selective V8 (Tootell et al.,
1998). It should be noted, however, that McKeefry & Zeki (1997)
mapped a similar colour selective region and found it to be localized
on the posterior fusiform gyrus, rather than on the lingual gyrus.
Furthermore, these various regions seem to be involved, at least at
the present level of spatial resolution, in multiple computational
processes. For example, the right fusiform cortex seems to be involved
in shape processing as well as spatial and temporal comparisons of
different attributes. This is reminiscent of properties of monkey
cortical areas such as V4, which contains neuronal populations
selective for orientation, size, colour and direction of motion
(Desimone & Schein, 1987). The relatively close association of
activations in the right lingual gyrus and the middle fusiform cortex
of the same hemisphere suggests that these two regions are functionally
linked, and that the lingual region is the gateway to the middle
fusiform cortex, in the way V4 is for inferotemporal cortex in monkey
(Felleman & Van Essen, 1991). Finally, combination of the different
discrimination experiments lends increasing support to the view that
the right hemisphere dominates in simple visual discrimination tasks.
Task-dependent processing
The present experiments indeed provide further evidence for task
dependent processing in the human visual cortex. One could argue,
however, that the difference between SSD and SID in fusiform cortex
reflects the greater difficulty of the simultaneous discrimination task,
as subject performance in SSD was poorer than in SID. The activity
profile of the right middle fusiform cortex does not support this view.
If fusiform activity were merely related to difficulty, one would
expect its activity to be higher than detection, only in SSD. The
profile (Fig. 4), however, shows that there was also relative activation
in TSD (see also Table 3) and TID. Furthermore, the comparison
(Table 7) of simultaneous and successive discrimination provides
© 1998 European Neuroscience Association, European Journal of Neuroscience, 10, 3689–3699
3698 P. Dupont et al.
further support for the hypothesis of task-dependent processing. Thus
the present results extend the previous observations (Colombo et al.,
1990; Orban et al., 1997; Vogels et al., 1997; Cornette et al., 1998a),
in showing that the principle applies not only to both human and
monkey, to visual and auditory systems and to several visual attributes,
but also to discrimination tasks other than successive tasks. The
recent experiments of Faillenot et al. (1997) comparing grasping and
recognition of the same visual objects also provide evidence for the
same hypothesis, although the nature of the tasks compared was more
widely different than in the discrimination experiments.
Acknowledgements
Cerebellar activation
Abbreviations
One region where the networks engaged by successive and simultaneous orientation discrimination differ is the cerebellum (Table 8).
There is growing evidence for the participation of the cerebellum in
sensory discrimination distinct from the execution of motor responses
(Dupont et al., 1993; Orban et al., 1995; 1997; Gao et al., 1996;
Allen et al., 1997; Rees et al., 1997). The present experiments provide
additional information in the sense that different discriminations
engage different parts of the cerebellum independently of motor
responses. Indeed, the cerebellar activation, putatively attributed to
the vermis in the simultaneous discrimination is not involved in the
successive discrimination (Fig. 6). This cerebellar site (8, –70, –16)
is located very close to the more posterior sites engaged in counting
visual stimuli (5, –65, –12, Orban et al., 1995). Its functional profile
suggests that it is distinct from the midline cerebellar region engaged
in the bimanual responses used in detection tasks (2, –64, –16,
Cornette et al. (1998a) and 0, –68, –16, Orban et al., 1998). Finally,
the simultaneous discrimination site is also distinct from the cerebellar
regions engaged in attention to visual stimuli during discrimination
tasks (28, –54, –26, Rees et al., 1997).
It is not clear which computational operation/cognitive process the
cerebellar activation in the simultaneous discrimination corresponds
to (De Schutter & Maex, 1996). It has recently been suggested
(Desmond et al., 1997) that the cerebellum plays a role in verifying
the quality of the phonological trace in auditory working memory by
comparing activity in different cortical regions. This is unlikely to
be the case here, since stimuli were presented simultaneously. It is
noteworthy that the presentation rate in the present experiments had
to be limited to 50 trials/min because simultaneous discrimination
was almost impossible at faster rates. Thus it is conceivable that the
cerebellar activation reflects preparation for a sensory-based response
when a rapid response is critical, a view not dissimilar to that of Gao
et al. (1996). This has been disputed by Allen et al. (1997), on the
basis of visual stimulus counting experiments which also activate the
cerebellum (Orban et al., 1995; Allen et al., 1997). We (Orban et al.,
1995), have suggested, however, that the preparation for a rapid,
sensory-based response might be extended to internal actions such as
those required by counting. These involve selection and planning
operations, which they may share with explicit motor responses.
Preparation for internal actions might also explain the observation of
Kim et al. (1994) who observed greater dentate activation during the
performance of an ‘insanity task’ compared with that of a simple
visually guided task.
Whatever the exact role of the cerebellum in sensory discriminations
may be, the present experiments show that different discrimination
tasks using the same attribute engage different parts of the cerebellum,
just as they do for the visual system. Thus these experiments provide
further evidence for the generality of task-dependent processing in
the visual system and extend the role of right fusiform cortex to
spatial comparisons in addition to temporal comparisons.
BA
PET
rCBF
AC
PC
SSD
The technical help of S. Vleugels, L. Verhaegen, W. Costermans, M. De Paep,
T. De Groot, D. Crombez, P. Kayenbergh, G. Meulemans is gratefully
acknowledged. We are very grateful to G. Marchal for access to the MRI
facilities and to J. Michiels for his help with the MRI measurements. We are
much indebted to professor R. Frackowiak for making the SPM software
available. This work was supported by grants 9.0007.88, 3.0043.89 and
3.0095.92 from the National Research Council.
R.V. is a research associate of the National Research Council. P.D. and
A.R. are postdoctoral fellows of the National Research Council and R.V.D.B.
and L.C. are research assistants of the National Research Council.
SID
SDET
TSD
TID
TDET
EOG
MRI
FWHM
Brodmann area
positron emission tomography
regional cerebral blood flow
anterior commissure
posterior commissure
spatial same different or simultaneous orientation
discrimination task
spatial identification task
detection task (pair of gratings)
temporal same different or successive orientation
discrimination task
identification (single grating)
detection task (single grating)
electro-oculography
magnetic resonance imaging
full width at half maximum
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