Erratum to “Voluntary orienting is dissociated from target detection in human posterior parietal cortex”

Abstract

Human ability to attend to visual stimuli based on their spatial locations requires the parietal cortex. One hypothesis maintains that parietal cortex controls the voluntary orienting of attention toward a location of interest. Another hypothesis emphasizes its role in reorienting attention toward visual targets appearing at unattended locations. Here, using event-related functional magnetic resonance (ER-fMRI), we show that distinct parietal regions mediated these different attentional processes. Cortical activation occurred primarily in the intraparietal sulcus when a location was attended before visual-target presentation, but in the right temporoparietal junction when the target was detected, particularly at an unattended location.

Full text

292 nature neuroscience • volume 3 no 3 • march 2000
articles
Acute structural damage to the right temporoparietal cortical
junction (TPJ; inferior parietal lobule and superior temporal
gyrus) in humans produces a complex clinical syndrome charac-
terized by the inability to attend and respond to objects positioned
in the left visual field (unilateral visual neglect)1–3. Some symp-
toms of neglect may reflect a deficit in reorienting attention toward
new stimuli in the visual field opposite to the lesion (contrale-
sional field)4–7. Patients with parietal lesions can detect visual stim-
uli in the contralesional field when correctly cued to their
locations, but they are slow or fail to detect the same stimuli when
attending to other locations4. This deficit is more severe after right
than left parietal lesions5, and is localized to the TPJ7. These find-
ings suggest that the posterior parietal cortex near TPJ may be
critical for reorienting the focus of attention toward visual stimuli
appearing at unattended locations (reorienting hypothesis).
Other data suggest that posterior parietal cortex near/along
the intraparietal sulcus (IPs), which separates the superior from
the inferior parietal lobule, is involved in voluntarily directing
attention to a spatial location (voluntary orienting hypothesis).
Neurons in the IPs increase firing rate when a monkey attends
to a location while preparing a response8–11. Human functional
brain imaging shows activations in the IPs (and superior pari-
etal lobule) when observers voluntarily pay attention to and detect
peripheral visual stimuli, with or without concurrent eye move-
ments12–16. It is unknown to what extent areas in human and
monkey IPs are homologous.
These complementary functional anatomical theories (reori-
enting to targets in the TPJ and voluntary orienting in the IPs)
make specific predictions about which regions should be acti-
vated while attending to a spatial location and, subsequently,
when detecting a visual target there. If IPs is preferentially
involved in voluntary orienting, then it should be activated when
an observer attends to a location before presentation/detection
of a visual target. If TPJ is necessary for reorienting to a visual
target, then its activation should follow the presentation/detec-
tion of the target, particularly when it is presented at an unat-
tended location. We tested these predictions using ER-fMRI and
an ANOVA-based procedure17. This method has two important
characteristics: it can separate the responses to events presented
within the same cognitive trial, and it is sensitive to differences
in both magnitude and timing of responses. Unlike other pub-
lished ER-fMRI methods18–21, this method makes no assump-
tions about the shape of the underlying response function.
R
ESULTS
Normal observers were given a cue indicating the most likely
location of a subsequent target stimulus they were required
to detect, according to a protocol modified from a published
procedure4. The stimulus display consisted of a central fixa-
tion cross flanked on either side by square boxes. The length of
each arm of the central fixation cross subtended 16 minutes
of visual angle. The boxes (size, 1°) were placed at 3.3° of visu-
al angle to either side of the fixation spot. Accurate fixation
of the central cross-hair was emphasized throughout the
experiment. At the beginning of a trial, a cue arrow pointing
to the left or right box was superimposed on the fixation cross.
The arrow indicated the most likely location of a subsequent
target stimulus, and leftward or rightward arrows were equal-
ly probable. The cue arrow remained on the screen for one
MR frame (2360 ms; cue period).
The sequence of events following presentation of the cue
arrow depended on the type of trial. On a cue trial (20% of the
Voluntary orienting is dissociated
from target detection in human
posterior parietal cortex
Maurizio Corbetta1,2,3, J. Michelle Kincade1,4, John M. Ollinger2, Marc P. McAvoy2and Gordon L.
Shulman1
1Department of Neurology and Neurological Surgery, Washington University School of Medicine, 4525 Scott Avenue, St. Louis, Missouri 63110, USA
2Mallinckrodt Institute of Radiology, Washington University School of Medicine, 4525 Scott Avenue, St. Louis, Missouri 63110, USA
3Department of Anatomy and Neurobiology, Washington University School of Medicine, 4525 Scott Avenue, St. Louis, Missouri 63110, USA
4Department of Psychology, Washington University School of Medicine, 4525 Scott Avenue, St. Louis, Missouri 63110, USA
Correspondence should be addressed to M.C. (mau@npg.wustl.edu)
Human ability to attend to visual stimuli based on their spatial locations requires the parietal cortex.
One hypothesis maintains that parietal cortex controls the voluntary orienting of attention toward a
location of interest. Another hypothesis emphasizes its role in reorienting attention toward visual
targets appearing at unattended locations. Here, using event-related functional magnetic resonance
(ER-fMRI), we show that distinct parietal regions mediated these different attentional processes.
Cortical activation occurred primarily in the intraparietal sulcus when a location was attended before
visual-target presentation, but in the right temporoparietal junction when the target was detected,
particularly at an unattended location.
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nature neuroscience • volume 3 no 3 • march 2000 293
trials), the trial ended immediately after cue presentation. The
end of the trial was signaled by a change in the color of the fix-
ation cross from green to red. During a cue trial, a subject shift-
ed attention toward the location indicated by the cue and
maintained it there until the end of the trial. On a noise trial
(20% of the trials), the cue period was followed by a test period
lasting 2 MR frames (4720 ms) in which no target was present-
ed. During noise trials, the subject presumably shifted and
maintained attention on the cued location for a longer time
than during a cue trial (7080 versus 2360 ms; Fig. 1). On a valid
trial (44% of the trials), the cue period was followed by a test
period during which a target appeared at the location indicated
by the cue. The target was a white asterisk that appeared in one
of the square boxes for 100 ms. On an invalid trial (16% of the
trials), a target appeared during the test period at the uncued
location. The cue arrow correctly predicted the target location
on 73% of the trials in which a target was presented. These four
trial types (cue, noise, valid, invalid) were randomly intermixed.
Subjects were instructed to press a button as quickly as possi-
ble upon detection of the target and to withhold responses on
cue or noise trials.
articles
We used this protocol during whole-brain measurements of
blood oxygenation level dependent (BOLD) responses on a
Siemens Vision 1.5 T magnet. The time course of the BOLD
response for each trial type and each trial period (cue period, test
period/noise, test period/valid, test period/invalid) was estimat-
ed in each subject using linear regression. Pixel-wise and region-
al ANOVAs were used for appropriate statistical contrasts17 (see
Methods).
Behavior
Reaction times for target detection were faster on valid than
invalid trials (380 ms versus 426 ms, F1,11 = 21.92, p = 0.0007),
indicating that subjects used the cue arrow to attend to the loca-
tion of the target. No responses were recorded during noise and
cue trials.
Imaging
During the cue period, a series of ventral and dorsal visual
regions were active; these included bilateral anterior fusiform
(Fus; x,y,zatlas coordinates, 35, –57, –20, right; –31, –55, –16,
left), lateral occipital (LO;–31, –83, 0, left; 27, –87, 0, right)22,23,
Fig. 1. Display, trial types and MR design. Each trial
lasted between 4 and 7 MR frames, and each MR
frame was 2.36 s long. MR frames are indicated by
elongated rectangles below displays. In a cue trial, a
cue arrow was presented for 1 MR frame (cue
period) at fixation (black rectangle) followed by an
intertrial interval (ITI) period signaled by a change in
the color of the fixation point (from green to red).
The ITI period lasted for 2, 3 or 4 MR frame duration
(white rectangles). A two-frame ITI is shown. In a
valid trial, the cue period was identical. During the
test period (2 MR frames or 4.72 s; crossed rectan-
gles), after a randomly selected time between
1500–3000 ms, a 100-ms target stimulus (asterisk)
was flashed in the box cued by the arrow. Subjects
indicated target detection with a key press. The ITI
period followed. Invalid trial, same as valid trials
except that the target was flashed at the uncued box
location. Noise trial, same as valid trials, except that
no target was flashed during the target period.
Table 1. List of parietal regions during cue and target periods (averaged over valid and invalid targets) and showing
significant validity effect.
Cue Target Valid – invalid
Regions xyzZ-score xy zZ-score xy zZ-score
L pos IPS –25 –67 48 7.58 –25 –65 48 6.94
L ant IPs –25 –57 46 7.55 –25 –57 42 7.27
L vIPS –23 –67 32 7.42
“–27 –75 26 7.23 –27 –77 20 7.16
R ant IPs 27 –59 52 6.75 33 –51 48 7.84 39 –47 48 4.09
R pos IPs 21 –65 52 6.62
R vIPS 29 –71 22 6.01 27 –71 30 6.63
R IPL 53 –45 20 8.53 53 –49 30 5.12
R STG 51 –55 4 5.41 57 –45 12 4.44
R PC 7 –73 32 7.71 7 –75 34 4.28
L PC –9 –71 40 7.37 –5 –71 34 4.27
See Figs. 2 and 3 for anatomical labels. Coordinates (x, y, z) correspond to the Talairach atlas.
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294 nature neuroscience • volume 3 no 3 • march 2000
MT+ (–45, –69, –2, left; 45, –69, –4, right)24,25 and ventral
(vIPs), anterior (ant IPs) and posterior intraparietal sulcus (pos
IPs; Fig. 2a, left; see Table 1 for coordinates of parietal foci). No
significant activation was detected in the TPJ during the cue
period (Fig. 2a, right). The IPs activity did not reach the sur-
face of the inferior parietal lobule, but spread into the superior
parietal lobule. The vIPs region was located at the junction of
dorsal occipital and parietal cortex, just dorsal and anterior to
the V3A representation26.
The time course of the BOLD response during the cue peri-
od was more sustained within the intraparietal sulcus (posIPs,
antIPs, vIPs) than in occipital regions (Fus, LO, MT+; ANOVA
regions ×frame, F56,672 = 5.18, p = 0.0001). We compared a tran-
sient time course in two occipital regions (LO, MT+) with more
sustained time courses in antIPs and posIPs (Fig. 2b). The dif-
ference in response duration cannot be explained by a differ-
ence in the peak magnitude. LO and posIPs, for example,
showed similar peak magnitudes on frame 3, but different
response duration (ANOVA regions ×frames 3 and 4 only,
F1,12 = 24.3, p = 0.0003). Similarly, antIPs and MT+ had similar
magnitudes, but the response was more sustained in antIPs
(ANOVA regions ×frames 3 and 4 only, F1,12 = 7.22, p = 0.02).
Whereas transient time courses in occipital regions probably
reflect visual processes related to the presentation of the foveal
cue, the more sustained time courses in intraparietal cortex may
reflect longer times required for processing cues related to ori-
enting toward and maintaining attention at the cued location.
To further test this idea, we compared BOLD responses of these
regions during the noise period, in which subjects maintained
attention at a peripheral location for 4.72 seconds after the offset
of the cue arrow. Across all regions active during the noise peri-
od, only antIPs and vIPs showed sustained activity (ANOVA
regions ×frame, F77,924 = 7.80, p = 0.0001), with responses of the
left hemisphere more sustained than those of the right hemi-
sphere (Ant IPs, F7,84 = 3.76 p = 0.0014; vIPs, F7,84 = 5.40,
p = 0.0001). Hence, after the presentation of the cue arrow, some
IPs regions (antIPs, vIPs) showed a sustained BOLD response
that was maintained during the noise period, during which sub-
jects attended to the cued location for almost five seconds while
waiting for the target stimulus.
During the target period, many visual and motor regions
were active for both valid and invalid targets (Fig. 2c). All occip-
ital regions that were active during the cue period also respond-
ed to target presentation, and these responses were significantly
stronger for targets in the contralateral visual field. In parietal
cortex, significant responses were recorded in antIPs, posIPs,
vIPs, precuneus (Precun) and the TPJ, where the activation was
much stronger in the right than the left hemisphere (Fig. 2c;
Tab l e 1 ). BOLD responses in these parietal regions during the
cue and valid-target periods were compared by ANOVAs. Data
articles
Fig. 2. BOLD responses during cue and target periods. (a) ANOVA Fmap transformed to Zmap for the cue period (averaged over subjects, cue
direction). Left, sagittal slice 25 mm left of midline. Right, coronal slice 45 mm posterior to center of atlas space. Yellow lines indicate corresponding
planes of section. Parietal regions with sustained responses to cues are labeled in red: posIPs, posterior intraparietal sulcus; antIPs, anterior intrapari-
etal sulcus; vIPs, ventral intraparietal sulcus. The lateral occipital region LO (labeled in white) showed a transient response to the cue. (b) BOLD time
courses in different regions (averaged over subjects, cue direction and hemispheres) during the cue period. Response typically peaked two frames
after onset of cue arrow on frame 1. ITI began on frame 4. (c) Target period (averaged over subjects, valid and invalid targets). Parietal regions with
prevalent responses to targets are shown in red label: TPJ, temporoparietal junction; Precun, precuneus. Several motor-related responses are also
shown: SMcx, sensory-motor cortex; Put, putamen; Cbl, cerebellum. (d) BOLD time courses in IPs and TPJ (averaged over subjects, cue direction,
target field and hemispheres) during cue and target (valid) periods. Target was randomly presented around frame 3. IPs/cue, IPs response during cue
period; IPs/valid, IPs response during valid-target period; TPJ/cue, TPJ response during cue period; TPJ/valid, TPJ response during valid-target period.
a
c
b
d
Pos IPs
Ant IPs
MT+
LO
IPS/cue
IPS/valid
TPJ/cue
TPJ/valid
MR frame number
Cue and target
period
cue ITI
MR frame number
Percent change BOLD signal Percent change BOLD signal
Cue period
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nature neuroscience • volume 3 no 3 • march 2000 295
for antIPs and posIPs were collapsed, as the borders between the
two regions could not be readily identified. Responses of IPs
(Ant + Pos) and vIPs regions were stronger during the cue peri-
od, whereas those of the TPJ and precuneus region were stronger
during the target period (F21,252 = 4.29 p = 0.0001; Fig. 2d). These
findings indicate that the voluntary orienting and maintenance
of attention to a location primarily recruited the cortex within
the IPs. In contrast, target detection recruited the TPJ (and pre-
cuneus), although a significant target response was also evident
in IPs (Fig. 2d).
To test whether TPJ was preferentially involved in reorient-
ing attention toward novel unattended stimuli, the time cours-
es of the BOLD responses during the target periods for valid
and invalid trials were compared. The strongest validity effect
(difference in the BOLD response for valid and invalid trials)
across the whole brain was localized in the right TPJ cortex,
with separate foci in the inferior parietal lobule (IPL) and supe-
rior temporal gyrus (STG; pixel-wise ANOVA Z-score = 5.12;
Fig. 3a; Table 1). A regional ANOVA confirmed that the valid-
ity effect was significant and independent of the visual field of
the target (ANOVA frame ×validity, R IPL, F7,84 = 8.13,
p < 0.0001; R STG, F7,84 = 7.53, p < 0.0001). The response in
right IPL and STG was more sustained for valid than invalid
targets in each visual field (Fig. 3b and c; ANOVA, frames 5 and
6 only ×validity, R IPL, F1,12 = 5.212, p = 0.041; R STG,
F1,12 = 6.51, p < 0.025). Significant validity effects were also
localized bilaterally in the precuneus and near the intersection
of the right IPs and postcentral sulcus. This latter region did
not overlap with the IPs regions active during the cue period
(vector distance, 17 mm; Table 1). Significant validity effects
not discussed here were also observed in other regions outside
of parietal cortex.
D
ISCUSSION
We tested two functional-anatomical theories about the role of
posterior parietal cortex in visuospatial attention. One theory,
supported by studies of neglect patients with parietal lesions,
proposes that the TPJ cortex is necessary for reorienting toward
visual targets appearing at unattended locations. Another theory,
based on single unit and imaging data, proposes that cortex along
the IPs is involved in the voluntary orienting of attention toward
a location. Our results provide direct confirmation of both views,
showing that IPs was active before target presentation when a
articles
location was voluntarily attended, independent of processes relat-
ed to target detection (for instance, visual responses and their
attentional modulation or motor responses). In contrast, the right
TPJ responded to target presentation more strongly when the
target occurred at an unattended location.
Voluntary orienting of attention
Two findings support a role for the IPs in the voluntary orient-
ing and maintenance of attention to a target location. First, the
presentation of a cue arrow indicating the most likely location
of a subsequent visual target triggered transient responses in
occipital cortex, but more sustained responses in IPs. Transient
responses in occipital cortex may reflect the encoding of the cue,
which was probably completed within a half second27. In con-
trast, sustained parietal responses may reflect the required shift
toward and maintenance of attention at the cued location for
the entire cue period (2360 ms). Second, when the delay after
the cue offset (noise period) was extended to 4.72 seconds, forc-
ing subjects to maintain attention at the cued location for longer,
IPs was the only brain region that showed a sustained response
during the delay.
Our results extend findings of increased activity in extras-
triate, frontal and IPs cortex without sensory stimulation when
subjects attend to a specific object at a specific location during
an identification task20. Those activations are thought to reflect
an attentional signal-biasing activity in visual cortex before
stimulus presentation28. Here we show, in a simpler detection
protocol, that IPs was uniquely active when attention was ori-
ented toward and maintained at a relevant location, suggesting
that IPs is the source of spatial biases observed in extrastriate
visual cortex.
Cue-related activity in IPs may underlie an attentional sig-
nal that ‘marks’ a location of interest9,11 or an intentional signal
that prepares a response (eye movement or hand reaching)
toward that location10. However, attending to direction of
motion also drives IPs regions before the presentation of any
motion target17. Activations are found in IPs during shifts in an
object feature (for instance, color or shape)29 or between per-
cepts during binocular rivalry30. These results suggest that the
intraparietal cortex may be involved in visual selection beyond
the selection of locations.
The BOLD response in IPs was time locked to different
processes in the course of a trial. Early in the trial (cue and noise
Fig. 3. BOLD responses for valid and invalid targets. (a) Coronal section through TPJ cortex (~47 mm posterior). ANOVA (validity ×frame) Fmap
transformed to Zmap. Voxels that show significant validity effect (different BOLD responses for valid and invalid targets) independent of the visual
field of the target. BOLD time courses (averaged over subjects) were estimated in right inferior parietal lobule (R IPL; b) and right superior temporal
gyrus (R STG; c) during the target period for valid and invalid targets in left and right visual field.
abL valid
L invalid
R valid
R invalid
c
Percent change BOLD signal
MR frame number MR frame number
L valid
L invalid
R valid
R invalid
Percent change BOLD signal
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296 nature neuroscience • volume 3 no 3 • march 2000
period), the response was controlled by voluntary orienting
processes, whereas the response was controlled by detection
processes later in the trial (target period; Fig. 2d). The coexis-
tence of different processes in human IPs resembled that observed
in monkey IPs, where neurons also manifest activity time locked
to different components of a task (for instance, visual, attentional,
memory, oculomotor or reaching)9–11,31,32. Although spatial over-
lap between regions of activation during different protocols (visu-
al attention versus eye movements) in previous imaging studies
was used to examine colocalization of processes16,33, the enhanced
temporal resolution of new ER-fMRI procedures17,20,34,35 allows
a richer and more realistic view of the temporal dynamics of acti-
vation in the human brain.
Target detection
The right TPJ (and precuneus) was specifically engaged during
target detection. Unlike other parietal regions that showed both
cue and target responses (antIPs, vIPs), right TPJ and precuneus
showed little if any response to the cue.
The pattern of activation in the TPJ cortex fits two main fea-
tures of unilateral spatial neglect. The much stronger activation
on the right than the left TPJ agrees with the clinical2,3,36 and neu-
ropsychological findings5that neglect is more severe following
right TPJ lesions. The stronger TPJ response following the pre-
sentation of a target at an invalid location is also consistent with
clinical and experimental4,7 observations that neglect is worse for
objects presented at unattended locations.
Activation of the right TPJ may underlie the process of spa-
tially redirecting the focus of attention toward the location of
unattended stimuli. This reorienting was indexed by longer reac-
tion times for invalid targets than for valid targets; thus, the sen-
sitivity of the TPJ activation to target validity implies that the
BOLD signal, despite its slow temporal resolution (seconds), can
track neuronal processes that yield behavioral differences of a
few tens of milliseconds (the difference in reaction time between
valid and invalid trials).
Another possibility, however, is that activity in right TPJ cor-
tex is related to spatially nonselective neural processes triggered
by reorienting to an unattended target. Interestingly, TPJ dam-
age reduces the amplitude of P300 scalp electrical potentials,
which are commonly elicited by the detection of infrequent visu-
al, auditory and somatosensory targets during spatial and non-
spatial tasks37. The right TPJ is also selectively activated when
observers monitor the environment for infrequent targets (for
example, vigilance38), and is the region most densely innervated
by noradrenergic projections from the locus coeruleus39 that are
thought to mediate vigilance and arousal. Damage to the right
TPJ can cause vigilance problems in patients with unilateral
neglect40. Changes in the level of vigilance have a slower time
course than shifts of attention41 and, therefore, might produce
stronger and more sustained right TPJ responses.
A dissociation of function between intraparietal and tem-
poroparietal cortex may explain why a verbal cue directing atten-
tion toward the contralesional field can transiently reduce neglect
in some neglect patients, who typically have damage in the right
TPJ region. This effect, extensively used by therapists to amelio-
rate unilateral visual neglect42,43, may reflect the preserved acti-
vation of the IPs, which mediates the allocation of attention by
cognitive cues. Normal orienting, however, can be maintained
only for a short time in neglect patients based on voluntary strate-
gies. Typically, orienting involves both cognitive and sensory-dri-
ven mechanisms27. This study, along with lesion analyses of
patients with TPJ damage, indicates that the right TPJ region is
articles
critical for visual reorienting, and dissociates this region from
voluntary orienting in nearby IPs.
M
ETHODS
Subjects. Thirteen subjects (6 female, 7 male; aged 18–38) were recruit-
ed from the Washington University community following procedures
approved by the local human studies committee. All subjects were strong-
ly right-handed as measured by the Edinburgh handedness inventory44
and had normal or corrected-to-normal visual acuity and no significant
abnormal neurological history. Informed consent was obtained from
each subject. Before the MR session, subjects participated in one behav-
ioral session during which they were trained to perform the task while
maintaining central fixation. Eye movements were monitored with elec-
tro-oculography, and all subjects were able to perform the task without
breaks of fixation (resolution, 1.5°). Eye movements were not recorded
during the fMRI session. Although we cannot rule out the occurrence
of small eye movements during the fMRI session, several arguments
diminish the likelihood of this possibility. The visual set-up was identi-
cal to the one used in the psychophysical session, in which eye move-
ments were monitored and found negligible. The detection task was not
demanding in terms of acuity. Subjects reported no problems in main-
taining fixation, in agreement with many studies involving detection
tasks27. Additionally, there was no activity in the frontal eye field during
the noise period, when the tendency to look at the peripheral box was
strongest. Reaction times were not collected from one subject because
of equipment malfunction.
Apparatus. Stimuli were generated by an Apple Power Macintosh com-
puter and projected onto a screen at the head of the bore by a Sharp LCD
projector. Subjects viewed the stimuli through a mirror attached to the
head coil. Subjects recorded behavioral responses by pressing an MRI-
compatible fiber-optic key held in the right hand.
Data analysis and fMRI scan acquisition. An asymmetric spin-echo,
echoplanar imaging sequence was used to measure blood oxygenation-
level-dependent (BOLD) contrast (TR = 2.36 s, TE = 50 ms, flip
angle = 90°). Each scan consisted of 128 frames during which 16 con-
tiguous 8 mm axial slices were acquired (3.75 ×3.75 mm in-plane res-
olution). Anatomical images were acquired using a sagittal MP-RAGE
sequence (TR = 97 ms, TE = 4 ms, flip angle = 12°, inversion time
T1 = 300 ms). Functional data were realigned within and across runs to
correct for head movement and coregistered with anatomical data.
Whole-brain normalization was applied to equalize signal intensity
across subjects. In each subject, hemodynamic responses (8 frames
long) were estimated at the voxel level using the general linear model.
The design matrix was defined using impulse-basis functions such that
at each frame, the data were modeled as the sum of the overlapping
hemodynamic response produced by each task effect and a linear trend.
The use of catch trials (trials in which only the cue stimulus was pre-
sent) made it possible to estimate unique responses for the cue, noise,
target-valid, and target-invalid periods even though the cue response
overlaps noise and target responses in each full trial17 (J.M.O. et al.,
Soc. Neurosci. Abstr. 24, 1178, 1998). A random-effects analysis was
performed by entering the individual time points of each hemodynamic
response into voxel-level ANOVAs45. These ANOVAs had two main
effects, time and task. The main effect of time was used to determine
which voxels were activated. The resulting F-maps were corrected for
multiple comparisons using a Gaussian random fields approach46. F-
statistics at each voxel were converted to equivalent Z-statistics. These
Z-maps were used to delineate regions of interest. Separate ANOVAs
were then run at the regional level to determine the task effect of cue
direction (left, right), noise direction (left, right), visual field of the
target (left, right) and target validity (valid, invalid).
A
CKNOWLEDGEMENTS
This research was supported by NIH EY00379 and EY12148 (M.C.). We thank
Thomas Conturo, Avi Snyder and Erbil Akbudak for technical support.
RECEIVED 27 SEPTEMBER 1999; ACCEPTED 6 JANUARY 2000
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nature neuroscience • volume 3 no 3 • march 2000 297
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