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The Journal of Neuroscience, July 16, 2003, 23(15):6209-6214
 Previous Article | Next Article 
BRIEF COMMUNICATION
Gaze-Centered Updating of Visual Space in Human Parietal Cortex
W. Pieter Medendorp,1,2,3,6
Herbert C. Goltz,1,4
Tutis Vilis,1,5 and
J. Douglas Crawford1,2,3
1Canadian Institutes of Health Research Group on
Action and Perception, 2Centre for Vision Research,
and 3Department of Psychology, York University,
Toronto, Ontario, Canada, M3J 1P3, Departments of
4Psychology and 5Physiology
and Pharmacology, University of Western Ontario, London, Ontario, Canada, N6A
5C1, and 6F. C. Donders Centre for Cognitive
Neuroimaging and Nijmegen Institute for Cognition and Information, University
of Nijmegen, NL 6500 HE, Nijmegen, The Netherlands
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Abstract
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Single-unit recordings have identified a region in the posterior parietal
cortex (PPC) of the monkey that represents and updates visual space in a
gaze-centered frame. Here, using event-related functional magnetic resonance
imaging, we identified an analogous bilateral region in the human PPC that
shows contralateral topography for memory-guided eye movements and arm
movements. Furthermore, when eye movements reversed the remembered horizontal
target location relative to the gaze fixation point, this PPC region exchanged
activity across the two cortical lobules. This shows that the human PPC
dynamically updates the spatial goals for action in a gaze-centered frame.
Key words: fMRI; remapping; spatial perception; arm; eye; human; spatial memory; spatial updating; parietal cortex; LIP; PRR
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Introduction
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The posterior parietal cortex (PPC) is important for spatial processing and
visually guided action (Goodale and
Milner, 1992 ; Jeannerod et
al., 1995; Andersen et al.,
1997; Colby and Goldberg,
1999). In the monkey, different regions within the PPC process
information for different actions. For example, the lateral intraparietal
sulcus (LIP) codes target location for eye movements called saccades
(Gnadt and Andersen, 1988;
Barash et al., 1991;
Duhamel et al. 1992a;
Colby et al., 1996;
Mazzoni et al., 1996), whereas
the adjacent parietal reach region (PRR) codes the same for impending reaching
movements (Galletti et al.,
1997; Snyder et al.,
1997; Batista et al.,
1999).
Both areas have been shown to encode this information explicitly in a
gaze-centered, eye-fixed frame of reference, which must be updated across eye
movements to remain accurate (Duhamel et
al., 1992a; Batista et al.,
1999) as opposed to a head or body-centered frame, which would be
independent of eye movements (Scherberger
et al., 2003). It is also thought that eye position, head
position, and vestibular signals in these regions may be used to transform
this gaze-centered information into other frames of reference
(Andersen et al., 1985;
Brotchie et al., 1995;
Snyder et al., 1998).
Human neuroimaging studies have also implicated the PPC in saccades and arm
movements (for review, see Corbetta et
al., 1998; Connolly et al.,
2000; DeSouza et al.,
2000; Culham and Kanwisher,
2001; Sereno et al.,
2001), with a topographic organization related to different
directions of target location (Sereno et
al., 2001). Without varying eye position, however, it is unclear
whether this topography is related to a gaze-centered frame of reference, let
alone whether the human PPC shows spatial updating across eye movements. Here
we performed two functional magnetic resonance imaging (fMRI) experiments to
investigate whether the human PPC demonstrates gaze-centered coding and
updating of spatial information for saccades and pointing movements.
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Materials and Methods
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MRI scanning and data analysis. Data were collected with a 4.0
Tesla Varian Siemens whole-body imaging system. Six male subjects (aged
21–35 years) viewed stimuli that were backprojected, using an NEC VT540
LCD projector (refresh rate, 70 Hz) with custom optics, onto the ceiling of
the magnet bore. All subjects but one were right-handed, and all pointing
movements were made using the right hand. During experiment 1, 19 contiguous
slices were used to image the entire parietal cortex using a quadrature
radio-frequency surface coil centered on the posterior parietal lobe.
Functional data were obtained using navigator echo corrected T2*-weighted
segmented gradient echoplanar imaging [echo time (TE), 15 msec; flip angle
(FA), 45°; field of view (FOV), 19.2 x 19.2 cm; repetition time
(TR), 2 sec; in-plane pixel size, 3 x 3 mm; thickness, 4 mm]. During
experiment 2, five of the initial 19 slices, which included the region of
interest identified in experiment 1, were scanned at a higher temporal
resolution (FA, 22°; TR, 0.5 sec). Functional data were superimposed on
high-resolution inversion prepared three-dimensional T1-weighted anatomical
images of the brain (typically 128 slices; 256 x 256; FOV, 19.2 x
19.2 cm; TE, 5.5 msec; TR, 10.0 msec) using a phase reference image that
corrected for high-field geometric distortions. In separate sessions, subjects
were rescanned using a birdcage-style head coil to obtain full brain
anatomical images. A high-resolution inversion prepared three-dimensional
T1-weighted sequence was used (FA, 15°; voxel size, 1.0 mm in-plane; 256
x 256; 164 slices; TR, 0.76 sec; TE, 5.3 msec). Analysis was performed
using Brain Voyager 4.6 software (Brain Innovation, Maastricht, The
Netherlands) and Matlab software (MathWorks, Natick, MA). Surface coil images
were realigned manually to head-coil images. Anatomical images for each
subject were segmented at the gray–white matter boundary, rendered and
inflated for visualization purposes only. For functional data analysis, we
excluded any scans in which motion artifacts were observed. Time courses
within each voxel were corrected for linear drift. Anatomical and functional
images were transformed to Talairach space to obtain coordinates for the
regions of interest.
Experiment 1: delayed-movement task. Subjects fixated a central
letter, S, P, or F, referring to a delayed-saccade task (S), a delayed
pointing task (P), or a fixation task (F), respectively (see
Fig. 1 A)
(Snyder et al., 1997;
Batista et al., 1999;
Sereno et al., 2001). Then, a
brief peripheral dot was presented for 250 msec, to either the left or right
at random horizontal eccentricities from the continuous interval between 10 to
25°. Subsequently, a band of distractors (70° horizontal x
8° vertical; eccentricity of the dot, 0.8°; density, 0.14 dots per
square degree) blinked (at 5 Hz) for 2.5 sec, during which the subjects
maintained central fixation (and pointed to the central letter P when in
pointing mode). Then, at distractor offset, subjects made either a saccade or
a pointing movement to the remembered target location and immediately back to
center. Subjects made no movement in the fixation (F) task. During the
pointing task, they were instructed to maintain central fixation of the eyes
at all times. Pointing movements consisted of wrist rotations, such that the
index finger pointed to the remembered target location
(DeSouza et al., 2000). The
subject's view of the hand was occluded with black cardboard during pointing
(DeSouza et al., 2000). The
time between successive movements was 5 sec.
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Figure 1. A, The delayed-movement task. Subjects fixated a central letter,
S, P or F, referring to a delayed-saccade task (S), a delayed pointing task
(P), or a fixation task (F), respectively. After a brief peripheral dot was
presented, a horizontal band of distractors blinked for 2.5 sec. Subsequently,
subjects made either a saccade or a pointing movement to the remembered target
location and immediately back to center. Subjects made no movement when in
fixation (F) task. B, The intervening saccade task. In this paradigm,
two targets were briefly flashed sequentially, a green (the goal target) and
red (refixation target for the first saccade) target. After a 6 sec delay,
subjects made a saccade to refixate at the remembered location of the red
target and, after a subsequent 12 sec period, made either a saccade
(Saccade–Saccade Task) or a pointing movement (Saccade–Point Task)
to the location of the remembered goal target and immediately back to
center.
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Scans to determine the movement-related activation maps comprised 17 blocks
(each 20 sec), in which saccade and pointing blocks (each including movements
to four different target locations) were alternated with fixation blocks.
Scans for topography comprised 12 blocks (each 20 sec), in which four leftward
targets were alternated with four rightward targets. Typically, three scans
for each task were obtained, which were averaged to improve the
signal-to-noise level. A general linear model analysis was then used to
determine activated and topographic voxels in the parietal and occipital
cortex. We used a higher statistical threshold to test between movement
activation and fixation (p < 10-5) than for
topography (p < 0.001).
Experiment 2: the intervening saccade task. The intervening
saccade task used the classical Hallett and Lightstone
(1976) double-step paradigm to
investigate how spatial information for saccades and pointing is stored and
updated during eye movements. As shown in
Figure 1 B, subjects
fixated centrally, and two brief peripheral dots, a green (the goal target)
and red dot (refixation target for the first saccade), were flashed (duration
of 250 msec) after 0.5 and 1.0 sec, respectively. Both targets were either
left or right of central fixation, at different random eccentricities, i.e.,
the red at 16–20° from central fixation and the green target at
7–10° on either side of the goal target. Subsequently, the
horizontal band of distractors blinked. Then, 6 sec after the start of the
flashing pattern, the central fixation was switched off, signaling a saccade
toward the remembered location of the red target. Then, after an additional 12
sec, the flashing distractors were turned off, and subjects made either a
saccade or a pointing movement to the remembered location of the goal target.
The paradigm had four different conditions (see
Fig. 3A). The location
of the remembered goal target after the intervening first saccade remained in
the right hemifield (RR) or remained in the left (LL), or it moved from the
left to the right hemifield (LR), or vice versa (RL). This test was designed
to discriminate between gaze-centered updating of signals as opposed to coding
in a saccade-independent frame such as head-, body-, or world-centered
coordinate frames.
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Figure 3. The remembered location of a target is transferred from one cerebral
hemisphere to the other within the PPC. A, RR, LL, RL, and LR signify
four possible conditions of the intervening saccade paradigm; the first letter
signifies initial location of the two targets (R, right hemifield; L, left
hemifield), and the second letter refers to the remapped location of the
remembered goal location. Left (B) and right (C) parietal
activation (mean ± SE across 6 subjects) for each of the four
conditions in the saccade–saccade task. SEs are plotted at the time
point at which they were computed. All time courses are shifted to compensate
for the fMRI hemodynamic lag. Dashed lines indicate presentation of stimuli,
time of first saccade, and time of second saccade, respectively. Gray areas
indicate the periods over which the differences between the LR and RL
condition were taken. D, E, Saccade–Point Task, The remembered
location of the goal target for hand pointing is transferred across cerebral
hemispheres within the human PPC after an intervening saccade. Data in same
format as B and C.
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Each scan contained 12 epochs (25 sec each), in which the four conditions
were pseudorandomly interleaved. Three to four scans were run for both the
saccade–saccade and the saccade–point task. For each scan, the
mean fMRI signal at volumes 6–8 across all 12 epochs was taken as
baseline. Data was temporally smoothed using a moving average filter with a
span of 5 (volumes). For additional data analysis, for each condition, a mean
signal and SD were computed at each volume. Furthermore, when differences
between conditions were computed, uncertainties were determined using standard
statistical rules for uncertainty combination.
Movement recordings. Each subject extensively practiced all tasks
before imaging to ensure that these were performed correctly. In addition, eye
movement recordings (Applied Science Laboratories, Bedford, MA) were performed
on three of our subjects outside the scanner for the saccade–saccade and
the saccade–point task to confirm that they followed the instructions
correctly. This also confirmed that subjects were able to keep fixation while
making pointing movements. Moreover, the fMRI experiment was self-controlling
for eye movements: the pseudorandom interleaving of the conditions was
designed so that positive results (like those we observed) would only be
obtained if subjects made eye movements to the correct locations. Pointing
movements were not recorded. However, in our fMRI analysis, we focused on
brain activity in the delay periods before the pointing movements, so any
small errors made during the subsequent movement would likely be reflected in
brain events that we did not record.
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Results
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In the first experiment, we used a delayed-movement task to identify human
parietal areas implicated in saccades or pointing movements
(Snyder et al., 1997;
Batista et al., 1999;
Sereno et al., 2001). In this
task, illustrated in Fig. 1, a
peripheral stimulus is flashed, and, after a 2.75 sec delay, the subject makes
either a saccade or a pointing movement toward its remembered location. As a
first step in our analysis, we compared movement (either saccade or pointing)
with no movement (fixation) to identify the cortical regions activated during
saccades and pointing movements.
Figure 2A shows the
complete set of regions activated in both the saccade and pointing tasks of
one subject, rendered onto an inflated representation of the cortical surface.
The majority of the parietal regions that were activated during saccades were
also activated during pointing movements (purple regions). Most of the
remaining voxels were exclusively activated during pointing movements (blue
regions), with fewer voxels activated exclusively by saccades (orange regions)
(Connolly et al., 2000;
DeSouza et al., 2000). We also
observed activation in occipital and frontal areas, but here we focus on
possible analogs of primate LIP and PRR in human PPC.
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Figure 2. A subset of areas within the PPC shows topography for both saccade and
pointing target locations. A, The regions showing higher activation
for saccades or pointing movements than for fixation (p <
10-5) in one subject, rendered onto an inflated
representation of the cortical surface. Orange, Voxels activated during
saccades. Blue, Voxels activated during pointing movements. Purple, Voxels
activated during both saccades and pointing movements. CS, Central sulcus;
IPS, intraparietal sulcus. B, Areas that show left–right
topography for saccades (p < 0.001). C, The same regions
show left–right topography for pointing movements (p <
0.001). D, The centers of the parietal maps marked in two slice
views: a coronal and sagittal view. Green cross indicates left hemisphere; red
cross indicates right hemisphere. Talairach coordinates were as follows (in
mm): x = -19, y = -58, z = 47 (left) and x
= 17, y = -68, z = 55 (right).
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Which of these regions are topographically organized for saccades? To
determine this at a simple level, we compared the PPC activation for leftward
and rightward target locations. Red regions
(Fig. 2B) indicate a
stronger activation for remembered target locations to the left than to the
right, whereas green voxels represent the opposite pattern. In the parietal
cortex, a topographic zone was located at a medial branch of the intraparietal
sulcus, in agreement with results by Sereno et al.
(2001). As can be seen, the
left hemisphere maps targets in the right hemifield, whereas the right
hemisphere maps targets in the left hemifield. All subjects tested
demonstrated an equivalently organized area in their PPC, mostly located
within a small sulcus running medially from the intraparietal sulcus.
Interestingly, the same set of PPC voxels also showed lateralized responses in
the delayed-pointing task, as demonstrated by
Figure 2C. Thus, the
same PPC region showed topography for both saccades and pointing
movements.
For anatomic reference, the center of this region of interest for this one
subject is indicated for the left and right hemispheres in D by the
red and green crosses. Across subjects, the average Talairach coordinates (in
millimeters) and their SDs of the peak parietal activation for saccades were
x = 21 (SD = 4), y = -62 (SD = 9), z = 42 (SD = 4)
(right hemisphere) and x = -19 (SD = 5), y = -63 (SD = 7),
z = 46 (SD = 5) (left hemisphere), consistent with but slightly more
medial than the location reported by Sereno et al.
(2001). For pointing, these
coordinates were x = 22 (SD = 4), y = -64 (SD = 9),
z = 44 (SD = 4) (right) and x = -19 (SD = 4), y =
-62 (SD = 7), z = 46 (SD = 6) (left). These were not significantly
different from the location for saccades in any dimension (paired t
test; p > 0.05).
The key question here is, does this area represent and update its
information in a gaze-centered frame of reference? We tested this in
experiment 2 using an event-related fMRI paradigm in which subjects produced
intervening saccades between seeing a goal target and generating an eye or arm
movement toward its remembered location
(Fig. 1B) (see
Materials and Methods). In all cases, the space-fixed goal target remained
stable relative to the head and body. However, the remembered location of this
target would have to be remapped during the intervening saccade to correctly
code its new location in eye-fixed coordinates. As illustrated by
Figure 3A, the
paradigm consisted of four different conditions with regard to the location of
the goal target relative to gaze direction before and after the first saccade;
the goal target started and remained in the right visual hemifield (RR), or it
started and remained in the left hemifield (LL), or it moved from the left
into the right hemifield (LR), or vice versa (RL).
Figure 3B shows the
mean response of the six subjects in the left parietal region for each of the
four conditions in the saccade–saccade task. As shown, after a brief
presentation of the two targets, those for the initial saccade and the final
goal, cortical activation builds up gradually during the first delay period,
leading to a higher activity when the two targets were in the right
(contralateral) hemifield than when they were in the left (ipsilateral)
hemifield. The reverse is true for initial responses in the right parietal
cortex (Fig. 3C). This
reflects the topographical nature of the region, as identified in experiment
1. The activity decays slightly during the first delay period, and then, when
the first saccade occurs (i.e., 7.5 sec after the start of the trial),
cortical activation increases again, in all four conditions.
What happens after this saccade, in the second delay period? The activation
of the region depends on the location of the remembered goal target relative
to current gaze direction. For the left cortex
(Fig. 3B), if the
remembered goal shifted from the left hemifield (ipislateral) into the right
(contralateral) hemifield (LR condition), a high sustained activation was
observed in the second delay period. However, if it shifted from the right to
left hemifield (RL condition), the level decreased. When the remembered goal
target remained in the same hemifield after the first saccade, the activation
was high if this location was contralateral (RR condition) and low if
ipsilateral (LL condition). The right parietal region
(Fig. 3C) showed a
similar, but mirrored, pattern of activation.
Figure 3, D and
E, shows a similar analysis for the four conditions in
the saccade–point task. Results in the saccade–point task were not
as homogeneous as those in the saccade–saccade task. The presaccadic
response is somewhat less clear compared with the saccade data in B
and C. However, the more important post-saccadic remapping response
is just as robust and clear in the pointing data as for the saccade data. In
other words, the saccade–point data showed the same gaze-centered
remapping pattern, with symmetrically yoked activation between the left and
right PPC. Together, these results suggest that, when the horizontal location
of a remembered saccade or pointing goal reverses (left–right) with
respect to gaze direction, these physical shifts are accompanied by dynamic
shifts in cortical activity from one hemisphere to the other. This suggests
that the parietal area identified here encodes and updates the remembered
location of the saccade and pointing goals in eye-fixed coordinates.
To analyze these findings quantitatively, in
Figure 4, A and
B, we plotted the difference in activation between the RL
condition and the LR condition after the first saccade (time period of
17–19.5 sec, second delay period) versus the difference before that
saccade (time period of 5–7.5 sec, first delay period). These volumes
are indicated schematically by the gray areas in
Figure 3B–E.
Gaze-centered updating requires that these differences have opposite sign.
Accordingly, data from the right parietal cortex should be represented in the
second quadrant, whereas left parietal data should be confined to the fourth
quadrant (gray zones). A failure to update this information (or representation
in a head–body fixed frame) would be indicated by data points in the
opposite white zones. As Figure
4A shows, the saccade–saccade data of all six
subjects fell within the gaze-centered gray zones (p < 0.001;
t test). Thus, all of our subjects encoded and updated visuospatial
information for saccades in a gaze-centered reference frame. When we applied
the same analysis to the saccade–point task data
(Fig. 4B), nearly all
of the data fell within the gaze-centered remapping zones (p <
0.05; t test).
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Figure 4. A comparison of activation before and after remapping in each subject.
A, Goal is target for saccades. B, Goal is target for
pointing movement. x-Axis, The difference (±SE) in the average
activation between the RL and LR conditions just before the first saccade.
y-Axis, Same but after remapping. Filled circles, Right PPC; open
squares, left PPC. Before the first saccade (first delay period) activation
should be contralateral (i.e., RL > LR for the left PPC, and LR > RL for
the right PPC). After the remapping (second delay period), activation should
switch hemispheres (i.e., LR > RL for left PPC, and RL > LR for right
PPC). Gaze-centered remapping requires right PPC data in the second quadrant
and left PPC data in the fourth quadrant (gray zones).
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Discussion
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Our results show that the topographic representation for goal-directed eye
and pointing movements in human PPC is organized in gaze-centered, eye-fixed
coordinates and is spatially updated across eye movements. This is consistent
with the known physiology of the monkey PPC
(Gnadt and Andersen, 1988;
Duhamel et al., 1992a;
Mazzoni et al., 1996;
Batista et al., 1999) and with
human psychophysics (McIntyre et al.,
1997; Henriques et al.,
1998; Medendorp and Crawford,
2002), which suggest an eye-fixed coordinate system for
representing reaching and pointing targets in both near and far space. This
result does not contradict the idea that the same parietal regions might also
be involved in implicitly transforming these gaze-centered signals into other
reference frames (Andersen et al.,
1985; Brotchie et al.,
1995), with the ultimate goal of formulating commands in
effector-centered coordinates.
There is currently debate as to whether the posterior parietal cortex is
more important for preparatory aspects of stimulus selection
(Colby and Goldberg, 1999;
Corbetta and Shulman, 2002;
Yantis et al., 2002) or for
response or action selection (Andersen et
al., 1997; Snyder et al.,
1997; Calton et al.,
2002). For example, a recent study by Yantis et al.
(2002) reported phasic, not
tonic, changes in activity in the human PPC, related to attentional shifts in
a visual recognition task. Our results show clear sustained responses
(Fig. 3). This suggests that
PPC responds in a more sustained manner for a task requiring an action to a
remembered target location (like our task) compared with a task that only
requires visual recognition.
The fact that the results are more apparent for the saccade–saccade
task than for the saccade–point task
(Fig. 4, compare A, B) might indicate that our activation arises from a saccade area, which is only
coactivated during arm movements (even when the saccade is suppressed) as part
of some eye–hand coordination strategy
(Snyder et al., 2000).
To distinguish whether our area specifically represents movements other
than saccades, it would be useful to perform experiments that explicitly
disassociate saccade planning from pointing movements
(Snyder et al., 1997).
Our results could also mean that saccade and arm movement neurons are
comingled in the same region with a greater preponderance of the former or a
lesser need to sustain gaze-centered activity for pointing. Consistent with
this, a recent report shows that neurons with arm-specific intention activity
lie on both banks of the monkey intraprietal sulcus
(Calton et al., 2002).
Moreover, it is clear that damage to the human PPC results in deficits in
programming both eye and arm movements
(Duhamel et al. 1992b;
Grea et al., 2002), and some
of these reaching deficits are best explained in terms of a gaze-centered
mechanism like that reported here (Khan et
al., 2002). In summary, although our fMRI measurements cannot
differentiate the proportion of cells involved in eye versus pointing
movements, the simplest interpretation of our data are that human PPC is
probably organized in a similar way as monkey PPC, i.e., there appears to be a
regional overlap for eye and pointing movements.
Why would the human PPC operate in eye-fixed coordinates? Perhaps it is not
surprising that the locations of targets for saccades are mapped relative to
the current fixation point. At first glance, however, it might be unexpected
that this also holds for pointing movements. It would seem more logical to
code these in egocentric coordinates relative to the body. If, however, the
function of the PPC is to select targets for action and the effectors to
perform these actions (Snyder et al.,
1997), it is critical that computations occur in a common
coordinate frame. It would appear that this common coordinate frame is gaze
centered (Batista et al.,
1999). Ultimately, however, even the remapped input
representations must be transferred into output coordinates. Indeed, this
seems to be the case at the level of the frontal cortex
(Graziano et al., 2002). From
primate studies, it is known that the PPC also possesses information on hand
position (possibly also in eye-fixed coordinates;
Buneo et al., 2002), eye
position (Andersen et al.,
1985), and head position
(Brotchie et al., 1995). This
places it in a unique position to begin the neural computations required for
an accurate arm movement in motor coordinates, without the need for
intervening high-level coordinate frames
(Smith and Crawford, 2001).
Together with the gaze-centered remapping mechanism described here, this
provides the substrate for an economical, biologically compact visuomotor
control system.
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Footnotes
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Received Feb. 25, 2003;
revised Mar. 13, 2003;
accepted Mar. 15, 2003.
This work was supported by grants from the Canadian Institutes of Health
Research and the Canadian Natural Sciences and Engineering Research Council.
W.P.M. and H.C.G. are supported by the Human Frontier Science Program. J.D.C.
is supported by the Canada Research Chair Program. We thank Drs. M. A.
Goodale, D. B. Tweed, and J. C. Culham for comments on this manuscript.
Correspondence should be addressed to Dr. W. P. Medendorp, F. C. Donders
Centre for Cognitive Neuroimaging and Nijmegen Institute for Cognition and
Information, University of Nijmegen, P.O. Box 9104, NL-6500 HE, Nijmegen, The
Netherlands. E-mail:
p.medendorp{at}nici.kun.nl.
Copyright © 2003 Society for Neuroscience
0270-6474/03/236209-06$15.00/0
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Top
Abstract
Introduction
