Abstract |
We applied the quantitative autoradiographic method of [14C]‐deoxyglucose
to study the location and extent of intraparietal and superior temporal cortical
activation in monkeys performing: i) fixation of a central visual target, ii)
visually‐guided saccades, and iii) memory‐guided saccades, of similar
amplitude and direction as in the visually‐guided paradigms, in complete
darkness. By eliminating visual stimulation in the memory‐guided task, we
dissociated the effects related to the sensory component from the effects
related to the motor component of saccadic behavior. This is the first study to
provide high‐resolution two‐dimensional functional and anatomical maps of
metabolic activity of the intraparietal and the superior temporal cortices that
allowed for direct comparisons between different experimental conditions.
In the intraparietal cortex, visually‐guided saccades induced enhanced
metabolic activation in approximately the middle and anterior third of the
lateral bank of the IPs. The effect spread in both the ventral and dorsal
subdivisions of area LIP (LIPv and LIPd, respectively), as we confirmed with
histological examination. Our results demonstrate that the oculomotor‐related
area LIP extends further rostrally than traditionally reported to include most of
the anterior part of the bank. Besides saccade execution, area LIPd requires
visual stimulation for maximal activation. The effect induced by memoryguided
saccades was equally robust as that induced in the visual‐guided
paradigm, but was confined to the middle third of the lateral bank within LIPv.
Active fixation of a visual target induced significant metabolic activation in the
border of LIPd/LIPv and extended in the anterior part of the lateral bank within
area LIPd, covering approximately one‐third of the neuronal space allocated to
visually‐guided saccades. We propose that the lateral intraparietal cortex
represents visual and motor space in functionally segregated subregions. A
rostral subregion located superficially (close to the crown) within the
cytoarchitectonically‐defined area LIPd is mainly dedicated to the visuo‐spatial
aspect of oculomotor behavior. A caudal one deeper in the bank (close to the
fundus) within the cytoarchitectonically‐defined area LIPv is predominantly
associated with the motor component of saccadic activity.
In the superior temporal cortex, oculomotor behavior activated a
constellation of brain areas, providing evidence for their involvement in
saccadic eye movements. All areas that comprise the motion‐complex
network traditionally implicated in the analysis of visual motion and in
smooth‐pursuit execution i.e., MT/V5, in the lower bank of the STs, MST, in
the upper bank of the STs, and FST, in the floor of the STs, were significantly
activated during visually‐guided saccades. Areas MT/V5 and MST were also
activated during memory‐guided saccades. Interestingly, we observed
enhanced metabolic activation in the rest of the regions of the caudal STs,
which are not usually implicated in oculomotor/fixational behavior. Both the
posterior and the anterior subdivisions of area V4t in the lower bank were
activated during the execution of visually‐ and memory‐guided saccades,
whereas the intermediate part of polysensory area TPO (TPOi) in the upper
bank displayed significant activation in the visually‐guided saccade task.
Fixation‐related metabolic increases were observed in the areas with central
field representations, i.e. the foveal part of MT (MTf), ventrally, in the anterior
part of V4t (V4ta), in FST and in area TEO. Our findings demonstrate that
areas MT/V5, MST and V4t receive and/or process extra‐retinal saccaderelated
information.
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