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Abstract:
Abstract: Single-unit studies of dorsolateral prefrontal
cortex in behaving monkeys by Goldman-Rakic and colleagues have
shown that neurons there maintain a temporary representation of the
metrics of an impending saccade. The prefrontal delay-period
activity appears to be retinotopically mapped. Similar results were
known from the superior colliculus and frontal eye fields, but it
was intriguing to see this mode of organization more anteriorly. We
used functional MRI, a phase-encoded task, and cortical surface
reconstruction to look for similarly organized areas in humans. A
brief peripheral target was presented while subjects fixated. A
ring of blinking distractors then appeared during the 3 second
delay period. At fixation dimming and distractor offset, subjects
made a saccade to the remembered location on a black screen. The
angle of the remembered location was stepped through 360 degrees so
that imaging data could be analyzed with the same Fourier-based
method used to map polar angle in visual areas (Sereno et al.,
1995). Initial scans revealed only a weak hemifield bias in frontal
areas, perhaps due to the predictability of target location.
However, a region of midline parietal cortex, near the medial end
of the intraparietal sulcus showed a more robust retinotopic
mapping of the remembered saccade angle. Reversing the stimulus
presentation order reversed the map. A weaker mapping was retained
for visually guided saccades (no delay). A double-step saccade task
generated a more widespread pattern of activation.
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