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Abstract:
Several studies in humans have linked oscillations in frontal and occipital cortex with working memory. In monkeys, working memory is thought to depend on delay actitity - elevated firing of neurons in prefrontal and other cortical areas. Whether or how these oscillatory phenomena and delay activity interact to support working memory is unknown. To address this question, we recorded both single-unit activity (SUA) and local field potentials (LFP) simultaneously from multiple sites in area V4. Monkeys performed a delayed-matching-to-sample task. After a 1sec fixation period, a sample object was briefly presented (300ms). Following a 1sec delay, a probe object was presented and monkeys had to release a lever if the probe matched the sample. Natural images (10°x10° at center of gaze) of varying contrast were used a sample stimuli, probe stimuli were always at full contrast. Monkeys performed above 90 correct at contrasts of 25 and higher, and near chance at low contrast. We assessed power in the theta band during the last 700ms of the delay period, and compared this to a similar period during fixation. Almost all sites showed a marked reduction of theta oscillations at 5 contrast, when monkeys were unable to discriminate the sample stimuli. We observed increased theta oscillations during the delay at many sites for contrasts of 25 and higher, where monkeys were performing accurately. Examining the relationship between theta phase and single unit activity, we found that many single neurons fired preferentially at a particular phase of the theta cycle. These findings suggest that increases in theta oscillations are present in monkey visual cortex during delays, and that these oscillations serve to structure single unit activity.
