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| Paper IPM / Cognitive / 14534 |
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Holding a location in working memory has been shown to produce perceptual benefits at that location. Our lab has found that prefrontal cortex sends a persistent signal indicating the content of spatial working memory to extrastriate visual cortex during the delay period of a spatial memory task. DelayÂperiod firing rates in extrastriate visual cortex, however, do not reflect the location held in memory, suggesting that any effect of this input from prefrontal cortex must be subÂthreshold. Such subÂthreshold input could nevertheless render extrastriate neurons more sensitive to incoming visual signals. To test this hypothesis, we recorded the neural activity of areas V4/MT in rhesus monkeys while they performed a memoryÂguided saccade task. In this task, the monkey fixated for 1 second (baseline period) to start the trial. Then the target was presented for 1 second (visual period) and the monkey remembered its location for another 1 second (delay period). The fixation point then disappeared, and the monkey moved his eyes to the remembered cue location to receive a reward. To test whether the location held in memory alters the gain of visual responses, we presented brief (200 ms) visual probes in a 7x7 grid of locations near the one of cue locations, during both the baseline and the delay period. We compared the probeÂevoked visual responses for trials with different target locations to see if the peak magnitude of the response changed based on the contents of spatial working memory. Remembering a location near the probe increased the peak response of single and multi units in areas V4/MT. Next, we used trials in which no visual probes were presented to examine the effect of spatial memory on the response reliability of single neurons in extrastriate cortex. The variability of neural responses in extrastriate cortex decreased when remembering a location near the neuron�??s RF. We hypothesize that these changes in reliability and sensitivity to visual signals are the result of subÂthreshold modulation by the persistent spatial signal from prefrontal cortex. These effects of memory maintenance on the neural activity in extrastriate cortex, which is similar to the effect of covert spatial attention, could contribute to the behavioral benefits seen at a location held in memory.
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