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| Paper IPM / Cognitive / 13964 |
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When subjects remember a location, visual processing at that location is enhanced compared to
elsewhere in space, as measured by visual discrimination performance. The feature selectivity of neurons in early visual areas makes these neurons a good candidate substrate for an enhanced visual
representation underlying these changes in discrimination. However, the firing rates of neurons in
extrastriate visual cortices such as V4 and V5 appear to be largely unaffected by spatial memory
maintenance. An alternative hypothesis is that although the delay-period activity of these neurons is unaltered, spatial memory can still enhance their sensitivity to incoming visual information. To examine this idea, we recorded the neuronal responses of area V4 in awake rhesus monkeys while they performed a memory-guided saccade task. In this task, the target was presented for one second and the monkey had to remember the target location throughout a one second delay period while maintaining his fixation. The fixation point then disappeared, and the monkey moved his eyes to the remembered cue location to receive a reward. We measured the responses and mapped the receptive field of V4 neurons by presenting brief visual probes in a 7x7 grid of locations near one of the cue locations, both during fixation and during memory maintenance. We found that the centers of V4 receptive fields shifted towards the remembered location. This resulted in a larger number of V4 neurons responding to a given visual stimuli when remembering a nearby location (compared to the number of neurons responding to the same stimulus when remembering a location in the opposite visual hemifield). We further examined the effect of memory maintenance on noise correlations between simultaneously recorded pairs of V4 neurons, and found that remembering a location common to both V4 receptive fields decorrelates their activity. This effect of memory maintenance on visual cortical responses, which closely resembles the effect of covert spatial attention, could underlie the behavioral effects of memory maintenance on visual discrimination.
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