Functional analysis of spatially discriminative neurons in prefrontal cortex of rhesus monkey

The present study addresses the question of whether prefrontal neurons that exhibit spatially selective patterns of discharge during the delay period in spatial delayed-response tasks code a mnemonic event. To examine this question, rhesus monkeys were trained to perform two variants of the classical spatial delayed-response task in both of which a delay intervened between cue presentation and response and the discriminative stimulus had to be recalled at the moment of response. They were also trained to perform two control tasks in which memory was not required since cues present throughout the delay informed the monkey of the correct response. Extracellular recordings were obtained from 192 neurons located in and around the principal sulcus of the frontal lobe during performance of both control and delay tasks. Comparison of the same neuron's activity across the 4 task conditions revealed a class of neuron that displayed spatially discriminative activity in the delay period only during delayed-response tasks and not during the same period of the control tasks. These neurons are candidates for units engaged in a central mnemonic process. Other neurons either exhibited similar activity in the delay period of control and delayed-response tasks or stronger discriminative behavior during this period in control tasks than in delayed response tasks. We conclude that delay-related spatially discriminative neurons found in the prefrontal association cortex are diversified and that certain of them play a specific role in mnemonic coding.