Spike count correlations between primate dorsolateral prefrontal cortex neurons during a spatial working memory task

Abstract

It has been shown that neurons in the primate prefrontal cortex encode information during the delay period of working memory tasks for spatial locations, however it remains unclear whether and how these units interact with each other. We examined this question by recording single cell responses from microelectrode arrays (each array = 96 microelectrodes, Blackrock Inc., UT) implanted in dorsolateral prefrontal cortex (area 8v) of two macaca fascicularis during a working memory task, and measuring spike count correlations between the recorded units. During trials the animals fixated on a central spot for 494 to 800 ms, then a circular sine wave grating was presented at one of 16 randomly selected locations for 507 ms. The offset of the grating was followed by a delay period that could last between 494 and 2000 ms and ended with the offset of the central fixation point. This latter event instructed the animals to make a saccade to the remembered location of the previously presented grating. The spiking activity of neurons was simultaneously recorded in blocks of 32 channels and sorted using Plexon software (Plexon Inc, TX). We recorded from 20 well isolated single units for a total of 190 neuronal pairs. We found that spike rate correlations between the pairs computed across all target locations were significantly higher during the delay period (r = 0.13) than during the stimulus presentation period (r = 0.04) (p < 0.01, paired t-test). Our results suggest that interactions between neurons in the dlPFC increase during working memory maintenance in the absence of visual inputs. Such interactions may be critical for working memory encoding by neuronal populations within the area.