The Role of Monkey Prefrontal Cortex in Encoding Multiple Items in Working Memory

As we interact with our environment we are confronted with a constant barrage of sensory information that has to be stored and manipulated, often within a very short period of time. Working memory is the mechanism that supports the active maintenance and manipulation of information. The ability to hold and use information in memory is fundamental for cognition. Many complex behaviors require that we store several pieces of information in working memory. The requirement to remember several things at once is ubiquitous to most of our day to day interactions, yet very little is known about its neural basis. \ud To explore the neural mechanisms underlying multi-item working memory we recorded activity of neurons in the prefrontal cortex (PFC) of two monkeys while they performed a Sternberg working memory task. On each trial, three samples were presented in succession, each followed by a delay. The monkeys were required to hold these three samples in memory and recall one of them at the end of the trial. The monkeys’ performance decreased when working memory load was high. The monkeys’ also exhibited a recency effect where performance was better when the item to be recalled was the most recent. \ud PFC neurons were selective for the identity of the sample during the delay and selectivity tended to be the same regardless of the ordinal position of the sample. However, the strongest determinant of activity during each delay-period was the identity of the immediately preceding sample. The degradation of information about earlier samples stood in striking contrast to the monkeys’ intact ability to perform the task.\ud We found that the mean population firing rate and power in the gamma frequency band of the LFP increased as samples were presented successively. However, these effects were not related solely to memory load, because a similar pattern was present under the control condition in which the same sample was presented three times. \ud From the results of these experiments we conclude that although monkeys were capable of performing a task requiring the memory for multiple items, delay-period activity in prefrontal neurons cannot easily explain the monkey’s working memory performance. \u