Neural correlates of memory retrieval in the prefrontal cortex

Abstract

Working memory includes short-term representations of information that were recently experienced or retrieved from long-term representations of sensory stimuli. Evidence is presented here that working memory activates the same dorsolateral prefrontal cortex neurons that: (a) maintained recently perceived visual stimuli; and (b) retrieved visual stimuli from long-term memory (LTM). Single neuron activity was recorded in the dorsolateral prefrontal cortex while trained monkeys discriminated between two orientated lines shown sequentially, separated by a fixed interstimulus interval. This visual task required the monkey to compare the orientation of the second line with the memory trace of the first and to decide the relative orientation of the second. When the behavioural task required the monkey to maintain in working memory a first stimulus that continually changed from trial to trial, the discharge in these cells was related to the parameters--the orientation--of the memorized item. Then, what the monkey had to recall from memory was manipulated by switching to another task in which the first stimulus was not shown, and had to be retrieved from LTM. The discharge rates of the same neurons also varied depending on the parameters of the memorized stimuli, and their response was progressively delayed as the monkey performed the task. These results suggest that working memory activates dorsolateral prefrontal cortex neurons that maintain parametrical visual information in short-term and LTM, and that the contents of working memory cannot be limited to what has recently happened in the sensory environment.