Prefrontal cortex delay activity: Sample code or reward code?

Abstract

Prefrontal cortex (PFC) delay activity found in working memory tasks has been said to be a neural correlate of memory. However, similar PFC delay activity has been found in reward tasks. Given that a reward usually follows a correct response in a working memory task, it is unclear whether delay activity found in these tasks is actually a memory trace. We examined whether delay activity in the avian equivalent of the prefrontal cortex represents a neural correlate of a to-be-remembered sample or an upcoming reward. Birds were trained on a directed forgetting paradigm in which sample stimuli (red and white) were either followed by cues to remember (high tone) or a cue to forget (low tone). In addition, the task also incorporated a differential outcomes procedure in which a correct response on the memory test following a red (remember) sample was rewarded with food, but correct responses on the memory test following the white (remember) sample were not. If delay activity represents a sample code, then it should be seen on both red-remember and white-remember trials. On the other hand, if delay activity represents a reward code, then delay activity should be seen only on red-remember trial, but not white-remember trials. The findings suggest that activity in the avian prefrontal cortex represents the outcome associated with each sample (reward or no reward) rather than memory for the sample itself.