Delay activity in avian prefrontal cortex--sample code or reward code?

In the current study, we examined whether delay activity in the avian equivalent of the prefrontal cortex (PFC) represents a neural correlate of a to-be-remembered sample stimulus or an upcoming reward. Birds were trained on a directed forgetting paradigm in which sample stimuli (red and white) were either followed by a cue to remember (high-frequency tone) or a cue to forget (low-frequency tone). 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 trials, but not white-remember trials. Our findings suggest that activity in the avian PFC represents the outcome associated with each sample (reward or no reward) rather than memory for the sample itself.