Selective Delay Activity In The Medial Prefrontal Cortex: The Contribution Of Sensory-Motor Information And Expectation

Abstract

The medial prefrontal cortex (mPFC) plays a critical role in the organization of goal directed behavior. This role is suggested by the anatomy of mPFC as the region rests at the top of a complex cortical and sub-cortical hierarchy, receives convergent sensory and motor information from multiple modalities, and is the target of modulatory brainstem nuclei that respond to prediction and reward. Given these observations, it was hypothesized that mPFC neurons store associations between stimuli when the stimuli contribute to the prediction of reward. To test this hypothesis, neural ensemble spiking activity was recorded in the mPFC as rats performed a paired-associate discrimination task. In one condition, both elements of the paired-associate stimulus-sequence provided information about reward delivery. In another condition, only the first stimulus contributed to the prediction. As hypothesized, stimulus-selective, prospective delay activity was observed during sequences in which both elements contributed to reward-prediction. Unexpectedly, however, selective delay responses were associated with slight variations in head position and thus were not necessarily generated by intrinsic mnemonic processes. Interestingly, the sensitivity of neurons to head position was greatest during intervals when reward delivery was certain. These result suggest that a major portion of delay activity in the rat mPFC reflects task-relevant sensory-motor activity, possibly related to behavioral strategies rather than to the local storage of stimulus-stimulus associations. These observations agree with evidence suggesting that mPFC neurons are particularly responsive during the performance of actions related to the acquisition of reward. These results also indicate that considerable attention must be given to the monitoring of sensory-motor variables during delay tasks as slight changes in position can produce activity that appears to be driven by intrinsic mechanisms. It is further suggested that such activity may perform an important role in memory guided behavior, although this role may contrast sharply with standard theories of delay activity and short term memory storage. In particular, it is suggested that delay activity observed in the prefrontal cortex may correspond to the maintenance of memories that are 'stored' in the body or in the environment in the forms of embodied or situated behaviors.