The medial prefrontal cortex (mPFC) plays a critical role in the organization of goal-directed behaviors and in the learning of reinforcement contingencies. Given these observations, it was hypothesized that mPFC neurons may store associations between sequentially paired stimuli when both stimuli contribute to the prediction of reward. To test this hypothesis, neural-ensemble spiking activity was recorded as rats performed a paired-associate discrimination task. Rats were trained to associate sequentially presented stimuli with probabilistic reward. In one condition, both elements of the 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 the prediction of reward. Unexpectedly, selective delay responses were associated with slight variations in head position and thus 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 results suggest that a significant portion of delay activity in the rat mPFC reflects task-relevant sensorimotor activity, possibly related to enhancing stimulus detection, rather than stimulus-stimulus associations. These observations agree with recent evidence that suggests that prefrontal neurons are particularly responsive during the performance of action sequences related to the acquisition of reward. These results also indicate that considerable attention must be given to the monitoring and analysis of sensorimotor variables during delay tasks because slight changes in position can produce activity in the mPFC that erroneously appears to be driven by intrinsic mechanisms.
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