Firing rate activity of a subset of neurons in the medial prefrontal cortex of the rat increases in response to cues that improve the prediction of future delivery of food reward.
NSMA, Univ of Arizona, Tucson, AZ, USA
The medial prefrontal cortex (anterior cingulate, prelimbic and infralimbic cortices) has been implicated in the acquisition of reinforcement learning tasks. Performance in such tasks depends critically on the acquisition of an accurate and rapidly updating estimate of future reward delivery. Alterations in these estimates may be signaled by neurons that respond to the degree to which a stimulus or event alters the current estimate of future reward delivery. We describe a subclass of neurons in the rat medial prefrontal cortex that exhibit short latency increases in firing rate that are proportional to the degree to which a stimulus improves the current estimate of food reward. Simultaneous activity from multiple neurons in the medial prefrontal cortex of the rat was recorded as the animal performed a stimulus sequence discrimination task. A delay of 700 msec separated the presentation of the first and second stimulus and an 800 msec delay separated the presentation of the second stimulus and reward delivery. The capacity of the second stimulus to predict the future delivery of reward was manipulated. Analysis of the activities of these neurons suggested two categories of short-latency (approximately 200 msec) neural responses. One category of neurons responded most robustly to the earliest cue that reliably predicted reward delivery. These neurons did not respond to non-predictive cues. A second category of neurons responded most robustly to cues that occurred immediately prior to the delivery of reward. Importantly, the strength of these responses was proportional to the degree to which the reward was unexpected. Cells with broadly similar properties have been observed in the VTA and parietal cortex of the primate. Somewhat similar responses have also been observed in response to cues predicting mfb stimulation in the mPFC of the rat. Neurons exhibiting these classes of response may facilitate adaptive behavior by signaling the degree to which cues alter current expectations of future reward delivery. Furthermore, such responses may be a critical component of associative accounts of causal learning.
Support Contributed By: MH046823
Key words: medial prefrontal cortex, reinforcement learning, attention, associative learning, reward, novelty