University of Lethbridge
Damage to medial prefrontal cortex (mPFC) leads to serious problems in making social and monetary decisions, often with catastrophic consequences for affected individuals. Moreover, this region of the brain has been implicated in a number of impulse-control disorders, including the various forms of addiction. What, exactly, is the role of the mPFC? One idea about mPFC function is that it enables flexible shifting between tasks. Another is that it plays a role in learning and signaling the value associated with courses of action. In support of the flexible shifting hypothesis, rats with mPFC lesions have difficulty switching from an odor-guided discrimination to a texture-guided discrimination. Critically, during this test, the previously rewarded dimension (odors) remains present and leads to reward half of the time. We suggest that the apparent impairment in set shifting may instead be due to an inability to detect the breakdown in contingency between a stimulus and reward. In support of this hypothesis, we have shown that rats with mPFC lesions have trouble with an odor reversal when the previously rewarded odor is still partially rewarded. Hence, the apparent task shifting impairment may actually be due to difficulty in assessing the value of a response in light of past reward contingencies. Further support for this hypothesis comes from an ongoing experiment which shows that rats with mPFC lesions are particularly impaired at flexibly shifting between responses leading to different quantities of reward. Further, electrophysiological data show that mPFC cells encode both the expectation of reward and the value of a reward just received. Interestingly, these two aspects of reward are encoded by separate populations of cells, suggesting separate systems within mPFC for reward anticipation and reward value assessment. Taken together with the work of others, these data support the view that the primary role of mPFC is to select the most adaptive response within a given behavioral context in light of past experience.