The role of monoamines in reward processing in the prefrontal cortex

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To clarify neuronal substrates for reward processing, we recorded neuronal activity in the dorsolateral prefrontal cortex (DLPFC) while the monkey performed various cognitive tasks. Using a variant of the oculomotor delayd-response task with two reward conditions (reward/non-reward), we found that the DLPFC contains distinct neuronal groups that code outcome following the differential spatial response, which should play a role in associating different response with its outcome. Such coding may be critical for flexible control of behavior according to a rule, which has been considered a critical role of the DLPFC. To address this issue, we examined neuronal activity while monkeys performed an oculomotor rule-shifting task that required them to shift behavioral rules based on response-outcome. A subset of DLPFC neurons showed differential activity by the rule that was used in the current trial (a retrospective code), and that would be used in the next trial (a prospective code). The acti … More vity of these neurons increased particularly when the monkey had to alter the rule. The retrospective and prospective coding of rules may be critical neuronal substrates for rule-shifting based on response-outcome and retention of the shifted rule, respectively. Next, to examine how the reward expectation affects neuronal processes of the sensory memory and that of motor preparation, we recorded neuronal activities from the DLPFC of monkeys that performed a variant of delayd anti-saccade task that can dissociate neuronal activity for sensory memory from that for motor preparation under reward and no-reward conditions. We found that the reward expectation enhances the neuronal process for sensory memory but not for motor preparation in the DLPFC. Further, to examine the involvement of monoamines in changing behavior based on the previous outcome, we injected dopaminergic and noradrenergic antagonists into the DLPFC, while the monkeys were performing an oculomotor rule-shifting task. It was found that D1-dopamine antagonist induced a deficit characterized by the increase in perseveration-errors. D1-dopamine receptors in the DLPFC are likely to be involved in a function of executive control ; i.e., alternating behavior based on outcome and rule. Less