Neural mechanisms of reward processing and emotion

National Institute Of Mental Health

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Abstract

Neurons in both the orbitofrontal cortex (OFC) and medial frontal cortex (MFC) encode the sensory properties, magnitude and subjective value of expected and received rewarding outcomes. In addition, the activity of neurons in these regions reflects anticipatory arousal, as measured by pupil diameter. Within the MFC, dysfunction of the anterior cingulate cortex (ACC) has been implicated in depression and obsessive-compulsive disorder. Evidence from studies in animals suggests that the ACC contributes to social cognition, decision making, and reward expectancy. Additionally, the ACC is involved in the regulation of autonomic states. Preliminary findings from our laboratory suggest that the ACC is essential for the expression of autonomic correlates of reward expectancy. Whereas control animals showed an increase in heart rate (HR) in anticipation of expected rewards, animals with bilateral neurotoxic lesions of the ACC did not. To better characterize the role of the ACC in this task setting, we investigated whether the change in HR was specific to anticipation of reward or due to a more general disruption of the HR control by behavioral events. To accomplish this, we added a small number of catch trials in which no reward was delivered despite otherwise identical stimulus conditions. The addition of catch trials allowed us to examine changes in HR in the context of reward omission. A total of 11 animals were studied: six received bilateral neurotoxic ACC lesions and five served as unoperated controls. In the subset of animals tested with catch trials (N = 2 ACC lesion; N = 5 control), animals with ACC lesions displayed a significant difference in HR during the time after trial outcome on trials with reward delivery versus reward omission, as did four of the five controls. These data suggest that the contribution of the ACC to HR increases in anticipation of reward in our original experimental setting is specific to expectation and not to a more general disruption of the control of HR by behavioral events. Prior studies in our laboratory found an ACC influence on anticipatory arousal as indexed by pupil diameter. The present study, which monitored arousal by measuring changes in HR, had no fixation requirement and no instrumental requirement of any sort. Thus, the ACC contributes to positive affect by generating arousal in anticipation of positive emotional events even in semi-naturalistic contexts with free-viewing conditions.