Behavior-related activity of primate dopamine neurons

In view of the behavioral deficits arising after lesions of midbrain dopamine systems, we recorded single dopamine neuron activity in monkeys which learned and performed reaction time tasks, delayed response tasks, and controlled, self-initiated movements. Dopamine neurons respond in a rather homogeneous fashion to salient external stimuli that attract the attention of the subject. Depending on the particular behavioral situation, dopamine neurons are activated by primary liquid and food rewards during learning or in the absence of predictive stimuli, by conditioned stimuli predicting reward and eliciting behavioral reactions, and by novel, unexpected stimuli. Thus, dopamine neurons signal the presence of reward-related, alerting stimuli that need to be processed by the subject with high priority. Besides these phasic responses, dopamine systems apparently operate also in a tonic mode, as inferred from the beneficial effects of dopamine receptor agonist drugs on Parkinsonian symptoms. Whereas the phasic responses may mediate alerting functions or possibly reward-directed learning, the tonic activity may be involved in maintaining states of behavioral alertness and thus enable-movements and cognitive processes. These data provide neurophysiological correlates for the involvement of dopamine neurons in central processes determining the behavioral reactivity of the subject to important environmental events, and possibly the learning of reward-directed behavior.