BEHAVIOR-RELATED ACTIVITY OF PRIMATE DOPAMINE NEURONS

In view of the behavioral deficits arising after lesions of midbrain d opamine systems, we recorded single dopamine neuron activity in monkey s which learned and performed reaction time tasks, delayed response ta sks, and controlled, self-initiated movements. Dopamine neurons respon d in a rather homogeneous fashion to salient external stimuli that att ract the attention of the subject. Depending on the particular behavio ral situation, dopamine neurons are activated by primary liquid and fo od rewards during learning or in the absence of predictive stimuli, by conditioned stimuli predicting reward and eliciting behavioral reacti ons, and by novel, unexpected stimuli. Thus, dopamine neurons signal t he presence of reward-related, alerting stimuli that need to be proces sed 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 Par kinsonian 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 en able-movements and cognitive processes. These data provide neurophysio logical correlates for the involvement of dopamine neurons in central processes determining the behavioral reactivity of the subject to impo rtant environmental events, and possibly the learning of reward-direct ed behavior.