Reward mechanisms in the brain and their role in dependence: evidence fromneurophysiological and neuroimaging studies

This article reviews neuronal activity related to reward processing in prim ate and human brains. In the primate brain, neurophysiological methods prov ide a differentiated view of reward processing in a limited number of brain structures. Dopamine neurons respond to unpredictable rewards and produce a global reinforcement signal. Some neurons in the striatum also react to t he expectation and detection of reward. Other striatal neurons show reward- related activities related to the preparation, initiation and execution of movement. Orbitofrontal neurons discriminate among different rewards and co de reward preferences. In the human brain, regions belonging to a meso-stri atal and meso-corticolimbic loop respond to reinforcement stimuli in contro l subjects. These observations corroborate results obtained in primates. Ad ditionally, reward induces activation in regions specific to task performan ce. Our results also show a similar pattern of reward-related activation in nicotine and opiate addicts. Thus, in contrast to healthy subjects, typica l reward-related regions respond in addicts to monetary reward but not to n onmonetary reinforcement. Reduced activation in performance-related regions is also observed in both groups of dependent subjects. The results of anim al and human studies suggest that dopamine and dopamine-related regions are associated with the integration of motivational information and movement e xecution. Dopamine-related pathological disorders can be associated with mo vement disorders, such as Parkinson's disease or with false motivational at tributions such as drug dependence. (C) 2001 Elsevier Science B.V. All righ ts reserved.