PREFERENTIAL ACTIVATION OF MIDBRAIN DOPAMINE NEURONS BY APPETITIVE RATHER THAN AVERSIVE STIMULI

MIDBRAIN dopamine systems are crucially involved in motivational proce sses underlying the learning and execution of goal-directed behaviour( 1-5). Dopamine neurons in monkeys are uniformly activated by unpredict ed appetitive stimuli such as food and liquid rewards and conditioned, reward-predicting stimuli. By contrast, fully predicted stimuli are i neffective(6-8), and the omission of predicted reward depresses their activity(9). These characteristics follow associative-learning rules(1 0,11), suggesting that dopamine responses report an error in reward pr ediction(12). Accordingly, neural network models are efficiently train ed using a dopamine-like reinforcement signal(13,14). However, it is u nknown whether the responses to environmental stimuli concern specific motivational attributes or reflect more general stimulus salience(4,1 5). To resolve this, we have compared dopamine impulse responses to mo tivationally opposing appetitive and aversive stimuli. In contrast to appetitive events, primary and conditioned non-noxious aversive stimul i either failed to activate dopamine neurons or, in cases of close res emblance with appetitive stimuli, induced weaker responses than appeti tive stimuli. Thus, dopamine neurons preferentially report environment al stimuli with appetitive rather than aversive motivational value.