Preferential activation of midbrain dopamine neurons by appetitive rather than aversive stimuli

MIDBRAIN dopamine systems are crucially involved in motivational processes underlying the learning and execution of goal-directed behaviour1–5. Dopamine neurons in monkeys are uniformly activated by unpredicted appetitive stimuli such as food and liquid rewards and conditioned, reward-predicting stimuli. By contrast, fully predicted stimuli are ineffective6–8, and the omission of predicted reward depresses their activity9. These characteristics follow associative-learning rules10,11, suggesting that dopamine responses report an error in reward prediction12. Accordingly, neural network models are efficiently trained using a dopamine-like reinforcement signal13,14. However, it is unknown whether the responses to environmental stimuli concern specific motivational attributes or reflect more general stimulus salience4,15. To resolve this, we have compared dopamine impulse responses to motivationally opposing appetitive and aversive stimuli. In contrast to appetitive events, primary and conditioned non-noxious aversive stimuli either failed to activate dopamine neurons or, in cases of close resemblance with appetitive stimuli, induced weaker responses than appetitive stimuli. Thus, dopamine neurons preferentially report environmental stimuli with appetitive rather than aversive motivational value.