Expectation modulates neural responses to pleasant and aversive stimuli in primate amygdala.

Animals and humans learn to approach and acquire pleasant stimuli and to avoid or defend against aversive ones. However, both pleasant and aversive stimuli can elicit arousal and attention, and their salience or intensity increases when they occur by surprise. Thus, adaptive behavior may require that neural circuits compute both stimulus valence--or value--and intensity. To explore how these computations may be implemented, we examined neural responses in the primate amygdala to unexpected reinforcement during learning. Many amygdala neurons responded differently to reinforcement depending upon whether or not it was expected. In some neurons, this modulation occurred only for rewards or aversive stimuli, but not both. In other neurons, expectation similarly modulated responses to both rewards and punishments. These different neuronal populations may subserve two sorts of processes mediated by the amygdala: those activated by surprising reinforcements of both valences-such as enhanced arousal and attention-and those that are valence-specific, such as fear or reward-seeking behavior.