A prospective code for value in the serotonin system

The in vivo responses of dorsal raphe nucleus serotonin neurons to emotionally salient stimuli are a puzzle¹. Existing theories centring on reward², surprise³, salience⁴ and uncertainty⁵ individually account for some aspects of serotonergic activity but not others. Merging ideas from reinforcement learning theory⁶ with recent insights into the filtering properties of the dorsal raphe nucleus⁷, here we find a unifying perspective in a prospective code for value. This biological code for near-future reward explains why serotonin neurons are activated by both rewards and punishments^3,4,8-13, and why these neurons are more strongly activated by surprising rewards but have no such surprise preference for punishments^3,9-observations that previous theories have failed to reconcile. Finally, our model quantitatively predicts in vivo population activity better than previous theories. By reconciling previous theories and establishing a precise connection with reinforcement learning, our work represents an important step towards understanding the role of serotonin in learning and behaviour.