Complementary roles of serotonin and dopamine in model-based learning

Model-based learning involves building mental representations of predictive relationships between events. Although there is growing evidence clarifying the role of dopamine in model-based learning, we know less about how serotonin contributes to this process. Here, we present evidence demonstrating a role for serotonin neurons in the dorsal raphe nucleus (DRN) in regulating model-based learning. We contrast this to the role of ventral tegmental area dopamine neurons in this process. We further propose a new framework, suggesting how dopamine and serotonin influence model-based learning in unique but complementary ways. Specifically, we know that the phasic firing of dopamine neurons acts as teaching signals to drive model-based learning. In contrast, we argue that the sustained activity seen in DRN serotonin neurons during waiting for rewards or during the presentation of reward-predictive cues reflects a precise expectation of future rewards across multiple dimensions. These two systems allow us to refine our cognitive maps by interplexing different types of information.