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 these processes. Here, we present evidence examining the role of serotonin neurons in the dorsal raphe nucleus (DRN) in regulating model-based learning. We contrast this to the role of ventral tegmental area (VTA) dopamine neurons. We further propose a new framework suggesting how dopamine and serotonin could 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 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.