Hippocampus and Decision Making

Little attention has been paid to the hippocampus so far in formulating neural mechanisms underlying decision making. However, theoretical considerations suggest potential involvement of the hippocampus in value-based decision making, especially during model-based reinforcement learning in which values can be updated according to the decision-maker’s knowledge or model of the environment. To investigate whether the hippocampus processes value-related information, we examined how activity of hippocampal neurons in rats performing a dynamic foraging task was related to reward values that were estimated using a reinforcement learning model. CA1 neurons carried robust signals for the value of chosen action and they temporally overlapped with reward signals, indicating that signals necessary to evaluate the outcome of an experienced event converged in CA1. Compared to CA1, value signals were substantially weaker in CA3 and subiculum, the two neighbouring structures of CA1. Furthermore, selective inactivation of CA1, but not CA3, CA2, or dentate gyrus, impaired value learning without affecting value-dependent action selection. These results suggest a major role of CA1 in integrating value information with other elements of episodic memory. This way, the retrieval of a memory for a previously experienced event will automatically entail the retrieval of its associated value. Convergent factual and value information represented in CA1 might also be essential for simulating most probable and rewarding scenario for maximizing value.