PT  - JOURNAL ARTICLE
AU  - Alexandre Y. Dombrovski
AU  - Beatriz Luna
AU  - Michael N. Hallquist
TI  - Encoding of reinforcement along the hippocampal long axis and the transition from exploration to exploitation
AID  - 10.1101/2020.01.02.893255
DP  - 2020 Jan 01
TA  - bioRxiv
PG  - 2020.01.02.893255
4099  - http://biorxiv.org/content/early/2020/01/02/2020.01.02.893255.short
4100  - http://biorxiv.org/content/early/2020/01/02/2020.01.02.893255.full
AB  - Hippocampal maps incorporate reward information, yet the functional contributions of the anterior and posterior hippocampal divisions (AH and PH) to reinforcement learning remain unclear. Here, we examined exploration and exploitation of a continuous unidimensional task with a basis function reinforcement learning model. In model-based fMRI analyses, we found doubly dissociated representations along the hippocampal long axis: state-wise reward prediction error signals in the PH (tail) and global value maximum signals in the AH (anterior body). PH prediction error signals predicted exploration whereas AH global value maximum signals predicted exploitation. AH-mediated exploitation depended on value representations compressed across episodes and options. PH responses to reinforcement were early and phasic while AH responses were delayed and evolved throughout learning. During choice, AH (head) displayed goal cell-like responses to the global value maximum. In summary, granular reinforcement representations in PH facilitate exploration and compressed representations of the value maximum in AH facilitate exploitation.