Complementary Task Structure Representations in Hippocampus and Orbitofrontal Cortex during an Odor Sequence Task

Both hippocampus (HPC) and orbitofrontal cortex (OFC) have been shown to be critical for behavioral tasks that require use of an internal model or cognitive map, composed of the states and the relationships between them, which define the current environment or task at hand. One general idea is that the HPC provides the cognitive map, which is then transformed by OFC to emphasize information of relevance to current goals. Our previous analysis of ensemble activity in OFC in rats performing an odor sequence task revealed a rich representation of behaviorally relevant task structure, consistent with this proposal. Here, we compared those data to recordings from single units in area CA1 of the HPC of rats performing the same task. Contrary to expectations that HPC ensembles would represent detailed, even incidental, information defining the full task space, we found that HPC ensembles-like those in OFC-failed to distinguish states when it was not behaviorally necessary. However, hippocampal ensembles were better than those in OFC at distinguishing task states in which prospective memory was necessary for future performance. These results suggest that, in familiar environments, the HPC and OFC may play complementary roles, with the OFC maintaining the subjects' current position on the cognitive map or state space, supported by HPC when memory demands are high.