Contributions of the medial etorhinal and postrhinal cortices to spatial and contextual processing

Abstract

The medial entorhinal and the postrhinal cortices are regions thought to critically mediate spatial and contextual processing in the medial temporal lobe. However, the manner by which these cortices contribute to spatial processing is not unequivocally established. In the first study, the entorhinal cortex was periodically disrupted utilizing the optogenetic inhibitory opsin, archaerhodopsin (ArchT) while the spatial firing of hippocampal neurons were recorded in adult rats. If the medial entorhinal cortex is an essential driver of spatial responses in the hippocampus, it would be expected that changes in hippocampal neuron firing would be specifically time-locked to when the inactivating laser is on. Instead, entorhinal disruption causes a subset of cells to remap only once during the repeated inactivations; once altered, the remapped cells maintain their new firing patterns irrespective of whether the laser is on or not. This remapping, however, does not lead to a net change in the spatial information coded across the hippocampal neuron population. This suggests that disrupting medial entorhinal inputs does not change the resolution of the spatial representation, but instead changes which hippocampal ensemble represents the environment. The participation of the postrhinal cortex in processing spatial contexts was examined in two experiments. The first experiment examined whether the spatial context of an object influences its perceived familiarity, and whether lesions of the postrhinal cortex diminish this effect. The second experiment of this study investigated whether animals with postrhinal ablations can use spatial context to conditionally discriminate which item contains a reward. No deficit was observed on either experiment, suggesting that the postrhinal cortex is not critical for processing spatial contexts. Though these results suggest that spatial processing is not the essential function of these cortices, they do not eliminate the possibility that spatial information may be one of several sources contributing to their computations. Overall, these studies suggest that the entorhinal and postrhinal cortices use multidimensional information to bias which ensembles are active in the medial temporal lobe, thereby dictating how objects and their relationships are processed.