Evidence for a gradient within the medial temporal lobes for flexible retrieval under hierarchical task rules

Environmental overlap influences goal-oriented coding of spatial sequences differently along the long-axis of hippocampus

When navigating our world we often first plan or retrieve a route to our goal, avoiding alternative paths to other destinations. Inspired by computational and animal models, we have recently demonstrated evidence that the human hippocampus supports prospective spatial coding, mediated by interactions with the prefrontal cortex. But the relationship between such signals and the need to discriminate possible routes based on their goal remains unclear. In the current study, we combined human fMRI, multi-voxel pattern analysis, and an established paradigm for contrasting memories of nonoverlapping routes with those of routes that cross paths and must be disambiguated. By classifying goal-oriented representations at the initiation of a navigational route, we demonstrate that environmental overlap modulates goal-oriented representations in the hippocampus. This modulation manifest through representational shifts from posterior to anterior components of the right hippocampus. Moreover, declines in goal-oriented decoding due to overlapping memories were predicted by the strength of the alternative memory, suggesting co-expression and competition between alternatives in the hippocampus during prospective thought. Moreover, exploratory whole-brain analyses revealed that a region of frontopolar cortex, which we have previously tied to prospective route planning, represented goal-states in new overlapping routes. Together, our findings provide insight into the influences of contextual overlap on the long-axis of the hippocampus and a broader memory and planning network that we have long-associated with such navigation tasks.