Spatial and temporal reference frames anchor cognitive maps for memory

Abstract

The hippocampal-entorhinal region forms cognitive maps of learned relations. Environmental boundaries are central to spatial orientation. However, trapezoidal boundary geometry distorts the regular firing patterns of entorhinal grid cells that proposedly provide a metric for cognitive maps. In this talk, I will first show that environmental geometry affects spatial memory in ways consistent with a grid-cell model based on the successor representation. Positional memory was degraded in a trapezoid compared to a square control environment and distance estimates between remembered positions were persistently biased. The reference frame provided by environmental geometry affected human spatial memory similarly to rodent grid-cell activity — thus strengthening the putative link between the grid metric of cognitive maps and behavior. Second, I will turn to cognitive maps of temporal relations in the hippocampus and anterior-lateral entorhinal cortex. I will address the question whether we flexibly reference sequence memories to a continuous clock as opposed to merely storing the sequential order of events or representing time as a passive “absolute”, akin to the readout of a stopwatch. In a sequence learning task, participants inferred when individual events took place based on infrequent umaskings of a hidden clock. We manipulated the clock’s speed between sequences to partially dissociate event times from their sequential order and the time objectively elapsing between them. After learning, multi-voxel patterns reflected the temporal relationships of event pairs in both hippocampus and entorhinal cortex. Hippocampal sequence memories were anchored to the reference frame of the hidden clock. Further, hippocampal and entorhinal sequence representations were organized in a way that generalized temporal relations across multiple sequences. Together, these findings suggest that cognitive maps built in service of episodic memory are tied to the external world by anchoring them to spatial and temporal reference frames.

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