Overview
Title: Constraints and Degrees of Freedom of Representational Drift in the Hippocampal-Entorhinal Network
Abstract:
Hippocampal spatial codes change gradually over days, even in familiar environments, but whether their entorhinal inputs also drift has been unknown. I will present two longitudinal calcium-imaging studies that ask which components of hippocampal and entorhinal codes drift and which remain fixed. In the first, we recorded CA1 neurons in mice living continuously in a familiar home cage for weeks. Representations of position, head direction, and time all drifted even though the animals never left the environment, and drift rates did not change when experience was divided into discrete daily visits. We found that distinct encoded variables (e.g., position and head direction) drifted at different rates, suggesting input-specific rather than purely cell-intrinsic mechanisms govern hippocampal drift. Drift persisted when tuning was measured relative to population-derived network states rather than predefined behavioral variables, so it cannot be explained by changes in behavior. In the second study, we imaged hundreds of entorhinal grid cells over weeks. On a linear track, population activity traced a slice through the two-dimensional grid lattice. Across days, this slice shifted in phase but kept its orientation. Grid orientation was coupled to head-direction codes, suggesting a role for head-directional inputs to MEC in constraining the drift in grid orientation. Simultaneous MEC and CA1 recordings showed that hippocampal maps remain spatially anchored despite drift in the spatial phase of their grid inputs. Together, these findings reveal distinct constraints on drift across encoded variables and interconnected brain regions, and challenge models in which hippocampal spatial anchoring is inherited directly from entorhinal grid inputs.
About the NITMB Seminar Series:
The NSF-Simons National Institute for Theory and Mathematics in Biology Seminar Series brings together a mix of mathematicians and biologists to foster discussion and collaboration between the two fields. The seminar series typically takes place on Fridays from 10am – 11am at NITMB. Talks are in-person with the ability to join remotely. Learn more at https://www.nitmb.org/nitmb-seminar-series
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