AN ORGANIZING PRINCIPLE FOR SPATIAL TRANSFORMATIONS ACROSS DIVERSE ROOMS IN THE HIPPOCAMPAL FORMATION

The organizing principle for remapping in the hippocampal formation cognitive map is unknown. . To study this, we used high-density chronic Neuropixels recordings of the subiculum and MEC in freely moving mice and investigated the structure of the transformation at the population level. We implemented an advanced decoder to estimate the existence and characteristics of the remapping transformation. We deliberately designed the decoder as non-linear and time-dependent to capture rich and complex transformations. To our surprise, we discovered that the ensemble activity often underwent a simple, smooth, low-dimensional transformation captured by an affine transformation (i.e., rotation, scaling, and shear). The population code is thus flexibly adapted to a new context while retaining a stable spatial representation at the network level. This principle was reproduced across brain regions, including the subiculum and the MEC , and across spatial cell types, including border, head direction, and other spatially modulated cells. These results provide new insights into the computational principles of hippocampal formation remapping, suggesting that spatial cognition is subserved by adaptive ensemble codes governed by a simple affine coordinate transformation. Our findings establish population structure as a critical organizing principle for spatial memory, suggesting avenues for decoding cognition in unvisited environments by embedding transformation rules into population-level decoders. We thus demonstrate for the first time a simple organizing principle for the representation of the transformation between diverse environments in the hippocampal formation.