Abstract
We investigated how entorhinal grid cells encode volumetric space. On a horizontal surface, grid cells usually produce multiple, spatially focal, approximately circular firing fields that are evenly sized and spaced to form a regular, close-packed, hexagonal array. This spatial regularity has been suggested to underlie navigational computations. In three dimensions, theoretically the equivalent firing pattern would be a regular, hexagonal close packing of evenly sized spherical fields. In the present study, we report that, in rats foraging in a cubic lattice, grid cells maintained normal temporal firing characteristics and produced spatially stable firing fields. However, although most grid fields were ellipsoid, they were sparser, larger, more variably sized and irregularly arranged, even when only fields abutting the lower surface (equivalent to the floor) were considered. Thus, grid self-organization is shaped by the environment’s structure and/or movement affordances, and grids may not need to be regular to support spatial computations.
Highlights
We investigated how entorhinal grid cells encode volumetric space
We recorded medial entorhinal cortex grid cells in seven rats freely foraging within a 3D climbing lattice maze[7,16,17,18] and a standard horizontal arena (1.2 m2; Fig. 1e and Extended Data Fig. 1a)
We found that rat grid cell firing fields filled a volumetric environment but did not make a regular (HCP or FCC) pattern[11,20]
Summary
Grid cells usually produce multiple, spatially focal, approximately circular firing fields that are evenly sized and spaced to form a regular, close-packed, hexagonal array. This spatial regularity has been suggested to underlie navigational computations. Entorhinal grid cells tile a horizontal environment’s surface with a hexagonal-close-packed (HCP) array of approximately circular firing fields, the regular spacing of which is widely thought to provide a distance metric supporting the brain’s spatial cognitive map[1]. 4), form spatially defined firing fields in a volumetric environment in both bats[5,6] and rats[7], suggesting a capacity for the vertebrate brain to fully map volumetric space We investigated whether this map could be founded on a regular 3D entorhinal grid.
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