The neuronal grid cells of the entorhinal cortex fire in a spatial grid pattern laid out across the surface of a familiar environment to provide the brain with an internal map of an animal's surroundings. The role of environmental boundaries in the construction of this pattern is not well understood. Early studies had suggested that properties such as symmetry, orientation and scale of grid cells' firing patterns were independent of an environment's shape. But now two separate papers in this issue of Nature — one from Edvard Moser and colleagues and the other from John O'Keefe and colleagues — demonstrate that grid orientation, scale, symmetry and homogeneity can be strongly affected by environmental geometry, with grid cells aligned with the borders of the environment at an offset of a few degrees such that it minimizes symmetry with boundaries. These findings suggest a mechanism by which the geometry of an environment causes local rotation and deformation of the hexagonal firing patterns of grid cells.
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