The functions of each cell type are yet to be determined, but the periodic fields of the grid cells provide a metric to the neural representation of space, in the same way that head direction cells provide a directional reference frame. Border cells may assist in the assessment of the allocentric distances by triangulation and perhaps in scaling the grid size to accommodate to the size of the discoverable environment. The availability of direction, position, distance and boundary information in the entorhinal cortex makes this brain structure an ideal candidate for computing the spatial metric of the surrounding environment22,25. Unlike hippocampal place cells, which fire differentially in different environments31, grid cells, border cells and head direction cells are active in all environments and often behave in a coherent manner27. The preservation of spatial and directional firing relationships in local ensembles of such cells implies that the entorhinal cortex generates a metric that can be applied universally across environments5,22,23.
