Microstructure of a spatial map in the entorhinal cortex

The ability to find one's way depends on neural algorithms that integrate information about place, distance and direction, but the implementation of these operations in cortical microcircuits is poorly understood. Here we show that the dorsocaudal medial entorhinal cortex (dMEC) contains a directionally oriented, topographically organized neural map of the spatial environment. Its key unit is the ‘grid cell’, which is activated whenever the animal's position coincides with any vertex of a regular grid of equilateral triangles spanning the surface of the environment. Grids of neighbouring cells share a common orientation and spacing, but their vertex locations (their phases) differ. The spacing and size of individual fields increase from dorsal to ventral dMEC. The map is anchored to external landmarks, but persists in their absence, suggesting that grid cells may be part of a generalized, path-integration-based map of the spatial environment. Maps in the mind We can find our way about, so somewhere in our brain there must be a neural equivalent of a three-dimensional map. Work on navigation in mammals points to the hippocampus as part of this ‘spatial learning’ system. Now an important advance shows that the entorhinal cortex, which inputs to the hippocampus, is the site where information about place, distance and direction is integrated into a neural map of the surroundings. Here a series of grid cells represents the space around the animal. Each grid cell is activated when an animal's position coincides with a vertex on a grid of equilateral triangles representing the environment. In answering so many questions about the perception of space, this raises the next question: how are these triangular-grid place fields constructed?