Hexagonal firing fields of entorhinal cortex neurons — the brain's metric map
Grid cells, discovered by Moser & Moser in the rat entorhinal cortex (Nobel Prize 2014), fire whenever an animal passes through any vertex of a periodic triangular lattice — creating a hexagonal pattern of "firing fields" that tiles the entire environment. Different grid cells vary in scale, orientation and phase offset, together forming a population code that can represent any position uniquely. The pattern arises from a continuous attractor network driven by path integration: head direction and velocity signals update an internal representation of position even in darkness, without external landmarks.