FIGURE 1.

Grid cells represent locations in environments. (A) An individual grid cell becomes active at multiple locations (green circles) in an environment. The locations of activation form a repeating grid-like lattice. (B) A grid cell module (left) is a set of cells that share the same lattice scale and orientation but which activate at different relative positions in the environment. If you sort the cells by their relative firing locations, it forms a rhombus-shaped tile. As the animal moves, as shown by the arrow, a bump of cell activity will move in some direction through this rhombus. Two grid cells and their firing locations (green and blue) are highlighted. The grid cell module will activate cells at every location in an environment, but because of tiling, a single grid cell module cannot represent a location uniquely. (C) This figure shows how a second module tiles the same space differently. Each cell’s firing lattice has a larger scale and a different orientation than the module in (A,B). The same movement of the animal as shown by the arrow causes the bump to move in a different direction and a different distance than the bump in the first module in (B). In this case, the bump overlaps the edge of the rhombus, so it wraps around. (D) Although a single module cannot represent locations in an environment uniquely, the activity across multiple modules can. Here, we superimpose the firing patterns of the two modules. Note that when the green and red cells fire together, only one location is possible. The larger the number of modules, the more locations that can be represented uniquely.