Grid Cell

A grid cell is a type of neuron that has been found in the brains of rats, mice, bats, and monkeys; and it is likely to exist in other animals including humans. In a typical experimental study, an electrode capable of recording the activity of an individual neuron is implanted in the cerebral cortex of a rat, in a part called the dorsomedial entorhinal cortex, and recordings are made as the rat moves around freely in an open arena. For a grid cell, if a dot is placed at the location of the rat's head every time the neuron emits an action potential, then as illustrated in the adjoining figure, these dots build up over time to form a set of small clusters, and the clusters form the vertices of a grid of equilateral triangles. This regular triangle-pattern is what distinguishes grid cells from other types of cells that show spatial firing correlates. By contrast, if a place cell from the rat hippocampus is examined in the same way (i.e., by placing a dot at the location of the rat's head whenever the cell emits an action potential), then the dots build up to form small clusters, but frequently there is only one cluster (one "place field") in a given environment, and even when multiple clusters are seen, there is no perceptible regularity in their arrangement.

Grid cells were discovered in 2005 by Edvard Moser, May-Britt Moser and their students Torkel Hafting, Marianne Fyhn and Sturla Molden at the Centre for the Biology of Memory (CBM) in Norway. The arrangement of spatial firing fields all at equal distances from their neighbors led to a hypothesis that these cells encode a cognitive representation of Euclidean space. The discovery also suggested a mechanism for dynamic computation of self-position based on continuously updated information about position and direction.

What makes grid cells especially interesting is that the regularity in grid spacing does not derive from any regularity in the environment or in the sensory input available to an animal. In other words, grid cells appear to encode a type of abstract spatial structure that is constructed inside the brain and imposed on the environment by the brain with no regard for the sensory features of the environment.

Thus, the discovery of grid cells may provide a verification of Immanuel Kant's theory that Euclidean space constitutes a synthetic a priori — a structure that is not purely logical but is constructed by the mind without requiring information from the environment. (In more modern terminology, a "synthetic a priori" is a structure that comes from nature rather than nurture; i.e., a structure that is innate rather than learned).