Several different classes of neurons are involved in the mapping of allocentric space. Two such classes, place cells and head-direction cells, have been found throughout the medial temporal lobe memory areas, including the presubiculum, parasubiculum and entorhinal cortex. Place cells represent spatial locations and features of the environment and head-direction cells are sensitive to the orientation of a rat's head with respect to the environment. A third major class of cells, known as grid cells, was recently identified in the medial entorhinal cortex (MEC). Grid cells, which encode abstract spatial structure, are of great interest because, unlike place cells, their response properties are independent of any particular environment, suggesting that they are involved in path integration mechanisms. On page 987, Boccara and colleagues report that grid cells are not unique to the MEC, where they were first found, but are also abundant in the pre- and parasubiculum.

Boccara and colleagues recorded from neurons throughout the presubiculum, parasubiculum and MEC of rats during food-motivated running in an open environment. They found grid cells in all three of these areas, interspersed with head-direction cells and border cells (another recently reported cell class that encodes the boundaries of a local environment). The relative proportions of each of these cell classes were comparable across presubiculum, parasubiculum and the deep layers of MEC. However, the rotational symmetry of the grid pattern and the theta modulation of presubiculum neurons were significantly weaker than those in MEC.

The pre- and parasubiculum project strongly to MEC, raising the possibility that they may be the sources of grid-cell properties in MEC neurons. However, it is also possible that grid properties are generated locally in each of these regions. Although the existence of grid cells in multiple areas does not definitively identify the mechanism for generating these unique response properties, it further specifies the neural network that supports the mapping of allocentric space.