Aims: 

The aim of this study was to determine whether grid cells on a population level display step-like or continuous increments in spatial frequency within animals. We also wanted to know whether grid cells in different mediolateral or dorsoventral locations associate with distinct or identical orientations.

Methods: 

In contrast to previous reports, we recorded grid cells in several locations simultaneously using a high density recording setup with 12 independently movable tetrodes. This allowed us to record grid cells over a large portion of the parahippocampal area concurrently. In addition, we sampled grid cells of increasing spatial frequency using a continuous dorsoventral recording technique involving 4 tetrodes advanced parallel to the grid cell layers.

Results: 

Within animals grid cells express step-like, not continuous, spatial frequencies. Grid orientation is similar at different mediolateral and dorsoventral sites of the grid cell area. However, systematic axis-specific differences in orientation and inter-node distance which introduce ellipticity to the grid occur across spatial frequency-defined populations of grid cells. These populations display distinct responses to environmental manipulations suggesting functionally segregated modules of grid cells.

Conclusions: 

Grid cells are modularly organized which is manifested in grid spatial frequency, orientation and response to environmental manipulation.