Neural Representations of Location Composed of Spatially Periodic Bands

How to Make a Place Cell Hippocampal place cells have been studied for more than 40 years, yet the mechanisms underlying their remarkable spatial tuning are still not established. Using whole-cell patch-clamp recordings in freely moving rats, Lee et al. (p. 849) changed the baseline membrane potential in hippocampal pyramidal neurons. Place fields arose spontaneously in otherwise silent pyramidal cells after depolarization when the membrane voltage reached a threshold. Place cells in the hippocampus and grid cells in the entorhinal cortex are thought to provide the framework for spatial navigation in an animal. However, it is still unclear how the hexagonal symmetry that is so prominent in grid cells emerges. While recording from the entorhinal cortex and in the pre- and parasubiculum during spatial behavior in the rat, Krupic et al. (p. 853) discovered that many forms of spatial periodicity exist in neurons in these structures. The grid cells seem to reflect a subset of this larger set, generated by self-organized dynamics. The brain performs a Fourier analysis of space to produce the representations of space in and around the hippocampus. The mammalian hippocampal formation provides neuronal representations of environmental location, but the underlying mechanisms are poorly understood. Here, we report a class of cells whose spatially periodic firing patterns are composed of plane waves (or bands) drawn from a discrete set of orientations and wavelengths. The majority of cells recorded in parasubicular and medial entorhinal cortices of freely moving rats belonged to this class and included grid cells, an important subset that corresponds to three bands at 60° orientations and has the most stable firing pattern. Occasional changes between hexagonal and nonhexagonal patterns imply a common underlying mechanism. Our results indicate a Fourier-like spatial analysis underlying neuronal representations of location, and suggest that path integration is performed by integrating displacement along a restricted set of directions.