Leave the navigation to the grid cells
Nerve cells of the type now named grid cells were first identified in the brains of rats several years ago. Their discharge rate increases as the animal explores its environment. Moreover, these peaks occur when the animal traverses positions in space that form the vertices of a hexagon. The new work from the Bernstein Centers in Berlin and Munich shows that the timing of the impulse trains is crucial. “By examining the temporal sequences of impulses discharged by grid cells, one can predict an animal’s location twice as accurately as one can by looking at the impulse rate,” says LMU’s Andreas Herz, the leader of the study.
Phase relationships and space
The mean rate of electrical activity in the grid cell population was known to encode a representation of the geometrical relationships in the animal’s environment. The new study shows that, even at the level of single runs of the animal, the signal spikes of individual grid cells show a shift in timing or phase relative to the activity of the surrounding brain region. This provides a further indication that finely tuned temporal relationships within discharge patterns, and not just the overall level of neural activity, is important for the encoding of spatial information.
This means that grid neurons can exploit both temporal patterns and impulse rates to encode spatial information. "The processing capacity of the rat brain is even larger than we suspected,” says Herz. “This is particularly intriguing, because knowledge of the neuronal systems that encode spatial memory in rodents gives us important clues to the neurobiological origins of the human ability to navigate.” Interestingly, the new study was based on the analysis of previously published data that are accessible online, and did not involve any additional animal experiments.
(PNAS online, 2 April 2012) suwe