Grid cells and phase coding in the human brain

Zoltan Nadasdy

Seton Brain & Spine Institute,

Dept. of Psychology, UT Austin,

Dept. of Cognitive Psychology, ELTE Budapest

Grid cells were discovered in the rodent brain but direct evidence for

their human analogue is still lacking. These neurons are active

whenever the animal passes equidistant spatial locations with a 60

degree rotational symmetry, making up a characteristic hexagonal

tessellation pattern. The spatially periodic activity is assumed to

serve as a scale free reference for spatial navigation. Searching for

the human analogue, we recorded single neurons from the entorhinal

cortex of epilepsy patients during spatial navigation in a virtual

reality environment. We found a broad repertoire of spatially periodic

firing patterns from each patient, including those of typical of grid

cells. In addition, we investigated the relationship of spatially

periodic activity to ongoing local field oscillations, in particular

to theta and gamma rhythms. Our preliminary data show a spatially

periodic modulation of spike phases relative to gamma oscillations,

which are consistent across multiple cells from the same patient. We

propose that a combination of firing rate and firing phase coding may

underlie spatial navigation in the human entorhinal cortex.