Grid cells are topographically organized in the sense that, within the dorsal part of the medial entorhinal cortex, the scale of the grid increases systematically with anatomical distance from the dorsal border of this brain area. The ventral limit of the spatial map is currently not known. To determine if the grid map extends into the intermediate and ventral parts of the medial entorhinal cortex, we recorded activity from entorhinal principal cells at multiple dorsoventral levels while rats shuttled back and forth on an 18 m long linear track. The recordings spanned a range of more than 3 mm, covering approximately three quarters of the dorsoventral extent of the medial entorhinal cortex. Distinct periodic firing fields were observed at all recording levels. The average interpeak distance between the fields increased from approximately 50 cm in the most dorsal part to approximately 3 m at the most ventral recording positions. The increase in grid scale was accompanied by a decrease in the frequency of theta modulation and the rate of phase precession. The increase in average spacing and field size was approximately linear but this relationship coincided with a substantial increase in the variability of each measure. Taken together, the observations suggest that the spatial scale of the grid representation increases progressively along most of the dorsoventral axis of the medial entorhinal cortex, mirroring the topographical scale expansion observed in place cells in the hippocampus.
Copyright 2008 Wiley-Liss, Inc.