ON THE ORIGIN OF VARIABLE SPATIAL SCALING ALONG THE SEPTO-TEMPORAL AXIS OF THE RODENT HIPPOCAMPUS

B.L. McNaughton¹; F.P. Battaglia²; S.R. VanRhoads¹; J.A. Dees¹; P. Lipa¹*

1. NSMA, Univ Arizona, Tucson, AZ, USA
2. LPPA, College de France, Paris, France

The spatial scaling of place specific activity in the rodent hippocampus varies systematically from the septal pole (high spatial resolution) to the temporal pole (low spatial resolution). In principle, this variable scaling permits the read-out of spatial proximity relationships from spatial population vector correlations over much larger spaces than would be possible from a fixed scale encoding scheme such as might be inferred from the majority of in vivo hippocampal recordings, which have been conducted only in the septal portion of the hippocampus. Decoupling movement in space from ambulatory motion, by having the animal activate and ride on a mobile platform, results in marked attenuation of the amplitude of the local theta rhythm and a corresponding enlargement of the spatial scale factor of 'place cells' in the dorsal hippocampus (Terrazas et al., 2003). These results lead to the hypothesis that the self-motion signal is embodied in the theta rhythm, whose gain may vary systematically along the septo-temporal axis of the hippocampus. To test this, multiple single neurons were recorded simultaneously from the dorsal, intermediate, and deep ventral regions of the hippocampus of rats running on a simple linear track. Depth of modulation in firing rate, referred to the theta rhythm recorded near the hippocampal fissure in the dorsal region, was substantially less in the deep ventral portion of the hippocampus.

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