SELF-MOTION AND THE ORIGIN OF THE HIPPOCAMPAL SPATIAL METRIC

S.R. VanRhoads; A.P. Maurer; G.R. Sutherland; P. Lipa; B.L. McNaughton

NSMA , University of Arizona, Tucson, AZ, USA

Scaling of place specific activity in the hippocampus varies systematically along the septo-temporal axis. Place fields get progressively larger and the probability of observing a field in a given environment gets smaller. Removing ambulatory and vestibular motion signals causes substantially larger dorsal place fields, suggesting that, the spatial metric is set by a self-motion signal. Thus, a gradient of this signal along the septo-temporal axis might also be responsible for the normal variation in place field scale. To test this, EEG theta rhythm and CA1 neural ensembles were recorded from the extreme dorsal and middle regions. Dorsal pyramidal cells had smaller place fields, higher mean and peak firing rates, and higher intrinsic oscillation frequencies during track running than middle pyramidal cells, and population vectors representing successive locations became decorrelated over much shorter distances in the dorsal region. Interneuron intrinsic oscillation frequencies did not differ between regions. Both cell classes had more elevated mean rates during running, compared to rest, and both cell classes increased their rates more as a function of speed in the dorsal region. There were no between-region mean firing rate differences for either cell class during rest epochs or at the lowest running speeds. There was no effect of track size on spatial scale or other firing parameters. The amplitude, but not the frequency, of the theta rhythm increased more as a function of running speed in the dorsal than middle hippocampus. We conclude that variation in the neuronal response to movement speed is the likely basis for the systematic variation in spatial scaling along the septo-temporal axis of the hippocampus. The origin of this speed signal and how it acts to set spatial scale remain to be determined.

Support Contributed By: NS020331

Keywords: place field, place cell, velocity, hippocampus