Self-Motion and the Hippocampal Spatial Metric

Self-motion signals are sufficient for animal navigation ("path integration") and for updating hippocampal location-specific firing. The contributions of ambulatory, vestibular, and optic self-motion signals to CA1 unit activity and EEG were studied while rats either walked or drove a car between locations on a circular track (referred to as WALK and CAR, respectively) or experienced pseudomotion, in which the animal was stationary and the environment was rotated (WORLD). Fewer pyramidal cells expressed place fields during CAR and those that did exhibited substantially larger place fields. The number of theta cycles required to traverse a place field increased, whereas the slope of the theta phase of firing versus position function was reduced. The presence and/or location of place fields were not well correlated between conditions. These effects were even more accentuated during WORLD. These results are not explainable by a simple "smearing out" of place fields but, in terms of size of place fields relative to the track size, are comparable with what would be observed if the track circumference was reduced and the animal moved around it at a correspondingly slower speed. Theta (and its 14-18 Hz harmonic) power were dependent on velocity, but the gain of this function was substantially reduced during CAR and WORLD, again as if the rat were moving more slowly. The spatial scale over which the hippocampal population vector is updated appears to be derived primarily from the gain of a self-motion velocity signal with approximately equal components derived from ambulation, vestibular, and optic-flow signals.