EFFECTS OF AMBULATION ON THE SCALE OF THE HIPPOCAMPAL REPRESENTATION OF SPACE: TAKING A LONG WALK OFF A SHORT PIER

K. Poneta; A. Terrazas; P. Lipa*; C.A. Barnes; B.L. McNaughton

NSMA, Univ Arizona, Tucson, AZ, USA

Information about angular and linear movement, coupled with a set of initial firing rates, can be sufficient to generate spatially-selective firing in the hippocampus(HC). Relative heading is likely conveyed through the head direction system and linear velocity or distance information through a combination of motor reafference, proprioception and vestibular activity. Velocity is reflected in the amplitude and shape of the theta wave recorded near the HC fissure and this velocity signal is attenuated during "car driving"(see Terrazas et al., this session). The hypothesis that these signals may, under some circumstances, be the major determinant of the updating of HC ensemble activity pattern, leading to place representations (e.g., McNaughton et al., 1996) predicts that changing the gain of the velocity signal will change the spatial scale of place fields. When ambulatory self-motion signals were eliminated by requiring rats to drive between goal locations on a circular track, place fields in CA1 units were unusually large, approximately 3 times the area of space compared to walking on the same track, (mean information per spike +/- SEM walking 1.47 + 0.06, car driving 1.04 +/- 0.04; P<<0.001). These large place fields maintained consistent locations on a lap-by-lap basis, and the distribution of firing rates for car driving was less positively skewed than during walking. Thus, movement through space by non-ambulatory means results in a population with fewer active cells and larger place fields. These results are consistent with a model in which the HC state-vector executes a circular trajectory on a planar manifold ( chart ), but with a smaller diameter circle during car-driving than during walking . The apparent circle diameter is further reduced when vestibular and ambulatory cues are eliminated by rotating the environment rather than the rat (Terrazas et al., 2001).
Support Contributed By: AG12609 & MH01565

hippocampus, self-motion, cognitive map, place cells