DISCORDANT HEAD DIRECTION INCREASES REMAPPING BETWEEN VISUALLY IDENTICAL ENVIRONMENTS IN RODENT CA1

S.R. VanRhoads¹; M.C. Fuhs²; A.E. Casale¹;
D.S. Touretzky²; B.L. McNaughton¹*

1. NSMA, Univ Arizona, Tucson, AZ, USA
2. Computer Sci, Carnegie Mellon Univ, Pittsburgh, PA, USA

To assess the influence of the head direction system on hippocampal "place-field" maps, three rats were induced to forage in two identical boxes with a doorway in one wall and a small light mounted opposite. In the side-by-side condition, in which the rats were pretrained, the boxes were connected by an external corridor, as reported by Skaggs and McNaughton (1998). In the other condition, the two boxes were abutted front-to-front by rotating each box by 90 . In both conditions, a barrier in the doorway controlled when the rat could move to the other box. Rats experienced both trial types on each of two days, in counterbalanced order. During a trial, the rat visited the boxes in an A-B-A-B sequence (20 min total). Trials were separated by 40 min.
In the side-by-side condition, most cells had similar fields in each box (i.e., weak partial remapping). In the front-to-front condition, few cells showed similar fields in both boxes (complete remapping); however, fields in box A in the two conditions were usually identical, independent of trial order.
The results indicate a powerful influence on hippocampal activity patterns by the internally maintained head-direction system (e.g., Knierim et al., 1995; 1998) and provide strong evidence against the hypothesis that hippocampal neurons selectively "represent" specific environmental features. Moreover, while the rat's head direction system may easily reset on the basis of visual input after passive transport into the 90 rotated apparatus (i.e., Box A), thereby inducing hippocampal recall of the side-by-side map, it is very resistant to reset when the animal travels actively into a situation in which the visual cues and head direction signals are discordant by 180 , with the result that a completely uncorrelated hippocampal firing pattern is induced.

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