Weak Path Integrator

A WEAK PATH INTEGRATOR BOTH CAUSES AND STABILIZES PARTIAL REMAPPING IN SIMILAR ENVIRONMENTS

D.S. Touretzky*

Computer Science Dept, Carnegie Mellon Univ, Pittsburgh, PA, USA

Skaggs and McNaughton (1998) reported that rats shuttling between two visually identical boxes connected by a corridor showed similar place codes in each box. The degree of "partial remapping" between boxes varied somewhat from day to day, but with no discernable trend. This poses a problem for place cell theories in which the hippocampal map is a crossproduct of extrinsic (local view) and intrinsic (path integrator, or PI) variables, and failure to properly reset the PI upen reentry into an environment leads to remapping. If the PI is reset to an identical value upon each box entry, why is there any remapping at all? One possibility is that the rat remembers whether it entered the current box via a left turn (north box) or a right turn (south box), but it would have to retain that information throughout its visit. On the other hand, if the PI is not reset, why isn't remapping complete?

We suggest that the PI is not reset upon box entry, but exerts a relatively weak influence on map selection when visual cues are identical in two environments. The primary contribution of the PI is to constrain the nascent place code to a proper 2D manifold, but the discrepancy in PI coordinates in the two boxes does induce some differences, hence partial remapping. Hebbian learning would then cause cells with fields in both boxes to become bound to two sets of path integrator coordinates. The PI thus both causes partial remapping and preserves it. We explore this hypothesis with computer simulations.

In Knierim's (2002) double cue rotation experiment, place code dissociation, individual place field splitting, and partial remapping are attributed to weak attractor dynamics. But they also imply a weak but nonzero PI influence. When some cells appear to follow local landmarks whle others follow distal, the weak PI may remain bound to the distal reference frame and compete with local landmarks for control of
place fields.

Support Contributed By: NIH MH59932