Conjoint Control of Hippocampal Place Cell Firing by Two Visual Stimuli

Abstract

Changing the angular separation between two visual stimuli attached to the wall of a recording cylinder causes the firing fields of place cells to move relative to each other, as though the representation of the floor undergoes a topological distortion. The displacement of the firing field center of each cell is a vector whose length is equal to the linear displacement and whose angle indicates the direction that the field center moves in the environment. Based on the observation that neighboring fields move in similar ways, whereas widely separated fields tend to move relative to each other, we develop an empirical vector-field model that accounts for the stated effects of changing the card separation. We then go on to show that the same vector-field equation predicts additional aspects of the experimental results. In one example, we demonstrate that place cell firing fields undergo distortions of shape after the card separation is changed, as though different parts of the same field are affected by the stimulus constellation in the same fashion as fields at different locations. We conclude that the vector-field formalism reflects the organization of the place-cell representation of the environment for the current case, and through suitable modification may be very useful for describing motions of firing patterns induced by a wide variety of stimulus manipulations.

Conjoint Control of Hippocampal Place Cell Firing by Two Visual Stimuli

“…Overall, it is clear that the magnitudes of the radial displacements are smaller for the together than for the apart manipulation. Testing this impression and considering its implications are deferred to the next paper (Fenton et al 2000). …”

“…The finding that the salience of a card depends on the distance between the card and the field means that fields cannot move relative to each other in arbitrary ways, but instead that the mapping of the environment onto the place cells has a distinct spatial flavor. The fact that field movements are systematic and smoothly distributed with position forms the basis of the theory presented in the next paper (Fenton et al 2000). …”

“…Specifically, the pattern of displacement vectors for reconfigurations is substantially the same for field peak as for field centroid (not shown). The importance of choice of field center measure is raised again in the next paper (Fenton et al 2000). …”

“…We have, however, already taken a parallel approach in which we model the effects of card removal and reconfiguration with a formal mathematical model (Fenton et al 2000). This strictly empirical model is built to account for the effects of removing one card or the other and for changing the separation between the cards.…”

What can the hippocampal representation of environmental geometry tell us about Hebbian learning?

“…There have been hints throughout the place cell literature that alterations in a familiar environment sometimes cause firing rate reduction in many cells in the altered environment, as though there were a learned template firing pattern. This was specifically studied in Fenton and Muller (2000) with clear results stating that in a circle with two cards on the internal wall, firing rates were significantly lower in the two manipulations where the cards were placed further apart and closer together than in the standard trained configuration.…”