DYNAMICS OF MISMATCH CORRECTION IN THE HIPPOCAMPAL ENSEMBLE CODE FOR SPACE - INTERACTION BETWEEN PATH INTEGRATION AND ENVIRONMENTAL CUES

Populations of hippocampal neurons were recorded simultaneously in rat s shuttling on a track between a fixed reward site at one end and a mo vable reward site, mounted in a sliding box, at the opposite end. Whil e the rat ran toward the fixed site, the box was moved. The rat return ed to the box in its new position. On the initial part of all journeys , cells fired at fixed distances from the origin, whereas on the final part, cells fired at fixed distances from the destination. Thus, on o utward journeys from the box, with the box behind the rat, the positio n representation must have been updated by path integration. Farther a long the journey, the place field map became aligned on the basis of e xternal stimuli. The spatial representation was quantified in terms of population vectors. During shortened journeys, the vector shifted fro m an alignment with the origin to an alignment with the destination. T he dynamics depended on the degree of mismatch with respect to the ful l-length journey. For small mismatches, the vector moved smoothly thro ugh intervening coordinates until the mismatch was corrected. For larg e mismatches, it jumped abruptly to the new coordinate. Thus, when mis matches occur, path integration and external cues interact competitive ly to control place-cell firing. When the same box was used in a diffe rent environment, it controlled the alignment of a different set of pl ace cells. These data suggest that although map alignment can be contr olled by landmarks, hippocampal neurons do not explicitly represent ob jects or events.