Spatial, behavioral and sensory correlates of hippocampal CA1 complex spike cell activity: implications for information processing functions.

The aim of this review is to better understand hippocampal function drawing almost entirely from single unit recording studies of pyramidal cells in areas CA1 and CA3 of behaving animals. Hippocampal location-selectivity ("place cell activity") as well as place-independent behavioral correlates and sensory-triggered discharges are demonstrated to have common features: (1) abstraction, that is, development within the hippocampal circuit of novel, cue-invariant supramodal representations; (2) varying degrees of generalization or specificity; (3) capacity for abrupt changes in discharge correlates of individual neurons as the animal changes its behavior pattern or its environment changes dramatically; (4) though individual neurons discharge when the subject occupies a certain place, or performs a certain behavior, the ensemble of hippocampal neurons comprehensively represent the whole environment and all behaviors required for the task at hand. A concordance is proposed: hippocampal neuronal discharge correlates represent elements partitioned from information abstracted along one or more systems of categorization or "information domains": the physical structure of the environment, regularities in the behavioral exigencies of the current situation. (Sensory stimuli can be considered as temporally varying features of the environment) Location-selectivity and behavioral correlates are extreme cases, and mixed correlates occur. The hippocampus is proposed to carry out several fundamental processes that transform information: abstraction, partitioning and recombination, that is, formation of conjunctive associations between events. Simultaneously activated neurons could then promote extrahippocampal associations linking together the diverse brain regions at the origin of these signals.