HIPPOCAMPAL PLACE FIELDS - RELATIONSHIP BETWEEN DEGREE OF FIELD OVERLAP AND CROSS-CORRELATIONS WITHIN ENSEMBLES OF HIPPOCAMPAL-NEURONS

The capacity to record from multiple neurons in awake freely moving an imals provides a means for characterizing organizational principles of place field encoding within ensembles of hippocampal neurons. In this study, cross-correlations between pairs of hippocampal place cells an d degree of overlap between their respective place fields were analyze d during behavioral performance of delayed matching (DMS) or nonmatchi ng sample (DNMS) tasks, or while the same rats chased pellets in a dif ferent environment. The relationship between field overlap and crossco rrelations of neural spike activity within ensembles was shown to be a positive, exponentially increasing, function. Place fields from the s ame neurons were markedly ''remapped'' between the Delay and Pellet-ch asing tasks, with respect to physical location and size of fields. How ever individual pairs of place cells within each ensemble retained nea rly the same degree of overlap and cross-correlation even though the s patial environment and the tasks differed markedly. This suggested tha t place cells were organized in functional ''clusters'' which exhibite d the same interrelations with respect to place field overlap and cros s-correlations, irrespective of actual field location. When cross-corr elations between place cells were compared to placement of the array r ecording electrodes within the hippocampus, the strongest correlations were found along previously defined posterior-projecting fiber gradie nts between CA3 and CA1 subfields (Ishizuka et al. [1990], J Comp Neur ol 295:580-623; Li et al. [1994] (J Comp Neurol 339:181-208). These fi ndings suggest that the functional organization of place fields confor ms to anatomical principles suspected to operate within hippocampal en sembles. (C) 1996 Wiley-Liss, Inc.