Replay and time compression of recurring spike sequences in the hippocampus

Information in neuronal networks may be represented by the spatiotemporal p atterns of spikes. Here we examined the temporal coordination of pyramidal cell spikes in the rat hippocampus during slow-wave sleep. In addition, rat s were trained to run in a defined position in space (running wheel) to act ivate a selected group of pyramidal cells. A template-matching method and a joint probability map method were used for sequence search. Repeating spik e sequences in excess of chance occurrence were examined by comparing the n umber of repeating sequences in the original spike trains and in surrogate trains after Monte Carlo shuffling of the spikes. Four different shuffling procedures were used to control for the population dynamics of hippocampal neurons. Repeating spike sequences in the recorded cell assemblies were pre sent in both the awake and sleeping animal in excess of what might be predi cted by random variations. Spike sequences observed during wheel running we re "replayed" at a faster timescale during single sharp-wave bursts of slow -wave sleep. We hypothesize that the endogenously expressed spike sequences during sleep reflect reactivation of the circuitry modified by previous ex perience. Reactivation of acquired sequences may serve to consolidate infor mation.