Firing rate modulation: a simple statistical view of memory trace reactivation.

Memory trace reactivation in hippocampal ensembles during sleep has been suggested as a coordinating mechanism for consolidation of new memories. Here we propose a simple statistical scheme allowing analysis of the reactivation of firing rate modulations, with a well-defined null hypothesis. This method allowed reliable detection of ensemble reactivation across three experimental settings. Reactivation of firing rate modulations mirrors several properties of commonly studied reactivation measures: it is stronger during hippocampal sharp waves, and decays over a period of 10-20 min. Moreover, in some conditions, firing rate reactivation covaries with reactivation of cell pair cross-correlations, suggesting the two phenomena reflect similar processes. We propose an attractor network model, with pre-wired attractors, in which experience selects and primes some attractors. Priming occurs by either experience dependent synaptic plasticity or changes in neuronal excitability. Primed attractors are more likely to activate in the following sleep, inducing reactivation of both rates and cross-correlations.