In PLoS, Sidarta Ribeiro, Damien Gervasoni , Ernesto S. Soares, Yi Zhou , Shih-Chieh Lin , Janaina Pantoja , Michael Lavine , Miguel A. L. Nicolelis report that after rats explored novel objects, patterns similar to the neural patterns evoked by those objects repeated in diverse areas of the brain for two days after exposure, especially during slow-wave sleep.
Experimenters recorded extracellularly from 59 to 159 neurons simultaineously in each animal, continuously throughout the experiment. They recorded from a variety of areas: HP, primary somatosensory ?barrel field? CX, ventral posteromedial thalamic nucleus (TH), and putamen (PU). They note that “…no sign of neuroanatomical specificity was found in the correlations measured, and in particular no significant differences between hippocampal and extrahippocampal areas could be detected.”
Rats were only exposed to the novel stimuli for 1 hour, but similar neural activity patterns “reverberated” for 48 hours after exposure. Reverberations (correlated patterns, using their chosen measure of correlation) occured even duration wakefulness, but were more pronounced during sleep.
Note that the replayed patterns were inexact, or “low-fidelity”:
“…neuronal ensemble correlations …. were typically small (on the order of 0.1?0.3)….Indeed, not a single template-to-target match (out of 979,200 matches sampled) yielded correlation values higher than 0.45, indicating that novelty-induced neuronal reverberation occurs at low fidelity. It has been proposed that a high-fidelity replay of neuronal firing patterns during sleep may be achieved assuming that replayed patterns can undergo time compression and expansion (Nadasdy et al. 1999; Louie and Wilson 2001; Lee and Wilson 2002). We assessed this possibility thoroughly, but found no evidence of such effects in any of the forebrain sites recorded.”
The authors notes that the reverberations seemed to be more intense when the mean firing rate of neurons was low (for example, in slow wave sleep). This argues that “…the importance of sleep for memory consolidation stems from the offline processing of memory traces, i.e., from the absence of sensory interference (Jenkins and Dallenbach 1924; Melton and Irwin 1940; Winson 1985)”
Also, “neural activity sampled when animals were aroused, but not touching the objects, yielded neuronal reverberation that was nearly identical to that obtained when animals made sensory contact with the objects.”
Ribeiro S, Gervasoni D, Soares ES, Zhou Y, Lin S, et al. (2003) Long-Lasting Novelty-Induced Neuronal Reverberation during Slow-Wave Sleep in Multiple Forebrain Areas. PLoS Biol 2(1): e24 DOI:10.1371/journal.pbio.0020024. Synposis.