Link to bioRxiv paper: http://biorxiv.org/cgi/content/short/2020.04.10.035980v1?rss=1 Authors: Deng, X., Chen, S., Sosa, M., Karlsson, M. P., Wei, X.-X., Frank, L. M. Abstract: Humans have the ability to retrieve memories with various degrees of specificity, and recent advances in reinforcement learning have identified benefits to learning when past experience is represented at different levels of temporal abstraction. How this flexibility might be implemented in the brain remains unclear. We analyzed the temporal organization of rat hippocampal population spiking to identify potential substrates for temporally flexible representations. We examined activity both during locomotion and during memory-retrieval-associated population events known as sharp wave-ripples (SWRs). We found that spiking during SWRs is rhythmically organized with higher event-to-event variability than spiking during locomotion-associated population events. Decoding analyses using clusterless methods further suggest that similar spatial experience can be replayed in multiple SWRs, each time with a different rhythmic structure whose periodicity is sampled from a lognormal distribution. This variability is preserved despite the decline in SWR rates that occurs as environments become more familiar: in more familiar environments the width of the lognormal distribution increases, further enhancing the range of temporal variability. We hypothesize that the variability in temporal organization of hippocampal spiking provides a mechanism for retrieving remembered experiences with various degrees of specificity. Copy rights belong to original authors. Visit the link for more info