Hippocampal gamma and sharp wave/ripples mediate bidirectional interactions with cortical networks during sleep

Hippocampus-neocortex interactions during sleep are critical for memory processes: hippocampally-initiated replay contributes to memory consolidation in the neocortex and hippocampal sharp wave/ripples are linked to generalized increases in neocortical cell activity and DOWN-UP state transitions. Yet, the spatial and temporal patterns of this exchange are unknown. With voltage imaging, electrocorticography, and laminarly-resolved hippocampal potentials, we characterized cortico-hippocampal interactions during anesthesia and NREM sleep. We observed neocortical activation transients spanning multiple spatial scales hinting at a quasi-critical regime. Transients were organized in a small number of functional networks matching known anatomical connectivity. A network overlapping with the default mode network and centered on retrosplenial cortex was the most associated with the hippocampus. Interestingly, hippocampal slow gamma was the oscillation that best correlated with this neocortical network, outpacing ripples. In fact, neocortical activity predicted hippocampal slow gamma and followed ripples, suggesting that consolidation processes rely on bi-directional exchanges between hippocampus and neocortex.