Hippocampus-retrosplenial cortex interaction is increased during phasic REM and contributes to memory consolidation

Abstract

Hippocampal (HPC) theta oscillation during post-training rapid eye movement (REM) sleep supports spatial learning. Theta also modulates neuronal and oscillatory activity in the retrosplenial cortex (RSC) during REM sleep. To investigate the relevance of theta-driven interaction between these two regions to memory consolidation, we computed the Granger causality within theta range on electrophysiological data recorded in freely behaving rats during REM sleep, both before and after contextual fear conditioning. We found a training-induced modulation of causality between HPC and RSC that was correlated with memory retrieval 24 h later. Retrieval was proportional to the change in the relative influence RSC exerted upon HPC theta oscillation. Importantly, causality peaked during theta acceleration, in synchrony with phasic REM sleep. Altogether, these results support a role for phasic REM sleep in hippocampo-cortical memory consolidation and suggest that causality modulation between RSC and HPC during REM sleep plays a functional role in that phenomenon.

Sleep scoring in rodents: Criteria, automatic approaches and outstanding issues

“…Similarly, one can distinguish phasic and tonic REM in rodents (S anchez-L opez & Escudero, 2011); however, because EMG and electrooculogram (EOG) electrodes are much more invasive and difficult to include in rodents, a differentiation is rarely done. It is possible to use recordings from the hippocampus for this separation because phasic REM displays a transient acceleration of the theta rhythm lasting 2 s, with increased hippocampal neuronal activity, increased power of high-frequency oscillations and enhanced theta-and gamma-band coherence within the hippocampal formation (de Almeida-Filho et al, 2021). Hippocampal recordings, whereas rare in humans, are more often included in rodent sleep studies, especially if the hippocampal ripple oscillations are within the scope of the study.…”

Is paradoxical sleep setting up innate and acquired complex sensorimotor and adaptive behaviours?: A proposed function based on literature review

“…Importantly, causality peaked during theta acceleration, in synchrony with phasic PS. Altogether, these results support a role for phasic REM sleep in hippocampo‐cortical memory consolidation, and suggest that causality modulation between RSC and the hippocampus during PS plays a functional role in that phenomenon (de Almeida‐Filho et al, 2021).…”

Memory reactivations during sleep: a neural basis of dream experiences?

“…Although much less frequently than in NREM sleep, SWRs may also be embedded into theta waves (4-12 Hz [19]) and sawtooth waves (2-6 Hz [135]) during REM sleep. Moreover, transient increases in theta activity during phasic REM periods are associated with increased coordination throughout the hippocampus and cortical areas [136,137]. In accordance with these observations, so-called targeted memory reactivation (TMR; Box 1) [138] protocols, which induce memory reactivation, may lead to increases in spindle (e.g., [139][140][141]), slow-wave (e.g., [142][143][144]), and/or theta activity [144][145][146].…”