The Latent Geometry of Memory Consolidation in Nocturnal Primates

 This study investigates the previously uncharted relationship between rapid eye movement sleep and spatial memory encoding in a nocturnal primate species, Galago senegalensis. Using high-density electrophysiological recordings over 18 months, we observed that hippocampal sharp-wave ripples occur in non-random, clustered sequences during post-prandial sleep phases. These clusters correlate with the reactivation of specific navigational routes the subjects explored during twilight hours. Unlike diurnal rodents, galagos exhibit a 37% longer persistence of place cell firing patterns, suggesting a phylogenetic adaptation to low-light foraging. We further identified a novel oscillatory band, termed "epsilon rhythm" (42–55 Hz), that appears exclusively during the transition from slow-wave to REM sleep. This rhythm phase-locks with thalamocortical spindles, potentially gating the transfer of episodic-like memories to the anterior cingulate cortex. Lesioning the suprachiasmatic nucleus abolished epsilon rhythms without disrupting basic sleep architecture, leading to fragmented memory recall in T-maze alternation tasks. Pharmacologically enhancing epsilon activity via retigabine (a KCNQ channel opener) improved memory retention by 62% compared to controls. Our results challenge the prevailing "sequential replay" model by proposing a parallel consolidation pathway that operates during micro-arousals. Computational modeling suggests that epsilon oscillations serve as a temporal scaffold, aligning hippocampal outputs with cortical plasticity windows. These findings have implications for understanding memory disorders tied to circadian disruption, such as shift work sleep disorder and certain dementias. Future work will explore whether epsilon rhythms exist in diurnal primates, including humans, under conditions of artificial light exposure. We conclude that memory consolidation is not a uniform process but is exquisitely tuned to ecological niche and sleep microarchitecture.