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Arousal state modulation of human hippocampal ripples across wake and sleep

Poster Session C - Sunday, March 8, 2026, 5:00 – 7:00 pm PDT, Fairview/Kitsilano Ballroom
Also presenting in Data Blitz Session 3 - Saturday, March 7, 2026, 10:30 am – 12:00 pm PST, Salon E.

Elizabeth M. Siefert¹ (), Yvonne Y. Chen², Kathryn A. Davis¹, H. Issac Chen¹, Anna C. Schapiro¹, Brett L. Foster¹; ¹University of Pennsylvania, ²University of Nevada, Las Vegas

Hippocampal replay involves the reactivation of spiking activity patterns from previous experiences and is thought to underlie memory consolidation. These replay events can be captured as high-frequency oscillations—termed ‘ripples’—in the hippocampal field potential. Ripples predominantly occur during states like sleep and awake rest, as these are offline periods thought to allow the replay of prior memories while limiting sensory interference. However, ripple rates vary dynamically across sleep stages and recent human studies have reported ripples during active tasks, suggesting their occurrence may not simply be driven by offline periods of inactivity. We propose a reframing of this offline view—whereby a common low arousal state is associated with ripple genesis, a state that is more prominent and persistent during certain stages of sleep and occurs transiently during waking. We recorded human hippocampal activity (iEEG) during overnight sleep and periods of wake, tracking arousal via sleep staging and pupillometry. Ripple rate varied profoundly across sleep stages, being maximal in NREM, reduced in REM, and minimal in wake. Interestingly, this modulation was stronger in anterior than posterior hippocampus. During wake, ripple rate similarly tracked arousal, increasing during small-pupil states, with the strongest modulation in anterior hippocampus. Altogether, across sleep and wake, ripple rate increased when arousal dipped. These results reframe offline periods as transient states within a continuous arousal spectrum, providing opportunistic moments for consolidation. Mechanistically, these results bridge arousal and ripple physiology, aligning with evidence implicating drops in acetylcholine—an arousal-related neurotransmitter—in ripple genesis and memory consolidation.

Topic Area: LONG-TERM MEMORY: Episodic