0080 Selective Brain and Eye Responses to Auditory Stimuli During Phasic and Tonic REM Sleep

Abstract Introduction Rapid eye movement (REM) sleep is divided into phasic and tonic episodes based on the presence or absence of eye movements, respectively. Phasic REM, often linked with dreaming, is traditionally seen as a phase of brain isolation from the surroundings. On the contrary, during tonic REM periods, the brain is thought to maintain a stronger connection to the environment and a higher sensitivity to external stimuli. However, the processing of external information during these distinct REM microstates is not well-understood. Here, we aim to investigate sensory processing during REM microstates by analyzing brain and eye responses to different auditory stimuli. Methods We analyzed high-density electroencephalography (EEG) data from 17 healthy subjects over a full night's sleep. Auditory stimuli included the subjects' own name (SON) and two unfamiliar names (UNs), spoken by familiar (FV) and unfamiliar voices (UFV). REM sleep was categorized into one-second epochs of either phasic and tonic episodes based on rapid eye movement occurrence. Our analysis encompassed multivariate decoding, event-related and time-frequency analyses, examination of aperiodic EEG activity, and measuring the velocity of the evoked eye movement responses. Results Tonic REM showed more microarousals and a shallower EEG slope than phasic REM, suggesting increased external processing and more excitatory cortical activity. Further, auditory brain responses during tonic REM revealed stronger alpha (8-12 Hz) and beta (13-30 Hz) responses to responses to UFVs as compared to FVs. In contrast, during phasic REM, we observed stronger desynchronization in the delta (1-4 Hz) frequency range in response to UFVs as compared to FVs. No difference in brain responses to the different names was observed. However, eye responses to SONs were notably stronger than to UNs during phasic REM periods. Conclusion Our findings support existing theories that suggest a preference for unfamiliar stimuli processing during sleep. We extend previous findings by showing that such preference is perserved in REM sleep as well. We also found that eye movements in phasic REM can distinguish between sounds, indicating partial brain responsiveness to the environment during dreaming episodes. These results enhance our understanding of REM sleep's complex nature and the neural activities occurring during this intriguing sleep stage. Support (if any)