This view has been recently challenged by a growing body of evidence indicating that many features of sleep are essentially local and that islands of sleep- and wake-like activity may coexist in different brain areas ( Siclari and Tononi, 2017).ĮEG slow waves of NREM sleep occur when neurons become bistable and oscillate between two states: a hyperpolarized down-state characterized by neuronal silence (off-period), and a depolarized up-state during which neurons fire (on-period) ( Steriade et al., 2001). Given these differences, it has been commonly assumed that wakefulness, NREM sleep, and REM sleep represent mutually exclusive “global” states. Although REM sleep is characterized by rapid eye movements (EMs) and muscular atonia and by a tonically “activated” (low-voltage, high-frequency) EEG resembling that of wakefulness ( Aserinsky and Kleitman, 1953 Dement and Kleitman, 1957), hallmarks of NREM sleep include high-amplitude, slow waves (≤4 Hz) and spindles (12–16 Hz) ( Steriade et al., 1993, 2001). Based on electrophysiological hallmarks, sleep is divided into non-rapid eye movement (NREM) sleep and REM sleep. Sleep is characterized by relative quiescence and reduced responsiveness to external stimuli. These findings indicate that REM sleep is a spatially and temporally heterogeneous state and may contribute to explaining its known functional and phenomenological properties. In particular, we identified two distinctive clusters of delta waves with different properties: a frontal-central cluster characterized by faster, activating “sawtooth waves” that share many characteristics with ponto-geniculo-occipital waves described in animals and a medial-occipital cluster containing slow waves that are more similar to NREM sleep slow waves. By analyzing high-density EEG recordings collected in healthy adult individuals, we show that REM sleep is characterized by prominent delta waves also in humans. SIGNIFICANCE STATEMENT The EEG slow wave is typically considered a hallmark of nonrapid eye movement (NREM) sleep, but recent work in mice has shown that it can also occur in REM sleep. Sawtooth waves, which are exclusive to REM sleep, share many characteristics with ponto-geniculo-occipital waves described in animals and may represent the human equivalent or a closely related event, whereas medial-occipital slow waves appear similar to NREM sleep slow waves. Therefore, delta waves are an integral part of REM sleep in humans and the two identified subtypes (sawtooth and medial-occipital slow waves) may reflect distinct generation mechanisms and functional roles. We identified two clusters of delta waves with distinctive properties: (1) a frontal-central cluster characterized by ∼2.5–3.0 Hz, relatively large, notched delta waves (so-called “sawtooth waves”) that tended to occur in bursts, were associated with increased gamma activity and rapid eye movements (EMs), and upon source modeling displayed an occipital-temporal and a frontal-central component and (2) a medial-occipital cluster characterized by more isolated, slower (<2 Hz), and smaller waves that were not associated with rapid EMs, displayed a negative correlation with gamma activity, and were also found in NREM sleep. Here, we investigated the presence and cortical distribution of negative delta (1–4 Hz) waves in human REM sleep by analyzing high-density EEG sleep recordings obtained in 28 healthy subjects. Although the EEG slow wave of sleep is typically considered to be a hallmark of nonrapid eye movement (NREM) sleep, recent work in mice has shown that slow waves can also occur in REM sleep.
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