Abstract
Sleep is characterized by a loss of consciousness, which has been attributed to a breakdown of functional connectivity between brain regions. Global field synchronization (GFS) can estimate functional connectivity of brain processes. GFS is a frequency-dependent measure of global synchronicity of multi-channel EEG data. Our aim was to explore and extend the hypothesis of disconnection during sleep by comparing GFS spectra of different vigilance states. The analysis was performed on eight healthy adult male subjects. EEG was recorded during a baseline night, a recovery night after 40 h of sustained wakefulness and at 3 h intervals during the 40 h of wakefulness. Compared to non-rapid eye movement (NREM) sleep, REM sleep showed larger GFS values in all frequencies except in the spindle and theta bands, where NREM sleep showed a peak in GFS. Sleep deprivation did not affect GFS spectra in REM and NREM sleep. Waking GFS values were lower compared with REM and NREM sleep except for the alpha band. Waking alpha GFS decreased following sleep deprivation in the eyes closed condition only. Our surprising finding of higher synchrony during REM sleep challenges the view of REM sleep as a desynchronized brain state and may provide insight into the function of REM sleep.
Highlights
Sleep is a behaviour observed in all organisms studied far
Sleep stages are shown in figure 1a, the power density and Global field synchronization (GFS) spectrograms across the night as a function of frequency in figure 1b and figure 1c, respectively
The increase in GFS in the spindle frequency during non-rapid eye movement (NREM) sleep is reflected in the all-night mean of stage 2 (S2), slow wave and NREM sleep depicted in figure 1d
Summary
Among the behavioural definition of sleep is a significant decline in responsiveness to external stimuli This reduction in responsiveness, or consciousness, during sleep has been attributed to the breakdown of connectivity between brain regions [1]. According to this idea, called the integrated information theory, consciousness arises when information is integrated across brain regions [1] Support for this conjecture comes from experimental studies that have used simultaneous high-density EEG and transcranial magnetic stimulation (TMS) to examine signal propagation during waking and sleep. These studies have found that when compared with waking, in which the TMS-induced signal propagates across brain regions for up to 300 ms, during early non-rapid eye movement (NREM) sleep the activity induced by TMS remains localized to the site of stimulation and lasts less than 150 ms [2]. The TMS response during REM sleep was more localized than that observed during waking, suggesting that the response to REM sleep is in between that of waking and NREM sleep
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