An electrophysiological marker of arousal level in humans

Deep non-rapid eye movement sleep (NREM) and general anesthesia with
propofol are prominent states of reduced arousal linked to the occurrence
of synchronized oscillations in the electroencephalogram (EEG). Although
rapid eye movement (REM) sleep is also associated with diminished arousal
levels, it is characterized by a desynchronized, 'wake-like'
EEG. This observation implies that reduced arousal states are not
necessarily only defined by synchronous oscillatory activity. Using
intracranial and surface EEG recordings in four independent data sets, we
demonstrate that the 1/f spectral slope of the electrophysiological power
spectrum, which reflects the non-oscillatory, scale-free component of
neural activity, delineates wakefulness from propofol anesthesia, NREM and
REM sleep. Critically, the spectral slope discriminates wakefulness from
REM sleep solely based on the neurophysiological brain state. Taken
together, our findings describe a common electrophysiological marker that
tracks states of reduced arousal, including different sleep stages as well
as anesthesia in humans.