Continued Evaluation of Environmental Stimuli in the Absence of Consciousness suggests the Human Brain is Standing Sentinel during Sleep

While it is well-known that subject’s own names (SON) or familiar voices are salient during wakefulness, we investigate stimulus processing during sleep including N3 and REM sleep. Additionally, we investigate how sleep EEG patterns (i.e. sleep spindles and slow oscillations [SOs]) relate to stimulus processing. Using 256-channel EEG we studied stimulus processing by means of event-related oscillatory responses (de-/synchronisation, ERD/ERS) and potentials (ERPs). We varied stimulus salience by manipulating subjective (SON vs. unfamiliar name) and paralinguistic emotional relevance (familiar vs. unfamiliar voice, FV/UFV). We show that evaluation of voice familiarity continues during all NREM sleep stages and even REM sleep suggesting a ‘sentinel processing mode’ in the absence of consciousness. Especially UFV stimuli elicit larger responses in a 1-15Hz range suggesting they are salient. Unlike previously suggested sleep spindles and the negative slope of SOs do not uniformly inhibit information processing but inhibition seems to be tuned to stimulus salience.