During REM sleep, the brain flexibly amplifies or suppresses external sounds depending on eye movements. Using EEG to reconstruct speech from brain responses in a multi-talker environment, meaningful speech was amplified over meaningless speech overall. However, at the precise moments of rapid eye movements, meaningful speech was selectively suppressed. This shows that eye movements during REM sleep act as a gate, selectively blocking informative external stimuli while allowing them at other times, resolving a long-standing debate about whether the sleeping brain processes or ignores the outside world.
The brain's ability to detect emotional signals persists during non-rapid eye movement (NREM) sleep. Using electroencephalography, neural responses to emotional voices differed from neutral voices both while awake and during NREM sleep. Emotional voices also triggered more EEG arousals (sleep perturbations) than neutral or control stimuli. The amplitude of sleep event-related potentials and the number of arousals after emotional voices increased linearly with how often participants recalled their dreams. Similar correlations with dream recall frequency appeared for beta and sigma brain oscillations, but not for theta. No such correlations were found for neutral or control stimuli. These findings suggest that emotion-specific neural reactivity during sleep, rather than general alertness, may contribute to encoding or retrieving dreams.