A new study led by scientists at the University of Salzburg shows the brain continues to respond selectively to stimuli in the environment, such as unfamiliar voices, during sleep.

The researchers conducted polysomnography recordings on participants of both sexes, who were presented with either their own or unfamiliar names, spoken in either a familiar or an unfamiliar voice, while they slept at night.

“Our findings highlight discrepancies in brain responses to auditory stimuli based on their relevance to the sleeper,” the authors note.

Based on their observations, the authors contend that the sleeping brain enters a “sentinel processing mode” during which it maintains the ability to process important external sensory information while engaged in internal sleep processes.

These new findings were published on January 17 in The Journal of Neuroscience in an article titled, “The brain selectively tunes to unfamiliar voices during sleep.”

Temporal aspects of the difference in the triggered K-complexes and micro-arousals. The difference between unfamiliar and familiar voices (UFV and FV) in the number of triggered K-complexes was significant from 100ms to 800ms (left). The difference in the number of micro-arousals between FVs and UFVs was significant in the periods from 200 to 400ms, and from 500 to 700ms (right) [Ameen et al., JNeurosci 2022]
Sleep and its neural mechanisms have confounded both scientists for ages. The new study indicates that the human brain balances the need to sleep and awake by screening the environment for unfamiliar voices.

Following earlier studies that have shown the brain continues to process sensory information during sleep, researchers evaluate the ability of the brain to extract and process relevant sensory information while asleep in the current study.

Although we are mostly concerned with the total hours of sleep we get at night, sleep occurs in phases distinguishable by specific brain waves and neuronal activity. These include rapid eye movement (REM) sleep and non-rapid eye movement (NREM) sleep.

In this study the research team detected K-complexes, sleep spindles, and micro-arousals, and assessed event-related responses and phase synchronizations between trials to different sensory stimuli during NREM sleep.

K-complexes are signature waveforms of approximately 1Hz frequency that can be seen on an EEG during the second phase of NREM sleep. These occur either spontaneously or in response to sensory stimuli and are believed to arise from the conflict between sensory activation and sleep-protecting influences.

“During NREM sleep, unfamiliar voices elicited more K-complexes and micro-arousals than familiar voices,” the authors note.

The researchers compared stimuli that evoked K-complexes, and show unfamiliar voices evoked larger and more synchronized brain responses as well as stronger power at frequencies higher than 16Hz, relative to familiar voices. The authors also showed that the differences in brain responses disappeared in the absence of K-complexes.

“Our results suggest a pivotal role for K-complexes in the selective processing of relevant information during NREM sleep,” the authors note.

While familiar voices also triggered K-complexes, only those triggered by unfamiliar voices were accompanied by large-scale changes in brain activity linked to sensory processing, the researchers observed.

That the brain is still able to learn during sleep was indicated by a reduction in brain responses to unfamiliar voices as the night progressed and the unfamiliar voices became more familiar.

“The decrease in the number of unfamiliar voice evoked K-complexes in the second half as compared to the first half of the night supports the notion that the sleeping brain continues to learn new information during sleep,” the authors write.

The new findings demonstrate the existence of a state where the brain retains the ability to respond to relevant stimuli while maintaining sleep processes. K-complexes, the authors believe, allow the brain to enter this “sentinel processing mode.”