Sleeping brains watch for unknown voices and learn

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

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

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

Based on their observations, the authors claim that the sleeping brain enters a “sentinel processing mode” during which it retains 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 attunes to unfamiliar voices during sleep.”

Sleep and its neural mechanisms have puzzled the two scientists for centuries. The new study indicates that the human brain balances the need to sleep and wake up by filtering the environment for unfamiliar voices.

Following previous studies that showed the brain continues to process sensory information during sleep, researchers are assessing the brain’s ability to retrieve and process relevant sensory information during sleep in the current study.

Although we are primarily concerned with the total number of hours of sleep we get at night, sleep occurs in phases that are distinguished by specific brain waves and neural 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 across trials to different sensory stimuli during NREM sleep.

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

“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 showed that unfamiliar voices evoked larger, more synchronized brain responses as well as stronger power at frequencies above 16 Hz, compared to familiar voices. The authors also showed that the differences in brain responses disappeared in the absence of K complexes.

“Our results suggest a central 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 related 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 K-complexes evoked by unfamiliar voices in the second half compared to the first half of the night supports the idea that the sleeping brain continues to learn new information during sleep,” the authors write.

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