Sleeping brain oscillates with intensity-induced auditory rhythm.

The rhythmic patterns embedded in auditory stimuli hold considerable significance, as individuals often exhibit a subconscious tendency to synchronize bodily movements with perceived rhythms during wakefulness. However, the extent to which the sleeping brain can discern specific acoustic attributes, such as sound intensity, and entrain to intensity-induced rhythms remains largely unexplored. Here, we employed a frequency-tagging paradigm with electroencephalographic (EEG) and magnetoencephalographic (MEG) simultaneous recording to investigate whether the sleeping brain could perceive the intensity-induced rhythm. Human participants were presented with a sequence of vocal syllables at a certain frequency with changed sound intensity while asleep and awake. The results showed that cortical activity oscillated periodically at the intensity-changed frequency, with a selective enhancement of spectral power at this frequency observed in the MEG spectrum during both rapid eye movement (REM) sleep and non-REM (NREM) sleep, although weaker than during wakefulness. Our findings further revealed increased engagement of the superior frontal gyrus and inferior parietal lobe during rhythm processing in sleep compared to wakefulness, suggesting a sleep-dependent enhancement of higher-order temporal organization of rhythmic information. Across different sleep stages, we found that the neural signals in light sleep were stronger than those in deep sleep and REM sleep at the vocal syllable frequency, and there was no significant difference in the rhythm frequency among the three sleep stages. These results suggest that processing of intensity-induced rhythms is preserved during both NREM and REM sleep and is associated with activity across frontal and parietal cortical regions.