Electrophysiology of vocalizations in Sleep Talking: Predictive marker or abnormal expression of a physiological mechanism?

Introduction: The literature about NREM parasomnias reports an increase in high and low EEG frequency before the behavioural episodes that occurred during slow-wave sleep. This EEG pattern could be an electrophysiological predictor of episode onset. However, recent evidence has shown that NREM awakenings, regardless of behavioural manifestations, are associated with a slow-wave synchronization correlated with rapid EEG activity predominantly in frontocentral areas. Sleep Talking (ST) is a parasomnia characterized by linguistic activations during sleep and other nonverbal sleep utterances (moaning, crying, and laughter. In its isolated form, it has been observed that vocal manifestations (verbal and nonverbal) occur most frequently in stage 2 of NREM sleep. Therefore, the present study analysed the EEG patterns preceding vocalizations from N2 aiming to identify the electrophysiological processes permissive to such activations in a stage of sleep that is not characterized by slow-wave activity. Method: We recorded 12 sleep talkers (F = 11, M = 1; 18–35 years) via 64-channel video-polysomnographic and performed a multiple awakenings protocol. We selected epochs of 4 s immediately prior to 32 vocalizations produced in N2 and 32 epochs extracted during a comparable continuous sleep period. Then, the Student's t-tests for paired samples were performed for each scalp location. Results: The comparisons showed a significant increase in the beta band in fronto-central areas preceding the onset of vocal activations compared with continuous sleep. Moreover, a parallel but not significant increase in the delta activity was observed in the same areas. Conclusion: The results are partially consistent with the literature on parasomnias. Rapid EEG activity, characterized the behavioural episodes onset in association with slow wave activity (SWA), has been repeatedly reported. In contrast to previous studies, we analysed the EEG before ST episodes onset during N2, which may explain the lack of differences concerning the delta activity. Although the increase of SWA was not significant, it seems that the arousal observed before the vocal productions, associated with a slower EEG background, is permissive to ST manifestations and could represent the abnormal expression of a physiological mechanism across different parasomnias and sleep stages.