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

Introduction: The literature about NREM parasomnias reports anincrease in high and low EEG frequency before the behavioural episodesthat occurred during slow-wave sleep. This EEG pattern could bean electrophysiological predictor of episode onset. However, recentevidence has shown that NREM awakenings, regardless of behaviouralmanifestations, are associated with a slow-wave synchronization correlatedwith rapid EEG activity predominantly in frontocentral areas.Sleep Talking (ST) is a parasomnia characterized by linguistic activationsduring 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 ofNREM sleep. Therefore, the present study analysed the EEG patternspreceding vocalizations from N2 aiming to identify the electrophysiologicalprocesses permissive to such activations in a stage of sleep thatis 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 multipleawakenings protocol. We selected epochs of 4 s immediately prior to32 vocalizations produced in N2 and 32 epochs extracted during acomparable continuous sleep period. Then, the Student's t-tests forpaired samples were performed for each scalp location.Results: The comparisons showed a significant increase in the betaband in fronto-central areas preceding the onset of vocal activationscompared with continuous sleep. Moreover, a parallel but not significantincrease in the delta activity was observed in the same areas.Conclusion: The results are partially consistent with the literature onparasomnias. Rapid EEG activity, characterized the behavioural episodesonset in association with slow wave activity (SWA), has been repeatedlyreported. In contrast to previous studies, we analysed the EEG before STepisodes onset during N2, which may explain the lack of differences concerningthe 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 manifestationsand could represent the abnormal expression of a physiologicalmechanism across different parasomnias and sleep stages.