Sleep Talking as Dream Enacting Behavior: a new perspective to study cognition during sleep

Introduction: Sleep Talking (ST) is characterized by the production ofunaware linguistic vocalizations (STs) during sleep. On the one hand, STcould allow researchers the direct observation of cognitive processesduring sleep. Interestingly, recent findings reported the replay of verbalmaterial learned during an episode of ST. This phenomenon was consid-ered as an “overt replay” of high cognitive processes and may help sleep-related memory consolidation. However, data on this issue are still lacking,and the role of ST in memory consolidation is poorly understood.On the other hand, the vocal activations also showed the incorporations ofwaking experiences. Besides, some studies reported a high concordancebetween vocalizations and oneiric contents (Dream Enactment Behavior),providing a potential access to mental activity during sleep. It is knownthat the waking-life experiences could be incorporated according to theday-residue effect or dream-lag effect. Therefore, the incorporations of theverbal task or daily experiences could represent a reprocessing of semanticand/or autobiographical memories. Overall, the STs might permit thedirect observation of these cognitive processes ongoing.Our explorative study had two independent aims:1. The investigation of autobiographical incorporations into dream reportsand STs. According to the “continuity hypothesis,” the incorporations intoSTs could represent “day-residue effect” or “the dream-lag effect”;2. The assessment of ST's impact on memory consolidation, according totwo alternative hypotheses: (A) the replay of verbal content on STs in-creases the sleep-dependent gain (defined as the difference betweenmorning and evening recall) in the ST group, or (B) the sleep fragmentationdue to STs is associated to a decreased gain.Materials and Methods: We recruited N¼28 participants with ST (F¼23;age mean: 23.71) and N¼27 controls (F¼21; age mean: 24.44). For eightdays, participants performed home monitoring. They were instructed tocomplete daily logs (for seven evenings), sleep logs, and record theironeiric contents every morning. On the 8th day, a word-pair task wasadministered. ST subjects audio-recorded their vocal activations.Results: Results showed a higher gain in the control than ST group (t¼2.103; p¼0.04), but no significant correlation was observed between thenumber of STs and gain. Notably, one ST subject produced a wordsemantically related to the task and revealed an increase in the gain.The incorporations of wake-experiences in dream content revealed adream lag effect of personally significant events in both groups (F¼ 3.510;p¼0.04). There was no correspondence between the daily activities, dreamcontent, and STs, although some semantic correspondences were observedbetween STs and dream recalls (N¼4).Conclusions: In conclusion, our results supported the idea that STs couldrepresent the overt expression of semantic memories consolidation.The worst performance of the ST group seems coherent with the hy-pothesis that sleep fragmentation due to STs is associated with a decreasedsleep-dependent gain, although a polysomnographic assessment isneeded.Consistently with literature, STs represent a window towards the dreamactivity, and multiple awaking protocols immediately after STs couldprovide more information about correspondence between STs and dreamreports and the reprocessing of autobiographical memories