Cortical activation during sleep predicts dream experience in narcolepsy.

Objective: Narcolepsy type 1 widely affects the architecture of sleep with frequent fast transition to REM sleep at both nighttime and daytime sleep onset. The occurrence of repeated sleep onset REM periods over the Multiple Sleep Latency Test offers a unique opportunity to identify EEG patterns predictive of successful dream recall after short periods composed of only REM or NREM sleep. It also permits to disentangle state- from trait-like differences in dream recall, by using a within-subjects design. Methods: A consecutive series of 115 first-diagnosed drug-free adult narcolepsy-type 1 patients underwent Multiple Sleep Latency Tests and were asked after each nap opportunity if they had or had not a dream experience. Scalp EEG power and a specific index of cortical activation (delta/beta power ratio), obtained from naps of 43 patients with both presence and absence of dream recall in the same sleep stage, were compared separately for REM and NREM sleep. Results: Successful dream recall was associated with an increased EEG desynchronization in both REM and NREM over partially overlapping cortical areas. Compared to unsuccessful recall, it showed (1) lower delta power over centro-parietal areas during both stages, (2) higher beta power in the same cortical areas during NREM, and (3) lower values in the delta/beta ratio during NREM in most scalp locations. Interpretation: A more activated electrophysiological milieu in both REM and NREM sleep promotes dream recall, strengthening the notion that the parietal areas are crucial not only in generating dream experience, as shown in brain-damaged patients, but also in the memory processing leading to recall.

Predicting Dream Recall: EEG Activation During NREM Sleep or Shared Mechanisms with Wakefulness?

The common knowledge of a uniqueness of REM sleep as a privileged scenario of dreaming still persists, although consolidated empirical evidence shows that the assumption that dreaming is just an epiphenomenon of REM sleep is no longer tenable. However, the brain mechanisms underlying dream generation and its encoding in memory during NREM sleep are still mostly unknown. In fact, only few studies have investigated on the mechanisms of dream phenomenology related to NREM sleep. For this reason, our study is specifically aimed to elucidate the electrophysiological (EEG) correlates of dream recall (DR) upon NREM sleep awakenings. Under the assumption that EEG activity predicts the presence/absence of DR also during NREM sleep, we have investigated whether DR from stage 2 NREM sleep shares similar brain mechanisms to those involved in the encoding of episodic memory during wakefulness, or it depends on the specific electrophysiological milieu of the sleep period along the desynchronized/synchronized EEG continuum. We collected DR from a multiple nap protocol in a within-subjects design. We found that DR is predicted by an extensive reduction of delta activity during the last segment of sleep, encompassing left frontal and temporo-parietal areas. The results could represent an update on the mechanisms underlying the sleep mentation during NREM sleep. In particular, they support the hypothesis that an increased cortical EEG activation is a prerequisite for DR, and they are not necessarily in conflict with the hypothesis of common wake-sleep mechanisms. We also confirmed that EEG correlates of DR depend on a state-like relationship.