Oscillatory EEG Activity During REM Sleep in Elderly People Predicts Subsequent Dream Recall After Awakenings

Abstract

Several findings underlined that the electrophysiological (EEG) background of the last segment of sleep before awakenings may predict the presence/absence of dream recall (DR) in young subjects. However, little is known about the EEG correlates of DR in elderly people. Only an investigation found differences between recall and non-recall conditions during NREM sleep EEG in older adults, while—surprisingly—no EEG predictor of DR was found for what concerns REM sleep. Considering REM sleep as a privileged scenario to produce mental sleep activity related to cognitive processes, our study aimed to investigate whether specific EEG topography and frequency changes during REM sleep in elderly people may predict a subsequent recall of mental sleep activity. Twenty-one healthy older volunteers (mean age 69.2 ± 6.07 SD) and 20 young adults (mean age 23.4 ± 2.76 SD) were recorded for one night from 19 scalp derivations. Dreams were collected upon morning awakenings from REM sleep. EEG signals of the last 5 min were analyzed by the Better OSCillation algorithm to detect the peaks of oscillatory activity in both groups. Statistical comparisons revealed that older as well as young individuals recall their dream experience when the last segment of REM sleep is characterized by frontal theta oscillations. No Recall (Recall vs. Non-Recall) × Age (Young vs. Older) interaction was found. This result replicated the previous evidence in healthy young subjects, as shown in within- and between-subjects design. The findings are completely original for older individuals, demonstrating that theta oscillations are crucial for the retrieval of dreaming also in this population. Furthermore, our results did not confirm a greater presence of the theta activity in healthy aging. Conversely, we found a greater amount of rhythmic theta and alpha activity in young than older participants. It is worth noting that the theta oscillations detected are related to cognitive functioning. We emphasize the notion that the oscillatory theta activity should be distinguished from the non-rhythmic theta activity identified in relation to other phenomena such as (a) sleepiness and hypoarousal conditions during the waking state and (b) cortical slowing, considered as an EEG alteration in clinical samples.

Local Sleep and Alzheimer’s Disease Pathophysiology

“…The brain expresses a complex and dynamic organization of oscillatory activities during sleep, cycling between NREM and REM sleep stages that each distinctly organize local brain activities across frequency, topography, and time (Landolt et al, 1996;Liscombe et al, 2002;Steriade, 2006;Diekelmann and Born, 2010;Staresina et al, 2015;Watson and Buzsaki, 2015;Sprecher et al, 2016;Helfrich et al, 2018;Scarpelli et al, 2019). The spatially circumscribed nature of the expression of these sleep-specific oscillatory activities and their frequency specificity highlight the local nature of some canonical sleep features.…”

“…In terms of spectral content, both alpha and theta power are particularly prominent in REM sleep (Landolt et al, 1996), with theta peaking over fronto-central derivations, and alpha peaking over occipito-parietal derivations (Scarpelli et al, 2019), Similar to theta activity, higher frequency content, including beta and gamma power, peak over fronto-central derivations, but while low frequency power, including delta and theta power decrease across successive REM sleep periods, high frequency content remains relatively stable (Liscombe et al, 2002). Little is known about the functional relevance of the organized expression of EEG activities during REM sleep, and this should be a topic of future study.…”

<p>Electrophysiological Correlates of Dream Recall During REM Sleep: Evidence from Multiple Awakenings and Within-Subjects Design</p>

“…Contrary to the initial continuity-hypothesis, we did not observe any differences between REC and NREC conditions in the theta range, recognized as crucial in the retrieving episodic memory and dream contents. 2,6,7,33 Notwithstanding, we have to underline that continuityand activation-hypothesis are not mutually exclusive. Actually, the local activation of the posterior cerebral cortex is a consolidated marker of consciousness and visual experiences also during wakefulness.…”

“…Besides, our sample is totally composed of young university students and this could represent a bias that does not allow us to generalize our results. For instance, since the literature on dreaming during lifespan is very poor, 7,8,35 within-subject studies will carry out on larger age ranges (eg, older adults; children). Although the difficulty of obtaining the NREC condition during REM sleep is well known, 1 our sample consisting of 10 subjects with within-subject comparisons is relatively small and further investigation should enlarge the sample size to strengthen the current findings.…”

“…Studies on neural correlates of dream experience suggested the potential influences of circadian, ultradian, and homeostatic factors on dream features. 1 Most investigations neglected the impact of these variables on EEG predictors of dream recall (DR) [2][3][4][5][6][7][8] and very few studies provided information about this issue.…”