Symposium: Cognitive processes and sleep disturbances: Sleep, dreams and memory: an overview

Whole-body procedural learning benefits from targeted memory reactivation in REM sleep and task-related dreaming

Sleep facilitates memory consolidation through offline reactivations of memory traces. Dreaming may play a role in memory improvement and may reflect these memory reactivations. To experimentally address this question, we used targeted memory reactivation (TMR), i.e., application, during sleep, of a stimulus that was previously associated with learning, to assess whether it influences task-related dream imagery (or task-dream reactivations). Specifically, we asked if TMR or task-dream reactivations in either slow-wave (SWS) or rapid eye movement (REM) sleep benefit whole-body procedural learning. Healthy participants completed a virtual reality (VR) flying task prior to and following a morning nap or rest period during which task-associated tones were readministered in either SWS, REM sleep, wake or not at all. Findings indicate that learning benefits most from TMR when applied in REM sleep compared to a Control-sleep group. REM dreams that reactivated kinesthetic elements of the VR task (e.g., flying, accelerating) were also associated with higher improvement on the task than were dreams that reactivated visual elements (e.g., landscapes) or that had no reactivations. TMR did not itself influence dream content but its effects on performance were greater when coexisting with task-dream reactivations in REM sleep. Findings may help explain the mechanistic relationships between dream and memory reactivations and may contribute to the development of sleep-based methods to optimize complex skill learning.

About sleep's role in memory

Over more than a century of research has established the fact that sleep benefits the retention of memory. In this review we aim to comprehensively cover the field of "sleep and memory" research by providing a historical perspective on concepts and a discussion of more recent key findings. Whereas initial theories posed a passive role for sleep enhancing memories by protecting them from interfering stimuli, current theories highlight an active role for sleep in which memories undergo a process of system consolidation during sleep. Whereas older research concentrated on the role of rapid-eye-movement (REM) sleep, recent work has revealed the importance of slow-wave sleep (SWS) for memory consolidation and also enlightened some of the underlying electrophysiological, neurochemical, and genetic mechanisms, as well as developmental aspects in these processes. Specifically, newer findings characterize sleep as a brain state optimizing memory consolidation, in opposition to the waking brain being optimized for encoding of memories. Consolidation originates from reactivation of recently encoded neuronal memory representations, which occur during SWS and transform respective representations for integration into long-term memory. Ensuing REM sleep may stabilize transformed memories. While elaborated with respect to hippocampus-dependent memories, the concept of an active redistribution of memory representations from networks serving as temporary store into long-term stores might hold also for non-hippocampus-dependent memory, and even for nonneuronal, i.e., immunological memories, giving rise to the idea that the offline consolidation of memory during sleep represents a principle of long-term memory formation established in quite different physiological systems.

Memory consolidation during sleep: interactive effects of sleep stages and HPA regulation

Sleep is critically involved in the consolidation of previously acquired memory traces. However, nocturnal sleep is not uniform but is subject to distinct changes in electrophysiological and neuroendocrine activity. Specifically, the first half of the night is dominated by slow wave sleep (SWS), whereas rapid eye movement (REM) sleep prevails in the second half. Concomitantly, hypothalamo-pituitary-adrenal (HPA) activity as indicated by cortisol release is suppressed to a minimum during early sleep, while drastically increasing during late sleep. We have shown that the different sleep stages and the concomitant glucocorticoid release are interactively involved in the consolidation of different types of memories. SWS-rich early sleep has been demonstrated to benefit mainly the consolidation of hippocampus-dependent declarative memories (i.e. facts and episodes). In contrast, REM sleep-rich late sleep was shown to improve in particular emotional memories involving amygdalar function, as well as procedural memories (for skills) not depending on hippocampal or amygdalar function. Enhancing plasma glucocorticoid concentrations during SWS-rich early sleep counteracted hippocampus-dependent declarative memory consolidation, but did not affect hippocampus-independent procedural memory. Preventing the increase in cortisol during late REM sleep-rich sleep by administration of metyrapone impaired hippocampus-dependent declarative memory but enhanced amygdala-dependent emotional aspects of memory. The data underscore the importance of pituitary-adrenal inhibition during early SWS-rich sleep for efficient consolidation of declarative memory. The increase in cortisol release during late REM sleep-rich sleep may counteract an overshooting consolidation of emotional memories.

Consolidation effect of repeated processing of declarative knowledge in mental experiences during human sleep

Sleep may positively influence declarative memory through the processing, which transforms items of declarative knowledge into contents of mental sleep experience (MSE). A prediction from this general hypothesis is that the consolidation level should be higher for the output of items repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs of the same night rather than for the output of items presumably processed once (that is, all other, non-interrelated contents). Two experiments examined whether and how far the frequency and long-term retention of interrelated contents depend on the repeated processing of given items rather than on the experimental procedure applied for detection of interrelated contents. This procedure entails both multiple awakenings and a verbal report of MSE after awakening. Multiple awakenings could facilitate the re-access and elaboration of some contents into the subsequent (i.e. contiguous) MSE rather than non-contiguous MSEs; verbal reports could enhance the delayed recall of interrelated contents in as much as repeatedly encoded. The first experiment showed that interrelated contents were more frequent and better retained than both non-interrelated and pseudo-interrelated (i.e. by-chance similar or identical) contents, and even more in pairs of contiguous than non-contiguous MSEs collected from the first four periods of REM sleep on each experimental night. The second experiment showed that the frequency and retention rate of interrelated contents, while higher than those of non-interrelated and pseudo-interrelated contents, were not significantly different in pairs of MSEs which were verbally or mentally recalled after awakening provoked during the first four periods of REM sleep in each experimental night. Taken together, these findings indicate that the advantage provided by repeated processing during REM sleep for the consolidation of the output of items of declarative knowledge is conspicuous and largely independent from the experimental procedure, as this only slightly enhances the frequency and retention rate of interrelated contents.

The relationships between memory systems and sleep stages

Sleep function remains elusive despite our rapidly increasing comprehension of the processes generating and maintaining the different sleep stages. Several lines of evidence support the hypothesis that sleep is involved in the off-line reprocessing of recently-acquired memories. In this review, we summarize the main results obtained in the field of sleep and memory consolidation in both animals and humans, and try to connect sleep stages with the different memory systems. To this end, we have collated data obtained using several methodological approaches, including electrophysiological recordings of neuronal ensembles, post-training modifications of sleep architecture, sleep deprivation and functional neuroimaging studies. Broadly speaking, all the various studies emphasize the fact that the four long-term memory systems (procedural memory, perceptual representation system, semantic and episodic memory, according to Tulving's SPI model; Tulving, 1995) benefit either from non-rapid eye movement (NREM) (not just SWS) or rapid eye movement (REM) sleep, or from both sleep stages. Tulving's classification of memory systems appears more pertinent than the declarative/non-declarative dichotomy when it comes to understanding the role of sleep in memory. Indeed, this model allows us to resolve several contradictions, notably the fact that episodic and semantic memory (the two memory systems encompassed in declarative memory) appear to rely on different sleep stages. Likewise, this model provides an explanation for why the acquisition of various types of skills (perceptual-motor, sensory-perceptual and cognitive skills) and priming effects, subserved by different brain structures but all designated by the generic term of implicit or non-declarative memory, may not benefit from the same sleep stages.

Consolidation effect of the processing of declarative knowledge during human sleep: evidence from long-term retention of interrelated contents of mental sleep experiences

Sleep may partly exert a positive influence on memory through the processing underlying the transformation of items of declarative knowledge into contents of mental sleep experience (MSE). This hypothesis implies that the level of consolidation (and thus, long-term retention) should be enhanced for those items which are repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs developed over the same night. To test this prediction, we examined accessibility at delayed recall (i.e., the next morning) of the interrelated contents of MSEs reported (immediate recall) by 14 subjects who were awakened during the first four periods of rapid eye-movement (REM) sleep in two experimental nights. Interrelated contents were much more frequent, and at delayed recall much better retained, than other, non-interrelated contents in report pairs of MSEs. Moreover, they were also more frequent and better retained than by-chance similar or identical contents, as estimated in report pairs of MSEs of different subjects. These findings provide partial but coherent evidence in favour of the hypothesis that a generation effect occurs during sleep, with a further consolidation of the input and the output of MSE processing (respectively, the items of declarative knowledge and the contents of MSEs resulting from their elaboration during sleep).

Incorporation of presleep stimuli into dream contents: evidence for a consolidation effect on declarative knowledge during REM sleep?

Presleep stimuli to be retained for further recall is often incorporated into dream contents. To establish whether processing for insertion into dream contents may improve consolidation, we compared the retention rate at delayed recall of contents resulting from incorporation of presleep sentence-stimuli with those of other contents of the same dream experiences. We hypothesized that association with a cognitive task of recall facilitates access to recently acquired items of declarative knowledge such as presleep stimuli, and triggers the deep elaboration of their semantic features, which involves rehearsal. Twelve subjects were given a task of delayed recall for three nonsense sentences delivered once a time before each of the sleep (re-)onsets over an experimental night. After each awakening in rapid eye movement sleep, subjects were asked to report dream experience and recall the sentence to be retained. In the morning, after spontaneous awakening, subjects were unexpectedly requested to again report their dream experiences and to recall the stimuli. Two pairs of judges independently identified possible incorporations of the stimuli, and parsed dream reports into propositional content units. The proportion of night reports with at least one incorporation of the stimulus delivered (i.e. valid incorporations) was higher than that of reports with contents similar to a stimulus(-i) not yet delivered (forward pseudo-incorporations) or delivered prior to an earlier sleep period (backward pseudo-incorporations). The proportion of content units common to night and morning reports (considered to be better consolidated) was significantly higher for incorporated contents than for other contents, including pseudo-incorporated contents. Instead, the retention at morning recall of words of sentence-stimuli corresponding to incorporated contents was not significantly higher than that of other words. The better retention of incorporated contents provides a partial confirmation (that is, limited to the output of the processing) that a generation effect, which benefits retention of actively processed information, is operative during sleep as well as in waking.

Increased periodic arousal fluctuations during non-REM sleep are associated to superior memory

Sleep has been implicated in the plastic cerebral changes that underlie learning and memory. The scientific investigation of people with exceptional memory has been relatively neglected. We report the results of a polysomnographic investigation of an individual with superior memory performance. The sleep structure, in terms of sleep induction and maintenance, as well as non-REM and REM sleep percentages, were normal. The main finding was an increased number of periodic arousal fluctuations during non-REM sleep (measured as cyclic alternating pattern, CAP) during two consecutive nights (7-8 S.D. units above that observed in age-matched controls). Since CAP rate reflects the structural organization of non-REM sleep, this observation supports the hypothesis of a link between non-REM sleep and declarative memory performance.

Continuity of the processing of declarative knowledge during human sleep: evidence from interrelated contents of mental sleep experiences

The positive influence of sleep on memory may partly depend on the processing which transforms items of declarative knowledge into contents of mental sleep experience (MSE). This view implies that the consolidation level should be more enhanced for those items which have been repeatedly processed and transformed into identical or very similar (so-called interrelated) contents of distinct MSEs in the same night. We examined here the occurrence of interrelated contents in the MSEs reported after an awakening provoked in stage 2 at sleep onset and the spontaneous awakening in the morning. Interrelated contents resulted much more frequently than the chance occurrence of contents with the same semantic features, regardless of the sleep stage in which morning awakening occurred. The accessibility of given items for transformation into MSE contents over the night makes it plausible that they are reprocessed, and thus further consolidated, during various stages and cycles of sleep.

Mental activity during sleep

Since remote antiquity humankind has believed in the supernatural origin of dreaming. The scientific approach to mental activity during sleep dates back to the eighteenth century. One hundred years ago, psychoanalysis introduced dream analysis for diagnostic and therapeutic purposes. Only 50 years ago psychophysiology made it possible to collect mental sleep activity by means of experimental awakenings while monitoring physiological variables; this approach encouraged investigation into the relationship between the features of sleep activity and sleep state (REM versus NREM). Advances in neurophysiology, in neurochemistry, and recently in brain imaging techniques, have shed light on the roles played by the different cerebral structures in determining specific characteristics of mental activity during sleep. The development of cognitive psychology has enabled investigation of dream generation in terms of output from a complex multilevel system of information processing. In addition to sleep state, other factors, such as the time of the night and the sequence of the NREM-REM cycles, have been shown to influence the characteristics of mental activity. The usefulness of investigation of mental sleep activity as a clinical tool is controversial. The psychophysiological approach to mental sleep activity in clinical contexts has enabled the exploration of adaptation processes and contributed to neuropsychological studies on focal and systemic brain pathology.

Awakening from sleep

Awakening is a crucial event for the organism. The transition from sleep to waking implies physiological processes which lead to a new behavioural state. Spontaneous awakenings have varying features which may change as a function of several factors. The latter include intrasleep architecture, circadian phase, time awake, age, or disordered sleep. Despite its clear theoretical and clinical importance, the topic of awakening (in humans) has received little attention so far. This contribution focuses on major issues which relate to awakening from both basic (experimental) and clinical research. Recent knowledge on neurophysiological mechanisms is reported. The experimental data which provide in the human suggestions on the regulation of awakening are discussed, mainly those concerning sleep architecture and homeostatic/circadian factors also in a life-span perspective, since age is a powerful factor which may influence awakening. Clinical contributions will examine two main sleep disorders: insomnia and hypersomnia. Daytime functioning is shown in insomniac patients and compared to other pathologies like sleep apnea. A final section evokes links between some types of night waking and psychological factors.

Early sleep triggers memory for early visual discrimination skills

Improvement after practicing visual texture discrimination does not occur until several hours after practice has ended. We show that this improvement strongly depends on sleep. To specify the process responsible for sleep-related improvement, we compared the effects of 'early' and 'late' sleep, dominated respectively by slow-wave and rapid eye movement (REM) sleep. Discrimination skills significantly improved over early sleep, improved even more over a whole night's sleep, but did not improve after late sleep alone. These findings suggest that procedural memory formation is prompted by slow-wave sleep-related processes. Late REM sleep may promote memory formation at a second stage, only after periods of early sleep have occurred.

Gedächtnisbildung im Schlaf: Die Bedeutung von Schlafstadien und Streßhormonfreisetzung

Zusammenfassung. Nach gängiger Auffassung finden Gedächtniskonsolidierungsprozesse während des Schlafs hauptsächlich im REM-Schlaf (REM - “rapid eye movement”) statt. Die hier dargestellten Befunde und methodischen Überlegungen zeigen, daß dieses Konzept zugunsten einer differenzierteren Sichtweise auf Schlaf-assoziierte Konsolidierungsprozesse revidiert werden muß, die neben den Schlafstadien (REM-Schlaf versus Tiefschlaf) die Art der Gedächtnisleistung (deklaratives versus non-deklaratives Gedächtnis) und die Cortisolfreisetzung des Hypothalamus-Hypophysen-Nebennierenrinden-Systems (HHN-System) berücksichtigt. REM-Schlaf findet vor allem in der zweiten Hälfte des nächtlichen Schlafs statt, während die erste Hälfte durch extensive Tiefschlafphasen geprägt wird. Der Vergleich der Gedächtnisleistung nach Phasen frühen und späten Schlafs zeigt, daß Konsolidierung sehr viel stärker durch die von Tiefschlaf geprägte erste Schlafhälfte als durch die von REM-Schlaf geprägte zweite Schlafhälfte gefördert wird. Dies gilt aber nur für hippocampal vermittelte deklarative Gedächtnisleistungen. Non-deklarative, prozedurale Gedächtnisleistungen scheinen dagegen stärker von der zweiten Schlafhälfte zu profitieren. Der Tiefschlaf in der ersten Schlafhälfte geht mit einer Hemmung der Cortisolfreisetzung einher, während die Cortisolspiegel in der zweiten Schlafhälfte stark ansteigen. Cortisol reguliert hippocampale Aktivität direkt über Bindung an Glucocorticoid- (GR) und Mineralocorticoidrezeptoren (MR). Der konsolidierungsfördernde Effekt des frühen Schlafs auf deklarative Gedächtnisinhalte kann durch experimentelle Erhöhung des Cortisolspiegels während dieser Schlafperiode vollständig gehemmt werden, ohne daß der Gehalt an Tiefschlaf verändert wird. Eine vergleichbare Hemmung tritt nach Gabe des selektiven GR-Agonisten Dexamethason auf. Die Ergebnisse zeigen, daß deklaratives Gedächtnis vor allem im frühen Schlaf zu Zeiten, die mit Tiefschlaf einhergehen, konsolidiert wird. Die Hemmung der Cortisolfreisetzung und die dadurch bedingte Inaktivierung hippocampaler Glucocorticoid-rezeptoren in dieser frühen Schlafperiode stellen eine notwendige Voraussetzung für diesen Konsolidierungsprozeß dar.

Morning recall of verbal material depends on prior sleep organization

Despite the evidence that sleep may facilitate memory, controversial findings concern the role of sleep states (NREM, REM). We put forward the hypothesis that sleep organization, i.e. the regular occurrence of NREM-REM cycles, more than sleep states per se, may be crucial for the retention of verbal material presented before sleep. An experiment was performed in which recall of verbal material was requested of young subjects after three different kinds of night sleep: undisturbed sleep, fragmented sleep without sleep cycles disorganization, and fragmented sleep interrupted with sleep disorganization. Morning recall of verbal material was impaired after the night with disturbed sleep cycles, whereas it was not after the night with preserved sleep cycles; the amount of REM was similar in both cases. We conclude that the recall of verbal material is greatly affected by sleep cycle disorganization.

Dexamethasone blocks sleep induced improvement of declarative memory

To investigate the role of glucocorticoids for effects of early and late nocturnal sleep on declarative and procedural memory, 2 mg dexamethasone (versus placebo) were administered to healthy men 7 h prior to retention sleep. The retention sleep interval covered either the early or late half of nocturnal sleep. Following placebo, recall of a paired associate list (declarative memory) benefitted more from early than late sleep and recall of mirror tracing skills (procedural memory) benefitted more from late than early sleep. Dexamethasone did not affect slow wave sleep dominating early sleep, but blocked the beneficial effect of early sleep on recall of paired associates. Conversely, dexamethasone reduced rapid eye movement sleep dominating late sleep, but did not affect late sleeps beneficial effect on mirror tracing skills. The natural inhibition of endogenous glucocorticoid secretion during early sleep seems to be essential for a sleep-related facilitation of declarative memory.

Scalp recorded direct current brain potentials during human sleep

The direct current (DC) potential recorded from the scalp of awake humans has been considered a reflection of general changes in cortical excitability. This study examined DC potential shifts in humans during a night of continuous sleep. Standard polysomnographic recordings and skin temperature were measured simultaneously. Contrary to expectations, average DC potential level indicated higher negativity during nonrapid eye movement (NREM) sleep than REM sleep and wakefulness. Moreover, a dynamic regulation of the DC potential level was revealed in association with the NREM-REM sleep cycle comprising four successive phases: (i) a steep 'NREM-transition-negative shift' during the initial 10-15 min of the NREM sleep period; (ii) a more subtle 'NREM-positive slope' during the subsequent NREM sleep period; (iii) a steep 'REM-transition-positive shift' starting shortly prior to the REM sleep period, and (iv) a 'REM-negative slope', characterizing the remaining greater part of the REM sleep period. DC potential changes were only weakly related to changes in slow-wave activity (r2 < 0.18). The NREM-negative slope and REM-positive slope could reflect, respectively, gradually increasing and decreasing cortical excitability resulting from widespread changes in the depolarization of apical dendrites. In contrast, the NREM-transition-negative shift and the REM-transition-positive shift may reflect the progression and retrogression, respectively, of a long-lasting hyperpolarization in deeply lying neurons.

Effects of Early and Late Nocturnal Sleep on Declarative and Procedural Memory

Recall of paired-associate lists (declarative memory) and mirror-tracing skills (procedural memory) was assessed after retention intervals defined over early and late nocturnal sleep. In addition, effects of sleep on recall were compared with those of early and late retention intervals filled with wakefulness. Twenty healthy men served as subjects. Saliva cortisol concentrations were determined before and after the retention intervals to determine pituitary-adrenal secretory activity. Sleep was determined somnopolygraphically. Sleep generally enhanced recall when compared with the effects of corresponding retention intervals of wakefulness. The benefit from sleep on recall depended on the phase of sleep and on the type of memory: Recall of paired-associate lists improved more during early sleep, and recall of mirror-tracing skills improved more during late sleep. The effects may reflect different influences of slow wave sleep (SWS) and rapid eye movement (REM) sleep since time in SWS was 5 times longer during the early than late sleep retention interval, and time in REM sleep was twice as long during late than early sleep (p &lt; 0.005). Changes in cortisol concentrations, which independently of sleep and wakefulness were lower during early retention intervals than late ones, cannot account for the effects of sleep on memory. The experiments for the first time dissociate specific effects of early and late sleep on two principal types of memory, declarative and procedural, in humans.