Remembering to Forget: A Dual Role for Sleep Oscillations in Memory Consolidation and Forgetting

Both slow wave and rapid eye movement sleep contribute to emotional memory consolidation

Sleep supports memory consolidation, but the specific roles of different sleep stages in this process remain unclear. While rapid eye movement sleep (REM) has traditionally been linked to the processing of emotionally charged material, recent evidence suggests that slow wave sleep (SWS) also plays a role in strengthening emotional memories. Here, we use targeted memory reactivation (TMR) during REM and SWS in a daytime nap to directly examine which sleep stage is primarily involved in consolidating emotional declarative memories. Contrary to our hypothesis, reactivating emotional stimuli during REM impairs memory. Meanwhile, TMR benefit in SWS is strongly correlated with the product of time spent in REM and SWS. The emotional valence of cued items modulates both delta/theta power and sleep spindles. Furthermore, emotional memories benefit more from TMR than neutral ones. Our findings suggest that SWS and REM have complementary roles in consolidating emotional memories, with REM potentially involved in forgetting them. These results also expand on recent evidence highlighting a connection between sleep spindles and emotional processing.

Neuronal theta oscillation of hippocampal ensemble and memory function

Memory is the ability to acquire and store information following an experience, which can be retrieved by related context exposure. Pioneering studies have demonstrated that sparsely distributed neuronal ensembles or engram cells can serve as neural substrates for storing and recalling memory traces. Many studies of neuronal ensembles have focused on the hippocampus, and increasing evidence has indicated that the neuronal oscillation is closely associated with hippocampal memory functions, including both encoding and retrieval processes. In particular, the theta synchronization of hippocampal ensembles with other brain regions mediates the retrieval of multiple types of memory. The recent progress of theta oscillations in the formation of memory engrams is reviewed, as well as the increased theta power and neurotransmitter regulation on memory function. Detailed information based on an analysis of hippocampal local theta rhythms is presented. Moreover, the hippocampus theta synchronization with the sensory cortex, prefrontal cortex and amygdala, which mediate different types of memory retrieval, are also reviewed. Together, these findings contribute to understanding the important role of hippocampal theta oscillation in the storage and recall of memory traces.

Selective Modulation of Fear Memory in Non-Rapid Eye Movement Sleep

Sleep stabilizes memories for their consolidation, but how to modify specific fear memory during sleep remains unclear. Here, it is reported that using targeted memory reactivation (TMR) to reactivate prior fear learning experience in non-slow wave sleep (NS) inhibits fear memory consolidation, while TMR during slow wave sleep (SWS) enhances fear memory in mice. Replaying conditioned stimulus (CS) during sleep affects sleep spindle occurrence, leading to the reduction or enhancement of slow oscillation-spindle (SO-spindle) coupling in NS and SWS, respectively. Optogenetic inhibition of pyramidal neurons in the frontal association cortex (FrA) during TMR abolishes the behavioral effects of NS-TMR and SWS-TMR by modulating SO-spindle coupling. Notably, calcium imaging of the L2/3 pyramidal neurons in the FrA shows that CS during SWS selectively enhances the activity of neurons previously activated during fear conditioning (FC+ neurons), which significantly correlates with CS-elicited spindle power spectrum density. Intriguingly, these TMR-induced calcium activity changes of FC+ neurons further correlate with mice freezing behavior, suggesting their contributions to the consolidation of fear memories. The findings indicate that TMR can selectively weaken or strengthen fear memory, in correlation with modulating SO-spindle coupling and the reactivation of FC+ neurons during substages of non-rapid eye movement (NREM) sleep.

Sensational Dreams: The Prevalence of Sensory Experiences in Dreaming

Dreaming, a widely researched aspect of sleep, often mirrors waking-life experiences. Despite the prevalence of sensory perception during wakefulness, sensory experiences in dreams remain relatively unexplored. Free recall dream reports, where individuals describe their dreams freely, may not fully capture sensory dream experiences. In this study, we developed a dream diary with direct questions about sensory dream experiences. Participants reported sensory experiences in their dreams upon awakening, over multiple days, in a home-based setting (n = 3476 diaries). Our findings show that vision was the most common sensory dream experience, followed by audition and touch. Olfaction and gustation were reported at equally low rates. Multisensory dreams were far more prevalent than unisensory dreams. Additionally, the prevalence of sensory dream experiences varied across emotionally positive and negative dreams. A positive relationship was found between on the one hand sensory richness and, on the other emotional intensity of dreams and clarity of dream recall, for both positive and negative dreams. These results underscore the variety of dream experiences and suggest a link between sensory richness, emotional content and dream recall clarity. Systematic registration of sensory dream experiences offers valuable insights into dream manifestation, aiding the understanding of sleep-related memory consolidation and other aspects of sleep-related information processing.

Does the brain sleep differently depending on intellectual abilities?

AIMS

To compare the children's sleep electroencephalogram according to their intellectual profile.

METHODS

Children were grouped according to their Wechsler Intelligence Scale for Children (WISC) scores (17 with normal intelligence quotient [IQ, NIQ] and 24 with high IQ [HIQ]). Comparisons of spectral power between groups and its relationship with WISC scores were assessed using analyses of variance and linear regression models, adjusted for age and sex.

RESULTS

Children with HIQ had more rapid eye movement (REM) sleep, especially late at night, and more power in slow-frequency bands during REM sleep than those with NIQ. There were also positive associations between the processing speed index and the spectral power in β bands in NREM sleep, and with the spectral power in α, σ, β, and γ bands in REM sleep, with different associations between groups.

CONCLUSION

The enhanced power in slow bands during REM sleep in children with HIQ overlaps with that of typical REM sleep oscillations thought to be involved in emotional memory consolidation. The dissimilar relationships between spectral power and WISC scores in NIQ and HIQ groups may underlie functional differences in brain activity related to cognitive efficiency, questioning the direction of the relationship between sleep and cognitive functioning.

Temporal cluster-based organization of sleep spindles underlies motor memory consolidation

Sleep benefits motor memory consolidation, which is mediated by sleep spindle activity and associated memory reactivations during non-rapid eye movement (NREM) sleep. However, the particular role of NREM2 and NREM3 sleep spindles and the mechanisms triggering this memory consolidation process remain unclear. Here, simultaneous electroencephalographic and functional magnetic resonance imaging (EEG-fMRI) recordings were collected during night-time sleep following the learning of a motor sequence task. Adopting a time-based clustering approach, we provide evidence that spindles iteratively occur within clustered and temporally organized patterns during both NREM2 and NREM3 sleep. However, the clustering of spindles in trains is related to motor memory consolidation during NREM2 sleep only. Altogether, our findings suggest that spindles' clustering and rhythmic occurrence during NREM2 sleep may serve as an intrinsic rhythmic sleep mechanism for the timed reactivation and subsequent consolidation of motor memories, through synchronized oscillatory activity within a subcortical-cortical network involved during learning.

Cognitive neuroscience perspective on memory: overview and summary

This paper explores memory from a cognitive neuroscience perspective and examines associated neural mechanisms. It examines the different types of memory: working, declarative, and non-declarative, and the brain regions involved in each type. The paper highlights the role of different brain regions, such as the prefrontal cortex in working memory and the hippocampus in declarative memory. The paper also examines the mechanisms that underlie the formation and consolidation of memory, including the importance of sleep in the consolidation of memory and the role of the hippocampus in linking new memories to existing cognitive schemata. The paper highlights two types of memory consolidation processes: cellular consolidation and system consolidation. Cellular consolidation is the process of stabilizing information by strengthening synaptic connections. System consolidation models suggest that memories are initially stored in the hippocampus and are gradually consolidated into the neocortex over time. The consolidation process involves a hippocampal-neocortical binding process incorporating newly acquired information into existing cognitive schemata. The paper highlights the role of the medial temporal lobe and its involvement in autobiographical memory. Further, the paper discusses the relationship between episodic and semantic memory and the role of the hippocampus. Finally, the paper underscores the need for further research into the neurobiological mechanisms underlying non-declarative memory, particularly conditioning. Overall, the paper provides a comprehensive overview from a cognitive neuroscience perspective of the different processes involved in memory consolidation of different types of memory.

Emotional memories are enhanced when reactivated in slow wave sleep, but impaired when reactivated in REM

Sleep supports memory consolidation. However, it is not completely clear how different sleep stages contribute to this process. While rapid eye movement sleep (REM) has traditionally been implicated in the processing of emotionally charged material, recent studies indicate a role for slow wave sleep (SWS) in strengthening emotional memories. Here, to directly examine which sleep stage is primarily involved in emotional memory consolidation, we used targeted memory reactivation (TMR) in REM and SWS during a daytime nap. Contrary to our hypothesis, reactivation of emotional stimuli during REM led to impaired memory. Consistent with this, REM% was correlated with worse recall in the group that took a nap without TMR. Meanwhile, cueing benefit in SWS was strongly correlated with the product of times spent in REM and SWS (SWS-REM product), and reactivation significantly enhanced memory in those with high SWS-REM product. Surprisingly, SWS-REM product was associated with better memory for reactivated items and poorer memory for non-reactivated items, suggesting that sleep both preserved and eliminated emotional memories, depending on whether they were reactivated. Notably, the emotional valence of cued items modulated both sleep spindles and delta/theta power. Finally, we found that emotional memories benefited from TMR more than did neutral ones. Our results suggest that emotional memories decay during REM, unless they are reactivated during prior SWS. Furthermore, we show that active forgetting complements memory consolidation, and both take place across SWS and REM. In addition, our findings expand upon recent evidence indicating a link between sleep spindles and emotional processing.

Global dissociation of the posterior amygdala from the rest of the brain during REM sleep

Rapid-eye-movement sleep (REMS) or paradoxical sleep is associated with intense neuronal activity, fluctuations in autonomic control, body paralysis and brain-wide hyperemia. The mechanisms and functions of these energy-demanding patterns remain elusive and a global picture of brain activation during REMS is currently missing. In the present work, we performed functional ultrasound imaging on rats over multiple coronal and sagittal brain sections during hundreds of spontaneous REMS episodes to provide the spatiotemporal dynamics of vascular activity in 259 brain regions spanning more than 2/3 of the total brain volume. We first demonstrate a dissociation between basal/midbrain and cortical structures, the first ones sustaining tonic activation during REMS while the others are activated in phasic bouts. Second, we isolated the vascular compartment in our recordings and identified arteries in the anterior part of the brain as strongly involved in the blood supply during REMS episodes. Finally, we report a peculiar activation pattern in the posterior amygdala, which is strikingly disconnected from the rest of the brain during most REMS episodes. This last finding suggests that the amygdala undergoes specific processing during REMS and may be linked to the regulation of emotions and the creation of dream content during this very state.

Sleep Biomarkers Help Predict the Development of Alzheimer Disease

SUMMARY

Middle-aged or older adults who self-report sleep-wake disorders are at an increased risk for incident dementia, mild cognitive impairment, and Alzheimer disease. Dementia in people with mild cognitive impairment and Alzheimer disease who complain of sleep-wake disorders progress faster than those without sleep-wake disorders. Removal of amyloid-beta and tau tangles occurs preferentially in non-rapid eye movement 3 sleep and fragmented or insufficient sleep may lead to accumulation of these neurotoxins even in preclinical stages. Selective atrophy in the medial temporal lobe on brain MRI has been shown to predict impaired coupling of slow oscillations and sleep spindles. Impaired slow wave-spindle coupling has been shown to correlate with impaired overnight memory consolidation. Whereas, a decrease in the amplitude of 0.6 to 1 Hz slow wave activity predicts higher cortical Aβ burden on amyloid PET scans. Overexpression of the wake-promoting neurotransmitter orexin may predispose patients with mild cognitive impairment and Alzheimer disease to increased wakefulness, decreasing time they need to clear from the brain the neurotoxic accumulation of amyloid-beta and especially tau. More research exploring these relationships is needed and continuing.

The Algorithms of Mindfulness

This paper analyzes notions and models of optimized cognition emerging at the intersections of psychology, neuroscience, and computing. What I somewhat polemically call the algorithms of mindfulness describes an ideal that determines algorithmic techniques of the self, geared at emotional resilience and creative cognition. A reframing of rest, exemplified in corporate mindfulness programs and the design of experimental artificial neural networks sits at the heart of this process. Mindfulness trainings provide cues as to this reframing, for they detail each in their own way how intermittent periods of rest are to be recruited to augment our cognitive capacities and combat the effects of stress and information overload. They typically rely on and co-opt neuroscience knowledge about what the brains of North Americans and Europeans do when we rest. Current designs for artificial neural networks draw on the same neuroscience research and incorporate coarse principles of cognition in brains to make machine learning systems more resilient and creative. These algorithmic techniques are primarily conceived to prevent psychopathologies where stress is considered the driving force of success. Against this backdrop, I ask how machine learning systems could be employed to unsettle the concept of pathological cognition itself.

Restraint stress potentiates neuropeptide Y-mediated impairment on spatial memory in rats

Memory is the ability to store, retrieve and use information that requires a progressive time-dependent stabilization process known as consolidation to be established. The hippocampus is essential for processing all the information that forms memory, especially spatial memory. Neuropeptide Y (NPY) affects memory, so in this study we investigated the participation and recruitment of NPY receptors during spatial memory consolidation in rats. Using the water maze test, we show that NPY (1 pmol) injected into the dorsal hippocampus impaired memory consolidation and that previous restraint stress (30 min) potentiates NPY effects, i.e. further impaired memory consolidation. Using selective antagonists for NPY Y₁ and Y₂ receptors we demonstrate that both receptors play a key role on spatial memory consolidation. Our data suggest that NPY modulates aversive and adaptive memory formation by NPY receptors activation.

REM Sleep Deprivation Alters Learning-Induced Cell Proliferation and Generation of Newborn Young Neurons in the Dentate Gyrus of the Dorsal Hippocampus

The hippocampus-dependent "trace-appetitive conditioning task" increases cell proliferation and the generation of newborn young neurons. Evidence suggests that adult hippocampal neurogenesis and rapid eye movement (REM) sleep play an essential role in memory consolidation. On the other hand, REM sleep deprivation (REM-SD) induces detrimental effects on training-induced cell proliferation in the hippocampus's dentate gyrus (DG). Nonetheless, the role of REM sleep in the trace-appetitive memory and fate determination of the newly proliferated cells is not known. Here, we have studied the following: (I) the effects of 24 h of REM-SD (soon after training) on trace- and delay-appetitive memory and cell proliferation in the adult DG and (II) the effects of chronic (96 h) REM-SD (3 days after the training, the period in which newly generated cells progressed toward the neuronal lineage) on trace-appetitive memory and the generation of newborn young neurons. We used a modified multiple platform method for the selective REM-SD without altering non-REM (NREM) sleep. We found that 24 h of REM-SD, soon after trace-conditioning, impaired the trace-appetitive memory and the training-induced cell proliferation. Nevertheless, 96 h of REM-SD (3 days after the training) did not impair trace memory. Interestingly, 96 h of REM-SD altered the generation of newborn young neurons. These results suggest that REM sleep plays an essential role in training-induced cell proliferation and the fate determination of the newly generated cells toward the neuronal lineage.

Molecular Mechanisms of Memory Consolidation That Operate During Sleep

The role of sleep for brain function has been in the focus of interest for many years. It is now firmly established that sleep and the corresponding brain activity is of central importance for memory consolidation. Less clear are the underlying molecular mechanisms and their specific contribution to the formation of long-term memory. In this review, we summarize the current knowledge of such mechanisms and we discuss the several unknowns that hinder a deeper appreciation of how molecular mechanisms of memory consolidation during sleep impact synaptic function and engram formation.

Modulating role of serotonergic signaling in sleep and memory

Serotonin is an important neurotransmitter with various receptors and wide-range effects on physiological processes and cognitive functions including sleep, learning, and memory. In this review study, we aimed to discuss the role of serotonergic receptors in modulating sleep-wake cycle, and learning and memory function. Furthermore, we mentioned to sleep deprivation, its effects on memory function, and the potential interaction with serotonin. Although there are thousands of research articles focusing on the relationship between sleep and serotonin; however, the pattern of serotonergic function in sleep deprivation is inconsistent and it seems that serotonin has not a certain role in the effects of sleep deprivation on memory function. Also, we found that the injection type of serotonergic agents (systemic or local), the doses of these drugs (dose-dependent effects), and up- or down-regulation of serotonergic receptors during training with various memory tasks are important issues that can be involved in the effects of serotonergic signaling on sleep-wake cycle, memory function, and sleep deprivation-induced memory impairments. This comprehensive review was conducted in the PubMed, Scopus, and ScienceDirect databases in June and July 2021, by searching keywords sleep, sleep deprivation, memory, and serotonin.

Bidirectional relationships of sleep and epilepsy in adults with epilepsy

This targeted review addresses the best accepted and most intriguing recent observations on the complex relationships between sleep and epilepsy. Ten to 15% of all epilepsies are sleep-related. Included in these is sleep-related hypermotor epilepsy, renamed from nocturnal frontal lobe epilepsy by a 2016 consensus conference since 30% of cases are extra-frontal, seizures are related to sleep rather than clock time, and the predominant semiology is hypermotor. Stereo-EEG is providing crucial insights into network activation in sleep-related epilepsies and definition of the epileptogenic zone. Pathologic high-frequency oscillations, a promising biomarker for identifying the epileptogenic zone, are most frequent in NREM sleep, lowest in wakefulness and REM sleep, similar to interictal epileptiform discharges (IEDs). Most sleep-related seizures are followed by awakening or arousal and IEDs cause arousals and increase after arousals, likely contributing to sleep/wake complaints. Sleep/wake disorders are 2-3 times more common in adults with epilepsy than the general population; these comorbidities are associated with poorer quality of life and may impact seizure control. Treatment of sleep apnea reduces seizures in many cases. An emerging area of research is in circadian biology and epilepsy. Over 90% of people with epilepsy have seizures with circadian periodicity, in part related to sleep itself, and the majority of SUDEP cases occur in sleep. Recognizing these bidirectional relationships is important for patient and caregiver education and counseling and optimizing epilepsy outcomes.

The nucleus reuniens, a thalamic relay for cortico-hippocampal interaction in recent and remote memory consolidation

The consolidation of declarative memories is believed to occur mostly during sleep and involves a dialogue between two brain regions, the hippocampus and the medial prefrontal cortex. The information encoded during experience by neuronal assemblies is replayed during sleep leading to the progressive strengthening and integration of the memory trace in the prefrontal cortex. The gradual transfer of information from the hippocampus to the medial prefrontal cortex for long-term storage requires the synchronization of cortico-hippocampal networks by different oscillations, like ripples, spindles, and slow oscillations. Recent studies suggest the involvement of a third partner, the nucleus reuniens, in memory consolidation. Its bidirectional connections with the hippocampus and medial prefrontal cortex place the reuniens in a key position to relay information between the two structures. Indeed, many topical works reveal the original role that the nucleus reuniens occupies in different recent and remote memories consolidation. This review aimed to examine these contributions, as well as its functional embedment in this complex memory network, and provide some insights on the possible mechanisms.

Varieties of graded forgetting

Forgetting is typically viewed as counterproductive in everyday life. However, it may mainly be harmful when it is complete, that is, all-encompassing and permanent, and not when it is graded, that is, partial and fluctuating. I propose that forgetting is in fact mostly graded, and that this is an essential reason that it is often helpful. I delineate three ways in which forgetting may be graded. First, it may occur with respect to one, but not another, part of a memory. Second, it may occur in one context, but not in another. Third, forgetting may be present at one point in time, but not at another. Also, I propose that different levels of forgetting are possible, based on whether an engram or a context is unavailable, silent, restricted, latent, or potent. Overall, I hypothesize that forgetting is often helpful because it can be flexible and tailored to the circumstances.

Can sleep protect memories from catastrophic forgetting?

Continual learning remains an unsolved problem in artificial neural networks. The brain has evolved mechanisms to prevent catastrophic forgetting of old knowledge during new training. Building upon data suggesting the importance of sleep in learning and memory, we tested a hypothesis that sleep protects old memories from being forgotten after new learning. In the thalamocortical model, training a new memory interfered with previously learned old memories leading to degradation and forgetting of the old memory traces. Simulating sleep after new learning reversed the damage and enhanced old and new memories. We found that when a new memory competed for previously allocated neuronal/synaptic resources, sleep replay changed the synaptic footprint of the old memory to allow overlapping neuronal populations to store multiple memories. Our study predicts that memory storage is dynamic, and sleep enables continual learning by combining consolidation of new memory traces with reconsolidation of old memory traces to minimize interference.

mPFC spindle cycles organize sparse thalamic activation and recently active CA1 cells during non-REM sleep

Sleep oscillations in the neocortex and hippocampus are critical for the integration of new memories into stable generalized representations in neocortex. However, the role of the thalamus in this process is poorly understood. To determine the thalamic contribution to non-REM oscillations (sharp-wave ripples, SWRs; slow/delta; spindles), we recorded units and local field potentials (LFPs) simultaneously in the limbic thalamus, mPFC, and CA1 in rats. We report that the cycles of neocortical spindles provide a key temporal window that coordinates CA1 SWRs with sparse but consistent activation of thalamic units. Thalamic units were phase-locked to delta and spindles in mPFC, and fired at consistent lags with other thalamic units within spindles, while CA1 units that were active during spatial exploration were engaged in SWR-coupled spindles after behavior. The sparse thalamic firing could promote an incremental integration of recently acquired memory traces into neocortical schemas through the interleaved activation of thalamocortical cells.

A sleep spindle framework for motor memory consolidation

Sleep spindle activity has repeatedly been found to contribute to brain plasticity and consolidation of both declarative and procedural memories. Here we propose a framework for motor memory consolidation that outlines the essential contribution of the hierarchical and multi-scale periodicity of spindle activity, as well as of the synchronization and interaction of brain oscillations during this sleep-dependent process. We posit that the clustering of sleep spindles in 'trains', together with the temporally organized alternation between spindles and associated refractory periods, is critical for efficient reprocessing and consolidation of motor memories. We further argue that the long-term retention of procedural memories relies on the synchronized (functional connectivity) local reprocessing of new information across segregated, but inter-connected brain regions that are involved in the initial learning process. Finally, we propose that oscillatory synchrony in the spindle frequency band may reflect the cross-structural reactivation, reorganization and consolidation of motor, and potentially declarative, memory traces within broader subcortical-cortical networks during sleep. This article is part of the Theo Murphy meeting issue 'Memory reactivation: replaying events past, present and future'.

Anesthesia and sleep disorders – a new problem in modern anesthesiology (literature review)

Sleep is an active state that is as complex as wakefulness. The main tasks of sleep are the adaptation and restoration of physical and mental strength. Sleep regulation is a complex multimodal process involving not only neurotransmitters, but also releasing­factors, hormones, cytokines, signaling molecules and metabolites. Having a lot of physiological effects, postoperative sleep plays a role not only in quality of life, but also in the recovery of the patient. The characteristics of the patient, the type of surgical intervention, the methods of anesthesia and their interaction affects postoperative sleep, but the relationship and the level of influence of these factors are not clear. Therefore, given the high prevalence of postoperative insomnia, this problem is relevant for modern anesthesiology.