Targeted Memory Reactivation during Sleep Depends on Prior Learning

Classical music, educational learning, and slow wave sleep: A targeted memory reactivation experiment

Poor sleep in college students compromises the memory consolidation processes necessary to retain course materials. A solution may lie in targeting reactivation of memories during sleep (TMR). Fifty undergraduate students completed a college-level microeconomics lecture (mathematics-based) while listening to distinctive classical music (Chopin, Beethoven, and Vivaldi). After they fell asleep, we re-played the classical music songs (TMR) or a control noise during slow wave sleep. Relative to the control condition, the TMR condition showed an 18% improvement for knowledge transfer items that measured concept integration (d = 0.63), increasing the probability of "passing" the test with a grade of 70 or above (OR = 4.68, 95%CI: 1.21, 18.04). The benefits of TMR did not extend to a 9-month follow-up test when performance dropped to floor levels, demonstrating that long-term-forgetting curves are largely resistant to experimentally-consolidated memories. Spectral analyses revealed greater frontal theta activity during slow wave sleep in the TMR condition than the control condition (d = 0.87), and greater frontal theta activity across conditions was associated with protection against long-term-forgetting at the next-day and 9-month follow-up tests (rs = 0.42), at least in female students. Thus, students can leverage instrumental music-which they already commonly pair with studying-to help prepare for academic tests, an approach that may promote course success and persistence.

Promoting memory consolidation during sleep: A meta-analysis of targeted memory reactivation

Targeted memory reactivation (TMR) is a methodology employed to manipulate memory processing during sleep. TMR studies have great potential to advance understanding of sleep-based memory consolidation and corresponding neural mechanisms. Research making use of TMR has developed rapidly, with over 70 articles published in the last decade, yet no quantitative analysis exists to evaluate the overall effects. Here we present the first meta-analysis of sleep TMR, compiled from 91 experiments with 212 effect sizes (N = 2,004). Based on multilevel modeling, overall sleep TMR was highly effective (Hedges' g = 0.29, 95% CI [0.21, 0.38]), with a significant effect for two stages of non-rapid-eye-movement (NREM) sleep (Stage NREM 2: Hedges' g = 0.32, 95% CI [0.04, 0.60]; and slow-wave sleep: Hedges' g = 0.27, 95% CI [0.20, 0.35]). In contrast, TMR was not effective during REM sleep nor during wakefulness in the present analyses. Several analysis strategies were used to address the potential relevance of publication bias. Additional analyses showed that TMR improved memory across multiple domains, including declarative memory and skill acquisition. Given that TMR can reinforce many types of memory, it could be useful for various educational and clinical applications. Overall, the present meta-analysis provides substantial support for the notion that TMR can influence memory storage during NREM sleep, and that this method can be useful for understanding neurocognitive mechanisms of memory consolidation. (PsycINFO Database Record (c) 2020 APA, all rights reserved).

Weakly encoded memories due to acute sleep restriction can be rescued after one night of recovery sleep

Sleep is thought to play a complementary role in human memory processing: sleep loss impairs the formation of new memories during the following awake period and, conversely, normal sleep promotes the strengthening of the already encoded memories. However, whether sleep can strengthen deteriorated memories caused by insufficient sleep remains unknown. Here, we showed that sleep restriction in a group of participants caused a reduction in the stability of EEG activity patterns across multiple encoding of the same event during awake, compared with a group of participants that got a full night's sleep. The decrease of neural stability patterns in the sleep-restricted group was associated with higher slow oscillation-spindle coupling during a subsequent night of normal sleep duration, thereby suggesting the instantiation of restorative neural mechanisms adaptively supporting cognition and memory. Importantly, upon awaking, the two groups of participants showed equivalent retrieval accuracy supported by subtle differences in the reinstatement of encoding-related activity: it was longer lasting in sleep-restricted individuals than in controls. In addition, sustained reinstatement over time was associated with increased coupling between spindles and slow oscillations. Taken together, these results suggest that the strength of prior encoding might be an important moderator of memory consolidation during sleep. Supporting this view, spindles nesting in the slow oscillation increased the probability of correct recognition only for weakly encoded memories. Current results demonstrate the benefit that a full night's sleep can induce to impaired memory traces caused by an inadequate amount of sleep.

Deciphering Age Differences in Experience-Based Decision-Making: The Role of Sleep

OBJECTIVE

Recent studies have demonstrated that sleep not only facilitates memory consolidation but also benefits more complex cognitive skills such as decision-making in young adults. Older adults use different decision strategies compared with young adults, which leaves the role of sleep in older adults' decision-making unclear. We investigated the age-by-sleep effect on decision-making.

METHODS

We recruited 67 young adults (ages 18 to 29 years) and 66 older adults (ages 60 to 79 years) and randomly assigned them into the "sleep" or "wake" study condition. They were given a modified Iowa gambling task to perform before and after a 12-hour interval with sleep or wakefulness.

RESULTS

Using the typical model-free analysis, we found that young adults' between-session performance improved greater than that of older adults regardless of the sleep/wake condition. Furthermore, older adults with longer total sleep time showed a greater improvement in the selection of one "good" deck. To further examine the sleep effect on age-related differences in cognitive processes underlying decision-making, we conducted computational modelling. This more fine-grained analysis revealed that sleep improved feedback sensitivity for both young and older adults while it increased loss aversion for older adults but not for young adults.

CONCLUSION

These findings indicate that sleep promotes learning-based decision-making performance via facilitating value representation, and such modulation is distinct in young compared to older adults.

Cued Memory Reactivation during Motor Imagery Practice Influences Early Improvement of Procedural Skill Learning

Reactivation processes are fundamental for procedural memory improvement. Targeted memory reactivation (TMR) influences memory consolidation through the re-exposure to certain perceptual components present in a previous phase of associative learning. On the other hand, motor imagery (MI) affects procedural skills through a repeated mental simulation of a pre-learned movement without physically moving. Both for TMR and MI, performance improvement has proven to be associated with an induction of reactivation processes. The positive effect of TMR is widely acknowledged in sleep. Here, we measured its impact on procedural learning during waking, in particular during MI practice, in line with the hypothesis that the exogenously induced involuntary reactivations through TMR could add up to the endogenous and voluntary reactivations induced by MI. Therefore, we assessed the influence on performance on a sequential finger tapping task of an auditory TMR during MI practice. It was compared to four conditions: (i) MI alone, (ii) MI during an incompatible sound stimulation, (iii) a mere video viewing and (iv) an auditory TMR during a video viewing. Results showed that the TMR + MI condition determined the largest early performance improvement as indexed by the combined measure of speed and accuracy (number of correct sequences typed in the task). We propose that TMR may enhance the effectiveness of MI protocols, and that MI could represent an optimal time window during wakefulness to take advantage of the effects of TMR.

Grappling With Implicit Social Bias: A Perspective From Memory Research

There is now widespread consensus that social biases often influence actions independently of the actor's intention or awareness. The notion that we are sometimes blind to the origins of our thoughts, attitudes, and behaviors also features prominently in research into domain-general human memory systems, which has a long history of distinguishing between implicit and explicit repercussions of past experience. A shared challenge across these fields of study is thus to identify techniques for effectively managing the contents of our memory stores, particularly those aspects into which we have limited metacognitive insight. In the present review, we examine recent developments in the cognitive neuroscience of human memory that speak to this challenge as it applies to the social domain. One area of progress pertains to the role of individuation, the process of attending to and representing in memory unique characteristics of individuals, which can limit the influence of generalizations based on social categories. A second body of work concerns breakthroughs in understanding memory consolidation, which determines the fate of newly encoded memories. We discuss the promise of each of these developments for identifying ways to become better stewards of our social minds. More generally, we suggest that, as with other forms of learning and memory, intentional practice and rehearsal may be critical in learning to minimize unwanted biases.

Effects of targeted memory reactivation during sleep at home depend on sleep disturbances and habituation

Targeted memory reactivation (TMR) during sleep improves memory consolidation. However, it is still unknown whether TMR also benefits memory in real-life conditions. We tested whether TMR during sleep enhances Dutch-German vocabulary learning when applied during multiple nights at home in an unsupervised fashion. During 3 consecutive nights, 66 healthy young participants used an mp3-player to play Dutch words during sleep, without any control of sleep or awakenings by tones (unsupervised TMR). Unsupervised TMR benefitted overall memory scores only in a subgroup of participants, who reported no disturbances by TMR during sleep. Participants who reported general disturbances of sleep showed no benefit, while TMR specifically impaired memory in a third group who reported specific disturbances by the played words during sleep. Separate analysis per night indicated that memory benefits by TMR were significant in the entire sample in the third night only. Our results indicate that sleep disturbances and habituation might be critical factors for the success of unsupervised TMR in a home setting. Habituation to the TMR process as well as automatic sleep monitoring and avoidance of auditory-induced awakenings might be a precondition to successful application of TMR to language learning in real-life.

Increased neuronal signatures of targeted memory reactivation during slow-wave up states

It is assumed that slow oscillatory up-states represent crucial time windows for memory reactivation and consolidation during sleep. We tested this assumption by utilizing closed-loop targeted memory reactivation: Participants were re-exposed to prior learned foreign vocabulary during up- and down-states of slow oscillations. While presenting memory cues during slow oscillatory up-states improved recall performance, down-state cueing did not result in a clear behavioral benefit. Still, no robust behavioral benefit of up- as compared to down-state cueing was observable. At the electrophysiological level however, successful memory reactivation during up-states was associated with a characteristic power increase in the theta and sleep spindle band. No oscillatory changes were observable for down-state cues. Our findings provide experimental support for the assumption that slow oscillatory up-states may represent privileged time windows for memory reactivation, while the interplay of slow oscillations, theta and sleep spindle activity promotes successful memory consolidation during sleep.

Use of targeted memory reactivation enhances skill performance during a nap and enhances declarative memory during wake in healthy young adults

Sleep is an important component of motor memory consolidation and learning, providing a critical tool to enhance training and rehabilitation. Following initial skill acquisition, memory consolidation is largely a result of non-rapid eye movement sleep over either a full night or a nap. Targeted memory reactivation is one method used to enhance this critical process, which involves the pairing of an external cue with task performance at the time of initial motor skill acquisition, followed by replay of the same cue during sleep. Application of targeted memory reactivation during sleep leads to increased functional connectivity within task-related brain networks and improved behavioural performance in healthy young adults. We have previously used targeted memory reactivation throughout the first two slow-wave sleep cycles of a full night of sleep to enhance non-dominant arm throwing accuracy in healthy young adults. Here, we aimed to determine whether application of targeted memory reactivation throughout a 1-hr daytime nap was sufficient to enhance performance on the same non-dominant arm throwing task in healthy young adults. Participants were allocated to either nap or no nap, and within those groups half received targeted memory reactivation throughout a 1-hr between-session period, leading to four groups. Only participants who slept between sessions while receiving targeted memory reactivation enhanced their throwing accuracy upon beginning the second session. Future studies will aim to use this technique as an adjunct to traditional physical rehabilitation with individuals with neurologic diagnoses such as stroke.

Targeted memory reactivation during sleep to strengthen memory for arbitrary pairings

A powerful way to investigate memory consolidation during sleep utilizes acoustic stimulation to reactivate memories. In multiple studies, Targeted Memory Reactivation (TMR) using sounds associated with prior learning improved later memory, as in recalling locations where objects previously appeared. In the present experiment, we examined whether a variant of the same technique could strengthen memory for the locations of pairs of objects. Each sound was naturally connected to one object from each pair, but we hypothesized that both memories could be improved with TMR. We first asked participants to memorize each of 50 pairs of objects by associating the two objects with each other and with the sound of one of the objects (e.g., cat-meow). Next, objects were presented in unique locations on a grid. Participants learned these locations in an adaptive procedure. During an afternoon nap, 25 of the sounds were quietly presented. In memory tests given twice before and twice after the nap, participants heard the sound for each object pair and were asked to recall the name of the second object and the locations of both objects. Forgetting scores were calculated using the mean difference between pre-nap and post-nap spatial recall errors. We found less forgetting after the nap for cued compared to non-cued objects. Additionally, the extent of forgetting tended to be similar for the two members of each pair, but only for cued pairs. Results thus substantiate the potential for sounds to reactivate spatial memories during sleep and thereby improve subsequent recall performance, even for multiple objects associated with a single sound and when participants must learn a novel sound-object association.

Stimulating the sleeping brain: Current approaches to modulating memory-related sleep physiology

BACKGROUND

One of the most audacious proposals throughout the history of psychology was the potential ability to learn while we sleep. The idea penetrated culture via sci-fi movies and inspired the invention of devices that claimed to teach foreign languages, facts, and even quit smoking by simply listening to audiocassettes or other devices during sleep. However, the promises from this endeavor didn't stand up to experimental scrutiny, and the dream was shunned from the scientific community. Despite the historic evidence that the sleeping brain cannot learn new complex information (i.e., words, images, facts), a new wave of current interventions are demonstrating that sleep can be manipulated to strengthen recent memories.

NEW METHOD

Several recent approaches have been developed that play with the sleeping brain in order to modify ongoing memory processing. Here, we provide an overview of the available techniques to non-invasively modulate memory-related sleep physiology, including sensory, vestibular and electrical stimulation, as well as pharmacological approaches.

RESULTS

N/A.

COMPARISON WITH EXISTING METHODS

N/A.

CONCLUSIONS

Although the results are encouraging, suggesting that in general the sleeping brain may be optimized for better memory performance, the road to bring these techniques in free-living conditions is paved with unanswered questions and technical challenges that need to be carefully addressed.

Cued reactivation during slow-wave sleep induces brain connectivity changes related to memory stabilization

Memory reprocessing following acquisition enhances memory consolidation. Specifically, neural activity during encoding is thought to be 'replayed' during subsequent slow-wave sleep. Such memory replay is thought to contribute to the functional reorganization of neural memory traces. In particular, memory replay may facilitate the exchange of information across brain regions by inducing a reconfiguration of connectivity across the brain. Memory reactivation can be induced by external cues through a procedure known as "targeted memory reactivation". Here, we analysed data from a published study with auditory cues used to reactivate visual object-location memories during slow-wave sleep. We characterized effects of memory reactivation on brain network connectivity using graph-theory. We found that cue presentation during slow-wave sleep increased global network integration of occipital cortex, a visual region that was also active during retrieval of object locations. Although cueing did not have an overall beneficial effect on the retention of cued versus uncued associations, individual differences in overnight memory stabilization were related to enhanced network integration of occipital cortex. Furthermore, occipital cortex displayed enhanced connectivity with mnemonic regions, namely the hippocampus, parahippocampal gyrus, thalamus and medial prefrontal cortex during cue sound presentation. Together, these results suggest a neural mechanism where cue-induced replay during sleep increases integration of task-relevant perceptual regions with mnemonic regions. This cross-regional integration may be instrumental for the consolidation and long-term storage of enduring memories.

Effects of early morning nap sleep on associative memory for neutral and emotional stimuli

Emotional events are preferentially retained in episodic memory. This effect is commonly attributed to enhanced consolidation and has been linked specifically to rapid eye movement (REM) sleep physiology. While several studies have demonstrated an enhancing effect of REM sleep on emotional item memory, it has not been thoroughly explored whether this effect extends to the retention of associative memory. Moreover, it is unclear how non-rapid eye movement (NREM) sleep contributes to these effects. The present study thus examined associative recognition of emotional and non-emotional material across an early morning nap (N = 23) and sustained wakefulness (N = 23). Nap group subjects demonstrated enhanced post-sleep associative memory performance, which was evident across both valence categories. Subsequent analyses revealed significant correlations between NREM spindle density and pre-sleep memory performance. Moreover, NREM spindle density was positively correlated with post-sleep neutral associative memory performance but not with post-sleep emotional associative memory. Accordingly, only neutral associative memory, but not emotional associative memory, was significantly correlated with spindle density after an additional night of sleep (+24 h). These results illustrate a temporally persistent relationship between spindle density and memory for neutral associations, whereas post-sleep emotional associative memory appears to be disengaged from NREM-sleep-dependent processes.

Retrieval and sleep both counteract the forgetting of spatial information

Repeatedly studying information is a good way to strengthen memory storage. Nevertheless, testing recall often produces superior long-term retention. Demonstrations of this testing effect, typically with verbal stimuli, have shown that repeated retrieval through testing reduces forgetting. Sleep also benefits memory storage, perhaps through repeated retrieval as well. That is, memories may generally be subject to forgetting that can be counteracted when memories become reactivated, and there are several types of reactivation: (i) via intentional restudying, (ii) via testing, (iii) without provocation during wake, or (iv) during sleep. We thus measured forgetting for spatial material subjected to repeated study or repeated testing followed by retention intervals with sleep versus wake. Four groups of subjects learned a set of visual object-location associations and either restudied the associations or recalled locations given the objects as cues. We found the advantage for restudied over retested information was greater in the PM than AM group. Additional groups tested at 5-min and 1-wk retention intervals confirmed previous findings of greater relative benefits for restudying in the short-term and for retesting in the long-term. Results overall support the conclusion that repeated reactivation through testing or sleeping stabilizes information against forgetting.

Shaping memory consolidation via targeted memory reactivation during sleep

Recent studies have shown that the reactivation of specific memories during sleep can be modulated using external stimulation. Specifically, it has been reported that matching a sensory stimulus (e.g., odor or sound cue) with target information (e.g., pairs of words, pictures, and motor sequences) during wakefulness, and then presenting the cue alone during sleep, facilitates memory of the target information. Thus, presenting learned cues while asleep may reactivate related declarative, procedural, and emotional material, and facilitate the neurophysiological processes underpinning memory consolidation in humans. This paradigm, which has been named targeted memory reactivation, has been successfully used to improve visuospatial and verbal memories, strengthen motor skills, modify implicit social biases, and enhance fear extinction. However, these studies also show that results depend on the type of memory investigated, the task employed, the sensory cue used, and the specific sleep stage of stimulation. Here, we present a review of how memory consolidation may be shaped using noninvasive sensory stimulation during sleep.

Mechanisms of Memory Retrieval in Slow-Wave Sleep

Study Objectives

Memories are strengthened during sleep. The benefits of sleep for memory can be enhanced by re-exposing the sleeping brain to auditory cues; a technique known as targeted memory reactivation (TMR). Prior studies have not assessed the nature of the retrieval mechanisms underpinning TMR: the matching process between auditory stimuli encountered during sleep and previously encoded memories. We carried out two experiments to address this issue.

Methods

In Experiment 1, participants associated words with verbal and nonverbal auditory stimuli before an overnight interval in which subsets of these stimuli were replayed in slow-wave sleep. We repeated this paradigm in Experiment 2 with the single difference that the gender of the verbal auditory stimuli was switched between learning and sleep.

Results

In Experiment 1, forgetting of cued (vs. noncued) associations was reduced by TMR with verbal and nonverbal cues to similar extents. In Experiment 2, TMR with identical nonverbal cues reduced forgetting of cued (vs. noncued) associations, replicating Experiment 1. However, TMR with nonidentical verbal cues reduced forgetting of both cued and noncued associations.

Conclusions

These experiments suggest that the memory effects of TMR are influenced by the acoustic overlap between stimuli delivered at training and sleep. Our findings hint at the existence of two processing routes for memory retrieval during sleep. Whereas TMR with acoustically identical cues may reactivate individual associations via simple episodic matching, TMR with nonidentical verbal cues may utilize linguistic decoding mechanisms, resulting in widespread reactivation across a broad category of memories.

The transformation of multi-sensory experiences into memories during sleep

Our everyday lives present us with a continuous stream of multi-modal sensory inputs. While most of this information is soon forgotten, sensory information associated with salient experiences can leave long-lasting memories in our minds. Extensive human and animal research has established that the hippocampus is critically involved in this process of memory formation and consolidation. However, the underlying mechanistic details are still only partially understood. Specifically, the hippocampus has often been suggested to encode information during experience, temporarily store it, and gradually transfer this information to the cortex during sleep. In rodents, ample evidence has supported this notion in the context of spatial memory, yet whether this process adequately describes the consolidation of multi-sensory experiences into memories is unclear. Here, focusing on rodent studies, I examine how multi-sensory experiences are consolidated into long term memories by hippocampal and cortical circuits during sleep. I propose that in contrast to the classical model of memory consolidation, the cortex is a "fast learner" that has a rapid and instructive role in shaping hippocampal-dependent memory consolidation. The proposed model may offer mechanistic insight into memory biasing using sensory cues during sleep.

Closed-Loop Targeted Memory Reactivation during Sleep Improves Spatial Navigation

Sounds associated with newly learned information that are replayed during non-rapid eye movement (NREM) sleep can improve recall in simple tasks. The mechanism for this improvement is presumed to be reactivation of the newly learned memory during sleep when consolidation takes place. We have developed an EEG-based closed-loop system to precisely deliver sensory stimulation at the time of down-state to up-state transitions during NREM sleep. Here, we demonstrate that applying this technology to participants performing a realistic navigation task in virtual reality results in a significant improvement in navigation efficiency after sleep that is accompanied by increases in the spectral power especially in the fast (12-15 Hz) sleep spindle band. Our results show promise for the application of sleep-based interventions to drive improvement in real-world tasks.

Brief targeted memory reactivation during the awake state enhances memory stability and benefits the weakest memories

Reactivation of representations corresponding to recent experience is thought to be a critical mechanism supporting long-term memory stabilization. Targeted memory reactivation, or the re-exposure of recently learned cues, seeks to induce reactivation and has been shown to benefit later memory when it takes place during sleep. However, despite recent evidence for endogenous reactivation during post-encoding awake periods, less work has addressed whether awake targeted memory reactivation modulates memory. Here, we found that brief (50 ms) visual stimulus re-exposure during a repetitive foil task enhanced the stability of cued versus uncued associations in memory. The extent of external or task-oriented attention prior to re-exposure was inversely related to cueing benefits, suggesting that an internally-orientated state may be most permissible to reactivation. Critically, cueing-related memory benefits were greatest in participants without explicit recognition of cued items and remained reliable when only considering associations not recognized as cued, suggesting that explicit cue-triggered retrieval processes did not drive cueing benefits. Cueing benefits were strongest for associations and participants with the poorest initial learning. These findings expand our knowledge of the conditions under which targeted memory reactivation can benefit memory, and in doing so, support the notion that reactivation during awake time periods improves memory stabilization.

Sleeping in a Brave New World: Opportunities for Improving Learning and Clinical Outcomes through Targeted Memory Reactivation

Neuroscientific insights into learning and memory have mostly concerned input and output, but intervening processing during the time between acquisition and retrieval is also critical. Indeed, intervening memory reactivation may regulate memory longevity, and a growing body of evidence implicates sleep in changing memory storage. For example, subtle auditory stimulation can be used experimentally to selectively encourage memory reactivation during sleep, which thereby improves learning. Much remains to be elucidated about how learning depends on sleep. Nevertheless, this methodology for modifying memory storage during sleep offers new opportunities for reinforcing learning to enhance clinical outcomes in conjunction with therapies engaged during waking. A variety of such possibilities must now be carefully investigated. Likewise, brain rhythms can be entrained to enhance sleep functions, facilitating further progress in understanding the neurophysiological basis of memory processing during sleep. Ultimately, empirical evidence may reveal the extent to which the way we behave when awake is a function of what our brains do when we are asleep. Through such research efforts, an advanced understanding of memory and sleep may allow us to both make better use of our time asleep and take steps toward better health.

Modulating influences of memory strength and sensitivity of the retrieval test on the detectability of the sleep consolidation effect

Emotionality can increase recall probability of memories as emotional information is highly relevant for future adaptive behavior. It has been proposed that memory processes acting during sleep selectively promote the consolidation of emotional memories, so that neutral memories no longer profit from sleep consolidation after learning. This appears as a selective effect of sleep for emotional memories. However, other factors contribute to the appearance of a consolidation benefit and influence this interpretation. Here we show that the strength of the memory trace before sleep and the sensitivity of the retrieval test after sleep are critical factors contributing to the detection of the benefit of sleep on memory for emotional and neutral stimuli. 228 subjects learned emotional and neutral pictures and completed a free recall after a 12-h retention interval of either sleep or wakefulness. We manipulated memory strength by including an immediate retrieval test before the retention interval in half of the participants. In addition, we varied the sensitivity of the retrieval test by including an interference learning task before retrieval testing in half of the participants. We show that a "selective" benefit of sleep for emotional memories only occurs in the condition with high memory strength. Furthermore, this "selective" benefit disappeared when we controlled for the memory strength before the retention interval and used a highly sensitive retrieval test. Our results indicate that although sleep benefits are more robust for emotional memories, neutral memories similarly profit from sleep after learning when more sensitive indicators are used. We conclude that whether sleep benefits on memory appear depends on several factors, including emotion, memory strength and sensitivity of the retrieval test.

Vocabulary learning benefits from REM after slow-wave sleep

Memory reactivation during slow-wave sleep (SWS) influences the consolidation of recently acquired knowledge. This reactivation occurs spontaneously during sleep but can also be triggered by presenting learning-related cues, a technique known as targeted memory reactivation (TMR). Here we examined whether TMR can improve vocabulary learning. Participants learned the meanings of 60 novel words. Auditory cues for half the words were subsequently presented during SWS in an afternoon nap. Memory performance for cued versus uncued words did not differ at the group level but was systematically influenced by REM sleep duration. Participants who obtained relatively greater amounts of REM showed a significant benefit for cued relative to uncued words, whereas participants who obtained little or no REM demonstrated a significant effect in the opposite direction. We propose that REM after SWS may be critical for the consolidation of highly integrative memories, such as new vocabulary. Reactivation during SWS may allow newly encoded memories to be associated with other information, but this association can include disruptive linkages with pre-existing memories. Subsequent REM sleep may then be particularly beneficial for integrating new memories into appropriate pre-existing memory networks. These findings support the general proposition that memory storage benefits optimally from a cyclic succession of SWS and REM.

Sleep to Remember

Scientific investigation into the possible role of sleep in memory consolidation began with the early studies of Jenkins and Dallenbach (1924). Despite nearly a century of investigation with a waxing and waning of interest, the role of sleep in memory processing remains controversial and elusive. This review provides the historical background for current views and considers the relative contribution of two sleep states, rapid eye movement sleep and slow-wave sleep, to offline memory processing. The sequential hypothesis, until now largely ignored, is discussed, and recent literature supporting this view is reviewed.

No effect of targeted memory reactivation during slow-wave sleep on emotional recognition memory

Recent work has suggested that the benefits of sleep for memory consolidation are enhanced for highly salient (versus non-salient) memories. Using a technique known as targeted memory reactivation, it is possible to selectively strengthen newly learned memories by re-exposing the sleeping brain to auditory cues. The aim of the current study was to examine whether emotionally salient memories are also more responsive to targeted memory reactivation in slow-wave sleep than neutral memories. In an initial training phase, participants memorised emotionally negative and neutral pictures, which were each paired with a semantically related sound. Recognition for the pictures was assessed before and after a 90-min nap opportunity, during which half the sounds were re-presented during slow-wave sleep (as assessed via online polysomnographic sleep monitoring). We observed no effect of targeted memory reactivation on the recognition of emotionally negative or neutral memories. Our results highlight the importance of the memory paradigm used to assess targeted memory reactivation, and suggest that the robust and durable nature of recognition memory may make it an insensitive measure of behavioural targeted memory reactivation benefits. To fully assess the impacts of targeted memory reactivation on emotional memory processing in sleep, future studies should adopt experimental paradigms that maximise the salience of emotional stimuli while also providing a sensitive index of memory accuracy.

Widespread reduction in sleep spindle activity in socially anxious children and adolescents

Social anxiety disorder (SAD) is one of the most prevalent psychiatric diseases typically emerging during childhood and adolescence. Biological vulnerabilities such as a protracted maturation of prefrontal cortex functioning together with heightened reactivity of the limbic system leading to increased emotional reactivity are discussed as factors contributing to the emergence and maintenance of SAD. Sleep slow wave activity (SWA, 0.75-4.5 Hz) and sleep spindle activity (9-16 Hz) reflect processes of brain maturation and emotion regulation. We used high-density electroencephalography to characterize sleep SWA and spindle activity and their relationship to emotional reactivity in children and adolescents suffering from SAD and healthy controls (HC). Subjectively rated arousal was assessed using an emotional picture-word association task. SWA did not differ between socially anxious and healthy participants. We found a widespread reduction in fast spindle activity (13-16 Hz) in SAD patients compared to HC. SAD patients rated negative stimuli to be more arousing and these arousal ratings were negatively correlated with fast spindle activity. These results suggest electrophysiological alterations that are evident at an early stage of psychopathology and that are closely linked to one core symptom of anxiety disorders such as increased emotional reactivity. The role of disturbed GABAergic neurotransmission is discussed as an underlying factor.

Sleep-based memory processing facilitates grammatical generalization: Evidence from targeted memory reactivation

Generalization-the ability to abstract regularities from specific examples and apply them to novel instances-is an essential component of language acquisition. Generalization not only depends on exposure to input during wake, but may also improve offline during sleep. Here we examined whether targeted memory reactivation during sleep can influence grammatical generalization. Participants gradually acquired the grammatical rules of an artificial language through an interactive learning procedure. Then, phrases from the language (experimental group) or stimuli from an unrelated task (control group) were covertly presented during an afternoon nap. Compared to control participants, participants re-exposed to the language during sleep showed larger gains in grammatical generalization. Sleep cues produced a bias, not necessarily a pure gain, suggesting that the capacity for memory replay during sleep is limited. We conclude that grammatical generalization was biased by auditory cueing during sleep, and by extension, that sleep likely influences grammatical generalization in general.

The beneficial role of memory reactivation for language learning during sleep: A review

Sleep is essential for diverse aspects of language learning. According to a prominent concept these beneficial effects of sleep rely on spontaneous reactivation processes. A series of recent studies demonstrated that inducing such reactivation processes by re-exposure to memory cues during sleep enhances foreign vocabulary learning. Building upon these findings, the present article reviews recent models and empirical findings concerning the beneficial effects of sleep on language learning. Consequently, the memory function of sleep, its neural underpinnings and the role of the sleeping brain in language learning will be summarized. Finally, we will propose a working model concerning the oscillatory requirements for successful reactivation processes and future research questions to advance our understanding of the role of sleep on language learning and memory processes in general.

Prior knowledge is essential for the beneficial effect of targeted memory reactivation during sleep

Prior knowledge speeds up system consolidation and accelerates integration of newly acquired memories into existing neocortical knowledge networks. By using targeted memory reactivations, we demonstrate that prior knowledge is also essential for successful reactivation and consolidation of memories during sleep, both on the behavioral and oscillatory level (i.e., theta and fast spindle activity). Thus, prior knowledge is a prerequisite for new memories to enter processes of system consolidation during sleep.

Sleep and memory consolidation: a common mechanism across species?

In humans, memory consolidation can be aided by the representation of an odor previously associated with target information during sleep. In an elegant study, Zwaka et al. ( Curr Biol 25: 2869–2874, 2015) have demonstrated that the same process occurs in honeybees, suggesting that the relationship between sleep and memory may be similar across different animal species.

Sleeping on the rubber-hand illusion: Memory reactivation during sleep facilitates multisensory recalibration

Plasticity is essential in body perception so that physical changes in the body can be accommodated and assimilated. Multisensory integration of visual, auditory, tactile, and proprioceptive signals contributes both to conscious perception of the body's current state and to associated learning. However, much is unknown about how novel information is assimilated into body perception networks in the brain. Sleep-based consolidation can facilitate various types of learning via the reactivation of networks involved in prior encoding or through synaptic down-scaling. Sleep may likewise contribute to perceptual learning of bodily information by providing an optimal time for multisensory recalibration. Here we used methods for targeted memory reactivation (TMR) during slow-wave sleep to examine the influence of sleep-based reactivation of experimentally induced alterations in body perception. The rubber-hand illusion was induced with concomitant auditory stimulation in 24 healthy participants on 3 consecutive days. While each participant was sleeping in his or her own bed during intervening nights, electrophysiological detection of slow-wave sleep prompted covert stimulation with either the sound heard during illusion induction, a counterbalanced novel sound, or neither. TMR systematically enhanced feelings of bodily ownership after subsequent inductions of the rubber-hand illusion. TMR also enhanced spatial recalibration of perceived hand location in the direction of the rubber hand. This evidence for a sleep-based facilitation of a body-perception illusion demonstrates that the spatial recalibration of multisensory signals can be altered overnight to stabilize new learning of bodily representations. Sleep-based memory processing may thus constitute a fundamental component of body-image plasticity.

Phase of Spontaneous Slow Oscillations during Sleep Influences Memory-Related Processing of Auditory Cues

Slow oscillations during slow-wave sleep (SWS) may facilitate memory consolidation by regulating interactions between hippocampal and cortical networks. Slow oscillations appear as high-amplitude, synchronized EEG activity, corresponding to upstates of neuronal depolarization and downstates of hyperpolarization. Memory reactivations occur spontaneously during SWS, and can also be induced by presenting learning-related cues associated with a prior learning episode during sleep. This technique, targeted memory reactivation (TMR), selectively enhances memory consolidation. Given that memory reactivation is thought to occur preferentially during the slow-oscillation upstate, we hypothesized that TMR stimulation effects would depend on the phase of the slow oscillation. Participants learned arbitrary spatial locations for objects that were each paired with a characteristic sound (eg, cat-meow). Then, during SWS periods of an afternoon nap, one-half of the sounds were presented at low intensity. When object location memory was subsequently tested, recall accuracy was significantly better for those objects cued during sleep. We report here for the first time that this memory benefit was predicted by slow-wave phase at the time of stimulation. For cued objects, location memories were categorized according to amount of forgetting from pre- to post-nap. Conditions of high versus low forgetting corresponded to stimulation timing at different slow-oscillation phases, suggesting that learning-related stimuli were more likely to be processed and trigger memory reactivation when they occurred at the optimal phase of a slow oscillation. These findings provide insight into mechanisms of memory reactivation during sleep, supporting the idea that reactivation is most likely during cortical upstates.

SIGNIFICANCE STATEMENT

Slow-wave sleep (SWS) is characterized by synchronized neural activity alternating between active upstates and quiet downstates. The slow-oscillation upstates are thought to provide a window of opportunity for memory consolidation, particularly conducive to cortical plasticity. Recent evidence shows that sensory cues associated with previous learning can be delivered subtly during SWS to selectively enhance memory consolidation. Our results demonstrate that this behavioral benefit is predicted by slow-oscillation phase at stimulus presentation time. Cues associated with high versus low forgetting based on analysis of subsequent recall performance were delivered at opposite slow-oscillation phases. These results provide evidence of an optimal slow-oscillation phase for memory consolidation during sleep, supporting the idea that memory processing occurs preferentially during cortical upstates.

The Benefits of Targeted Memory Reactivation for Consolidation in Sleep are Contingent on Memory Accuracy and Direct Cue-Memory Associations

STUDY OBJECTIVES

To investigate how the effects of targeted memory reactivation (TMR) are influenced by memory accuracy prior to sleep and the presence or absence of direct cue-memory associations.

METHODS

30 participants associated each of 50 pictures with an unrelated word and then with a screen location in two separate tasks. During picture-location training, each picture was also presented with a semantically related sound. The sounds were therefore directly associated with the picture locations but indirectly associated with the words. During a subsequent nap, half of the sounds were replayed in slow wave sleep (SWS). The effect of TMR on memory for the picture locations (direct cue-memory associations) and picture-word pairs (indirect cue-memory associations) was then examined.

RESULTS

TMR reduced overall memory decay for recall of picture locations. Further analyses revealed a benefit of TMR for picture locations recalled with a low degree of accuracy prior to sleep, but not those recalled with a high degree of accuracy. The benefit of TMR for low accuracy memories was predicted by time spent in SWS. There was no benefit of TMR for memory of the picture-word pairs, irrespective of memory accuracy prior to sleep.

CONCLUSIONS

TMR provides the greatest benefit to memories recalled with a low degree of accuracy prior to sleep. The memory benefits of TMR may also be contingent on direct cue-memory associations.

Memory cueing during sleep modifies the interpretation of ambiguous scenes in adolescents and adults

The individual tendency to interpret ambiguous situations negatively is associated with mental disorders. Interpretation biases are already evident during adolescence and due to the greater plasticity of the developing brain it may be easier to change biases during this time. We investigated in healthy adolescents and adults whether stabilizing memories of positive or negative scenes modulates the later interpretation of similar scenes. In the evening, participants learnt associations between ambiguous pictures and words that disambiguate the valence of the pictures in a positive or negative direction. Half of the words were acoustically presented (i.e. cued) during post-learning sleep which is known to benefit memory consolidation by inducing reactivation of learned information. Cued compared to un-cued stimuli were remembered better the next morning. Importantly, cueing positively disambiguated pictures resulted in more positive interpretations whereas cueing negatively disambiguated pictures led to less positive interpretations of new ambiguous pictures with similar contents the next morning. These effects were not modulated by participants' age indicating that memory cueing was as efficient in adolescents as in adults. Our findings suggest that memory cueing during sleep can modify interpretation biases by benefitting memory stabilization and generalization. Implications for clinical settings are discussed.