Generalizing memories over time: sleep and reinforcement facilitate transitive inference

Integration of overlapping sequences emerges with consolidation through medial prefrontal cortex neural ensembles and hippocampal-cortical connectivity

Systems consolidation theories propose two mechanisms that enable the behavioral integration of related memories: coordinated reactivation between hippocampus and cortex, and the emergence of cortical traces that reflect overlap across memories. However, there is limited empirical evidence that links these mechanisms to the emergence of behavioral integration over time. In two experiments, participants implicitly encoded sequences of objects with overlapping structure. Assessment of behavioral integration showed that response times during a recognition task reflected behavioral priming between objects that never occurred together in time but belonged to overlapping sequences. This priming was consolidation-dependent and only emerged for sequences learned 24 hr prior to the test. Critically, behavioral integration was related to changes in neural pattern similarity in the medial prefrontal cortex and increases in post-learning rest connectivity between the posterior hippocampus and lateral occipital cortex. These findings suggest that memories with a shared predictive structure become behaviorally integrated through a consolidation-related restructuring of the learned sequences, providing insight into the relationship between different consolidation mechanisms that support behavioral integration.

A visual paired associate learning (vPAL) paradigm to study memory consolidation during sleep

Sleep improves the consolidation and long-term stability of newly formed memories and associations. Most research on human declarative memory and its consolidation during sleep uses word-pair associations requiring exhaustive learning. In the present study, we present the visual paired association learning (vPAL) paradigm, in which participants learn new associations between images of celebrities and animals. The vPAL is based on a one-shot exposure that resembles learning in natural conditions. We tested if vPAL can reveal a role for sleep in memory consolidation by assessing the specificity of memory recognition, and the cued recall performance, before and after sleep. We found that a daytime nap improved the stability of recognition memory and discrimination abilities compared to identical intervals of wakefulness. By contrast, cued recall of associations did not exhibit significant sleep-dependent effects. High-density electroencephalography during naps further revealed an association between sleep spindle density and stability of recognition memory. Thus, the vPAL paradigm opens new avenues for future research on sleep and memory consolidation across ages and heterogeneous populations in health and disease.

Manipulated overlapping reactivation of multiple memories promotes explicit gist abstraction

Sleep is considered to promote gist abstraction on the basis of spontaneous memory reactivation. As speculated in the theory of 'information overlap to abstract (iOtA)', 'overlap' between reactivated memories, beyond reactivation, is crucial to gist abstraction. Yet so far, empirical research has not tested this theory by manipulating the factor of 'overlap'. In the current study, 'overlap' itself was manipulated by targeted memory reactivation (TMR), through simultaneously reactivating multiple memories that either contain or do not contain spatially overlapped gist information, to investigate the effect of overlapping reactivation on gist abstraction. This study had a factorial design of 2 factors with 2 levels respectively (spatial overlap/no spatial overlap, TMR/no-TMR). Accordingly, 82 healthy college students (aged 19 ∼ 25, 57 females) were randomized into four groups. After learning 16 pictures, paired with 4 auditory cues (4 pictures - 1 cue) according to the grouping, participants were given a 90-minute nap opportunity. Then TMR cueing was conducted during N2 and slow wave sleep of the nap. Performance in memory task was used to measure gist abstraction. The results showed a significant main effect of TMR on both implicit and explicit gist abstraction, and a marginally significant interaction effect on explicit gist abstraction. Further analyses showed that explicit gist abstraction in the spatial overlap & TMR group was significantly better than in the control group. Moreover, explicit gist abstraction was positively correlated with spindle density. The current study thus indicates that TMR facilitates gist abstraction, and explicit gist abstraction may benefit more from overlapping reactivation.

Memory reactivation in slow wave sleep enhances relational learning in humans

Sleep boosts the integration of memories, and can thus facilitate relational learning. This benefit may be due to memory reactivation during non-REM sleep. We set out to test this by explicitly cueing reactivation using a technique called targeted memory reactivation (TMR), in which sounds are paired with learned material in wake and then softly played during subsequent sleep, triggering reactivation of the associated memories. We specifically tested whether TMR in slow wave sleep leads to enhancements in inferential thinking in a transitive inference task. Because the Up-phase of the slow oscillation is more responsive to cues than the Down-phase, we also asked whether Up-phase stimulation is more beneficial for such integration. Our data show that TMR during the Up-Phase boosts the ability to make inferences, but only for the most distant inferential leaps. Up-phase stimulation was also associated with detectable memory reinstatement, whereas Down-phase stimulation led to below-chance performance the next morning. Detection of memory reinstatement after Up-state stimulation was negatively correlated with performance on the most difficult inferences the next morning. These findings demonstrate that cueing memory reactivation at specific time points in sleep can benefit difficult relational learning problems.

Sleep shapes the associative structure underlying pattern completion in multielement event memory

Sleep supports the consolidation of episodic memory. It is, however, a matter of ongoing debate how this effect is established, because, so far, it has been demonstrated almost exclusively for simple associations, which lack the complex associative structure of real-life events, typically comprising multiple elements with different association strengths. Because of this associative structure interlinking the individual elements, a partial cue (e.g., a single element) can recover an entire multielement event. This process, referred to as pattern completion, is a fundamental property of episodic memory. Yet, it is currently unknown how sleep affects the associative structure within multielement events and subsequent processes of pattern completion. Here, we investigated the effects of post-encoding sleep, compared with a period of nocturnal wakefulness (followed by a recovery night), on multielement associative structures in healthy humans using a verbal associative learning task including strongly, weakly, and not directly encoded associations. We demonstrate that sleep selectively benefits memory for weakly associated elements as well as for associations that were not directly encoded but not for strongly associated elements within a multielement event structure. Crucially, these effects were accompanied by a beneficial effect of sleep on the ability to recall multiple elements of an event based on a single common cue. In addition, retrieval performance was predicted by sleep spindle activity during post-encoding sleep. Together, these results indicate that sleep plays a fundamental role in shaping associative structures, thereby supporting pattern completion in complex multielement events.

Transitive inference and the testing effect: Retrieval practice impairs transitive inference

There is substantial interest in the extent to which the testing effect (the finding that retrieval practice enhances memory) extends to more complex forms of learning, especially those entailing greater element interactivity. Transitive inference (TI) requires just such interactivity, in which information must be combined across multiple learning elements or premises to extract an underlying structure. Picklesimer et al. provided preliminary evidence that retrieval practice fails to enhance, and actually disrupts, TI. This study assessed the generality of that result. The current experiments employed a seven- or eight-element TI paradigm in which participants initially learned a set of premise pairs (e.g., A > B, B > C, and C > D) and then engaged in either restudy or retrieval practice of the premise pairs before taking a final test that assessed memory for the original premise pairs and one's ability to make TIs (e.g., to infer that B > D). Experiments 1 and 2 used pictorial materials and simultaneous presentation of premises during learning, a form of presentation that has induced testing effects on other forms of inference. For TI, the results were unchanged from Picklesimer et al.-TI was worse for retrieval practice than restudy. Experiment 3 used verbal materials and likewise found worse TI for retrieval practice. A small-scale meta-analysis combining the current experiments with those of Picklesimer et al. revealed a significant negative testing effect on TI (d = -0.37). Although retrieval practice enhances many aspects of memory, this fundamental aspect of human reasoning may be impaired by retrieval practice.

Sleep-related benefits to transitive inference are modulated by encoding strength and joint rank

Transitive inference is a measure of relational learning that has been shown to improve across sleep. Here, we examine this phenomenon further by studying the impact of encoding strength and joint rank. In experiment 1, participants learned adjacent premise pairs and were then tested on inferential problems derived from those pairs. In line with prior work, we found improved transitive inference performance after retention across a night of sleep compared with wake alone. Experiment 2 extended these findings using a within-subject design and found superior transitive inference performance on a hierarchy, consolidated across 27 h including sleep compared with just 3 h of wake. In both experiments, consolidation-related improvement was enhanced when presleep learning (i.e., encoding strength) was stronger. We also explored the interaction of these effects with the joint rank effect, in which items were scored according to their rank in the hierarchy, with more dominant item pairs having the lowest scores. Interestingly, the consolidation-related benefit was greatest for more dominant inference pairs (i.e., those with low joint rank scores). Overall, our findings provide further support for the improvement of transitive inference across a consolidation period that includes sleep. We additionally show that encoding strength and joint rank strongly modulate this effect.

Advantage conferred by overnight sleep on schema-related memory may last only a day

Study Objectives

Sleep contributes to declarative memory consolidation. Independently, schemas benefit memory. Here we investigated how sleep compared with active wake benefits schema consolidation 12 and 24 hours after initial learning.

Methods

Fifty-three adolescents (age: 15-19 years) randomly assigned into sleep and active wake groups participated in a schema-learning protocol based on transitive inference (i.e. If B > C and C > D then B > D). Participants were tested immediately after learning and following 12-, and 24-hour intervals of wake or sleep for both the adjacent (e.g. B-C, C-D; relational memory) and inference pairs: (e.g.: B-D, B-E, and C-E). Memory performance following the respective 12- and 24-hour intervals were analyzed using a mixed ANOVA with schema (schema, no-schema) as the within-participant factor, and condition (sleep, wake) as the between-participant factor.

Results

Twelve hours after learning, there were significant main effects of condition (sleep, wake) and schema, as well as a significant interaction, whereby schema-related memory was significantly better in the sleep condition compared to wake. Higher sleep spindle density was most consistently associated with greater overnight schema-related memory benefit. After 24 hours, the memory advantage of initial sleep was diminished.

Conclusions

Overnight sleep preferentially benefits schema-related memory consolidation following initial learning compared with active wake, but this advantage may be eroded after a subsequent night of sleep. This is possibly due to delayed consolidation that might occur during subsequent sleep opportunities in the wake group.

Clinical Trial Information

Name: Investigating Preferred Nap Schedules for Adolescents (NFS5) URL: https://clinicaltrials.gov/ct2/show/NCT04044885. Registration: NCT04044885.

The unique contributions of day and night sleep to infant motor problem solving

The current study sought to tease apart the unique contributions of napping and nighttime sleep to infant learning, specifically in the context of motor problem solving. We challenged 54 walking infants to solve a novel locomotor problem at three time points-training, test, and follow-up the next morning. One group of infants napped during the delay between training and test. Another group did not sleep during the delay. A third group received the test immediately after training with no delay. Only the Nap group's strategy choices continued to improve through the follow-up session, suggesting that daytime sleep has an active role in strengthening otherwise fragile memory. Although group did not affect strategy maintenance, walk experience did, suggesting that task difficulty may shape the impact of sleep on learning. Thus, day sleep and night sleep make independent contributions to the consolidation of motor problem-solving strategies during infancy.

Reward Uncertainty and Expected Value Enhance Generalization of Episodic Memory

Previous research has revealed some mechanisms underlying the generalization of reward expectation of generalization stimuli, but little is known about the generalization of episodic memory for rewarding events, its consolidation, and how reward components such as expected value and reward uncertainty affect it. Participants underwent a Pavlovian reward-conditioning task to test whether reward conditioning would enhance episodic memory generalization and which reward components would directly affect it. Counterbalanced across participants, one semantic category was paired with a reward, while the other was never paired. Following a delay of either 5 min or 24 h, participants took a memory test consisting of old, highly similar, and new items. We found that participants were more likely to falsely recognize lure items as old in the reward-paired category after 5 min and 24 h delays. These results indicate that reward conditioning enhanced the generalization of episodic memory, but this effect was not necessarily dependent on consolidation. The composite score and raw data of generalization further showed that the uncertainty and expected value enhanced generalization. Together, these findings revealed an effect of reward conditioning on episodic memory generalization and supported the enhancement effects of expected value and uncertainty.

A sleep schedule incorporating naps benefits the transformation of hierarchical knowledge

Study Objectives

The learning brain establishes schemas (knowledge structures) that benefit subsequent learning. We investigated how sleep and having a schema might benefit initial learning followed by rearranged and expanded memoranda. We concurrently examined the contributions of sleep spindles and slow-wave sleep to learning outcomes.

Methods

Fifty-three adolescents were randomly assigned to an 8 h Nap schedule (6.5 h nocturnal sleep with a 90-minute daytime nap) or an 8 h No-Nap, nocturnal-only sleep schedule. The study spanned 14 nights, simulating successive school weeks. We utilized a transitive inference task involving hierarchically ordered faces. Initial learning to set up the schema was followed by rearrangement of the hierarchy (accommodation) and hierarchy expansion (assimilation). The expanded sequence was restudied. Recall of hierarchical knowledge was tested after initial learning and at multiple points for all subsequent phases. As a control, both groups underwent a No-schema condition where the hierarchy was introduced and modified without opportunity to set up a schema. Electroencephalography accompanied the multiple sleep opportunities.

Results

There were main effects of Nap schedule and Schema condition evidenced by superior recall of initial learning, reordered and expanded memoranda. Improved recall was consistently associated with higher fast spindle density but not slow-wave measures. This was true for both nocturnal sleep and daytime naps.

Conclusion

A sleep schedule incorporating regular nap opportunities compared to one that only had nocturnal sleep benefited building of robust and flexible schemas, facilitating recall of the subsequently rearranged and expanded structured knowledge. These benefits appear to be strongly associated with fast spindles.

Clinical Trial registration

NCT04044885 (https://clinicaltrials.gov/ct2/show/NCT04044885).

Examining the effects of time of day and sleep on generalization

Extracting shared structure across our experiences allows us to generalize our knowledge to novel contexts. How do different brain states influence this ability to generalize? Using a novel category learning paradigm, we assess the effect of both sleep and time of day on generalization that depends on the flexible integration of recent information. Counter to our expectations, we found no evidence that this form of generalization is better after a night of sleep relative to a day awake. Instead, we observed an effect of time of day, with better generalization in the morning than the evening. This effect also manifested as increased false memory for generalized information. In a nap experiment, we found that generalization did not benefit from having slept recently, suggesting a role for time of day apart from sleep. In follow-up experiments, we were unable to replicate the time of day effect for reasons that may relate to changes in category structure and task engagement. Despite this lack of consistency, we found a morning benefit for generalization when analyzing all the data from experiments with matched protocols (n = 136). We suggest that a state of lowered inhibition in the morning may facilitate spreading activation between otherwise separate memories, promoting this form of generalization.

Memory consolidation as an adaptive process

We rely on our long-term memories to guide future behaviors, making it adaptive to prioritize the retention of goal-relevant, salient information in memory. In this review, we discuss findings from rodent and human research to demonstrate that active processes during post-encoding consolidation support the selective stabilization of recent experience into adaptive, long-term memories. Building upon literatures focused on dynamics at the cellular level, we highlight that consolidation also transforms memories at the systems level to support future goal-relevant behavior, resulting in more generalized memory traces in the brain and behavior. We synthesize previous literatures spanning animal research, human cognitive neuroscience, and cognitive psychology to propose an integrative framework for adaptive consolidation by which goal-relevant memoranda are "tagged" for subsequent consolidation, resulting in selective transformations to the structure of memories that support flexible, goal-relevant behaviors.

Neuronal Computation Underlying Inferential Reasoning in Humans and Mice

Every day we make decisions critical for adaptation and survival. We repeat actions with known consequences. But we also draw on loosely related events to infer and imagine the outcome of entirely novel choices. These inferential decisions are thought to engage a number of brain regions; however, the underlying neuronal computation remains unknown. Here, we use a multi-day cross-species approach in humans and mice to report the functional anatomy and neuronal computation underlying inferential decisions. We show that during successful inference, the mammalian brain uses a hippocampal prospective code to forecast temporally structured learned associations. Moreover, during resting behavior, coactivation of hippocampal cells in sharp-wave/ripples represent inferred relationships that include reward, thereby "joining-the-dots" between events that have not been observed together but lead to profitable outcomes. Computing mnemonic links in this manner may provide an important mechanism to build a cognitive map that stretches beyond direct experience, thus supporting flexible behavior.

Active transitive inference: When learner control facilitates integrative encoding

A growing body of research indicates that active control of learning improves episodic memory for material experienced during study. It is less clear how active learning impacts the integration of those experiences into flexible, generalizable knowledge. This study uses a novel active transitive inference task to investigate how people learn a relational hierarchy through active selection of premise pairs. Active control improved memory for studied premises as well as transitive inferences involving items that were never experienced together during study. Active learners also exhibited a systematic search preference, generating sequences of overlapping premises that may facilitate relational integration. Critically, however, advantages from active control were not universal: Only participants with higher working memory capacity benefited from the opportunity to select premise pairs during learning. These findings suggest that active control enhances integrative encoding of studied material, but only among individuals with sufficient cognitive resources.

Sleep Promotes Phonological Learning in Children Across Language and Autism Spectra

Purpose Establishing stable and flexible phonological representations is a key component of language development and one which is thought to vary across children with neurodevelopmental disorders affecting language acquisition. Sleep is understood to support the learning and generalization of new phonological mappings in adults, but this remains to be examined in children. This study therefore explored the time course of phonological learning in childhood and how it varies by structural language and autism symptomatology. Method Seventy-seven 7- to 13-year-old children, 30 with high autism symptomatology, were included in the study; structural language ability varied across the sample. Children learned new phonological mappings based on synthesized speech tokens in the morning; performance was then charted via repetition (without feedback) over 24 hr and followed up 4 weeks later. On the night following learning, children's sleep was monitored with polysomnography. Results A period of sleep but not wake was associated with improvement on the phonological learning task in childhood. Sleep was associated with improved performance for both trained items and novel items. Structural language ability predicted overall task performance, though language ability did not predict degree of change from one session to the next. By contrast, autism symptomatology did not explain task performance. With respect to sleep architecture, rapid eye movement features were associated with greater phonological generalization. Conclusions Children's sleep was associated with improvement in performance on both trained and novel items. Phonological generalization was associated with brain activity during rapid eye movement sleep. This study furthers our understanding of individual differences in the acquisition of new phonological mappings and the role of sleep in this process over childhood. Supplemental Material https://doi.org/10.23641/asha.11126732.

The Effect of Retrieval Practice on Transitive Inference

Abstract. Compared to restudying, retrieval practice has often been found to enhance memory (the testing effect). However, it has been proposed that materials with high element interactivity may not benefit from retrieval practice. Transitive inference (TI) requires just such interactivity, in which information must be combined across multiple learning elements or premises. The current study employed a 7-element TI paradigm in which participants initially learned a set of premises (e.g., A > B, B > C, C > D, etc.), then engaged in either restudy or retrieval practice with the premises, and then were given a final test that assessed memory for the original premises and one’s ability to make transitive inferences about them (e.g., to infer that B > D). Three experiments examined TI on final tests with retention intervals of a few minutes (Experiment 1), 2 days (Experiment 2), or up to a week (Experiment 3). Retrieval practice consistently failed to enhance transitive inference. Furthermore, retrieval practice significantly reduced TI in Experiment 1. Across experiments, TI was numerically worse in the retrieval-practice than restudy condition in 4 of 5 comparisons, and a small-scale meta-analysis revealed a significant negative effect of retrieval practice on TI.

Sleep and the extraction of hidden regularities: A systematic review and the importance of temporal rules

As part of its role in memory consolidation, sleep has been repeatedly identified as critical for the extraction of regularities from wake experiences. However, many null results have been published as well, with no clear consensus emerging regarding the conditions that yield this sleep effect. Here, we systematically review the role of sleep in the extraction of hidden regularities, specifically those involving associative relations embedded in newly learned information. We found that the specific behavioral task used in a study had far more impact on whether a sleep effect was discovered than either the category of the cognitive processes targeted, or the particular experimental design employed. One emerging pattern, however, was that the explicit detection of hidden rules is more likely to happen when the rules are of a temporal nature (i.e., event A at time t predicts a later event B) than when they are non-temporal. We discuss this temporal rule sensitivity in reference to the compressed memory replay occurring in the hippocampus during slow-wave-sleep, and compare this effect to what happens when the extraction of regularities depends on prior knowledge and relies on structures other than the hippocampus.

The hippocampus is critical for value-based decisions guided by dissociative inference

The hippocampus supports flexible decision-making through memory integration: bridging across episodes and inferring associations between stimuli that were never presented together ('associative inference'). A pre-requisite for memory integration is flexible representations of the relationships between stimuli within episodes (AB) but also of the constituent units (A,B). Here we investigated whether the hippocampus is required for parsing experienced episodes into their constituents to infer their re-combined within-episode associations ('dissociative inference'). In three experiments male rats were trained on an appetitive conditioning task using compound auditory stimuli (AB+, BA+, CD-, DC-). At test either the compound or individual stimuli were presented as well as new stimuli. Rats with hippocampal lesions acquired and retained the compound discriminations as well as controls. Single constituent stimuli (A, B, C, D) were presented for the first time at test, so the only value with which they could be associated was the one from the compound to which they belonged. Controls inferred constituent tones' corresponding values while hippocampal rats did not, treating them as merely familiar stimuli with no associated value. This finding held whether compound training occurred before or after hippocampal lesions, suggesting that hippocampus-dependent inferential processes more likely occur at retrieval. The findings extend recent discoveries about the role of the hippocampus in intrinsic value representation, demonstrating hippocampal contributions to allocating value from primary rewards to individual stimuli. Importantly, we discovered that dissociative inferences serve to restructure or reparse patterns of directly acquired associations when animals are faced with environmental changes and need to extract relevant information from a multiplex memory. The hippocampus is critical for this fundamental flexible use of associations.

The malleability of executive function in early childhood: Effects of schooling and targeted training

Executive function (EF), its importance for scholastic achievement and the question of whether or not EF is malleable, have become a topic of intense interest. Education or schooling is often seen as effective approaches to enhance EF due to the specific school-related requirements as compared to kindergarten or pre-school. However, no study to date has investigated whether targeted training focusing on those domains might be comparable with regular schooling in improving EF and fluid intelligence (Gf). The aim of the present study was to replicate and extend the previously demonstrated schooling effects on EF by using a school-cutoff design, and to further investigate whether a theoretically motivated intervention targeting specific EF, i.e., working memory (WM) or inhibitory control (IC), could achieve comparable effects with schooling in both, WM and IC, as well as Gf. 91 6-year-old kindergarteners and first-graders with similar chronological age participated the study. We compared the performance of a first-grade schooling group with that of two kindergarten training groups as well as a business-as-usual kindergarten control group. Participants were assessed in WM, IC and Gf at baseline, immediately after the intervention (posttest), as well as 3 months after training completion (follow-up). The results showed that the schooling group indeed outperformed the kindergarten groups at baseline in several cognitive tasks. Furthermore, both the WM and IC training showed pronounced gains in the trained tasks, as well as varying degrees of improvement in non-trained outcome measures. Most importantly, both training groups achieved comparable performance with the schooling group, which was especially apparent in Gf at follow-up. Our findings provide further evidence for the malleability of EF demonstrating that both, long-term and short-term interventions can facilitate the acquisition of those important skills, and as such, our work has important implications for educational practice.

Curiosity-driven memory enhancement persists over time but does not benefit from post-learning sleep

Sleep-dependent memory processing is dependent on several factors at learning, including emotion, encoding strength, and knowledge of future relevance. Recent work documents the role of curiosity on learning, showing that memory associated with high-curiosity encoding states is retained better and that this effect may be driven by activity within the dopaminergic circuit. Here, we examined whether this curiosity effect was enhanced by or dependent on sleep-related consolidation. Participants learned the answers to trivia questions that they had previously rated on a curiosity scale, and they were shown faces between each question and answer presentation. Memory for these answers and faces was tested either immediately or after a 12-hour delay containing sleep or wakefulness, and polysomnography data was collected for a subset of the sleep participants. Although the curiosity effect for both the answers and incidentally-learned faces was replicated in immediate tests and after the 12-hour delay, the effect was not impacted by the presence of sleep in either case, nor did the effect show a relationship with total sleep time or time in slow-wave sleep. This study suggests that curiosity may be a learning factor that is not subsequently affected by sleep-dependent memory consolidation, but more work ought to examine the role of sleep on curiosity-driven memory in other contexts.

Sculpting memory during sleep: concurrent consolidation and forgetting

There is compelling evidence that sleep actively supports the formation of long-lasting memory representations. Experimental cuing of memories proved that neural replay of representations during sleep plays a causal role for this consolidation, which has also been shown to promote neocortical synaptic plasticity and spine formation. Concurrently, sleep has been proposed to facilitate forgetting through processes of synaptic renormalisation. This view received indirect support by findings in humans of sleep enhancing TMS-evoked plasticity and capabilities for encoding new information. First direct behavioural evidence of sleep inducing forgetting has only recently emerged after encoding large amounts of stimuli in adults. We propose forgetting complements sleep-dependent consolidation and facilitates gist abstraction especially at high memory loads, when reactivation-based consolidation reaches capacity limits.

Does Sleep Improve Your Grammar? Preferential Consolidation of Arbitrary Components of New Linguistic Knowledge

We examined the role of sleep-related memory consolidation processes in learning new form-meaning mappings. Specifically, we examined a Complementary Learning Systems account, which implies that sleep-related consolidation should be more beneficial for new hippocampally dependent arbitrary mappings (e.g. new vocabulary items) relative to new systematic mappings (e.g. grammatical regularities), which can be better encoded neocortically. The hypothesis was tested using a novel language with an artificial grammatical gender system. Stem-referent mappings implemented arbitrary aspects of the new language, and determiner/suffix+natural gender mappings implemented systematic aspects (e.g. tibscoiffesh+ ballerina, tibmofeem + bride; kedjorool + cowboy, kedheefaff + priest). Importantly, the determiner-gender and the suffix-gender mappings varied in complexity and salience, thus providing a range of opportunities to detect beneficial effects of sleep for this type of mapping. Participants were trained on the new language using a word-picture matching task, and were tested after a 2-hour delay which included sleep or wakefulness. Participants in the sleep group outperformed participants in the wake group on tests assessing memory for the arbitrary aspects of the new mappings (individual vocabulary items), whereas we saw no evidence of a sleep benefit in any of the tests assessing memory for the systematic aspects of the new mappings: Participants in both groups extracted the salient determiner-natural gender mapping, but not the more complex suffix-natural gender mapping. The data support the predictions of the complementary systems account and highlight the importance of the arbitrariness/systematicity dimension in the consolidation process for declarative memories.

When Delays Improve Memory: Stabilizing Memory in Children May Require Time

Memory is critical for learning, cognition, and cognitive development. Recent work has suggested that preschool-age children are vulnerable to catastrophic levels of memory interference, in which new learning dramatically attenuates memory for previously acquired knowledge. In the work reported here, we investigated the effects of consolidation on children's memory by introducing a 48-hr delay between learning and testing. In Experiment 1, the delay improved children's memory and eliminated interference. Results of Experiment 2 suggest that the benefit of this delay is limited to situations in which children are given enough information to form complex memory structures. These findings have important implications for understanding consolidation processes and memory development.

A nap to recap or how reward regulates hippocampal-prefrontal memory networks during daytime sleep in humans

Sleep plays a crucial role in the consolidation of newly acquired memories. Yet, how our brain selects the noteworthy information that will be consolidated during sleep remains largely unknown. Here we show that post-learning sleep favors the selectivity of long-term consolidation: when tested three months after initial encoding, the most important (i.e., rewarded, strongly encoded) memories are better retained, and also remembered with higher subjective confidence. Our brain imaging data reveals that the functional interplay between dopaminergic reward regions, the prefrontal cortex and the hippocampus contributes to the integration of rewarded associative memories. We further show that sleep spindles strengthen memory representations based on reward values, suggesting a privileged replay of information yielding positive outcomes. These findings demonstrate that post-learning sleep determines the neural fate of motivationally-relevant memories and promotes a value-based stratification of long-term memory stores.

DOI:http://dx.doi.org/10.7554/eLife.07903.001

Fresh memories are strengthened while we sleep. However, we don’t remember every detail of our daily life experiences. Instead, it is essential that we retain information that promotes our survival, such as what we call "rewards" (including food, money or sex) and dangers that we should avoid.

Igloi et al. sought to find out how the human brain picks out important memories to be consolidated during sleep, while discarding irrelevant information. Healthy participants learned series of pictures associated with either high or low rewards. After learning, some of the participants had a nap, while others remained awake. Directly after this and three months later, all the participants returned for a memory test. Igloi et al. found that the highly rewarded pictures were better remembered at both time points (at the expense of lowly rewarded ones), but only for participants who had slept after learning.

Further analysis revealed that distinctive bursts of brain activity occurring during sleep, so-called “sleep spindles", favor the reorganization of memories stored in a region of the brain called the hippocampus, often considered to be the organ of memory.

These findings uncover how sleep enhances long-term memory selectivity thus demonstratethat sleep does not just passively increase the retention of all memories. In the future, this work may inspire educational strategies that combine the careful use of rewards followed by an overnight period of sleep.

DOI:http://dx.doi.org/10.7554/eLife.07903.002

Memory integration: neural mechanisms and implications for behavior

Everyday behaviors require a high degree of flexibility, in which prior knowledge is applied to inform behavior in new situations. Such flexibility is thought to be supported in part by memory integration, a process whereby related memories become interconnected in the brain through recruitment of overlapping neuronal populations. Recent advances in cognitive and behavioral neuroscience highlight the importance of a hippocampal-medial prefrontal circuit in memory integration. Emerging evidence suggests that abstracted representations in medial prefrontal cortex guide reactivation of related memories during new encoding events, thus promoting hippocampal integration of related experiences. Moreover, recent work indicates that integrated memories are called upon during novel situations to facilitate a host of behaviors, from spatial navigation to imagination.

Relational memory generalization and integration in a transitive inference task with and without instructed awareness

Two experiments investigated the potential facilitative effects of prior instructed awareness and predetermined learning criteria on humans' ability to make transitive inference (TI) judgments. Participants were first exposed to a learning phase and required to learn five premise pairs (A+B-, B+C-, C+D-, D+E-, E+F-). Testing followed, where participants made judgments on novel non-endpoint (BD, BE and CE) and endpoint inferential pairs (AC, AD, AE, AF, BF, CF and DF), as well as learned premise pairs. Across both experiments, one group were made aware that the stimuli could be arranged in a hierarchy, while another group were not given this instruction. Results demonstrated that prior instructional task awareness led to a minor performance advantage, but that this difference was not significant. Furthermore, in Experiment 2, inferential test trial accuracy was not correlated with a post-experimental measure of awareness. Thus, the current findings suggest that successful TI task performance may occur in the absence of awareness, and that repeated exposure to learning and test phases may allow weak inferential performances to emerge gradually. Further research and alternative methods of measuring awareness and its role in TI are needed.

Wakefulness (not sleep) promotes generalization of word learning in 2.5-year-old children

Sleep enhances generalization in adults, but this has not been examined in toddlers. This study examined the impact of napping versus wakefulness on the generalization of word learning in toddlers when the contextual background changes during learning. Thirty 2.5-year-old children (M = 32.94, SE = 0.46) learned labels for novel categories of objects, presented on different contextual backgrounds, and were tested on their ability to generalize the labels to new exemplars after a 4-hr delay with or without a nap. The results demonstrated that only children who did not nap were able to generalize learning. These findings have critical implications for the functions of sleep versus wakefulness in generalization, implicating a role for forgetting during wakefulness in generalization.

Neurophysiological Basis of Sleep's Function on Memory and Cognition

A wealth of recent studies support a function of sleep on memory and cognitive processing. At a physiological level, sleep supports memory in a number of ways including neural replay and enhanced plasticity in the context of reduced ongoing input. This paper presents behavioral evidence for sleep's role in selective remembering and forgetting of declarative memories, in generalization of these memories, and in motor skill consolidation. Recent physiological data reviewed suggests how these behavioral changes might be supported by sleep. Importantly, in reviewing these findings, an integrated view of how distinct sleep stages uniquely contribute to memory processing emerges. This model will be useful in developing future behavioral and physiological studies to test predictions that emerge.