Sleep consolidation effects are well established in the cognitive domains of memory and language learning, however very little is known regarding sleep effects in social contexts. Across three experiments, we investigated whether sleep enhances the accuracy of socially formed memories and communicative efficiency, including the use of partner-specific common ground between interlocutors in conversation. Participants completed a computerised (Experiment 1) or face-to-face (Experiment 2) matcher-director task with two other people before and after 12 h of sleep/wake. Both experiments showed more accurate item recognition and source memory after sleep vs. wake. The Sleep group also used fewer words to describe old vs. new items in Experiment 2, suggesting an influence of sleep on communicative behaviour based on prior discourse, but this effect was not partner specific. In Experiment 3, the retention of item/source memory and common ground was measured across a two-hour, polysomnographically recorded nap. Results revealed no significant relationships between consolidation-linked sleep electrophysiology (e.g., slow oscillation-spindle coupling) and memory/common ground retention. Together, these findings indicate that sleep plays a key role in stabilising the social-cognitive foundations of communication but cannot elucidate the mechanism(s) underpinning this support. Regardless, by preserving conversational memories and the efficiency of shared understanding, sleep may help sustain smooth coordination in everyday interaction and social relationships.

Sleep may play a key role in consolidating social interactions by transforming brief communicative experiences into stable social memories. In this paper, we used the lexical alignment task to investigate whether sleep enhances the consolidation of shared linguistic representations between partners. Lexical alignment, where participants converge their word choices with those produced by a partner, served as a marker of successful communication and shared understanding. Eighty-two participants completed a picture-matching and picture-naming task before and after 12 h of sleep or wake. Results showed the lexical alignment effect persisted 12 h after the initial picture-matching interaction. However, this effect was not influenced by whether participants slept or remained awake during the retention interval. These findings suggest that while participants encoded enduring representations of partner-specific lexical information during interaction, sleep-based consolidation did not confer an additional benefit. One possibility is that a higher level of social relevance is needed for these memories to be susceptible to sleep consolidation. Alternatively, limited hippocampal involvement during encoding may have reduced the extent to which information was reactivated during sleep. Future research should employ richer and more interactive tasks to clarify how sleep supports the consolidation of social experiences and relationship perceptions.

Encountering a homonym in a sentential context that biases interpretation towards its subordinate meaning makes that meaning easier to access later. This word-meaning priming effect is not restricted to homonyms and may be supported by general episodic memory processes. Such an account predicts that word-meaning priming may be affected by factors that affect episodic learning (e.g., predictability). Specifically, the contextual predictability of an incoming stimulus has been shown to affect episodic memory in that both highly expected as well as highly unexpected input leads to better memory for that input, via two different underlying neurocognitive mechanisms. We hypothesised that if word-meaning priming is supported by episodic memory processes, both highly expected as well as highly unexpected target words should lead to stronger word-meaning priming effects than target words of intermediate predictability. Four pre-registered online experiments tested whether contextual predictability affects word-meaning priming and incidental memory for language. We exposed participants to sentences that emphasised a particular aspect of a sentence-final target word’s meaning. Importantly, target words differed in how predictable they were based on the sentence context (e.g., “You can get in a good workout by riding / lifting a bicycle”). Associate production and semantic relatedness judgement assessed the strength of word-meaning priming. Contrary to our hypotheses, while there was evidence for priming across experiments, we found no significant effects of contextual predictability in any experiments. Our findings suggest that predictability-guided updating is rarely triggered during language comprehension, and we outline the theoretical implications of these results for the architecture of the language system.

Sleep has been shown to play an important role in the acquisition of novel vocabulary in an individual's first language. In this entry, we review evidence suggesting that sleep also supports the learning and consolidation of novel second language words, enabling their integration into existing lexical networks. However, there are several differences between the consolidation processes of first and second language material. While prior studies have shown variable time courses for the consolidation of first language vocabulary, the evidence reviewed here suggests that overall, second language acquisition involves consolidation processes that are more protracted. This may be due to the increased difficulty of integrating novel words that are inconsistent with (phonological or semantic) patterns of a learner's first language. Moreover, lexical integration may be particularly slow if a learner's second language is phonologically distant from their first language. Finally, we review some preliminary evidence suggesting that it may be possible to not only consolidate but also acquire novel vocabulary during sleep, but further research is needed to be able to draw clear conclusions regarding the educational utility of sleep‐based interventions to support second language vocabulary acquisition.

Recent experience with a word significantly influences its subsequent interpretation. For instance, encountering bank in a river-related context biases future interpretations toward 'side of a river' (vs. 'financial bank'). To explain this effect, the episodic context account posits that episodic memory helps bind word meanings in the language input, creating a temporary, context-specific representation that can bias subsequent lexical interpretation. This account predicts that even unrelated words would be linked together in episodic memory, potentially altering their interpretation. In Experiments 1-3, participants read unrelated word pairs (e.g., sword-microwave, privacy-export) embedded in meaningful sentences, then completed a speeded relatedness judgement task after delays of 5 min, 20 min, or 12 h (including sleep). Results showed that sentence exposure increased the likelihood of the unrelated pairs being judged as related-a robust effect observed across all delay intervals. Experiment 4 showed that this exposure effect was abolished when words in a target pair were read in separate sentences, suggesting that the exposure effect may be dependent on lexical co-occurrence. Experiment 5, also with a 12-h delay (including sleep), additionally used an innovative word arrangement task to assess word relatedness without presenting the target pairs simultaneously or successively. In line with relatedness judgement, sentence exposure pushed the unrelated words closer in semantic space. Overall, our findings suggest that a context-specific representation, supported by episodic memory, is generated during language comprehension, and in turn, these representations can influence lexical interpretation for at least 12 h and across different linguistic circumstances. We argue that these representations endow the mental lexicon with the efficiency to deal with word burstiness and the dynamic nature of language.

Fast sleep spindles and slow-wave sleep (SWS) have been linked to memory consolidation, however, their associations with learning and longer term retention of different aspects of language remain unclear. We investigated the temporal dynamics of consolidation of vocabulary and grammar, and their links with these sleep metrics. Young adult participants were trained in the evening on an artificial language that used plural inflections with an underlying morpho-phonological regularity that was not taught explicitly. Some of the words were presented frequently and others infrequently. Polysomnographic measures were collected during the night following learning; participants were tested on the vocabulary, trained inflections, and generalisation to untrained words at four time points across nine days. Accuracy on the vocabulary test improved across the first night following learning, and the change was positively associated with SWS duration. Memory for infrequent words declined towards Day 9, but greater spindle density during the first night was associated with a smaller decline. Although mean group accuracy on trained inflections did not significantly change overnight, individually, the change was negatively correlated with spindle density. Generalisation accuracy showed no change across time and no correlations with sleep characteristics. Overall, the results demonstrate that vocabulary and grammar learning have different temporal dynamics of consolidation and distinct patterns of association with sleep metrics. The findings suggest a protective role of spindles for long-term retention of memory, particularly of weakly encoded items, and emphasise the need to dissociate the benefits of SWS from those of spindles.

Sleep plays a crucial role in consolidating recently acquired memories and preparing the brain for learning new ones, but the relationship between these two processes is currently unclear. According to the prominent Active Systems Consolidation model, memory representations that are initially reliant on the hippocampus are redistributed to neocortex during sleep for long-term storage. An indirect assumption of this model is that sleep-associated memory processing paves the way for next-day learning by freeing up hippocampal encoding resources. In this review, we evaluate two central tenets of this ‘resource reallocation hypothesis’: (i) sleep-associated memory consolidation reduces hippocampal engagement during retrieval, and (ii) this reduction in hippocampal burden enhances the brain's capacity for new learning. We then describe recent work that has directly tested the relationship between sleep-associated memory processing and next-day learning. In the absence of clear evidence supporting the resource reallocation hypothesis, we consider alternative accounts in which efficient learning is not contingent on prior overnight memory processing, but rather that sleep-associated consolidation and post-sleep learning rely on overlapping or independent mechanisms. We conclude by outlining how future research can rigorously test the resource reallocation hypothesis.

Precise temporal coordination, or coupling, of two neural oscillations during non-REM sleep – slow oscillations and sleep spindles – is recognized as a fundamental characteristic of healthy sleep and cortico-thalamic circuitry. Coupling is tightly linked to memory consolidation, further underscoring the need to understand its development. In this preregistered cross-sectional study using existing polysomnographic datasets, we tested associations between coupling metrics and age in typically developing (N = 84) and neurodiverse (N = 40) children and adolescents (~7.5-14.5 years old), and linked coupling precision to slow oscillation and spindle characteristics. The analyses substantially advance current understanding of the coupling mechanism during sleep: 1) We report an increase with age in coupling density in typically developing children that was mostly driven by an increase in overall spindle density. 2) Using a statistical model of coupling consistency – a key metric of coupling – we showed that slow-oscillation amplitude and spindle density were its strongest predictors in typically developing children. 3) We found no group differences in any coupling metric between neurodiverse (with dyslexia or autism) and typically developing children when collapsing across age. Furthermore, only coupling consistency, but not prevalence, was significantly associated with age in the neurodiverse sample. and we were not able to fit a model for coupling consistency for neurodiverse data. The results show that it is important to examine how age relates to the prevalence of coupling events, not only to coupling precision, and that developmental trajectories of coupling may be important markers of brain health - pending confirmation in longitudinal work.

Sentence comprehension involves the rapid decoding of semantic and grammatical information, a process fundamental to communication. As with other cognitive processes, language comprehension relies partly on long-term memory. However, the electrophysiological mechanisms underpinning the initial encoding and generalisation of higher-order linguistic knowledge remains elusive, particularly from a sleep-based consolidation perspective. One candidate mechanism that may subserve the consolidation of language is the temporal coordination of slow oscillations (SO) and sleep spindles during non-rapid eye movement sleep (NREM). To examine this hypothesis, we analysed electroencephalographic (EEG) data recorded from 35 participants (Mage = 25.4, SD = 7.10; 16 males) during an artificial language learning task, contrasting performance between individuals who were given an 8hr nocturnal sleep period or an equivelant period of wake. We found that sleep relative to wake was associated with superior performance for rules that followed a sequence-based word order. Post-sleep sequence-based word order processing was associated with an increase in task-related theta power, an electrophysiological signature of successful memory consolidation. Frontal NREM SO-spindle coupling was also positively associated with behavioural sensitivity to sequence-based word order rules, as well as with task-related theta power. As such, theta activity during retrieval of previously learned information correlates with SO-spindle coupling, thus linking neural activity in the sleeping and waking brain. Taken together, this study presents converging behavioural and neurophysiological evidence for a role of NREM SO-spindle coupling and task-related theta activity as signatures of successful memory consolidation and retrieval in the context of higher-order language learning. SIGNIFICANCE STATEMENT The endogenous temporal coordination of neural oscillations supports information processing during both wake and sleep states. Here we demonstrate that slow oscillation-spindle coupling during non-rapid eye movement sleep predicts the consolidation of complex grammatical rules and modulates task-related oscillatory dynamics previously implicated in sentence processing. We show that increases in theta power predict enhanced sensitivity to grammatical violations after a period of sleep and strong slow oscillation-spindle coupling modulates subsequent task-related theta activity to influence behaviour. Our findings reveal a complex interaction between both wake- and sleep-related oscillatory dynamics during the early stages of language learning beyond the single word level.

Certain aspects of lexical knowledge can be primed by recent usage, with effects observed up to 24 hours later in some circumstances. Here, we used syntactically ambiguous sentences (“The man hit/chose the dog with the stick”) to explore the longevity of priming of syntactic structure. Some verbs provide a bias towards an instrument interpretation (the stick was used to hit the dog), whilst others are biased towards the modifier interpretation (the man chose the dog that possessed the stick). Experiment 1 revealed an effect of pre-existing verb bias on resolving syntactic ambiguities. In Experiment 2, we primed specific verbs towards their dispreferred interpretation in a study phase (e.g., hit was primed to the modifier interpretation). ~20 minutes later, the same verbs, along with unprimed verbs, were encountered in syntactically ambiguous contexts in a test phase. Exposure to the dispreferred interpretation in the study phase increased the preference for the same interpretation in the test phase, particularly for instrument-biased verbs. In Experiment 3, the study and test phases were separated by a ~12-hour interval that included sleep. No overall effect of exposure was found, but again a simple effect of priming was found for instrument-biased verbs. Our results suggest verb-bias priming is maintained over relatively long periods such as 20 minutes, and possibly as long as 12 hours, consistent with a contribution of episodic memory to maintenance of verb-specific syntactic biases.

The subordinate meaning of a homonym becomes temporarily more accessible after it is encountered, an effect termed word-meaning priming. Over the longer-term, word-meaning priming is better maintained across periods of sleep compared with wakefulness. This has been explained as sleep actively consolidating episodic memories related to recent linguistic events (Gaskell et al., 2019). Here, we tested this hypothesis by investigating whether word-meaning priming can be boosted following sleep using targeted memory reactivation (TMR), a technique of biassing specific memories for sleep-based consolidation by presenting information-associated sensory cues during sleep. Experiment 1 confirmed that word-meaning priming is elicited with the inclusion of encoding instructions that are necessary for a TMR manipulation. In Experiment 2, 40 (of 80) homonyms were primed toward their subordinate meaning in an exposure phase via a sentence, which was also associated with an auditory cue (the homonym) for TMR. Participants then took a ~2 hour nap, where half of the cues from exposure (memory cues) were replayed with the aim of strengthening the subordinate sentence meaning, along with 20 cues that had not been encountered previously (control cues). After sleep, there was an overall word-meaning priming effect, however there was no additional benefit of TMR on priming, nor did TMR benefit the recall of contextual information. Interestingly, there was an increased sleep spindle/beta band power response to memory cues relative to control cues, indicating cue-evoked memory reprocessing during sleep. These findings imply a bounded role of sleep in actively consolidating linguistic-related memories.

Sleep supports memory consolidation and next-day learning. The Active Systems model of consolidation proposes that sleep facilitates a shift in the memory retrieval network from hippocampus to neocortex in service of long-term storage. Accordingly, overnight consolidation may support efficient next-day learning. We tested this hypothesis across two preregistered behavioural experiments. In both experiments, participants learned a set of word pairs and recall was assessed before and after a 12-h delay containing overnight sleep or daytime wakefulness. Participants then learned and were immediately tested on a new set of word pairs. Word pair retention was better after the delay of sleep than wakefulness, suggesting a benefit of sleep for memory consolidation, but there was no sleep-related learning advantage for the new set of word pairs. Sleep-associated consolidation was not associated with next-day learning in our preregistered analyses, although a significant positive relationship with learning did emerge in an exploratory analysis that accounted for performance at pre-sleep recall. Taken together, our findings provide exploratory evidence that overnight consolidation may be linked to new learning, with pre-sleep retrieval performance influencing the magnitude of this relationship.

In this article, I review the evidence on the involvement of sleep and consolidation in word learning and processing during language comprehension, focusing on implications for theory. The theoretical basis for the review is a complementary systems account of word learning involving flexible (hippocampal) and stable (cortical) pathways to lexical knowledge. I argue that the accumulated data are consistent with a role for both pathways in both learning and recognition of lexical items, with sleep and consolidation supporting the transfer of recent experience between the pathways. The level of involvement of each pathway is dependent on key factors, such as consistency with prior knowledge in the case of learning, and reliance on context and/or automaticity in the case of recognition. As a consequence, the notion of a mental lexicon cannot really be restricted to just the listener's stable knowledge about words: flexible knowledge and recent experiences are also important. Furthermore, I argue that the flexible pathway plays a critical role even in the absence of new lexical items. The available evidence suggests that this pathway encodes (and potentially consolidates) recent linguistic experiences, providing potential benefits to interpretation of subsequent language and the long-term retention of knowledge. In conclusion, I propose that a dual-pathway account incorporating both flexibility and stability is necessary to explain the learning, recognition, and interpretation of words.

The episodic context account (Gaskell et al., 2019) proposes that the act of language comprehension gives rise to an episodic discourse representation, and that this representation is prone to sleep-related memory effects. In three experiments, we tested this prediction by asking participants to read/listen to naturalistic stories before their memory was tested after a 12-hr interval, which included either daytime wakefulness or overnight sleep. To assess discourse memory, we used sentence recognition (Experiment 1; N = 386), free story recall (Experiment 2; N = 96), and cued recall (Experiments 2 and 3; N = 192). We found no evidence of sleep-related effects in sentence recognition or free recall, but cued recall (aka fill-in-the-blank) showed that the degree of time-related distortion, as indexed by both a subjective categorisation measure and Latent Semantic Analysis, was lower after sleep than after wake. Overall, our experiments suggest that the effect of sleep on discourse memory is modest but observable and may [1] be constrained by the retrieval processes (recollection vs. familiarity & associative vs. item), [2] lie on a qualitative level that is difficult to detect in an all-or-nothing scoring metric, and [3] primarily situated in the textbase level of the tripartite model of discourse processing.

Conceptual knowledge is known to modulate episodic memory, but it remains unclear whether and how verbal labels shape event learning and recollection over time. To investigate this issue, we asked participants to study and memorise unfamiliar animations and their titles. The titles conveyed fast or slow motion speed (e.g., a bus vs ambulance travelling ). Event memory was assessed at different time points -soon after learning and after 12 h of sleep or wakefulness -using a timed mental event reproduction task and verbal recall. Unlike previous findings with these stimuli, we found that intentional title study elicited title -related biases on reproduced durations soon after learning. Post -sleep but not post -wakefulness recollection also showed title -related biases and systematically longer reproduced durations. Nevertheless, reproduced durations correlated with stimulus segments, stimulus durations and verbal recall, indicating that event memories combined episodic and verbal conceptual features. Results suggest that intentional verbal learning promoted conceptual influences at encoding and that sleep -dependent consolidation enhanced these influences. We argue that the degree of integration between conceptual and episodic features determines the extent of conceptual influences and, more generally, the role of verbal labels in event learning and memory.

Episodic memory retrieval is associated with the holistic neocortical reinstatement of all event information; an effect driven by hippocampal pattern completion. However, whether holistic reinstatement occurs, and whether hippocampal pattern completion continues to drive reinstatement, after a period of consolidation is unclear. Theories of systems consolidation predict either a time-variant or -invariant role of the hippocampus in the holistic retrieval of episodic events. Here, we assessed whether episodic events continue to be reinstated holistically and whether hippocampal pattern completion continues to facilitate holistic reinstatement following a period of consolidation. Female and male human participants learnt ‘events’ that were composed of multiple overlapping pairs of event elements (e.g., person-location, object-location, location-person). Importantly, encoding occurred either immediately before or 24-hours before retrieval. Using fMRI during the retrieval of events, we show evidence for holistic reinstatement, as well as a correlation between reinstatement and hippocampal pattern completion, regardless of whether retrieval occurred immediately or 24-hours after encoding. Thus, hippocampal pattern completion continues to contribute to holistic reinstatement after a delay. However, our results also revealed that some holistic reinstatement can occur without evidence for a corresponding signature of hippocampal pattern completion after a delay (but not immediately after encoding). We therefore show that hippocampal pattern completion, in addition to a non-hippocampal process, has a role in holistic reinstatement following a period of consolidation. Our results point to a consolidation process where the hippocampus and neocortex may work in an additive, rather than compensatory, manner to support episodic memory retrieval.Significance StatementThe hippocampus plays a critical role in the recollection of episodic events. However, its role in episodic memory retrieval over time is still debated with theories of systems consolidation predicting both a time-limited and time-invariant role. We used fMRI and an experimental assay of holistic episodic memory retrieval to assess hippocampal pattern completion and its relationship to neocortical reinstatement across delay. We provide evidence that hippocampal pattern completion is associated with neocortical reinstatement following delay, but that some reinstatement can occur without a corresponding signature of hippocampal pattern completion. Our results suggest that hippocampal pattern completion, in addition to an emergent non-hippocampal process, may play an additive role in the recollection of episodic memories following a period of consolidation.