Mechanisms of sleep-associated memory consolidation and next-day learning

Sleep is linked to overnight memory consolidation and next-day learning. However, it is unclear which mechanisms of sleep support these memory processes. The Active Systems Consolidation model postulates that during sleep, newly formed hippocampus-dependent memories are reactivated and transformed into stable representations within neocortex. This transformation may, in turn, refresh new learning capacity within hippocampus. With a basis in these assumptions, the present thesis aimed to investigate how sleep facilitates offline consolidation and whether sleep-associated consolidation might contribute to learning the following day. Firstly, a targeted memory reactivation paradigm investigated the oscillatory signatures of reactivation during sleep elicited by verbal and non-verbal memory cues. Increases in theta and spindle power were linked to memory reactivation and stabilization during sleep, and furthermore, verbal cues evoked stronger spindle-mediated memory processes as compared to non-verbal memory cues. Secondly, three experiments investigated the benefits of sleeping before and after learning as compared to staying awake, either overnight or during the day. The results suggested that sleep benefits memory consolidation, and that losing sleep disrupts a neural signature of successful learning, namely, beta desynchrony. However, no benefits of sleeping prior to learning were observed when compared to daytime wakefulness. Addressing the novel hypothesis of a potential relationship between sleep-associated consolidation and next-day learning, three experiments consistently found no evidence to support this hypothesis. Surprisingly, an association was reported between forgetting during daytime wakefulness and subsequent learning of similar materials. Overall, this thesis provides insights into how sleep supports consolidation and raises novel questions about which processes during both sleep and wake may support new memory formation

Sleep loss disrupts the neural signature of successful learning

Sleep supports memory consolidation as well as next-day learning. The influential Active Systems account of offline consolidation suggests that sleep-associated memory processing paves the way for new learning, but empirical evidence in support of this idea is scarce. Using a within-subjects (N = 30), crossover design, we assessed behavioural and electrophysiological indices of episodic encoding after a night of sleep or total sleep deprivation in healthy adults (aged 18-25 years), and investigated whether behavioural performance was predicted by the overnight consolidation of episodic associations formed the previous day. Sleep supported memory consolidation and next-day learning, as compared to sleep deprivation. However, the magnitude of this sleep-associated consolidation benefit did not significantly predict the ability to form novel memories after sleep. Interestingly, sleep deprivation prompted a qualitative change in the neural signature of encoding: whereas 12-20 Hz beta desynchronization – an established marker of successful encoding – was observed after sleep, sleep deprivation disrupted beta desynchrony during successful learning. Taken together, these findings suggest that effective learning depends on sleep, but not necessarily sleep-associated consolidation

Memory consolidation is linked to spindle-mediated information processing during sleep

How are brief encounters transformed into lasting memories? Previous research has established the role of non-rapid eye movement (NREM) sleep, along with its electrophysiological signatures of slow oscillations (SOs) and spindles, for memory consolidation [1–4]. In related work, experimental manipulations have demonstrated that NREM sleep provides a window of opportunity to selectively strengthen particular memory traces via the delivery of auditory cues [5–10], a procedure known as targeted memory reactivation (TMR). It has remained unclear, however, whether TMR triggers the brain's endogenous consolidation mechanisms (linked to SOs and/or spindles) and whether those mechanisms in turn mediate effective processing of mnemonic information. We devised a novel paradigm in which associative memories (adjective-object and adjective-scene pairs) were selectively cued during a post-learning nap, successfully stabilizing next-day retention relative to non-cued memories. First, we found that, compared to novel control adjectives, memory cues evoked an increase in fast spindles. Critically, during the time window of cue-induced spindle activity, the memory category linked to the verbal cue (object or scene) could be reliably decoded, with the fidelity of this decoding predicting the behavioral consolidation benefits of TMR. These results provide correlative evidence for an information processing role of sleep spindles in service of memory consolidation. Sleep spindles play a crucial role in memory consolidation, but the underlying mechanisms are not well understood. Using an auditory memory-cueing technique and EEG analysis in humans, Cairney et al. show that sleep spindles mediate the informational content of reactivated memory traces in service of offline mnemonic processing