Memory consolidation involves the reactivation of memory traces during sleep. If different memories are reactivated each night, how much do they interfere with one another? We examined whether reactivating multiple memories incurs a cost to sleep-related benefits by contrasting reactivation of multiple memories versus single memories during sleep. First, participants learned the on-screen location of different objects. Each object was part of a semantically coherent group comprised of either one, two, or six items (e.g., six different cats). During sleep, sounds were unobtrusively presented to reactivate memories for half of the groups (e.g., “meow”). Memory benefits for cued versus non-cued items were independent of the number of items in the group, suggesting that reactivation occurs in a simultaneous and promiscuous manner. Intriguingly, sleep spindles and delta-theta power modulations were sensitive to group size, reflecting the extent of previous learning. Our results demonstrate that multiple memories may be consolidated in parallel without compromising each memory’s sleep-related benefit. These findings highlight alternative models for parallel consolidation that should be considered in future studies.

Sleep's role in memory consolidation is widely acknowledged, but its role in weakening memories is still debated. Memory weakening is evolutionary beneficial and makes an integral contribution to cognition. We sought evidence on whether sleep-based memory reactivation can facilitate memory suppression. Participants learned pairs of associable words (e.g., DIET–CREAM) and were then exposed to hint words (e.g., DIET) and instructed to either recall (“think”) or suppress (“no-think”) the corresponding target words (e.g., CREAM). As expected, suppression impaired retention when tested immediately after a 90-min nap. To test if reactivation could selectively enhance memory suppression during sleep, we unobtrusively presented one of two sounds conveying suppression instructions during sleep, followed by hint words. Results showed that targeted memory reactivation did not enhance suppression-induced forgetting. Although not predicted, post-hoc analyses revealed that sleep cues strengthened memory, but only for suppressed pairs that were weakly encoded before sleep. The results leave open the question of whether memory suppression can be augmented during sleep, but suggest strategies for future studies manipulating memory suppression during sleep. Additionally, our findings support the notion that sleep reactivation is particularly beneficial for weakly encoded information, which may be prioritized for consolidation.

Memory consolidation during sleep involves reactivation of memory traces. Targeting specific memories by presenting learning-related cues during sleep selectively enhances memory, but the mechanism behind this benefit is not fully understood. To better characterize the consolidation process in humans, we tested whether multiple memories can be reactivated in parallel using a spatial-memory task. After learning the locations of images belonging to semantically related sets of one, two, or six items, half of the sets were reactivated during a nap. Results showed a selective benefit in location recall for cued versus non-cued items regardless of set size, implying that reactivation may occur in a simultaneous, promiscuous manner. Intriguingly, sleep spindles and delta power modulations were sensitive to set-size and reflected the extent of previous learning. Taken together, our results refute the notion that resource availability strictly reduces the capacity of simultaneous sleep-reactivation and bring forward alternative testable models for sleep-related consolidation.More than a century after researchers started to explore the beneficial effects of sleep on memory 1 , the mechanism by which this benefit is achieved in still debated 2 . The leading hypothesis, termed active systems consolidation 3 , postulates that memories are stored in the hippocampus and then reactivated during sleep, subsequently shaping neocortical memory traces. Reactivation of memories during sleep was first observed in rodents 4, 5 . Sequential learning-related spiking activity was shown to "replay" during sleep, and this phenomenon has since been connected to the off-line consolidation process 6 . In Humans, recent studies using multivariate pattern classification analysis in fMRI have also shown evidence for reactivation of cortical and hippocampal memory-related patterns during sleep and awake rest 7,8,9,10 .An important milestone in the study of sleep-related reactivation has been the development of targeted memory reactivation (TMR), a paradigm designed to reactivate specific memories by unobtrusively presenting learning-related cues during sleep 11 . TMR has been shown to improve various forms of learning, including spatial 12, 13 , skill 14, 15 and vocabulary 16 learning. A recent TMR-fMRI study 17 demonstrated that cuing reactivated category-level learning-related cortical activity, further establishing the link between TMR-related memory reactivation and spontaneous reactivation during sleep. TMR spatial-learning paradigms have predominantly used olfactory and auditory cues to reactivate memories. Whereas olfactory designs have associated multiple learned items to a single odor (e.g., 15items 13,18,19 ), auditory designs have commonly used sounds that were associated with a single item (and more recently, with two items 20, 21 ). Both techniques have consistently shown benefits for cued items, but the question of whether effect sizes rely on the number of items reactivated has never been directly addressed. The relationship betwee...

Although we experience thousands of distinct events on a daily basis, relatively few are committed to memory. the human capacity to intentionally control which events will be remembered has been demonstrated using learning procedures with instructions to purposely avoid committing specific items to memory. in this study, we used a variant of the item-based directed-forgetting procedure and instructed participants to memorize the location of some images but not others on a grid. these instructions were conveyed using a set of auditory cues. then, during an afternoon nap, we unobtrusively presented a cue that was used to instruct participant to avoid committing the locations of some images to memory. After sleep, memory was worse for to-be-forgotten image locations associated with the presented sound relative to those associated with a sound that was not presented during sleep. We conclude that memory processing during sleep can serve not only to secure memory storage but also to weaken it. Given that intentional suppression may be used to weaken unpleasant memories, such sleep-based strategies may help accelerate treatments for memory-related disorders such as post-traumatic stress disorder. Out of the multitude of events humans experience each day, relatively few are retained as declarative memories to support subsequent recall or recognition. Whereas forgetting has often been viewed as a negative, its adaptive role has gained prominence 1. Additionally, control over memory processes, including the retrieval of intrusive memories, has been hypothesized to support emotional regulation 2. Deficits in the capacity to actively suppress intrusive memories has been associated with disorders such as post-traumatic stress disorder 3. Forgetting involves the passive decay of memories 4,5 , but inhibitory neurocognitive mechanisms may also contribute to declining recollective abilities. This effect, termed active forgetting, is supported by adaptive, flexible processes that suppress memory-related brain networks 6-8. Possible mechanisms for active forgetting range from prefrontal inhibition 6 on the systems level to dopaminergic forgetting cells 7 and neurogenesis 8 on the cellular level. Most of these models are similar in that they involve a newly acquired, learning-like process that targets core memory structures or cells and suppresses them. An outstanding question is whether these inhibitory circuits, established through suppression learning, are strengthened during sleep, as is the case for declarative and nondeclarative memories generally 9. Several memory paradigms, such as extinction and "Think-No Think" 10 , have attempted to model active forgetting by using intentional, motivated suppression of memories. Another paradigm, item-based directed forgetting, involves exposure to remember-or forget-instructions directly after exposure to an item. To-be-forgotten (TBF) items are later recalled at lower rates relative to to-be-remembered (TBR) items 11 , an effect putatively mediated, in part, by active inhibitory proce...

An amendment to this paper has been published and can be accessed via a link at the top of the paper.