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Learning never stops. As we navigate life, we continuously acquire and update knowledge to optimize memory-guided action, with a gradual shift from the former to the latter as we master our environment. How are these learning dynamics expressed in the brain and in behavioral patterns? Here, we devised a spatiotemporal image learning task ("Memory Arena") in which participants learn a set of 50 items to criterion across repeated exposure blocks. Critically, brief task-free periods between successive image presentations allowed us to assess multivariate electroencephalogram (EEG) patterns representing the previous and/or upcoming image identity, as well as anticipatory eye movements toward the upcoming image location. As expected, participants eventually met the performance criterion, albeit with different learning rates. During task-free periods, we were able to readily decode representations of both previous and upcoming image identities. Importantly though, decoding strength followed opposing slopes for previous vs. upcoming images across time, with a gradual decline of evidence for the previous image and a gradual increase of evidence for the upcoming image. Moreover, the ratio of upcoming vs. previous image evidence directly followed behavioral learning rates. Finally, eye movement data revealed that participants increasingly used the task-free period to anticipate upcoming image locations, with target-precision slopes paralleling both behavioral performance measures as well as EEG decodability of the upcoming image across time. Together, these results unveil the neural and behavioral dynamics underlying the gradual transition from learning to memory-guided action., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Advancing treatment to resolve human cognitive disorders requires a comprehensive understanding of the molecular signaling pathways underlying learning and memory. While most organ systems evolved to maintain homeostasis, the brain developed the capacity to perceive and adapt to environmental stimuli through the continuous modification of interactions within a gene network functioning within a broader neural network. This distinctive characteristic enables significant neural plasticity, but complicates experimental investigations. A thorough examination of the mechanisms underlying behavioral plasticity must integrate multiple levels of biological organization, encompassing genetic pathways within individual neurons, interactions among neural networks providing feedback on gene expression, and observable phenotypic behaviors. Model organisms, such as Drosophila melanogaster , which possess more simple and manipulable nervous systems and genomes than mammals, facilitate such investigations. The evolutionary conservation of behavioral phenotypes and the associated genetics and neural systems indicates that insights gained from flies are pertinent to understanding human cognition. Rather than providing a comprehensive review of the entire field of Drosophila memory research, we focus on olfactory associative reward memories and their related neural circuitry in fly brains, with the objective of elucidating the underlying neural mechanisms, thereby advancing our understanding of brain mechanisms linked to cognitive systems.

Filial imprinting, a crucial ethological paradigm, provides insights into the neurobiology of early learning and its long-term impact on behaviour. To date, invasive techniques like autoradiography or lesions have been used to study it, limiting the exploration of whole brain networks. Recent advances in fMRI for avian brains now open new windows to explore bird's brain functions at the network level. We developed an fMRI technique for awake, newly hatched chicks, capturing BOLD signal changes during imprinting experiments. While early memory acquisition phases are understood, long-term storage and retrieval remain unclear. Our findings identified potential long-term storage of imprinting memories across a neural network, including the hippocampal formation, the medial striatum, the arcopallium, and the prefrontal-like nidopallium caudolaterale. This paradigm opens up new avenues for exploring the broader landscape of learning and memory in neonatal vertebrates, enhancing our understanding of behaviour and brain networks., (© 2024. The Author(s).)

What determines when neural representations of memories move together (integrate) or apart (differentiate)? Classic supervised learning models posit that, when two stimuli predict similar outcomes, their representations should integrate. However, these models have recently been challenged by studies showing that pairing two stimuli with a shared associate can sometimes cause differentiation, depending on the parameters of the study and the brain region being examined. Here, we provide a purely unsupervised neural network model that can explain these and other related findings. The model can exhibit integration or differentiation depending on the amount of activity allowed to spread to competitors - inactive memories are not modified, connections to moderately active competitors are weakened (leading to differentiation), and connections to highly active competitors are strengthened (leading to integration). The model also makes several novel predictions - most importantly, that when differentiation occurs as a result of this unsupervised learning mechanism, it will be rapid and asymmetric, and it will give rise to anticorrelated representations in the region of the brain that is the source of the differentiation. Overall, these modeling results provide a computational explanation for a diverse set of seemingly contradictory empirical findings in the memory literature, as well as new insights into the dynamics at play during learning., Competing Interests: VR, AN, NT, KN No competing interests declared, (© 2023, Ritvo et al.)

Telomere length (TL) is an important cellular marker of biological aging impacting the brain and heart. However, how it is related to the brain (e.g., cognitive function and neuroanatomic architecture), and how these relationships may vary by sex and reproductive status, is not well established. Here we assessed the association between leukocyte TL and memory circuitry regional brain volumes and memory performance in early midlife, in relation to sex and reproductive status. Participants (N = 198; 95 females, 103 males; ages 45-55) underwent structural MRI and neuropsychological assessments of verbal, associative, and working memory. Overall, shorter TL was associated with smaller white matter volume in the parahippocampal gyrus and dorsolateral prefrontal cortex. In males, shorter TL was associated with worse working memory performance and corresponding smaller white matter volumes in the parahippocampal gyrus, anterior cingulate cortex, and dorsolateral prefrontal cortex. In females, the impact of cellular aging was revealed over the menopausal transition. In postmenopausal females, shorter TL was associated with poor associative memory performance and smaller grey matter volume in the right hippocampus. In contrast, TL was not related to memory performance or grey and white matter volumes in any memory circuitry region in pre/perimenopausal females. Results demonstrated that shorter TL is associated with worse memory function and smaller volume in memory circuitry regions in early midlife, an association that differs by sex and reproductive status. Taken together, TL may serve as an early indicator of sex-dependent brain abnormalities in early midlife., Competing Interests: Declaration of competing interest The authors declare no competing financial interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

Recent and pioneering animal research has revealed the brain utilizes a variety of molecular, cellular, and network-level mechanisms used to forget memories in a process referred to as "active forgetting". Active forgetting increases behavioral flexibility and removes irrelevant information. Individuals with impaired active forgetting mechanisms can experience intrusive memories, distressing thoughts, and unwanted impulses that occur in neuropsychiatric diseases. The current evidence indicates that active forgetting mechanisms degrade, or mask, molecular and cellular memory traces created in synaptic connections of "engram cells" that are specific for a given memory. Combined molecular genetic/behavioral studies using Drosophila have uncovered a complex system of cellular active-forgetting pathways within engram cells that is regulated by dopamine neurons and involves dopamine-nitric oxide co-transmission and reception, endoplasmic reticulum Ca 2+ signaling, and cytoskeletal remodeling machinery regulated by small GTPases. Some of these molecular cellular mechanisms have already been found to be conserved in mammals. Interestingly, some pathways independently regulate forgetting of distinct memory types and temporal phases, suggesting a multi-layering organization of forgetting systems. In mammals, active forgetting also involves modulation of memory trace synaptic strength by altering AMPA receptor trafficking. Furthermore, active-forgetting employs network level mechanisms wherein non-engram neurons, newly born-engram neurons, and glial cells regulate engram synapses in a state and experience dependent manner. Remarkably, there is evidence for potential coordination between the network and cellular level forgetting mechanisms. Finally, subjects with several neuropsychiatric diseases have been tested and shown to be impaired in active forgetting. Insights obtained from research on active forgetting in animal models will continue to enrich our understanding of the brain dysfunctions that occur in neuropsychiatric diseases., (© 2024. The Author(s).)

Previous studies on the chess game demonstrated that chess experts strongly rely on the activation of memory chunks to manifest accurate decision-making. Although the chunk memory might be affected by temporal constraints, it is unclear why the performance of chess experts is not significantly dropped under time pressure. In this study, our objective is to examine the variations in cognitive neural mechanisms between chess experts and novices under time pressure. The underlying cognitive neural mechanism was carefully inspected by accessing the chess game performance between 20 local experienced and 20 inexperienced chess players with 1-minute and 5-minute time constraints. In addition, functional near-infrared spectroscopy (fNIRS) recordings were carried out for each individual from the two groups while playing a 1-minute or 5-minute chess game. It was discovered that under temporal constraints, players exhibited different patterns of functional connectivity in frontal-parietal regions, suggesting that temporal stress can enhance segmentation processes in chess games. In particular, the experienced group exhibited significantly enhanced functional connectivity networks under time pressure including the dorsolateral prefrontal cortex, inferior frontal gyrus, supramarginal gyrus, and postcentral gyrus, which demonstrated the important role of the segmentation process for experienced players under time pressure. Our study found that experienced players were able to enhance recall, reorganize, and integrate chunks to improve chess performance under time pressure., (Copyright © 2024 IBRO. Published by Elsevier Inc. All rights reserved.)

Electroencephalography (EEG) based Brain Computer Interface (BCI) systems play a significant role in facilitating how individuals with neurological impairments effectively interact with their environment. In real world applications of BCI system for clinical assistance and rehabilitation training, the EEG classifier often needs to learn on sequentially arriving subjects in an online manner. As patterns of EEG signals can be significantly different for different subjects, the EEG classifier can easily erase knowledge of learnt subjects after learning on later ones as it performs decoding in online streaming scenario, namely catastrophic forgetting. In this work, we tackle this problem with a memory-based approach, which considers the following conditions: (1) subjects arrive sequentially in an online manner, with no large scale dataset available for joint training beforehand, (2) data volume from the different subjects could be imbalanced, (3) decoding difficulty of the sequential streaming signal vary, (4) continual classification for a long time is required. This online sequential EEG decoding problem is more challenging than classic cross subject EEG decoding as there is no large-scale training data from the different subjects available beforehand. The proposed model keeps a small balanced memory buffer during sequential learning, with memory data dynamically selected based on joint consideration of data volume and informativeness. Furthermore, for the more general scenarios where subject identity is unknown to the EEG decoder, aka. subject agnostic scenario, we propose a kernel based subject shift detection method that identifies underlying subject changes on the fly in a computationally efficient manner. We develop challenging benchmarks of streaming EEG data from sequentially arriving subjects with both balanced and imbalanced data volumes, and performed extensive experiments with a detailed ablation study on the proposed model. The results show the effectiveness of our proposed approach, enabling the decoder to maintain performance on all previously seen subjects over a long period of sequential decoding. The model demonstrates the potential for real-world applications., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Testing is more beneficial for memory retention than restudying the same content. However, the effect of the initial encoding method on the testing effect remains unclear. In this study, a classical testing effect paradigm was employed, along with event-related potentials (ERP), to investigate the electrophysiological processes underlying the effect of enactment encoding on the testing effect. Participants were randomly assigned to the Self-Performed Test (SPT) or Verbalized Test (VT) groups. Both groups underwent three stages: an initial encoding phase, an initial test phase (comprising a source memory task and a restudy task), and a final test phase. During the initial encoding phase, the SPT group encoded action phrases through enactment, while the VT group encoded information through reading. During the initial test phase, the SPT group exhibited superior recognition performance in item memory compared with the VT group. Both groups exhibited significant parietal old/new effects in the source memory task, with only the SPT group displaying parietal positivity during the restudy task. During the final test phase, the behavioral testing effect was exclusively observed in the VT group. Furthermore, the VT group displayed a more pronounced parietal positivity in the test condition compared to the restudy condition, while the parietal positivity between the two conditions was comparable in the SPT group. In summary, the absence of a final behavioral testing effect in the SPT group may be attributed to both enactment and testing primarily enhancing memory performance through recollection-based retrieval, as indicated by the parietal positivity. Consequently, the initial enactment encoding method leaves limited scope for further improvements through subsequent testing. These findings suggest that initial enactment encoding, and subsequent testing may be redundant in improving episodic memory performance., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

The synaptic tagging and capture (STC) hypothesis lays the framework on the synapse-specific mechanism of protein synthesis-dependent long-term plasticity upon synaptic induction. Activated synapses will display a transient tag that will capture plasticity-related products (PRPs). These two events, tag setting and PRP synthesis, can be teased apart and have been studied extensively-from their electrophysiological and pharmacological properties to the molecular events involved. Consequently, the hypothesis also permits interactions of synaptic populations that encode different memories within the same neuronal population-hence, it gives rise to the associativity of plasticity. In this review, the recent advances and progress since the experimental debut of the STC hypothesis will be shared. This includes the role of neuromodulation in PRP synthesis and tag integrity, behavioural correlates of the hypothesis and modelling in silico . STC, as a more sensitive assay for synaptic health, can also assess neuronal aberrations. We will also expound how synaptic plasticity and associativity are altered in ageing-related decline and pathological conditions such as juvenile stress, cancer, sleep deprivation and Alzheimer's disease. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

How is new information organized in memory? According to latent state theories, this is determined by the level of surprise, or prediction error, generated by the new information: a small prediction error leads to the updating of existing memory, large prediction error leads to encoding of a new memory. We tested this idea using a protocol in which rats were first conditioned to fear a stimulus paired with shock. The stimulus was then gradually extinguished by progressively reducing the shock intensity until the stimulus was presented alone. Consistent with latent state theories, this gradual extinction protocol (small prediction errors) was better than standard extinction (large prediction errors) in producing long-term suppression of fear responses, and the benefit of gradual extinction was due to updating of the conditioning memory with information about extinction. Thus, prediction error determines how new information is organized in memory, and latent state theories adequately describe the ways in which this occurs., Competing Interests: NK, JL, SK, RW, NH No competing interests declared, (© 2024, Kennedy et al.)

Sequence memory is subject to age-related decline, but the underlying processes are not yet fully understood. We analyzed electroencephalography (EEG) in 21 healthy older (60-80 years) and 26 young participants (20-30 years) and compared time-frequency spectra and theta-gamma phase-amplitude-coupling (PAC) during encoding of the order of visually presented items. In older adults, desynchronization in theta (4-8 Hz) and synchronization in gamma (30-45 Hz) power did not distinguish between subsequently correctly and incorrectly remembered trials, while there was a subsequent memory effect for young adults. Theta-gamma PAC was modulated by item position within a sequence for older but not young adults. Specifically, position within a sequence was coded by higher gamma amplitude for successive theta phases for later correctly remembered trials. Thus, deficient differentiation in theta desynchronization and gamma oscillations during sequence encoding in older adults may reflect neurophysiological correlates of age-related memory decline. Furthermore, our results indicate that sequences are coded by theta-gamma PAC in older adults, but that this mechanism might lose precision in aging., (Copyright © 2024 The Authors. Published by Elsevier Inc. All rights reserved.)

Semantic memory representations are generally well maintained in aging, whereas semantic control is thought to be more affected. To explain this phenomenon, this study tested the predictions of the Compensation-Related Utilization of Neural Circuits Hypothesis (CRUNCH), focusing on task demands in aging as a possible framework. The CRUNCH effect would manifest itself in semantic tasks through a compensatory increase in neural activation in semantic control network regions but only up to a certain threshold of task demands. This study compares 39 younger (20-35 years old) with 39 older participants (60-75 years old) in a triad-based semantic judgment task performed in an fMRI scanner while manipulating task demand levels (low versus high) through semantic distance. In line with the CRUNCH predictions, differences in neurofunctional activation and behavioral performance (accuracy and response times) were expected in younger versus older participants in the low- versus high-demand conditions, which should be manifested in semantic control Regions of Interest (ROIs). Our older participants had intact behavioral performance, as proposed in the literature for semantic memory tasks (maintained accuracy and slower response times (RTs)). Age-invariant behavioral performance in the older group compared to the younger one is necessary to test the CRUNCH predictions. The older adults were also characterized by high cognitive reserve, as our neuropsychological tests showed. Our behavioral results confirmed that our task successfully manipulated task demands: error rates, RTs and perceived difficulty increased with increasing task demands in both age groups. We did not find an interaction between age group and task demand, or a statistically significant difference in activation between the low- and high-demand conditions for either RTs or accuracy. As for brain activation, we did not find the expected age group by task demand interaction, or a significant main effect of task demand. Overall, our results are compatible with some neural activation in the semantic network and the semantic control network, largely in frontotemporoparietal regions. ROI analyses demonstrated significant effects (but no interactions) of task demand in the left and right inferior frontal gyrus, the left posterior middle temporal gyrus, the posterior inferior temporal gyrus and the prefrontal gyrus. Overall, our test did not confirm the CRUNCH predictions., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Haitas et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

To understand the neurocognitive mechanisms that underlie heterogeneity in cognitive ageing, recent scientific efforts have led to a growing public availability of imaging cohort data. The Advanced BRain Imaging on ageing and Memory (ABRIM) project aims to add to these existing datasets by taking an adult lifespan approach to provide a cross-sectional, normative database with a particular focus on connectivity, myelinization and iron content of the brain in concurrence with cognitive functioning, mechanisms of reserve, and sleep-wake rhythms. ABRIM freely shares MRI and behavioural data from 295 participants between 18-80 years, stratified by age decade and sex (median age 52, IQR 36-66, 53.20% females). The ABRIM MRI collection consists of both the raw and pre-processed structural and functional MRI data to facilitate data usage among both expert and non-expert users. The ABRIM behavioural collection includes measures of cognitive functioning (i.e., global cognition, processing speed, executive functions, and memory), proxy measures of cognitive reserve (e.g., educational attainment, verbal intelligence, and occupational complexity), and various self-reported questionnaires (e.g., on depressive symptoms, pain, and the use of memory strategies in daily life and during a memory task). In a sub-sample (n = 120), we recorded sleep-wake rhythms using an actigraphy device (Actiwatch 2, Philips Respironics) for a period of 7 consecutive days. Here, we provide an in-depth description of our study protocol, pre-processing pipelines, and data availability. ABRIM provides a cross-sectional database on healthy participants throughout the adult lifespan, including numerous parameters relevant to improve our understanding of cognitive ageing. Therefore, ABRIM enables researchers to model the advanced imaging parameters and cognitive topologies as a function of age, identify the normal range of values of such parameters, and to further investigate the diverse mechanisms of reserve and resilience., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Jansen et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)

Memories are thought to be stored in neuronal ensembles referred to as engrams. Studies have suggested that when two memories occur in quick succession, a proportion of their engrams overlap and the memories become linked (in a process known as prospective linking) while maintaining their individual identities. In this Review, we summarize the key principles of memory linking through engram overlap, as revealed by experimental and modelling studies. We describe evidence of the involvement of synaptic memory substrates, spine clustering and non-linear neuronal capacities in prospective linking, and suggest a dynamic somato-synaptic model, in which memories are shared between neurons yet remain separable through distinct dendritic and synaptic allocation patterns. We also bring into focus retrospective linking, in which memories become associated after encoding via offline reactivation, and discuss key temporal and mechanistic differences between prospective and retrospective linking, as well as the potential differences in their cognitive outcomes., (© 2024. Springer Nature Limited.)

Fear extinction is a biological process in which learned fear behavior diminishes without anticipated reinforcement, allowing the organism to re-adapt to ever-changing situations. Based on the behavioral hypothesis that extinction is new learning and forms an extinction memory, this new memory is more readily forgettable than the original fear memory. The brain's cellular and synaptic traces underpinning this inherently fragile yet reinforceable extinction memory remain unclear. Intriguing questions are about the whereabouts of the engram neurons that emerged during extinction learning and how they constitute a dynamically evolving functional construct that works in concert to store and express the extinction memory. In this review, we discuss recent advances in the engram circuits and their neural connectivity plasticity for fear extinction, aiming to establish a conceptual framework for understanding the dynamic competition between fear and extinction memories in adaptive control of conditioned fear responses., (© 2023. The Author(s).)

The brain is designed not only with molecules and cellular processes that help to form memories but also with molecules and cellular processes that suppress the formation and retention of memory. The latter processes are critical for an efficient memory management system, given the vast amount of information that each person experiences in their daily activities and that most of this information becomes irrelevant with time. Thus, efficiency dictates that the brain should have processes for selecting the most critical information for storage and suppressing the irrelevant or forgetting it later should it escape the initial filters. Such memory suppressor molecules and processes are revealed by genetic or pharmacologic insults that lead to enhanced memory expression. We review here the predominant memory suppressor molecules and processes that have recently been discovered. They are diverse, as expected, because the brain is complex and employs many different strategies and mechanisms to form memories. They include the gene-repressive actions of small noncoding RNAs, repressors of protein synthesis, cAMP-mediated gene expression pathways, inter- and intracellular signaling pathways for normal forgetting, and others. A deep understanding of memory suppressor molecules and processes is necessary to fully comprehend how the brain forms, stabilizes, and retrieves memories and to reveal how brain disorders disrupt memory., Competing Interests: Declaration of Conflicting InterestsThe author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

In this study, we investigated the electrical brain responses in a high-density EEG array (64 electrodes) elicited specifically by the word memory cue in the Think/No-Think paradigm in 46 participants. In a first step, we corroborated previous findings demonstrating sustained and reduced brain electrical frontal and parietal late potentials elicited by memory cues following the No-Think (NT) instructions as compared to the Think (T) instructions. The topographical analysis revealed that such reduction was significant 1000 ms after memory cue onset and that it was long-lasting for 1000 ms. In a second step, we estimated the underlying brain generators with a distributed method (swLORETA) which does not preconceive any localization in the gray matter. This method revealed that the cognitive process related to the inhibition of memory retrieval involved classical motoric cerebral structures with the left primary motor cortex (M1, BA4), thalamus, and premotor cortex (BA6). Also, the right frontal-polar cortex was involved in the T condition which we interpreted as an indication of its role in the maintaining of a cognitive set during remembering, by the selection of one cognitive mode of processing, Think, over the other, No-Think, across extended periods of time, as it might be necessary for the successful execution of the Think/No-Think task., (© 2024. The Author(s).)

Information storage in the brain involves different memory types and stages that are processed by several brain regions. Cholinergic pathways through acetylcholine receptors actively participate on memory modulation, and their disfunction is associated with cognitive decline in several neurological disorders. During the last decade, the role of α7 subtype of nicotinic acetylcholine receptors in different memory stages has been studied. However, the information about their role in memory processing is still scarce. In this review, we attempt to identify brain areas where α7 nicotinic receptors have an essential role in different memory types and stages. In addition, we discuss recent work implicating-or not-α7 nicotinic receptors as promising pharmacological targets for memory impairment associated with neurological disorders., (© 2023 Federation of European Neuroscience Societies and John Wiley & Sons Ltd.)

Intrusive memories are a core symptom of posttraumatic stress disorder. Compared with memories of everyday events, they are characterized by several seemingly contradictory features: intrusive memories contain distinct sensory and emotional details of the traumatic event and can be triggered by various perceptually similar cues, but they are poorly integrated into conceptual memory. Here, we conduct exploratory whole-brain analyses to investigate the neural representations of trauma-analog experiences and how they are reactivated during memory intrusions. We show that trauma-analog movies induce excessive processing and generalized representations in sensory areas but decreased blood-oxygen-level-dependent (BOLD) responses and highly distinct representations in conceptual/semantic areas. Intrusive memories activate generalized representations in sensory areas and reactivate memory traces specific to trauma-analog events in the anterior cingulate cortex. These findings provide the first evidence of how traumatic events could distort memory representations in the human brain, which may form the basis for future confirmatory research on the neural representations of traumatic experiences., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2024 The Author(s). Published by Elsevier Inc. All rights reserved.)

Expressions such as 'sleep on it' refer to the resolution of distressing experiences across a night of sound sleep. Sleep is an active state during which the brain reorganizes the synaptic connections that form memories. This Perspective proposes a model of how sleep modifies emotional memory traces. Sleep-dependent reorganization occurs through neurophysiological events in neurochemical contexts that determine the fates of synapses to grow, to survive or to be pruned. We discuss how low levels of acetylcholine during non-rapid eye movement sleep and low levels of noradrenaline during rapid eye movement sleep provide a unique window of opportunity for plasticity in neuronal representations of emotional memories that resolves the associated distress. We integrate sleep-facilitated adaptation over three levels: experience and behaviour, neuronal circuits, and synaptic events. The model generates testable hypotheses for how failed sleep-dependent adaptation to emotional distress is key to mental disorders, notably disorders of anxiety, depression and post-traumatic stress with the common aetiology of insomnia., (© 2024. Springer Nature Limited.)

Memory is vital and indispensable for organisms and brain-inspired intelligence to gain complete sensation and cognition of the environment. In this work, a memristive bionic memory circuit inspired by human memory model is proposed, which includes 1) receptor and sensory neuron (SN), 2) short-term memory (STM) module, and 3) long-term memory (LTM) module. By leveraging the in-memory computing characteristic of memristors, various functions such as sensation, learning, forgetting, recall, consolidation, reconsolidation, retrieval, and reset are realized. Besides, a multisensory mutual associative learning network is constructed with several bionic memory units to memorize and associate sensory information of different modalities bidirectionally. Except for association establishment, enhancement, and extinction, we also mimicked multisensory integration to manifest the synthetic process of information from different sensory channels. According to the simulation results in PSPICE, the proposed circuit performs high robustness, low area overhead, and low power consumption. Combining associative memory with human memory model, this work provides a possible idea for further research in associative learning networks.

Throughout development, the neuronal epigenome is highly sensitive to external stimuli, yet capable of safeguarding cellular memory for a lifetime. In the adult brain, memories of fearful experiences are rapidly instantiated, yet can last for decades, but the mechanisms underlying such longevity remain unknown. Here, we showcase how fear memory formation and storage - traditionally thought to exclusively affect synapse-based events - elicit profound and enduring changes to the chromatin, proposing epigenetic regulation as a plausible molecular template for mnemonic processes. By comparing these to mechanisms occurring in development and differentiation, we notice that an epigenetic machinery similar to that preserving cellular memories might be employed by brain cells so as to form, store, and retrieve behavioral memories., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Certain memories resist extinction to continue invigorating maladaptive actions. The robustness of these memories could depend on their widely distributed implementation across populations of neurons in multiple brain regions. However, how dispersed neuronal activities are collectively organized to underpin a persistent memory-guided behavior remains unknown. To investigate this, we simultaneously monitored the prefrontal cortex, nucleus accumbens, amygdala, hippocampus, and ventral tegmental area (VTA) of the mouse brain from initial recall to post-extinction renewal of a memory involving cocaine experience. We uncover a higher-order pattern of short-lived beta-frequency (15-25 Hz) activities that are transiently coordinated across these networks during memory retrieval. The output of a divergent pathway from upstream VTA glutamatergic neurons, paced by a slower (4-Hz) oscillation, actuates this multi-network beta-band coactivation; its closed-loop phase-informed suppression prevents renewal of cocaine-biased behavior. Binding brain-distributed neural activities in this temporally structured manner may constitute an organizational principle of robust memory expression., Competing Interests: Declaration of interests C.G.M. and A.S. are inventors on a pending patent application related to the phase-tracking algorithm used in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Neural plasticity can be defined as the ability of neural circuits to be shaped by external and internal factors. It provides the brain with a capacity for functional and morphological remodelling, with many lines of evidence indicating that these changes are vital for learning and memory formation. The basis of this brain plasticity resides in activity- and experience-driven modifications of synaptic strength, including synaptic formation, elimination or weakening, as well as of modulation of neuronal population, which drive the structural reorganization of neural networks. Recent evidence indicates that brain-resident glial cells actively participate in these processes, suggesting that mechanisms underlying plasticity in the brain are multifaceted. Establishing the 'tripartite' synapse, the role of astrocytes in modulating synaptic transmission in response to neuronal activity was recognized first. Further redefinition of the synapse as 'quad-partite' followed to acknowledge the contribution of microglia which were revealed to affect numerous brain functions via dynamic interactions with synapses, acting as 'synaptic sensors' that respond to neuronal activity and neurotransmitter release, as well as crosstalk with astrocytes. Early studies identified microglial ability to dynamically survey their local brain environment and established their integral role in the active interfacing of environmental stimuli (both internal and external), with brain plasticity and remodelling. Following the introduction to neurogenesis, this chapter details the role that microglia play in regulating neurogenesis in adulthood, specifically as it relates to learning and memory, as well as factors involved in modulation of microglia. Further, a microglial perspective is introduced for the context of environmental enrichment impact on neurogenesis, learning and memory across states of stress, ageing, disease and injury., (© 2024. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

Memory engrams in mice brains are potentially related to groups of concept cells in human brains. A single concept cell in human hippocampus responds, for example, not only to different images of the same object or person but also to its name written down in characters. Importantly, a single mental concept (object or person) is represented by several concept cells and each concept cell can respond to more than one concept. Computational work shows how mental concepts can be embedded in recurrent artificial neural networks as memory engrams and how neurons that are shared between different engrams can lead to associations between concepts. Therefore, observations at the level of neurons can be linked to cognitive notions of memory recall and association chains between memory items., (© 2024. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

One of the most fascinating aspects of the brain is its ability to acquire new information from experience and retain it over time as memory. The search for physical correlates of memory, the memory engram, has been a longstanding priority in modern neurobiology. Advanced genetic approaches have led to the localization of engram cells in a few brain regions, including the hippocampus and cortex. Additionally, engram cells exhibit learning-induced, persistent modifications and have at least two states, active and silent. However, it has been hypothesized that engrams for a specific memory are distributed among multiple brain regions that are functionally connected, referred to as a unified engram complex. Recent tissue-clearing techniques have permitted high-throughput analyses of intact brain samples, which have been used to obtain a map of the engram complex for a contextual fear memory. Careful examination of these engram complex maps has revealed a potentially underappreciated contribution of subcortical regions, specifically thalamic nuclei, to memory function. These more holistic studies support the unified engram complex hypothesis for memory storage and have important implications for understanding dysfunctional engrams in the context of human disease., (© 2024. The Author(s), under exclusive license to Springer Nature Switzerland AG.)

Memory is the persisting consequence of cognitive activities instigated by and engrossed on exterior information from the environment and commenced by an intensive on internal mental representations. Establishing a gut-brain axis (GBA) in health and disease has recently brought the gut, the main portal of communication with the external environment, to the forefront of this interaction. Dietary stimuli have long been linked to brain development, behavioral responses, and memory reflections. Vagus nerve, immune system, bacterial metabolites and products are just a few of the linkages that make up the GBA, a bidirectional arrangement of signaling pathways that connects the neurological system with the gastrointestinal tract. GBA involves two-way communication between central and enteric neural systems, connecting the brain's affective and cognitive regions to peripheral activities of the intestine. Recent scientific progress has highlighted the significance of gut microbiota in affecting these relationships. By controlling myelination at the prefrontal cortex, a crucial area for multifaceted cognitive behavior forecast and decision-making, this axis influences social behavior, including memory reflections. Humans may experience late myelination of the prefrontal cortex's axonal projections into the third decade of life, making it vulnerable to outside factors like microbial metabolites. It has been demonstrated that changes in the gut microbiome can change the microbial metabolome's composition, impacting highly permeable bioactive chemicals like p-cresol that may hinder oligodendrocyte differentiation. This review will discuss the memory reflections of the microbiota-gut and oligodendrocyte axis. Adopting this concept should encourage a new arena of thinking that recognizes the intricate central and periphery dynamics influencing behavior and uses that knowledge to develop novel therapies and interventions for maladjusted memory and learning systems., (Copyright© Bentham Science Publishers; For any queries, please email at epub@benthamscience.net.)

How memories are organized in the brain influences whether they are remembered discretely versus linked with other experiences or whether generalized information is applied to entirely novel situations. Here, we used scFLARE2 (single-chain fast light- and activity-regulated expression 2), a temporally precise tagging system, to manipulate mouse lateral amygdala neurons active during one of two 3 min threat experiences occurring close (3 h) or further apart (27 h) in time. Silencing scFLARE2-tagged neurons showed that two threat experiences occurring at distal times are dis-allocated to orthogonal engram ensembles and remembered discretely, whereas the same two threat experiences occurring in close temporal proximity are linked via co-allocation to overlapping engram ensembles. Moreover, we found that co-allocation mediates memory generalization applied to a completely novel stimulus. These results indicate that endogenous temporal evolution of engram ensemble neuronal excitability determines how memories are organized and remembered and that this would not be possible using conventional immediate-early gene-based tagging methods., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2023 The Author(s). Published by Elsevier Inc. All rights reserved.)

Sleep promotes memory consolidation: the process by which newly acquired memories are stabilised, strengthened, and integrated into long-term storage. Pioneering research in rodents has revealed that memory reactivation in sleep is a primary mechanism underpinning sleep's beneficial effect on memory. In this review, we consider evidence for memory reactivation processes occurring in human sleep. Converging lines of research support the view that memory reactivation occurs during human sleep, and is functionally relevant for consolidation. Electrophysiology studies have shown that memory reactivation is tightly coupled to the cardinal neural oscillations of non-rapid eye movement sleep, namely slow oscillation-spindle events. In addition, functional imaging studies have found that brain regions recruited during learning become reactivated during post-learning sleep. In sum, the current evidence paints a strong case for a mechanistic role of neural reactivation in promoting memory consolidation during human sleep., (© 2023 The Author(s).)

Memory retrieval is strikingly susceptible to external states (environment) and internal states (mood states and alcohol), yet we know little about the underlying mechanisms. We examined how internally generated states influence successful memory retrieval using the functional magnetic resonance imaging (fMRI) of laboratory mice during memory retrieval. Mice exhibited a strong tendency to perform memory retrieval correctly only in the reinstated mammillary body-inhibited state, in which mice were trained to discriminate auditory stimuli in go/no-go tasks. fMRI revealed that distinct auditory cues engaged differential brain regions, which were primed by internal state. Specifically, a cue associated with a reward activated the lateral amygdala, while a cue signaling no reward predominantly activated the postsubiculum. Modifying these internal states significantly altered the neural activity balance between these regions. Optogenetic inhibition of those regions in the precue period blocked the retrieval of type-specific memories. Our findings suggest that memory retrieval is under the control of two interrelated neural circuits underlying the neural basis of state-dependent memory retrieval., (© 2023 Wang et al.; Published by Cold Spring Harbor Laboratory Press.)

Sleep and memory are highly intertwined, yet the integrative neural network of these two fundamental physiological behaviors remains poorly understood. Multiple cell types and structures of the Drosophila brain have been shown involved in the regulation of sleep and memory, and recent efforts are focusing on bridging them at molecular and circuit levels. Here, we briefly review 1) identified neurons as key nodes of olfactory-associative memory circuits involved in different memory processes; 2) how neurons of memory circuits participate in sleep regulation; and 3) other cell types and circuits besides the mushroom body in linking sleep and memory. We also attempt to provide the remaining gaps of circuitry integration of sleep and memory, which may spark some new thinking for future efforts., Competing Interests: Declaration of Competing Interest None., (Copyright © 2023 Elsevier Inc. All rights reserved.)

Background: Since the late 1930s, electric brain stimulation (EBS) in awake patients has been known to occasionally elicit patient descriptions of a form of memory flashbacks, known as experiential phenomena. One understanding of these sensations are as caused by an augmentation of the capacity for memory retrieval. However, an alternative hypothesis holds that memory flashbacks during EBS are "synthetic constructions" in the form of mental events, falsely interpreted as memories., Methods: A critical narrative review is used to discuss the false memory hypothesis in relation to the current empirical literature and source attribution theory., Results: EBS as well as situational demands in the form of interaction between patient and neurosurgeon may both lead to the creation of mental events and influence their interpretation in a way that may create false memories. The false memory hypothesis provides a potential explanation for several apparent inconsistencies in the current literature such as (a) the fragmented nature of experiential reports, (b) the ability of EBS to induce memory retrieval errors in controlled studies, (c) that Penfield's elicitations of experiential phenomena are so rarely replicated in the modern era, and (d) the limited utility of techniques that elicit experiential phenomena in the treatment of memory disorders., Conclusions: The hypothesis that experiential phenomena may largely be "synthetic constructions" deserves serious consideration by neurosurgeons., (© 2022. The Author(s).)

The same visual input can serve as the target of perception or as a trigger for memory retrieval depending on whether cognitive processing is externally oriented (perception) or internally oriented (memory retrieval). While numerous human neuroimaging studies have characterized how visual stimuli are differentially processed during perception versus memory retrieval, perception and memory retrieval may also be associated with distinct neural states that are independent of stimulus-evoked neural activity. Here, we combined human fMRI with full correlation matrix analysis (FCMA) to reveal potential differences in "background" functional connectivity across perception and memory retrieval states. We found that perception and retrieval states could be discriminated with high accuracy based on patterns of connectivity across (1) the control network, (2) the default mode network (DMN), and (3) retrosplenial cortex (RSC). In particular, clusters in the control network increased connectivity with each other during the perception state, whereas clusters in the DMN were more strongly coupled during the retrieval state. Interestingly, RSC switched its coupling between networks as the cognitive state shifted from retrieval to perception. Finally, we show that background connectivity (1) was fully independent from stimulus-related variance in the signal and, further, (2) captured distinct aspects of cognitive states compared to traditional classification of stimulus-evoked responses. Together, our results reveal that perception and memory retrieval are associated with sustained cognitive states that manifest as distinct patterns of connectivity among large-scale brain networks., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier Inc.)

Modulation of cognitive functions supporting human declarative memory is one of the grand challenges of neuroscience, and of vast importance for a variety of neuropsychiatric, neurodegenerative and neurodevelopmental diseases. Despite a recent surge of successful attempts at improving performance in a range of memory tasks, the optimal approaches and parameters for memory enhancement have yet to be determined. On a more fundamental level, it remains elusive as to how delivering electrical current in a given brain area leads to enhanced memory processing. Starting from the local and distal physiological effects on neural populations, the mechanisms of enhanced memory encoding, maintenance, consolidation or recall in response to direct electrical stimulation are only now being unravelled. With the advent of innovative neurotechnologies for concurrent recording and stimulation intracranially in the human brain, it becomes possible to study both acute and chronic effects of stimulation on memory performance and the underlying neural activities. In this review, we summarize the effects of various invasive stimulation approaches for modulating memory functions. We first outline the challenges that were faced in the initial studies of memory enhancement and the lessons learnt. Electrophysiological biomarkers are then reviewed as more objective measures of the stimulation effects than behavioural outcomes. Finally, we classify the various stimulation approaches into continuous and phasic modulation with an open or closed loop for responsive stimulation based on analysis of the recorded neural activities. Although the potential advantage of closed-loop responsive stimulation over the classic open-loop approaches is inconclusive, we foresee the emerging results from ongoing longitudinal studies and clinical trials will shed light on both the mechanisms and optimal strategies for improving declarative memory. Adaptive stimulation based on the biomarker analysis over extended periods of time is proposed as a future direction for obtaining lasting effects on memory functions. Chronic tracking and modulation of neural activities intracranially through adaptive stimulation opens tantalizing new avenues to continually monitor and treat memory and cognitive deficits in a range of brain disorders. Brain co-processors created with machine-learning tools and wireless bi-directional connectivity to seamlessly integrate implanted devices with smartphones and cloud computing are poised to enable real-time automated analysis of large data volumes and adaptively tune electrical stimulation based on electrophysiological biomarkers of behavioural states. Next-generation implantable devices for high-density recording and stimulation of electrophysiological activities, and technologies for distributed brain-computer interfaces are presented as selected future perspectives for modulating human memory and associated mental processes., (© The Author(s) 2022. Published by Oxford University Press on behalf of the Guarantors of Brain. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Associating multiple sensory cues with objects and experience is a fundamental brain process that improves object recognition and memory performance. However, neural mechanisms that bind sensory features during learning and augment memory expression are unknown. Here we demonstrate multisensory appetitive and aversive memory in Drosophila. Combining colours and odours improved memory performance, even when each sensory modality was tested alone. Temporal control of neuronal function revealed visually selective mushroom body Kenyon cells (KCs) to be required for enhancement of both visual and olfactory memory after multisensory training. Voltage imaging in head-fixed flies showed that multisensory learning binds activity between streams of modality-specific KCs so that unimodal sensory input generates a multimodal neuronal response. Binding occurs between regions of the olfactory and visual KC axons, which receive valence-relevant dopaminergic reinforcement, and is propagated downstream. Dopamine locally releases GABAergic inhibition to permit specific microcircuits within KC-spanning serotonergic neurons to function as an excitatory bridge between the previously 'modality-selective' KC streams. Cross-modal binding thereby expands the KCs representing the memory engram for each modality into those representing the other. This broadening of the engram improves memory performance after multisensory learning and permits a single sensory feature to retrieve the memory of the multimodal experience., (© 2023. The Author(s).)

The ability to suppress unwelcome memories is important for productivity and well-being. Successful memory suppression is associated with hippocampal deactivations and a concomitant disruption of this region's functionality. Much of the previous neuroimaging literature exploring such suppression-related hippocampal modulations has focused on the region's negative coupling with the prefrontal cortex. Task-based changes in functional connectivity between the hippocampus and other brain regions still need further exploration. In the present study, we utilize psychophysiological interactions and seed connectome-based predictive modeling to investigate the relationship between the hippocampus and the rest of the brain as 134 participants attempted to suppress unwanted memories during the Think/No-Think task. The results show that during retrieval suppression, the right hippocampus exhibited decreased functional connectivity with visual cortical areas (lingual and cuneus gyrus), left nucleus accumbens and the brain-stem that predicted superior forgetting of unwanted memories on later memory tests. Validation tests verified that prediction performance was not an artifact of head motion or prediction method and that the negative features remained consistent across different brain parcellations. These findings suggest that systemic memory suppression involves more than the modulation of hippocampal activity-it alters functional connectivity patterns between the hippocampus and visual cortex, leading to successful forgetting., (© The Author(s) 2022. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permission@oup.com.)

Studying the underlying neural mechanisms of cognitive functions of the brain is one of the central questions in modern biology. Moreover, it has significantly impacted the development of novel technologies in artificial intelligence. Spontaneous activity is a unique feature of the brain and is currently lacking in many artificially constructed intelligent machines. Spontaneous activity may represent the brain's idling states, which are internally driven by neuronal networks and possibly participate in offline processing during awake, sleep, and resting states. Evidence is accumulating that the brain's spontaneous activity is not mere noise but part of the mechanisms to process information about previous experiences. A bunch of literature has shown how previous sensory and behavioral experiences influence the subsequent patterns of brain activity with various methods in various animals. It seems, however, that the patterns of neural activity and their computational roles differ significantly from area to area and from function to function. In this article, I review the various forms of the brain's spontaneous activity, especially those observed during memory processing, and some attempts to model the generation mechanisms and computational roles of such activities., (Copyright © 2023 Elsevier Ltd and Japan Neuroscience Society. All rights reserved.)

Increasing evidence suggests that reactivation of newly acquired memory traces during postlearning wakefulness plays an important role in memory consolidation. Here, we sought to boost the reactivation of a motor memory trace during postlearning wakefulness (quiet rest) immediately following learning using somatosensory targeted memory reactivation (TMR). Using functional magnetic resonance imaging, we examined the neural correlates of the reactivation process as well as the effect of the TMR intervention on brain responses elicited by task practice on 24 healthy young adults. Behavioral data of the post-TMR retest session showed a faster learning rate for the motor sequence that was reactivated as compared to the not-reactivated sequence. Brain imaging data revealed that motor, parietal, frontal, and cerebellar brain regions, which were recruited during initial motor learning, were specifically reactivated during the TMR episode and that hippocampo-frontal connectivity was modulated by the reactivation process. Importantly, the TMR-induced behavioral advantage was paralleled by dynamical changes in hippocampal activity and hippocampo-motor connectivity during task practice. Altogether, the present results suggest that somatosensory TMR during postlearning quiet rest can enhance motor performance via the modulation of hippocampo-cortical responses., (© The Author(s) 2022. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Humans have the ability to intentionally forget information via different strategies, included suppression of encoding (directed forgetting) and mental replacement of the item to encode (thought substitution). These strategies may rely on different neural mechanisms; namely, encoding suppression may induce prefrontally mediated inhibition, whereas thought substitution is potentially accomplished through modulating contextual representations. Yet, few studies have directly related inhibitory processing to encoding suppression, or tested its involvement in thought substitution. Here, we directly tested whether encoding suppression recruits inhibitory mechanisms with a cross-task design, relating the behavioral and neural data from male and female participants in a Stop Signal task (a task specifically testing inhibitory processing) to a directed forgetting task with both encoding suppression (Forget) and thought substitution (Imagine) cues. Behaviorally, Stop Signal task performance (stop signal reaction times) was related to the magnitude of encoding suppression, but not thought substitution. Two complementary neural analyses corroborated the behavioral result. Namely, brain-behavior analysis demonstrated that the magnitude of right-frontal beta activity following stop signals was related to stop signal reaction times and successful encoding suppression, but not thought substitution; and classifiers trained to discriminate successful and unsuccessful stopping in the Stop Signal task could also classify successful and unsuccessful forgetting following Forget cues, but not Imagine cues. Importantly, inhibitory neural mechanisms were engaged following Forget cues at a later time than motor stopping. These findings not only support an inhibitory account of directed forgetting, and that thought substitution engages separate mechanisms, but also potentially identify a specific time in which inhibition occurs when suppressing encoding. SIGNIFICANCE STATEMENT Forgetting often seems like an unintended experience, but forgetting can be intentional, and can be accomplished with multiple strategies. These strategies, including encoding suppression and thought substitution, may rely on different neural mechanisms. Here, we test the hypothesis that encoding suppression engages domain-general prefrontally driven inhibitory control mechanisms, while thought substitution does not. Using cross-task analyses, we provide evidence that encoding suppression engages the same inhibitory mechanisms used for stopping motor actions, but these mechanisms are not engaged by thought substitution. These findings not only support the notion that mnemonic encoding processes can be directly inhibited, but also have broad relevance, as certain populations with disrupted inhibitory processing may be more successful accomplishing intentional forgetting through thought substitution strategies., (Copyright © 2023 the authors.)

Memories are thought to be stored in ensembles of neurons across multiple brain regions. However, whether and how these ensembles are coordinated at the time of learning remains largely unknown. Here, we combined CREB-mediated memory allocation with transsynaptic retrograde tracing to demonstrate that the allocation of aversive memories to a group of neurons in one brain region directly affects the allocation of interconnected neurons in upstream brain regions in a behavioral- and brain region-specific manner in mice. Our analysis suggests that this cross-regional recruitment of presynaptic neurons is initiated by downstream memory neurons through a retrograde mechanism. Together with statistical modeling, our results indicate that in addition to the anterograde flow of information between brain regions, the establishment of interconnected, brain-wide memory traces relies on a retrograde mechanism that coordinates memory ensembles at the time of learning., Competing Interests: Declaration of interests The authors declare no competing interests., (Copyright © 2022 Elsevier Inc. All rights reserved.)

A large body of evidence suggests that cognitive functions rely on the coordination of ensembles of neurons across brain circuits. One example is social memory, the ability to recognize and remember other conspecifics. A broad range of brain regions have been implicated in social behaviors and memory processes. At the single-cell level, neurons from different brain areas have responded to specific social features. The coordination of these ensembles both within a region and across structures is required to support social memory and decision-making. The synchronous activation of these neuronal ensembles could allow for the integration of different aspects of a social episode into a unified representation of experience. In this review, recent results on the circuit basis and physiological mechanisms of social memory are discussed, from a systems neuroscience perspective. An integrative framework of the neuronal ensemble dynamics supporting this fundamental cognitive ability is proposed., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

The human brain has limited storage capacity often challenging the encoding and recall of a long series of multiple items. Different encoding strategies are therefore employed to optimize performance in memory processes such as chunking where particular items are 'grouped' to reduce the number of items to store artificially. Additionally, related to the position of an item within a series, there is a tendency to remember the first and last items on the list better than the middle ones, which calls the "serial position effect". Although relatively well-established in behavioral research, the neuronal mechanisms underlying such encoding strategies and memory effects remain poorly understood. Here, we used event-related EEG oscillation analyses to unravel the neuronal substrates of serial encoding strategies and effects during the behaviorally controlled execution of the digit span task. We recorded EEG in forty-four healthy young-adult participants during a backward digit span (ds) task with two difficulty levels (i.e., 3-ds and 5-ds). Participants were asked to recall the digits in reverse order after the presentation of each set. We analyzed the pattern of event-related delta and theta oscillatory power in the time-frequency domain over fronto-central and parieto-occipital areas during the item (digit) list encoding, focusing on how these oscillatory responses changed with each subsequent digit being encoded in the series. Results showed that the development of event-related delta power evoked by digits in each series matched the 'serial position curve', with higher delta power being present during the first, and especially last, digits as compared to digits presented in the middle of a set, for both difficulty levels. Event-related theta power, in contrast, rather resembled a neural correlate of a chunking pattern where, during the 5-ds encoding, a clear change in event-related theta occurred around the third/fourth positions, with decreasing power values for later digits. This suggests that different oscillatory mechanisms linked to different frequency bands may code for the different encoding strategies and effects in serial item presentation. Furthermore, recall-EEG correlations suggested that participants with higher fronto-central delta responses during digit encoding showed also higher recall scores. The here presented findings contribute to our understanding of the neural oscillatory mechanisms underlying multiple item encoding, directly informing recent efforts towards memory enhancement through targeted oscillation-based neuromodulation., Competing Interests: Declaration of Competing Interest None., (Copyright © 2022 The Authors. Published by Elsevier Inc. All rights reserved.)

To efficiently process information, the brain shifts between encoding and retrieval states, prioritizing bottom-up or top-down processing accordingly. Expectation violation before or during learning has been shown to trigger an adaptive encoding mechanism, resulting in better memory for unexpected events. Using fMRI, we explored (1) whether this encoding mechanism is also triggered during retrieval, and if so, (2) what the temporal dynamics of its mnemonic consequences are. Male and female participants studied object images, then, with new objects, they learned a contingency between a cue and a semantic category. Rule-abiding (expected) and violating (unexpected) targets and similar foils were used at test. We found interactions between previous and current similar events' expectation, such that when an expected event followed a similar but unexpected event, its performance was boosted, underpinned by activation in the hippocampus, midbrain, and occipital cortex. In contrast, a sequence of two unexpected similar events also triggered occipital engagement; however, this did not enhance memory performance. Taken together, our findings suggest that when the goal is to retrieve, encountering surprising events engages an encoding mechanism, supported by bottom-up processing, that may enhance memory for future related events. SIGNIFICANCE STATEMENT Optimizing the balance between new learning and the retrieval of existing knowledge is an ongoing process, at the core of human cognition. Previous research into memory encoding suggests experiencing surprise leads to the prioritization of the learning of new memories, forming an adaptive encoding mechanism. We examined whether this mechanism is also engaged when the current goal is to retrieve information. Our results demonstrate that an expectation-driven shift toward an encoding state, supported by enhanced perceptual processing, is beneficial for the correct identification of subsequent expected similar events. These findings have important implications for our understanding of the temporal dynamics of the adaptive encoding of information into memory., (Copyright © 2022 the authors.)

During sleep, the brain undergoes transitions of activity states and reorganization of neural circuits. Recent in vivo optical imaging and manipulation techniques are revealing spatiotemporal and multiscale brain dynamics during sleep and their functional roles in awake state cognition, including learning and memory. First, along with electrophysiological recordings, Ca 2+ imaging is becoming the gold standard for tracking the activity of large neural ensembles to analyze memory replay during the sleep/wake cycle in freely moving and head-fixed animals. Comparable to the speed of electrophysiological recordings, voltage indicators can monitor neural activity at millisecond resolution. While one-photon systems have advantages in temporal resolution, two-photon lasers can image microstructures such as dendritic spines and axonal terminals in vivo. Also, optogenetic manipulation is used to perform loss- or gain-of-function experiments with temporal precision, cell-type, and pathway specificity without the artifacts observed with electrophysiological stimulation. In particular, closed-loop optogenetic manipulation is well suited to reveal the functional roles of neural activity and synaptic plasticity in each sleep/wake state. Here, I describe the recent advances in optical imaging and manipulation approaches for sleeping-brain dynamics in memory processing., (Copyright © 2022 Japan Neuroscience Society and Elsevier B.V. All rights reserved.)

Background: Treating memory and cognitive deficits requires knowledge about anatomical sites and neural activities to be targeted with particular therapies. Emerging technologies for local brain stimulation offer attractive therapeutic options but need to be applied to target specific neural activities, at distinct times, and in specific brain regions that are critical for memory formation., Methods: The areas that are critical for successful encoding of verbal memory as well as the underlying neural activities were determined directly in the human brain with intracranial electrophysiological recordings in epilepsy patients. We recorded a broad range of spectral activities across the cortex of 135 patients as they memorised word lists for subsequent free recall., Findings: The greatest differences in the spectral power between encoding subsequently recalled and forgotten words were found in low theta frequency (3-5 Hz) activities of the left anterior prefrontal cortex. This subsequent memory effect was proportionally greater in the lower frequency bands and in the more anterior cortical regions. We found the peak of this memory signal in a distinct part of the prefrontal cortex at the junction between the Broca's area and the frontal pole. The memory effect in this confined area was significantly higher (Tukey-Kramer test, p<0.05) than in other anatomically distinct areas., Interpretation: Our results suggest a focal hotspot of human verbal memory encoding located in the higher-order processing region of the prefrontal cortex, which presents a prospective target for modulating cognitive functions in the human patients. The memory effect provides an electrophysiological biomarker of low frequency neural activities, at distinct times of memory encoding, and in one hotspot location in the human brain., Funding: Open-access datasets were originally collected as part of a BRAIN Initiative project called Restoring Active Memory (RAM) funded by the Defence Advanced Research Project Agency (DARPA). CT, ML, MTK and this research were supported from the First Team grant of the Foundation for Polish Science co-financed by the European Union under the European Regional Development Fund., Competing Interests: Declaration of interests The authors declare that they have no competing interests., (Copyright © 2022 The Authors. Published by Elsevier B.V. All rights reserved.)