Episodic Memory Consolidation Research Articles

Transcranial direct current stimulation applied after encoding facilitates episodic memory consolidation in older adults.

Episodic memory shows the largest degree of age-related memory decline. There is evidence that consolidation, the process that stabilizes memories after encoding, is reduced in older adults. Previous studies have shown that transcranial direct current stimulation (tDCS) applied during intentional encoding or immediately after a contextual reminder enhanced delayed episodic memory performance, suggesting a potential interaction between tDCS and consolidation or reconsolidation processes. The present randomized, double-blind, sham-controlled study addressed the question whether tDCS applied immediately after verbal encoding enhances episodic memory recall through consolidation in healthy older adults. Twenty-eight participants received tDCS (Active or Sham) over the prefrontal cortex (anode over the left dorsolateral prefrontal cortex and cathode over the contralateral supraorbital region), a brain region contributing to episodic memory function. Verbal recall was tested two days and one month later. The results showed that recall performance at one month was enhanced in the Active tDCS group relative to the Sham group. These findings suggest that tDCS applied off-line immediately after encoding over the prefrontal cortex interacts with the processes promoting consolidation of episodic memories in healthy older adults. Targeting consolidation by means of tDCS might be a novel strategy for reducing episodic memory decline.

0234 Role of Midday Naps on Episodic Memory Consolidation in Preschool Children

0234 Role of Midday Naps on Episodic Memory Consolidation in Preschool Children

Young and Older Adults Benefit From Sleep, but Not From Active Wakefulness for Memory Consolidation of What-Where-When Naturalistic Events.

An extensive psychological literature shows that sleep actively promotes human episodic memory (EM) consolidation in younger adults. However, evidence for the benefit of sleep for EM consolidation in aging is still elusive. In addition, most of the previous studies used EM assessments that are very different from everyday life conditions and are far from considering all the hallmarks of this memory system. In this study, the effect of an extended period of sleep was compared to the effect of an extended period of active wakefulness on the EM consolidation of naturalistic events, using a novel (What-Where-When) EM task, rich in perceptual details and spatio-temporal context, presented in a virtual environment. We investigated the long-term What-Where-When and Details binding performances of young and elderly people before and after an interval of sleep or active wakefulness. Although we found a noticeable age-related decline in EM, both age groups benefited from sleep, but not from active wakefulness. In younger adults, only the period of sleep significantly enhanced the capacity to associate different components of EM (binding performance) and more specifically the free recall of what-when information. Interestingly, in the elderly, sleep significantly enhanced not only the recall of factual elements but also associated details and contextual information as well as the amount of high feature binding (i.e., What-Where-When and Details). Thus, this study evidences the benefit of sleep, and the detrimental effect of active wakefulness, on long-term feature binding, which is one of the core characteristics of EM, and its effectiveness in normal aging. However, further research should investigate whether this benefit is specific to sleep or more generally results from the effect of a post-learning period of reduced interference, which could also concern quiet wakefulness.

Glycogen distribution in mouse hippocampus.

The hippocampus is a limbic structure involved in the consolidation of episodic memory. In the recent decade, glycogenolysis in the rodent hippocampus has been shown critical for synaptic plasticity and memory formation. Astrocytes are the primary cells that store glycogen which is subject to degradation in hypoglycemic conditions. Focused microwave application to the brain halts metabolic activities, and therefore preserves brain glycogen. Immunohistochemistry against glycogen on focused microwave-assisted brain samples is suitable for both macroscopic and microscopic investigation of glycogen distribution. Glycogen immunohistochemistry in the hippocampus showed a characteristic punctate signal pattern that depended on hippocampal layers. In particular, the hilus is the most glycogen-rich subregion of the hippocampus. Moreover, large glycogen puncta (>0.5µm in diameter) observed in neuropil areas are organized in a patchy pattern consisting of puncta-rich and -poor astrocytes. These observations are discussed with respect to distinct hippocampal neural activity states observed in live animals.

Respiration Modulates Olfactory Memory Consolidation in Humans.

In mammals respiratory-locked hippocampal rhythms are implicated in the scaffolding and transfer of information between sensory and memory networks. These oscillations are entrained by nasal respiration and driven by the olfactory bulb. They then travel to the piriform cortex where they propagate further downstream to the hippocampus and modulate neural processes critical for memory formation. In humans, bypassing nasal airflow through mouth-breathing abolishes these rhythms and impacts encoding as well as recognition processes thereby reducing memory performance. It has been hypothesized that similar behavior should be observed for the consolidation process, the stage between encoding and recognition, were memory is reactivated and strengthened. However, direct evidence for such an effect is lacking in human and nonhuman animals. Here we tested this hypothesis by examining the effect of respiration on consolidation of episodic odor memory. In two separate sessions, female and male participants encoded odors followed by a 1 h awake resting consolidation phase where they either breathed solely through their nose or mouth. Immediately after the consolidation phase, memory for odors was tested. Recognition memory significantly increased during nasal respiration compared with mouth respiration during consolidation. These results provide the first evidence that respiration directly impacts consolidation of episodic events, and lends further support to the notion that core cognitive functions are modulated by the respiratory cycle.SIGNIFICANCE STATEMENT Memories pass through three main stages in their development: encoding, consolidation, and retrieval. Growing evidence from animal and human studies suggests that respiration plays an important role in the behavioral and neural mechanisms associated with encoding and recognition. Specifically nasal, but not mouth, respiration entrains neural oscillations that enhance encoding and recognition processes. We demonstrate that respiration also affects the consolidation stage. Breathing through the nose compared with the mouth during consolidation enhances recognition memory. This demonstrates, first, that nasal respiration is important during the critical period were memories are reactivated and strengthened. Second, it suggests that the neural mechanisms responsible may emerge from nasal respiration.

Promotion and suppression of autobiographical thinking differentially affect episodic memory consolidation.

During a post-encoding delay period, the ongoing consolidation of recently acquired memories can suffer interference if the delay period involves encoding of new memories, or sensory stimulation tasks. Interestingly, two recent independent studies suggest that (i) autobiographical thinking also interferes markedly with ongoing consolidation of recently learned wordlist material, while (ii) a 2-Back task might not interfere with ongoing consolidation, possibly due to the suppression of autobiographical thinking. In this study, we directly compare these conditions against a quiet wakeful rest baseline to test whether the promotion (via familiar sound-cues) or suppression (via a 2-Back task) of autobiographical thinking during the post-encoding delay period can affect consolidation of studied wordlists in a negative or a positive way, respectively. Our results successfully replicate previous studies and show a significant interference effect (as compared to the rest condition) when learning is followed by familiar sound-cues that promote autobiographical thinking, whereas no interference effect is observed when learning is followed by the 2-Back task. Results from a post-experimental experience-sampling questionnaire further show significant differences in the degree of autobiographical thinking reported during the three post-encoding periods: highest in the presence of sound-cues and lowest during the 2-Back task. In conclusion, our results suggest that varying levels of autobiographical thought during the post-encoding period may modulate episodic memory consolidation.

Pacing Hippocampal Sharp-Wave Ripples With Weak Electric Stimulation.

Sharp-wave ripples (SWRs) are spontaneous neuronal population events that occur in the hippocampus during sleep and quiet restfulness, and are thought to play a critical role in the consolidation of episodic memory. SWRs occur at a rate of 30–200 events per minute. Their overall abundance may, however, be reduced with aging and neurodegenerative disease. Here we report that the abundance of SWR within murine hippocampal slices can be increased by paced administration of a weak electrical stimulus, especially when the spontaneously occurring rate is low or compromised. Resultant SWRs have large variations in amplitude and ripple patterns, which are morphologically indistinguishable from those of spontaneous SWRs, despite identical stimulus parameters which presumably activate the same CA3 neurons surrounding the electrode. The stimulus intensity for reliably pacing SWRs is weaker than that required for inducing detectable evoked field potentials in CA1. Moreover, repetitive ~1 Hz stimuli with low intensity can reliably evoke thousands of SWRs without detectable LTD or “habituation.” Our results suggest that weak stimuli may facilitate the spontaneous emergence of SWRs without significantly altering their characteristics. Pacing SWRs with weak electric stimuli could potentially be useful for restoring their abundance in the damaged hippocampus.

Considering selection pressures for identity fusion and self-sacrifice in small-scale societies.

Whitehouse's view that our ultrasocial species evolved in small, warring bands is questioned, and alternative social selection pressures for the evolution of identity fusion and self-sacrifice in small-scale societies are proposed. Short durations of states of fusion allow for re-evaluation of risks; the consolidation of episodic memories into collective oral traditions elicits cooperation. Dysmorphic memories may be more powerful in generating identity fusion when followed by euphoric ones, as in "rites of torture" in male initiations.

Enhancing early consolidation of human episodic memory by theta EEG neurofeedback

Consolidation of newly formed memories is readily disrupted, but can it be enhanced? Given the prominent role of hippocampal theta oscillations in memory formation and retrieval, we hypothesized that upregulating theta power during early stages of consolidation might benefit memory stability and persistence. We used EEG neurofeedback to enable participants to selectively increase theta power in their EEG spectra following episodic memory encoding, while other participants engaged in low beta-focused neurofeedback or passively viewed a neutral nature movie. Free recall assessments immediately following the interventions, 24h later and 7d later all indicated benefit to memory of theta neurofeedback, relative to low beta neurofeedback or passive movie-viewing control conditions. The degree of benefit to memory was correlated with the extent of theta power modulation, but not with other spectral changes. Theta enhancement may provide optimal conditions for stabilization of new hippocampus-dependent memories.

Treatment of multiple sclerosis during pregnancy and lactation – Updated national treatment guidelines in Norway

L’atteinte des processus d’encodage et de consolidation en mémoire épisodique est précoce dans le syndrome démentiel de type Alzheimer. Or, il existe peu d’outils diagnostiques adaptés aux personnes âgées, en termes de facilité d’administration et de données normatives.Le but de cette étude était de développer une tâche évaluant la mémoire épisodique verbale chez les personnes âgées de 70 à 89 ans.La tâche (GERIA-12) est basée sur la même procédure que l’épreuve de rappel libre/rappel indicé à 16 items mais ne comporte que 12 items et 2 essais d’apprentissage. Afin d’évaluer les processus de consolidation, des rappels différés ont lieu après 20 minutes et 24 h. L’étalonnage a porté sur 220 personnes âgées de 70 à 89 ans, issues de 3 niveaux de scolarité différents.Les performances diminuent avec l’âge et augmentent avec le niveau scolaire. L’étalonnage a été effectué avec la méthode Barona pour les rappels libres, et sous forme de percentiles pour les rappels totaux, la reconnaissance et les indices (encodage, consolidation et récupération).Cette tâche évalue les processus d’encodage, de stockage et de récupération, et dispose de données normatives précises et d’une procédure plus adaptée aux personnes âgées (facilité d’administration en consultation).Early damage to episodic memory encoding and consolidation processes has been demonstrated in dementia of the Alzheimer type. However, in the domain of verbal episodic memory assessment, there are few diagnostic tools adapted to the old and oldest old as far as ease of administration and accuracy of normative data are concerned. Classic tasks are either too effortful (like the free recall/cued recall of 16 items), not sensitive enough (like the 5 words test), or insufficiently accurate for people above 70 years old in terms of normative data.The aim of this study was to develop a reduced task (in terms of number of items and number of trials) assessing verbal episodic memory in people aged between 70 and 89 years old.The task (GERIA-12) used the same procedure as the RL/RI-16 task but the list comprised only 12 words and there were only 2 learning trials. In order to assess consolidation processes, we included 2 delayed recall trials, one after 20 minutes and the other after 24 hours. We also calculated indexes adapted from the Item-Specific Deficit Approach developed by Wright et al., which has the advantage of providing measures specific to encoding, consolidation and retrieval processes. Standardization was done with data from 220 people aged between 70 and 89 years old and belonging to 3 education levels.We obtained a significant effect of age and education level: scores decrease with age and increase with education. Norms have thus been calculated taking those two variables into consideration. Concerning the standardization, Barona method has been used for free recall scores while percentiles have been used for all other scores (total recall, free recall, encoding, consolidation and retrieval indexes). Normative data are also provided for intrusions and perseverations.This new task allows encoding, consolidation and retrieval processes assessment in older people and has the following advantages: the procedure is more suitable (ease and time of administration), there are accurate normative data for old and oldest old people, and there are normative data for two delayed recalls (at 20 minutes and at 24 hours).

The effect of sleep deprivation on retrieval of emotional memory: a behavioural study using film stimuli.

Although the deleterious effects of sleep deprivation on memory consolidation are well documented, it is still unclear how the facilitating effect of emotions on memory consolidation processes could be modulated by the lack of sleep. In this study, we investigated the effects of sleep deprivation on episodic memory using emotional and non-emotional film stimuli. Forty-eight healthy subjects, divided into a sleep group (SG) and a sleep-deprived (SD) group, completed an Encoding and a Recall phase. Participants in the SD group were sleep deprived the night immediately following the Encoding phase, whereas the control group slept at home. The Recall phase was administered to all subjects 48h after the Encoding. During the Encoding phase, six film clips of different valence (two positive, two neutral and two negative) were presented to the participants. During the Recall phase, episodic memory was assessed by a recognition task. Results showed that the SD group had a lower discrimination memory performance for all stimuli compared to the SG, confirming the deleterious effect of sleep deprivation on episodic memory consolidation. Therefore, lack of sleep severely impairs the consolidation of both emotional and neutral memories, as valence-related effects on memory consolidation were not observed after sleep deprivation.

Non-Interfering Effects of Active Post-Encoding Tasks on Episodic Memory Consolidation in Humans.

So far, studies that investigated interference effects of post-learning processes on episodic memory consolidation in humans have used tasks involving only complex and meaningful information. Such tasks require reallocation of general or encoding-specific resources away from consolidation-relevant activities. The possibility that interference can be elicited using a task that heavily taxes our limited brain resources, but has low semantic and hippocampal related long-term memory processing demands, has never been tested. We address this question by investigating whether consolidation could persist in parallel with an active, encoding-irrelevant, minimally semantic task, regardless of its high resource demands for cognitive processing. We distinguish the impact of such a task on consolidation based on whether it engages resources that are: (1) general/executive, or (2) specific/overlapping with the encoding modality. Our experiments compared subsequent memory performance across two post-encoding consolidation periods: quiet wakeful rest and a cognitively demanding n-Back task. Across six different experiments (total N = 176), we carefully manipulated the design of the n-Back task to target general or specific resources engaged in the ongoing consolidation process. In contrast to previous studies that employed interference tasks involving conceptual stimuli and complex processing demands, we did not find any differences between n-Back and rest conditions on memory performance at delayed test, using both recall and recognition tests. Our results indicate that: (1) quiet, wakeful rest is not a necessary prerequisite for episodic memory consolidation; and (2) post-encoding cognitive engagement does not interfere with memory consolidation when task-performance has minimal semantic and hippocampally-based episodic memory processing demands. We discuss our findings with reference to resource and reactivation-led interference theories.

Sleep Supports the Slow Abstraction of Gist from Visual Perceptual Memories

Sleep benefits the consolidation of individual episodic memories. In the long run, however, it may be more efficient to retain the abstract gist of single, related memories, which can be generalized to similar instances in the future. While episodic memory is enhanced after one night of sleep, effective gist abstraction is thought to require multiple nights. We tested this hypothesis using a visual Deese-Roediger-McDermott paradigm, examining gist abstraction and episodic-like memory consolidation after 20 min, after 10 hours, as well as after one year of retention. While after 10 hours, sleep enhanced episodic-like memory for single items, it did not affect gist abstraction. One year later, however, we found significant gist knowledge only if subjects had slept immediately after encoding, while there was no residual memory for individual items. These findings indicate that sleep after learning strengthens episodic-like memories in the short term and facilitates long-term gist abstraction.

Resting-state fMRI evidence for early episodic memory consolidation: effects of age

Aging-related episodic memory decline is often attributed to insufficient encoding of new information, although the underlying neural processes remain elusive. We here tested the hypothesis that impaired memory consolidation contributes to aging-related memory decline. To this end, we used resting state functional magnetic resonance imaging in healthy young and older adults and investigated neural network connectivity underlying episodic memory consolidation and the effects of aging thereon. During postencoding rest, connectivity increased in subregions of temporobasal and temporo-occipital networks but decreased in a precuneal network. These connectivity changes predicted subsequent memory performance thereby constituting functional correlates of early memory consolidation. Furthermore, these consolidation-related regional connectivity changes partially overlapped with encoding-related neural activity changes, suggesting a close relationship between encoding- and consolidation-related activity. Older when compared to young participants failed to increase connectivity in the right lingual gyrus as part of an extended default mode network during consolidation, thereby providing a functional correlate for spatial contextual memory deficits. In conclusion, results are consistent with previous reports of persistent activity in regions mediating memory encoding as a core mechanism underlying episodic memory consolidation. Our data extend previous findings suggesting that aging-related memory decline results from a reduction of consolidation processes.

Decoupling of large-scale brain networks supports the consolidation of durable episodic memories

At a large scale, the human brain is organized into modules of interconnected regions, some of which play opposing roles in supporting cognition. In particular, the Default-Mode Network (DMN) has been linked to operations on internal representations, while task-positive networks are recruited during interactions with the external world. Here, we test the hypothesis that the generation of durable long-term memories depends on optimal recruitment of such antagonistic large-scale networks. As long-term memory consolidation is a process ongoing for days and weeks after an experience, we propose that individuals characterized by strong decoupling of the DMN and task-positive networks at rest operate in a mode beneficial for the long-term stabilization of episodic memories. To capture network connectivity unaffected by transient task demands and representative of brain behavior outside an experimental setting, 87 participants were scanned during rest before performing an associative encoding task. To link individual resting-state functional connectivity patterns to time-dependent memory consolidation processes, participants were given an unannounced memory test, either after a brief interval or after a retention period of ~6 weeks. We found that participants with a resting state characterized by high synchronicity in a DMN-centered network system and low synchronicity between task-positive networks showed superior recollection weeks after encoding. These relationships were not observed for information probed only hours after encoding. Furthermore, the two network systems were found to be anticorrelated. Our results suggest that this memory-relevant antagonism between DMN and task-positive networks is maintained through complex regulatory interactions between the systems.

Alzheimer's disease: Lost memories found.

Enhancing synaptic connections between neurons in the brain's hippocampus that are normally activated during memory formation rescues memory deficits in a mouse model of early Alzheimer's disease. See Letter p.508 The hippocampus plays a crucial role in the encoding, consolidation, and retrieval of episodic memories, which are the first to go missing in the early stages of Alzheimer's disease. This study shows in transgenic mouse models of early Alzheimer's disease that the amnesia is due to a defect in memory retrieval rather than in encoding. Importantly, the 'forgotten' memories can be rescued by direct activation of hippocampal dentate gyrus engram cells, and the amnesia correlates with a progressive reduction of dentate gyrus engram cell spine density. The authors suggest that selective rescue of dentate gyrus engram cells and their spine density may lead to new therapeutic strategies to recoup lost memories in early Alzheimer's disease.

A Unified Dynamic Model for Learning, Replay, and Sharp-Wave/Ripples.

During phases of rest and sleep, the hippocampus, the "memory center" of the brain, generates intermittent patterns of strongly increased overall activity with high-frequency oscillations, the so-called sharp-wave/ripples. We investigate their role in learning and memory processing. They occur together with replay of activity sequences reflecting previous behavior. Developing a unifying computational model, we propose that both phenomena are tightly linked, by mutually generating and supporting each other. The underlying mechanism depends on nonlinear amplification of synchronous inputs that has been prominently found in the hippocampus. Besides assigning sharp-wave/ripples a crucial role for replay generation and thus memory processing, the proposed mechanism also explains their characteristic features, such as the oscillation frequency and the overall wave form.

Dopamine and Consolidation of Episodic Memory: Timing is Everything.

Memory consolidation underpins adaptive behavior and dopaminergic networks may be critical for prolonged, selective information storage. To understand the time course of the dopaminergic contribution to memory consolidation in humans, here we investigate the effect of dopaminergic medication on recall and recognition in the short and longer term in Parkinson disease (PD). Fifteen people with PD were each tested on or off dopaminergic medication during learning/early consolidation (Day 1) and/or late consolidation (Day 2). Fifteen age-matched healthy participants were tested only once. On Day 1 participants learned new information, and early episodic memory was tested after 30 min. Then on Day 2, recall and recognition were retested after a 24-hr delay. Participants on medication on Day 1 recalled less information at 30 min and 24 hr. In contrast, patients on medication on Day 2 (8-24 hr after learning) recalled more information at 24 hr than those off medication. Although recognition sensitivity was unaffected by medication, response bias was dependent on dopaminergic state: Medication during learning induced a more liberal bias 24 hr later, whereas patients off medication during learning were more conservative responders 24 hr later. We use computational modeling to propose possible mechanisms for this change in response bias. In summary, dopaminergic medication in PD patients during learning impairs early consolidation of episodic memory and makes delayed responses more liberal, but enhances late memory consolidation presumably through a dopamine-dependent consolidation pathway that may be active during sleep.

Bi-frontal transcranial alternating current stimulation in the ripple range reduced overnight forgetting

High frequency oscillations in the hippocampal structures recorded during sleep have been proved to be essential for long-term episodic memory consolidation in both animals and in humans. The aim of this study was to test if transcranial Alternating Current Stimulation (tACS) of the dorsolateral prefrontal cortex (DLPFC) in the hippocampal ripple range, applied bi-frontally during encoding, could modulate declarative memory performance, measured immediately after encoding, and after a night's sleep. An associative word-pair learning test was used. During an evening encoding phase, participants received 1 mA 140 Hz tACS or sham stimulation over both DLPFCs for 10 min while being presented twice with a list of word-pairs. Cued recall performance was investigated 10 min after training and the morning following the training session. Forgetting from evening to morning was observed in the sham condition, but not in the 140 Hz stimulation condition. 140 Hz tACS during encoding may have an effect on the consolidation of declarative material.

Different temporal patterns of memory incorporations into dreams for laboratory and virtual reality experiences: relation to dreamed locus of control

Previous work demonstrates that memories about a target experience are incorporated into dream content according to a U-shaped temporal pattern with a peak of incorporations around 1-2 days following the experience (day-residue effect), a diminution on days 3-4, and a recurrence of incorporations on days 5-7 (dream-lag effect). This temporal pattern has been observed for many types of experiences, but no study has yet investigated either whether the pattern occurs differentially for qualitatively distinct, yet temporally proximal, experiences or whether the pattern also manifests by generic changes not specifically related to the target memory. The current study traced memory source incorporation patterns of two qualitatively distinct but temporally overlapping events: a laboratory overnight stay (LAB), considered to be an interpersonal and passive experience; and a virtual reality maze task (VR), considered to be a solitary and active experience. Hypotheses were that: 1) elements of LAB and VR experiences would be incorporated independently and exhibit different temporal patterns of incorporation into dream content; and 2) these incorporation patterns would be associated with different generic changes in dream content, i.e., a primarily external dreamed Locus of Control (Dream LoC) for dreams incorporating LAB experiences and a primarily internal Dream LoC for dreams incorporating VR experiences. Twenty-six participants each spent 1 night in the sleep laboratory, underwent a VR maze task in the morning, and then kept a home dream log for 10 days. Judges rated dreams for evidence of incorporation of LAB and VR experiences. Results were consistent with expectations: 1) LAB and VR experiences showed independent and opposite temporal incorporation patterns; LAB incorporations showed both day-residue and dream-lag effects whereas VR incorporations revealed a peak only on day 4; 2) Dream LoC scores were more external for Day 1 (peak of LAB incorporations), and more internal for Day 4 (peak of VR incorporations). Different incorporation patterns for these two experiences may reflect separate underlying processes of memory consolidation responding to different types of stimulus events. Further, temporal patterning of both specific dream incorporations and general dream content changes (LoC) may reflect qualitatively different aspects of episodic memory consolidation.