Dynamic neural systems enable adaptive, flexible memories

Motivation and prediction-driven processing of social memoranda

Social semantic memory guides many aspects of behavior. Individuals rely on acquired and inferred knowledge about personal characteristics and group membership to predict the behavior and character of social targets. These predictions then determine the expectations from, the behavior in, and the interpretations of social interactions. According to predictive processing accounts, mnemonic and attentional mechanisms should enhance the processing of prediction-violating events. However, empirical findings suggest that prediction-consistent social events are often better remembered. This mini-review integrates recent evidence from social and non-social memory research to highlight the role of motivation in explaining these discrepancies. A particular emphasis is given to the continuous nature of prediction-(in)consistency, the epistemic tendency of perceivers to maintain or update their knowledge, and the dynamic influences of motivation on multiple steps in prediction-driven social memory. The suggested framework provides a coherent outlook of existing work and offers promising future directions to better understand the ebb and flow of social memoranda.

Mystery of the memory engram: History, current knowledge, and unanswered questions

The quest to understand the memory engram has intrigued humans for centuries. Recent technological advances, including genetic labelling, imaging, optogenetic and chemogenetic techniques, have propelled the field of memory research forward. These tools have enabled researchers to create and erase memory components. While these innovative techniques have yielded invaluable insights, they often focus on specific elements of the memory trace. Genetic labelling may rely on a particular immediate early gene as a marker of activity, optogenetics may activate or inhibit one specific type of neuron, and imaging may capture activity snapshots in a given brain region at specific times. Yet, memories are multifaceted, involving diverse arrays of neuronal subpopulations, circuits, and regions that work in concert to create, store, and retrieve information. Consideration of contributions of both excitatory and inhibitory neurons, micro and macro circuits across brain regions, the dynamic nature of active ensembles, and representational drift is crucial for a comprehensive understanding of the complex nature of memory.

Is there selective retroactive memory enhancement in humans?: a meta-analysis

Memory is an adaptive and flexible system that preferentially stores motivationally relevant information. However, in some cases information that is initially irrelevant can become relevant at a later time. The question arises whether and to what extent the memory system can retroactively boost memories of the initially irrelevant information. Experimental studies in animals and humans have provided evidence for such retroactive memory boosting. Additionally, these studies suggest that retroactive memory enhancement (RME) can be selective to the semantic meaning of the material. Nonetheless, recent experimental work could not replicate these findings, posing the question whether the selective RME effect is reliable. To synthesize the available evidence, we conducted meta-analyses of 14 experiments. Although the classical meta-analytic procedure suggested a small selective RME effect, Cohen's dz = 0.16, when accounting for small-study bias using robust Bayesian meta-analysis the null hypothesis was supported, Cohen's dz = 0.02, BF01 = 3.03. Furthermore strong evidence was found for a bias due to small-study effects, BF10 = 11.39. Together, this calls the reliability of a selective RME effect into question.

The Role of Autobiographical Resilience Memories in Emotion Regulation: An Account of Age Differences in Mnemonic and Positive Reappraisal

A lifetime of resilience through emotionally challenging experiences may benefit older adults, lending to emotion regulation mastery with time. Yet the influence of autobiographical experiences on momentary reappraisal, the reinterpretation of negative stimuli as more positive, has never been empirically tested. This online study examined the extent to which associating life memories of resilience with novel negative scenarios enhanced reappraisal efficacy and reduced difficulty to reappraise. Younger and older adults reappraised negative images by associating reappraisals to freely selected autobiographical resilience memories, cued autobiographical resilience memories, or by finding situational silver linings without mnemonic association (control). Changes in image emotional intensity ratings revealed no difference across reappraisal conditions for younger adults, while older adults most effectively down-regulated emotional intensity using the control reappraisal strategy. Older adults found autobiographical memories more helpful for mood regulation and less difficult to implement, and identified greater similarities between novel negative scenarios and their memories than younger adults. Surprisingly, greater similarity between resilience memories and negative images was associated with lower reappraisal efficacy for both age groups. Findings demonstrate the age-equivalent benefits of utilizing reappraisals associated with past narratives of resilience and suggest a sacrifice of immediate hedonic benefit for disproportionately greater subjective benefits with age.

Neural distinctiveness and discriminability underlying unitization and associative memory in aging

Previous work has suggested unitized pairs behave as a single unit and more critically, are processed neurally different than those of associative memories. The current works examines the neural differences between unitization and non-unitized memory using fMRI and multivoxel analyses. Specifically, we examined the differences across face-occupation pairings as a function of whether the pairing was viewed as a person performing the given job (unitized binding) or a person saying they knew someone who had a particular job (non-unitized binding). The results show that at encoding and retrieval, the angular gyrus can discriminate between unitized and non-unitized target trials. Additionally, during encoding, the medial temporal lobe (hippocampus and perirhinal cortex), frontal parietal regions (angular gyrus and medial frontal gyrus) and visual regions (middle occipital cortex) exhibit distinct neural patterns to recollected unitized and non-unitized targets. Furthermore, the perirhinal cortex and medial frontal gyrus show greater neural similarity within subsequently recollected unitized trials compared to non-unitized trials. We conclude that an encoding based strategy to elicit unitization can produce greater associative memory compared to non-unitized trials in older adults. Additionally, when unitized trials are subsequently recollected in the perirhinal cortex older adults show greater neural similarity within unitized trials compared to non-unitized trials.

Measuring flexibility: A text-mining approach

In creativity research, ideational flexibility, the ability to generate ideas by shifting between concepts, has long been the focus of investigation. However, psychometric work to develop measurement procedures for flexibility has generally lagged behind other creativity-relevant constructs such as fluency and originality. Here, we build from extant research to theoretically posit, and then empirically validate, a text-mining based method for measuring flexibility in verbal divergent thinking (DT) responses. The empirical validation of this method is accomplished in two studies. In the first study, we use the verbal form of the Torrance Test of Creative Thinking (TTCT) to demonstrate that our novel flexibility scoring method strongly and positively correlates with traditionally used TTCT flexibility scores. In the second study, we conduct a confirmatory factor analysis using the Alternate Uses Task to show reliability and construct validity of our text-mining based flexibility scoring. In addition, we also examine the relationship between personality facets and flexibility of ideas to provide criterion validity of our scoring methodology. Given the psychometric evidence presented here and the practicality of automated scores, we recommend adopting this new method which provides a less labor-intensive and less costly objective measurement of flexibility.

Sleep as a window to target traumatic memories

Post-traumatic stress disorder (PTSD) is a severe psychiatric disorder in which traumatic memories result in flashbacks and nightmares. With one-third of patients not responding to standard exposure-based psychotherapy, new treatment strategies are needed. Sleep offers a unique time window to enhance therapeutic efficacy. Traumatic memories that are neutralized in therapy need to be stored back into memory (consolidated) during sleep to solidify the treatment effect. New basic research shows that memory consolidation can be enhanced by presenting sounds or scents that were linked to the memory at encoding, again during sleep. This procedure, termed targeted memory reactivation (TMR), has, despite its clinical potential, not been tested in (PTSD) patients. In this narrative review, we explore the potential of TMR as a new sleep-based treatment for PTSD. First we provide the necessary background on the memory and sleep principles underlying PTSD as well as the present applications and conditional factors of TMR. Then, we will discuss the outstanding questions and most promising experimental avenues when testing TMR to treat traumatic memories.

Schematic information influences memory and generalisation behaviour for schema-relevant and -irrelevant information

Schemas modulate memory performance for schema-congruent and -incongruent information. However, it is assumed they do not influence behaviour for information irrelevant to themselves. We assessed memory and generalisation behaviour for information related to an underlying pattern, where a schema could be extracted (schema-relevant), and information that was unrelated and therefore irrelevant to the extracted schema (schema-irrelevant). Using precision measures of long-term memory, where participants learnt associations between words and locations around a circle, we assessed memory and generalisation for schema-relevant and -irrelevant information. Words belonged to two semantic categories: human-made and natural. For one category, word-locations were clustered around one point on the circle (clustered condition), while the other category had word-locations randomly distributed (non-clustered condition). The presence of an underlying pattern in the clustered condition allows for the extraction of a schema that can support both memory and generalisation. At test, participants were presented with old (memory) and new (generalisation) words, requiring them to identify a remembered location or make a best guess. The presence of the clustered pattern modulated memory and generalisation. In the clustered condition, participants placed old and new words in locations consistent with the underlying pattern. In contrast, for the non-clustered condition, participants were less likely to place old and new non-clustered words in locations consistent with the clustered condition. Therefore, we provide evidence that the presence of schematic information modulates memory and generalisation for schema-relevant and -irrelevant information. Our results highlight the need to carefully construct appropriate schema-irrelevant control conditions such that behaviour in these conditions is not modulated by the presence of a schema. Theoretically, models of schema processing need to account for how the presence of schematic information can have consequences for information that is irrelevant to itself.

Dorsolateral Prefrontal Cortex Enables Updating of Established Memories

Updating established memories in light of new information is fundamental for memory to guide future behavior. However, little is known about the brain mechanisms by which existing memories can be updated. Here, we combined functional magnetic resonance imaging and multivariate representational similarity analysis to elucidate the neural mechanisms underlying the updating of consolidated memories. To this end, participants first learned face-city name pairs. Twenty-four hours later, while lying in the MRI scanner, participants were required to update some of these associations, but not others, and to encode entirely new pairs. Updating success was tested again 24 h later. Our results showed increased activity of the dorsolateral prefrontal cortex (dlPFC) specifically during the updating of existing associations that was significantly stronger than when simple retrieval or new encoding was required. The updating-related activity of the dlPFC and its functional connectivity with the hippocampus were directly linked to updating success. Furthermore, neural similarity for updated items was markedly higher in the dlPFC and this increase in dlPFC neural similarity distinguished individuals with high updating performance from those with low updating performance. Together, these findings suggest a key role of the dlPFC, presumably in interaction with the hippocampus, in the updating of established memories.

Investigating the efficacy of the reminder-extinction procedure to disrupt contextual threat memories in humans using immersive Virtual Reality

Upon reactivation, consolidated memories can enter a temporary labile state and require restabilisation, known as reconsolidation. Interventions during this reconsolidation period can disrupt the reactivated memory. However, it is unclear whether different kinds of memory that depend on distinct brain regions all undergo reconsolidation. Evidence for reconsolidation originates from studies assessing amygdala-dependent memories using cue-conditioning paradigms in rodents, which were subsequently replicated in humans. Whilst studies providing evidence for reconsolidation of hippocampus-dependent memories in rodents have predominantly used context conditioning paradigms, studies in humans have used completely different paradigms such as tests for wordlists or stories. Here our objective was to bridge this paradigm gap between rodent and human studies probing reconsolidation of hippocampus-dependent memories. We modified a recently developed immersive Virtual Reality paradigm to test in humans whether contextual threat-conditioned memories can be disrupted by a reminder-extinction procedure that putatively targets reconsolidation. In contrast to our hypothesis, we found comparable recovery of contextual conditioned threat responses, and comparable retention of subjective measures of threat memory, episodic memory and exploration behaviour between the reminder-extinction and standard extinction groups. Our result provide no evidence that a reminder before extinction can prevent the return of context conditioned threat memories in humans.

The role of intrinsic excitability in the evolution of memory: Significance in memory allocation, consolidation, and updating

Memory is a dynamic process that is continuously regulated by both synaptic and intrinsic neural mechanisms. While numerous studies have shown that synaptic plasticity is important in various types and phases of learning and memory, neuronal intrinsic excitability has received relatively less attention, especially regarding the dynamic nature of memory. In this review, we present evidence demonstrating the importance of intrinsic excitability in memory allocation, consolidation, and updating. We also consider the intricate interaction between intrinsic excitability and synaptic plasticity in shaping memory, supporting both memory stability and flexibility.

The update of semantic memories in amnestic mild cognitive impairment

It is still controversial to what extent neocortical consolidated memories are susceptible of change by processes of reconsolidation and transformation throughout experience, and whether the medial temporal lobes are necessary for this update of semantic consolidated memories, as they are for episodic remembering. We hypothesize that patients with amnestic mild cognitive impairment (aMCI) who have deficits in episodic memory may also have difficulties in updating information on added new features of objects. Sixteen participants with aMCI and 20 healthy control participants performed a semantic word-to-picture task, in which they were asked to identify as belonging to a given semantic category NEW objects, that have incorporated novel features, as well as OLD items, semantically and visually SIMILAR items and UNRELATED items. Patients with aMCI made a greater percentage of errors than healthy controls. Participants globally made greater percentages of errors in difficult types of items, namely NEW and SIMILAR, as compared to easier ones, OLD and UNRELATED. Importantly, an item by diagnostic group interaction effect was observed, and post hoc analysis showed that patients with aMCI made a higher percentage of errors than controls in NEW items only. In conclusion, patients with aMCI had a particular difficulty in identifying the NEW items of the word-to-picture task as compared to the control participants, supporting the concept of a flexible and dynamic conceptual knowledge system, involving the update of semantic memories and the integration of new attributes in a constant transformation process, which is impaired in these patients.

How to optimize knowledge construction in the brain

Well-structured knowledge allows us to quickly understand the world around us and make informed decisions to adequately control behavior. Knowledge structures, or schemas, are presumed to aid memory encoding and consolidation of new experiences so we cannot only remember the past, but also guide behavior in the present and predict the future. However, very strong schemas can also lead to unwanted side effects such as false memories and misconceptions. To overcome this overreliance on a schema, we should aim to create robust schemas that are on the one hand strong enough to help to remember and predict, but also malleable enough to avoid such undesirable side effects. This raises the question as to whether there are ways to deliberately influence knowledge construction processes, with the goal to reach such optimally balanced schemas. Here, we will discuss how the mnemonic processes in our brains build long-term knowledge and, more specifically, how different phases of memory formation (encoding, consolidation, retrieval, and reconsolidation) contribute to this schema build-up. We finally provide ways how to best keep a balance between generalized semantic and detailed episodic memories, which can prove very useful in, e.g., educational settings.

The Predictive Processing Model of EMDR

Eye Movement Desensitization and Reprocessing Therapy (EMDR) is an effective treatment for Post-traumatic Stress Disorder (PTSD). The Adaptive Information Processing Model (AIP) guides the development and practice of EMDR. The AIP postulates inadequately processed memory as the foundation of PTSD pathology. Predictive Processing postulates that the primary function of the brain is prediction that serves to anticipate the next moment of experience in order to resist the dissipative force of entropy thus facilitating continued survival. Memory is the primary substrate of prediction, and is optimized by an ongoing process of precision weighted prediction error minimization that refines prediction by updating the memories on which it is based. The Predictive Processing model of EMDR postulates that EMDR facilitates the predictive processing of traumatic memory by overcoming the bias against exploration and evidence accumulation. The EMDR protocol brings the traumatic memory into an active state of re-experiencing. Defensive responding and/or low sensory precision preclude evidence accumulation to test the predictions of the traumatic memory in the present. Sets of therapist guided eye movements repeatedly challenge the bias against evidence accumulation and compel sensory sampling of the benign present. Eye movements reset the theta rhythm organizing the flow of information through the brain, facilitating the deployment of both overt and covert attention, and the mnemonic search for associations. Sampling of sensation does not support the predictions of the traumatic memory resulting in prediction error that the brain then attempts to minimize. The net result is a restoration of the integrity of the rhythmic deployment of attention, a recalibration of sensory precision, and the updating (reconsolidation) of the traumatic memory. Thus one prediction of the model is a decrease in Attention Bias Variability, a core dysfunction in PTSD, following successful treatment with EMDR.

Structural and Functional MRI Evidence for Distinct Medial Temporal and Prefrontal Roles in Context-dependent Relational Memory

Declarative memory is supported by distributed brain networks in which the medial-temporal lobes (MTLs) and pFC serve as important hubs. Identifying the unique and shared contributions of these regions to successful memory performance is an active area of research, and a growing literature suggests that these structures often work together to support declarative memory. Here, we present data from a context-dependent relational memory task in which participants learned that individuals belonged in a single room in each of two buildings. Room assignment was consistent with an underlying contextual rule structure in which male and female participants were assigned to opposite sides of a building and the side assignment switched between buildings. In two experiments, neural correlates of performance on this task were evaluated using multiple neuroimaging tools: diffusion tensor imaging (Experiment 1), magnetic resonance elastography (Experiment 1), and functional MRI (Experiment 2). Structural and functional data from each individual modality provided complementary and consistent evidence that the hippocampus and the adjacent white matter tract (i.e., fornix) supported relational memory, whereas the ventromedial pFC/OFC (vmPFC/OFC) and the white matter tract connecting vmPFC/OFC to MTL (i.e., uncinate fasciculus) supported memory-guided rule use. Together, these data suggest that MTL and pFC structures differentially contribute to and support contextually guided relational memory.

Imaging object-scene relations processing in visible and invisible natural scenes

Integrating objects with their context is a key step in interpreting complex visual scenes. Here, we used functional Magnetic Resonance Imaging (fMRI) while participants viewed visual scenes depicting a person performing an action with an object that was either congruent or incongruent with the scene. Univariate and multivariate analyses revealed different activity for congruent vs. incongruent scenes in the lateral occipital complex, inferior temporal cortex, parahippocampal cortex, and prefrontal cortex. Importantly, and in contrast to previous studies, these activations could not be explained by task-induced conflict. A secondary goal of this study was to examine whether processing of object-context relations could occur in the absence of awareness. We found no evidence for brain activity differentiating between congruent and incongruent invisible masked scenes, which might reflect a genuine lack of activation, or stem from the limitations of our study. Overall, our results provide novel support for the roles of parahippocampal cortex and frontal areas in conscious processing of object-context relations, which cannot be explained by either low-level differences or task demands. Yet they further suggest that brain activity is decreased by visual masking to the point of becoming undetectable with our fMRI protocol.

Electrophysiological mechanisms of human memory consolidation

Consolidation stabilizes memory traces after initial encoding. Rodent studies suggest that memory consolidation depends on replay of stimulus-specific activity patterns during fast hippocampal “ripple” oscillations. Here, we measured replay in intracranial electroencephalography recordings in human epilepsy patients, and related replay to ripples. Stimulus-specific activity was identified using representational similarity analysis and then tracked during waking rest and sleep after encoding. Stimulus-specific gamma (30–90 Hz) activity during early (100–500 ms) and late (500–1200 ms) encoding is spontaneously reactivated during waking state and sleep, independent of later memory. Ripples during nREM sleep, but not during waking state, trigger replay of activity from the late time window specifically for remembered items. Ripple-triggered replay of activity from the early time window during nREM sleep is enhanced for forgotten items. These results provide the first electrophysiological evidence for replay related to memory consolidation in humans, and point to a prominent role of nREM ripple-triggered replay in consolidation processes.

It is believed that fast “ripple” oscillations in the hippocampus play a role in consolidation, a process by which memory traces are stabilized. Here, the authors show that ripples occuring during non-REM sleep trigger “replay” of brain activity associated with previously experienced stimuli.

iPlasticity: Induced juvenile-like plasticity in the adult brain as a mechanism of antidepressants

The network hypothesis of depression proposes that mood disorders reflect problems in information processing within particular neural networks. Antidepressants (AD), including selective serotonin reuptake inhibitors (SSRI), function by gradually improving information processing within these networks. AD have been shown to induce a state of juvenile-like plasticity comparable to that observed during developmental critical periods: Such critical-period-like plasticity allows brain networks to better adapt to extrinsic and intrinsic signals. We have coined this drug-induced state of juvenile-like plasticity 'iPlasticity.' A combination of iPlasticity induced by chronic SSRI treatment together with training, rehabilitation, or psychotherapy improves symptoms of neuropsychiatric disorders and issues underlying the developmentally or genetically malfunctioning networks. We have proposed that iPlasticity might be a critical component of AD action. We have demonstrated that iPlasticity occurs in the visual cortex, fear erasure network, extinction of aggression caused by social isolation, and spatial reversal memory in rodent models. Chronic SSRI treatment is known to promote neurogenesis and to cause dematuration of granule cells in the dentate gyrus and of interneurons, especially parvalbumin interneurons enwrapped by perineuronal nets in the prefrontal cortex, visual cortex, and amygdala. Brain-derived neurotrophic factor (BDNF), via its receptor tropomyosin kinase receptor B, is involved in the processes of synaptic plasticity, including neurogenesis, neuronal differentiation, weight of synapses, and gene regulation of synaptic formation. BDNF can be activated by both chronic SSRI treatment and neuronal activity. Accordingly, the BDNF/tropomyosin kinase receptor B pathway is critical for iPlasticity, but further analyses will be needed to provide mechanical insight into the processes of iPlasticity.

Similarity in functional brain architecture between rest and specific task modes: A model of genetic and environmental contributions to episodic memory

The ability to keep a mental record of specific past events, dubbed episodic memory (EM), is key to lifespan adaptation. Nonetheless, the neural mechanisms underlying its typical inter-individual variability remain poorly understood. To address this issue, we tested whether individual differences in EM could be predicted from levels of functional brain re-organization between rest and task modes relevant to the transformation of perceptual information into mental representations (relational processing, meaning extraction, online maintenance versus updating of bound perceptual features). To probe the trait specificity of our model, we included three additional core mental functions, processing speed, abstract reasoning, and cognitive control. Finally, we investigated the extent to which our proposed model reflected genetic versus environmental contributions to EM variability. Hypotheses were tested by applying graph theoretical analysis and structural equation modeling to resting state and task fMRI data from two samples of participants in the Human Connectome Project (Sample 1: N = 338 unrelated individuals; Sample 2: N = 268 monozygotic vs. dizygotic twins [134 same-sex pairs]). Levels of functional brain reorganization between rest and the scrutinized task modes, particularly relational processing and online maintenance of bound perceptual features, contributed substantially to variations in both EM and abstract reasoning (but not in cognitive control or processing speed) among the younger adults in our sample, implying a substantial neurofunctional overlap, at least during this life stage. Similarity in functional organization between rest and each of the scrutinized task modes drew on distinguishable neural resources and showed differential susceptibility to genetic versus environmental influences. Our results suggest that variability on complex traits, such as EM, is supported by neural mechanisms comprising multiple components, each reflecting a distinct pattern of genetic versus environmental contributions and whose relative importance may vary across typical versus psychopathological development.

Behavioral interventions to eliminate fear responses

Fear memory underlies anxiety-related disorders, including posttraumatic stress disorder (PTSD). PTSD is a fear-based disorder, characterized by difficulties in extinguishing the learned fear response and maintaining extinction. Currently, the first-line treatment for PTSD is exposure therapy, which forms an extinction memory to compete with the original fear memory. However, the extinguished fear often returns under numerous circumstances, suggesting that novel methods are needed to eliminate fear memory or facilitate extinction memory. This review discusses research that targeted extinction and reconsolidation to manipulate fear memory. Recent studies indicate that sleep is an active state that can regulate memory processes. We also discuss the influence of sleep on fear memory. For each manipulation, we briefly summarize the neural mechanisms that have been identified in human studies. Finally, we highlight potential limitations and future directions in the field to better translate existing interventions to clinical settings.

Ventromedial Prefrontal Cortex Is Necessary for Normal Associative Inference and Memory Integration

The ability to flexibly combine existing knowledge in response to novel circumstances is highly adaptive. However, the neural correlates of flexible associative inference are not well characterized. Laboratory tests of associative inference have measured memory for overlapping pairs of studied items (e.g., AB, BC) and for nonstudied pairs with common associates (i.e., AC). Findings from functional neuroimaging and neuropsychology suggest the ventromedial prefrontal cortex (vmPFC) may be necessary for associative inference. Here, we used a neuropsychological approach to test the necessity of vmPFC for successful memory-guided associative inference in humans using an overlapping pairs associative memory task. We predicted that individuals with focal vmPFC damage (n = 5; 3F, 2M) would show impaired inferential memory but intact non-inferential memory. Performance was compared with normal comparison participants (n = 10; 6F, 4M). Participants studied pairs of visually presented objects including overlapping pairs (AB, BC) and nonoverlapping pairs (XY). Participants later completed a three-alternative forced-choice recognition task for studied pairs (AB, BC, XY) and inference pairs (AC). As predicted, the vmPFC group had intact memory for studied pairs but significantly impaired memory for inferential pairs. These results are consistent with the perspective that the vmPFC is necessary for memory-guided associative inference, indicating that the vmPFC is critical for adaptive abilities that require application of existing knowledge to novel circumstances. Additionally, vmPFC damage was associated with unexpectedly reduced memory for AB pairs post-inference, which could potentially reflect retroactive interference. Together, these results reinforce an emerging understanding of a role for the vmPFC in brain networks supporting associative memory processes.SIGNIFICANCE STATEMENT We live in a constantly changing environment, so the ability to adapt our knowledge to support understanding of new circumstances is essential. One important adaptive ability is associative inference which allows us to extract shared features from distinct experiences and relate them. For example, if we see a woman holding a baby, and later see a man holding the same baby, then we might infer that the two adults are a couple. Despite the importance of associative inference, the brain systems necessary for this ability are not known. Here, we report that damage to human ventromedial prefrontal cortex (vmPFC) disproportionately impairs associative inference. Our findings show the necessity of the vmPFC for normal associative inference and memory integration.

The effect of prior knowledge on post-encoding brain connectivity and its relation to subsequent memory

It is known that prior knowledge can facilitate memory acquisition. It is unclear, however, whether prior knowledge can affect post-encoding brain activity to facilitate memory consolidation. In this fMRI study, we asked participants to associate novel houses with famous/nonfamous faces and investigated how associative-encoding tasks with/without prior knowledge differentially affected post-encoding brain connectivity during rest. Besides memory advantages in the famous condition, we found that post-encoding hippocampal connectivity with the fusiform face area (FFA) and ventral-medial-prefrontal cortex (vmPFC) was stronger following encoding of associations with famous than non-famous faces. Importantly, post-encoding functional connectivity between the hippocampus (HPC) and FFA, and between the anterior temporal pole region (aTPL) and posterior perceptual regions (i.e., FFA and the parahippocampal place area), together predicted a large proportion of the variance in subsequent memory performance. This prediction was specific for face-house associative memory, not face/house item memory, and only in the famous condition where prior knowledge was involved. These results support the idea that when prior knowledge is involved, the HPC, vmPFC, and aTPL, which support prior episodic, social-evaluative/schematic, and semantic memories, respectively, continue to interact with each other and posterior perceptual brain regions during the post-encoding rest to facilitate off-line processing of the newly formed memory, and enhance memory consolidation.

A reminder before extinction strengthens episodic memory via reconsolidation but fails to disrupt generalized threat responses

A reminder can temporarily renew flexibility of consolidated memories, referred to as reconsolidation. Pavlovian threat-conditioning studies suggest that a reminder can renew flexibility of threat responses but that episodic memories remain stable. In contrast, outside the threat-conditioning domain, studies testing memory for word lists or stories find that a reminder can renew flexibility of episodic memory. This discrepancy in findings leaves it unclear if episodic memories reconsolidate, or only Pavlovian responses. Here we unite the different approaches in the field and show that a reminder can retroactively strengthen episodic memory for Pavlovian threat-conditioned events, but that, in contrast to threat-conditioning studies with simple sensory stimuli, extinction after a reminder fails to prevent recovery of generalized threat responses. Our results indicate the episodic memories also reconsolidate, allowing strengthening of relevant memories. These findings also suggest that generalized threat responses and episodic memories are less susceptible to be modified by reminder-interventions procedures.

Effects of Prior-Knowledge on Brain Activation and Connectivity During Associative Memory Encoding

Forming new associations is a fundamental process of building our knowledge system. At the brain level, how prior-knowledge influences acquisition of novel associations has not been thoroughly investigated. Based on recent cognitive neuroscience literature on multiple-component memory processing, we hypothesize that prior-knowledge triggers additional evaluative, semantic, or episodic-binding processes, mainly supported by the ventromedial prefrontal cortex (vmPFC), anterior temporal pole (aTPL), and hippocampus (HPC), to facilitate new memory encoding. To test this hypothesis, we scanned 20 human participants with functional magnetic resonance imaging (fMRI) while they associated novel houses with famous or nonfamous faces. Behaviorally, we found beneficial effects of prior-knowledge on associative memory. At the brain level, we found that the vmPFC and HPC, as well as the parahippocampal place area (PPA) and fusiform face area, showed stronger activation when famous faces were involved. The vmPFC, aTPL, HPC, and PPA also exhibited stronger activation when famous faces elicited stronger emotions and memories, and when associations were later recollected. Connectivity analyses also suggested that HPC connectivity with the vmPFC plays a more important role in the famous than nonfamous condition. Taken together, our results suggest that prior-knowledge facilitates new associative encoding by recruiting additional perceptual, evaluative, or associative binding processes.

Translational Approaches Targeting Reconsolidation

Maladaptive learned responses and memories contribute to psychiatric disorders that constitute a significant socio-economic burden. Primary treatment methods teach patients to inhibit maladaptive responses, but do not get rid of the memory itself, which explains why many patients experience a return of symptoms even after initially successful treatment. This highlights the need to discover more persistent and robust techniques to diminish maladaptive learned behaviours. One potentially promising approach is to alter the original memory, as opposed to inhibiting it, by targeting memory reconsolidation. Recent research shows that reactivating an old memory results in a period of memory flexibility and requires restorage, or reconsolidation, for the memory to persist. This reconsolidation period allows a window for modification of a specific old memory. Renewal of memory flexibility following reactivation holds great clinical potential as it enables targeting reconsolidation and changing of specific learned responses and memories that contribute to maladaptive mental states and behaviours. Here, we will review translational research on non-human animals, healthy human subjects, and clinical populations aimed at altering memories by targeting reconsolidation using biological treatments (electrical stimulation, noradrenergic antagonists) or behavioural interference (reactivation-extinction paradigm). Both approaches have been used successfully to modify aversive and appetitive memories, yet effectiveness in treating clinical populations has been limited. We will discuss that memory flexibility depends on the type of memory tested and the brain regions that underlie specific types of memory. Further, when and how we can most effectively reactivate a memory and induce flexibility is largely unclear. Finally, the development of drugs that can target reconsolidation and are safe for use in humans would optimize cross-species translations. Increasing the understanding of the mechanism and limitations of memory flexibility upon reactivation should help optimize efficacy of treatments for psychiatric patients.

Hippocampal Mismatch Signals Are Modulated by the Strength of Neural Predictions and Their Similarity to Outcomes

The hippocampus is thought to compare predicted events with current perceptual input, generating a mismatch signal when predictions are violated. However, most prior studies have only inferred when predictions occur without measuring them directly. Moreover, an important but unresolved question is whether hippocampal mismatch signals are modulated by the degree to which predictions differ from outcomes. Here, we conducted a human fMRI study in which subjects repeatedly studied various word-picture pairs, learning to predict particular pictures (outcomes) from the words (cues). After initial learning, a subset of cues was paired with a novel, unexpected outcome, whereas other cues continued to predict the same outcome. Critically, when outcomes changed, the new outcome was either "near" to the predicted outcome (same visual category as the predicted picture) or "far" from the predicted outcome (different visual category). Using multivoxel pattern analysis, we indexed cue-evoked reactivation (prediction) within neocortical areas and related these trial-by-trial measures of prediction strength to univariate hippocampal responses to the outcomes. We found that prediction strength positively modulated hippocampal responses to unexpected outcomes, particularly when unexpected outcomes were close, but not identical, to the prediction. Hippocampal responses to unexpected outcomes were also associated with a tradeoff in performance during a subsequent memory test: relatively faster retrieval of new (updated) associations, but relatively slower retrieval of the original (older) associations. Together, these results indicate that hippocampal mismatch signals reflect a comparison between active predictions and current outcomes and that these signals are most robust when predictions are similar, but not identical, to outcomes.

SIGNIFICANCE STATEMENT

Although the hippocampus is widely thought to signal "mismatches" between memory-based predictions and outcomes, previous research has not linked hippocampal mismatch signals directly to neural measures of prediction strength. Here, we show that hippocampal mismatch signals increase as a function of the strength of predictions in neocortical regions. This increase in hippocampal mismatch signals was particularly robust when outcomes were similar, but not identical, to predictions. These results indicate that hippocampal mismatch signals are driven by both the active generation of predictions and the similarity between predictions and outcomes.

How Administration of the Beta-Blocker Propranolol Before Extinction can Prevent the Return of Fear

Combining beta-blockers with exposure therapy has been advocated to reduce fear, yet experimental studies combining beta-blockers with memory reactivation have had contradictory results. We explored how beta-blockade might affect the course of safety learning and the subsequent return of fear in a double-blind placebo-controlled functional magnetic resonance imaging study in humans (N=46). A single dose of propranolol before extinction learning caused a loss of conditioned fear responses, and prevented the subsequent return of fear and decreased explicit memory for the fearful events in the absence of drug. Fear-related neural responses were persistently attenuated in the dorsal medial prefrontal cortex (dmPFC), increased in the hippocampus 24 h later, and correlated with individual behavioral indices of fear. Prediction error-related responses in the ventral striatum persisted during beta-blockade. We suggest that this pattern of results is most consistent with a model where beta-blockade can prevent the return of fear by (i) reducing retrieval of fear memory, via the dmPFC and (ii) increasing contextual safety learning, via the hippocampus. Our findings suggest that retrieval of fear memory and contextual safety learning form potential mnemonic target mechanisms to optimize exposure-based therapy with beta-blockers.

Neural correlates of event clusters in past and future thoughts: How the brain integrates specific episodes with autobiographical knowledge

When remembering the past or envisioning the future, events often come to mind in organized sequences or stories rather than in isolation from one another. The aim of the present fMRI study was to investigate the neural correlates of such event clusters. Participants were asked to consider pairs of specific past or future events: in one condition, the two events were part of the same event cluster (i.e., they were thematically and/or causally related to each other), whereas in another condition the two events only shared a surface feature (i.e., their location); a third condition was also included, in which the two events were unrelated to each other. The results showed that the processing of past and future events that were part of a same cluster was associated with higher activation in the medial prefrontal cortex (PFC), rostrolateral PFC, and left lateral temporal and parietal regions, compared to the two other conditions. Furthermore, functional connectivity analyses revealed an increased coupling between these cortical regions. These findings suggest that largely similar processes are involved in organizing events in clusters for the past and the future. The medial and rostrolateral PFC might play a pivotal role in mediating the integration of specific events with conceptual autobiographical knowledge 'stored' in more posterior regions. Through this integrative process, this set of brain regions might contribute to the attribution of an overarching meaning to representations of specific past and future events, by contextualizing them with respect to personal goals and general knowledge about one's life story.

Hippocampal-medial prefrontal circuit supports memory updating during learning and post-encoding rest

Learning occurs in the context of existing memories. Encountering new information that relates to prior knowledge may trigger integration, whereby established memories are updated to incorporate new content. Here, we provide a critical test of recent theories suggesting hippocampal (HPC) and medial prefrontal (MPFC) involvement in integration, both during and immediately following encoding. Human participants with established memories for a set of initial (AB) associations underwent fMRI scanning during passive rest and encoding of new related (BC) and unrelated (XY) pairs. We show that HPC-MPFC functional coupling during learning was more predictive of trial-by-trial memory for associations related to prior knowledge relative to unrelated associations. Moreover, the degree to which HPC-MPFC functional coupling was enhanced following overlapping encoding was related to memory integration behavior across participants. We observed a dissociation between anterior and posterior MPFC, with integration signatures during post-encoding rest specifically in the posterior subregion. These results highlight the persistence of integration signatures into post-encoding periods, indicating continued processing of interrelated memories during rest. We also interrogated the coherence of white matter tracts to assess the hypothesis that integration behavior would be related to the integrity of the underlying anatomical pathways. Consistent with our predictions, more coherent HPC-MPFC white matter structure was associated with better performance across participants. This HPC-MPFC circuit also interacted with content-sensitive visual cortex during learning and rest, consistent with reinstatement of prior knowledge to enable updating. These results show that the HPC-MPFC circuit supports on- and offline integration of new content into memory.

A Prefrontal-Hippocampal Comparator for Goal-Directed Behavior: The Intentional Self and Episodic Memory

The hypothesis of this article is that the interactions between the prefrontal cortex and the hippocampus play a critical role in the modulation of goal-directed self-action and the strengthening of episodic memories. We describe various theories that model a comparator function for the hippocampus, and then elaborate the empirical evidence that supports these theories. One theory which describes a prefrontal-hippocampal comparator for voluntary action is emphasized. Action plans are essential for successful goal-directed behavior, and are elaborated by the prefrontal cortex. When an action plan is initiated, the prefrontal cortex transmits an efference copy (or corollary discharge) to the hippocampus where it is stored as a working memory for the action plan (which includes the expected outcomes of the action plan). The hippocampus then serves as a response intention-response outcome working memory comparator. Hippocampal comparator function is enabled by the hippocampal theta rhythm allowing the hippocampus to compare expected action outcomes to actual action outcomes. If the expected and actual outcomes match, the hippocampus transmits a signal to prefrontal cortex which strengthens or consolidates the action plan. If a mismatch occurs, the hippocampus transmits an error signal to the prefrontal cortex which facilitates a reformulation of the action plan, fostering behavioral flexibility and memory updating. The corollary discharge provides the self-referential component to the episodic memory, affording the personal and subjective experience of what behavior was carried out, when it was carried out, and in what context (where) it occurred. Such a perspective can be applied to episodic memory in humans, and episodic-like memory in non-human animal species.

Schematic memory components converge within angular gyrus during retrieval

Mental schemas form associative knowledge structures that can promote the encoding and consolidation of new and related information. Schemas are facilitated by a distributed system that stores components separately, presumably in the form of inter-connected neocortical representations. During retrieval, these components need to be recombined into one representation, but where exactly such recombination takes place is unclear. Thus, we asked where different schema components are neuronally represented and converge during retrieval. Subjects acquired and retrieved two well-controlled, rule-based schema structures during fMRI on consecutive days. Schema retrieval was associated with midline, medial-temporal, and parietal processing. We identified the multi-voxel representations of different schema components, which converged within the angular gyrus during retrieval. Critically, convergence only happened after 24-hour-consolidation and during a transfer test where schema material was applied to novel but related trials. Therefore, the angular gyrus appears to recombine consolidated schema components into one memory representation.

DOI:http://dx.doi.org/10.7554/eLife.09668.001

To make sense of the world around us, we constantly try to work out the relationship of new information to other things that we already know, and sort our knowledge into pre-existing mental frameworks, or “schemas”. This makes learning new things that are related to a schema, as well as remembering this knowledge, easier. The process of making these mental connections is thought to involve an extensive brain network. Separate types of information are stored in different brain regions within this network, yet to link this information together, the brain must combine them into a single representation.

Wagner et al. have now investigated which brain regions are involved in recombining separate information. Human volunteers were trained to interpret the positions or colors of pairs of circles with different rules. The combination of these separate types of information formed a mental schema that could be used as a “weather forecast”. The design of the experiment meant that measuring the brain activity of the volunteers during the task (using a technique called functional magnetic resonance imaging) allowed the brain regions involved in retrieving the different parts of such a schema to be distinguished.

Twenty-four hours later volunteers returned to use the mental schemas that they had learned to predict the weather. Retrieving which weather conditions the circle pairs represented activated a network of regions in the volunteers’ brains. Further analysis revealed that some of these regions showed specific activity patterns in response to remembering information about only one element of the task (for example, only the rules or only the visual information). However, the different aspects of the task all appeared to be integrated by a brain region called the angular gyrus. This suggests that the angular gyrus is responsible for combining separate memory parts and pieces of information into a single representation. It is able to do so by connecting to brain regions that code for such specific aspects, although this only occurs 24 hours after the mental schemas have been established.

Future studies could investigate the result of damage to the angular gyrus: different pieces of information might not be combined, or could result in an incorrect memory during retrieval. Finally, since the angular gyrus has been related to a wealth of different mental processes, it remains a challenge for future research to "converge" these findings and to understand the underlying computations.

DOI:http://dx.doi.org/10.7554/eLife.09668.002

Neural correlates of personal goal processing during episodic future thinking and mind-wandering: An ALE meta-analysis

The ability to imagine the future is a complex mental faculty that depends on an ensemble of cognitive processes supported by an extended set of brain regions. Our aim here was to shed light on one key component of future thinking--personal goal processing--and to determine its neural correlates during both directed and spontaneous forms of thoughts. To address this question, we performed separate ALE meta-analyses of neuroimaging studies of episodic future thinking (EFT), mind-wandering, and personal goal processing, and then investigated the commonalities and differences in brain activity between these three domains. The results showed that the three domains activated a common set of brain regions within the default network and, most notably, the medial prefrontal cortex. This finding suggests that the medial prefrontal cortex mediates the processing of personal goals during both EFT and mind-wandering. Differences in activation were also observed, and notably regions supporting cognitive control processes (the dorsolateral prefrontal cortex) were recruited to a lesser extent during mind-wandering than experimentally directed future thinking, suggesting that different kinds of self-generated thoughts may recruit varying levels of attentional control abilities.

Reconsolidation and the regulation of plasticity: moving beyond memory

Memory reconsolidation is a protein synthesis-dependent process that preserves, in some form, memories that have been destabilized through recall. Reconsolidation is a nearly universal phenomenon, occurring in a diverse array of species and learning tasks. The function of reconsolidation remains unclear but it has been proposed as a mechanism for updating or strengthening memories. Observations of an analog of reconsolidation in vitro and in sensory systems indicate that reconsolidation is unlikely to be a learning-specific phenomenon and may serve a broader function. We propose that reconsolidation arises from the activity-dependent induction of two coincident but opposing processes: the depotentiation and repotentiation of strengthened synapses. These processes suggest that reconsolidation reflects a fundamental mechanism that regulates and preserves synaptic strength.

O après-coup e a reconsolidação da memória

<p>Em diferentes áreas do conhecimento é reconhecido que a memória está sujeita a transformações ao longo do tempo. Partindo desse pressuposto, o objetivo do presente estudo é apresentar uma discussão compreendendo a ideia de que nossas recordações são suscetíveis a transformações e a concepção de <italic>après-coup</italic>. Há o delineamento de algumas ideias acerca da temporalidade em Psicanálise e do mecanismo de reconsolidação da memória segundo as Neurociências. Utilizando-se tais conceitos, evidenciou-se a viabilidade de se pensar uma interlocução entre a Psicanálise e as Neurociências.</p>

Initial investigation of the effects of an experimentally learned schema on spatial associative memory in humans

Networks of interconnected neocortical representations of prior knowledge, "schemas," facilitate memory for congruent information. This facilitation is thought to be mediated by augmented encoding and accelerated consolidation. However, it is less clear how schema affects retrieval. Rodent and human studies to date suggest that schema-related memories are differently retrieved. However, these studies differ substantially as most human studies implement pre-experimental world-knowledge as schemas and tested item or nonspatial associative memory, whereas animal studies have used intraexperimental schemas based on item-location associations within a complex spatial layout that, in humans, could engage more strategic retrieval processes. Here, we developed a paradigm conceptually linked to rodent studies to examine the effects of an experimentally learned spatial associative schema on learning and retrieval of new object-location associations and to investigate the neural mechanisms underlying schema-related retrieval. Extending previous findings, we show that retrieval of schema-defining associations is related to activity along anterior and posterior midline structures and angular gyrus. The existence of such spatial associative schema resulted in more accurate learning and retrieval of new, related associations, and increased time allocated to retrieve these associations. This retrieval was associated with right dorsolateral prefrontal and lateral parietal activity, as well as interactions between the right dorsolateral prefrontal cortex and medial and lateral parietal regions, and between the medial prefrontal cortex and posterior midline regions, supporting the hypothesis that retrieval of new, schema-related object-location associations in humans also involves augmented monitoring and systematic search processes.

State-dependencies of learning across brain scales

Learning is a complex brain function operating on different time scales, from milliseconds to years, which induces enduring changes in brain dynamics. The brain also undergoes continuous “spontaneous” shifts in states, which, amongst others, are characterized by rhythmic activity of various frequencies. Besides the most obvious distinct modes of waking and sleep, wake-associated brain states comprise modulations of vigilance and attention. Recent findings show that certain brain states, particularly during sleep, are essential for learning and memory consolidation. Oscillatory activity plays a crucial role on several spatial scales, for example in plasticity at a synaptic level or in communication across brain areas. However, the underlying mechanisms and computational rules linking brain states and rhythms to learning, though relevant for our understanding of brain function and therapeutic approaches in brain disease, have not yet been elucidated. Here we review known mechanisms of how brain states mediate and modulate learning by their characteristic rhythmic signatures. To understand the critical interplay between brain states, brain rhythms, and learning processes, a wide range of experimental and theoretical work in animal models and human subjects from the single synapse to the large-scale cortical level needs to be integrated. By discussing results from experiments and theoretical approaches, we illuminate new avenues for utilizing neuronal learning mechanisms in developing tools and therapies, e.g., for stroke patients and to devise memory enhancement strategies for the elderly.

Memory integration: neural mechanisms and implications for behavior

Everyday behaviors require a high degree of flexibility, in which prior knowledge is applied to inform behavior in new situations. Such flexibility is thought to be supported in part by memory integration, a process whereby related memories become interconnected in the brain through recruitment of overlapping neuronal populations. Recent advances in cognitive and behavioral neuroscience highlight the importance of a hippocampal-medial prefrontal circuit in memory integration. Emerging evidence suggests that abstracted representations in medial prefrontal cortex guide reactivation of related memories during new encoding events, thus promoting hippocampal integration of related experiences. Moreover, recent work indicates that integrated memories are called upon during novel situations to facilitate a host of behaviors, from spatial navigation to imagination.

New automated procedure to assess context recognition memory in mice

RATIONALE AND OBJECTIVES

Recognition memory is an important aspect of human declarative memory and is one of the routine memory abilities altered in patients with amnestic syndrome and Alzheimer's disease. In rodents, recognition memory has been most widely assessed using the novel object preference paradigm, which exploits the spontaneous preference that animals display for novel objects. Here, we used nose-poke units instead of objects to design a simple automated method for assessing context recognition memory in mice.

METHODS

In the acquisition trial, mice are exposed for the first time to an operant chamber with one blinking nose-poke unit. In the choice session, a novel nonblinking nose-poke unit is inserted into an empty spatial location and the number of nose poking dedicated to each set of nose-poke unit is used as an index of recognition memory.

RESULTS

We report that recognition performance varies as a function of the length of the acquisition period and the retention delay and is sensitive to conventional amnestic treatments. By manipulating the features of the operant chamber during a brief retrieval episode (3-min long), we further demonstrate that reconsolidation of the original contextual memory depends on the magnitude and the type of environmental changes introduced into the familiar spatial environment.

CONCLUSIONS

These results show that the nose-poke recognition task provides a rapid and reliable way for assessing context recognition memory in mice and offers new possibilities for the deciphering of the brain mechanisms governing the reconsolidation process.

High gamma oscillations in medial temporal lobe during overt production of speech and gestures

The study of the production of co-speech gestures (CSGs), i.e., meaningful hand movements that often accompany speech during everyday discourse, provides an important opportunity to investigate the integration of language, action, and memory because of the semantic overlap between gesture movements and speech content. Behavioral studies of CSGs and speech suggest that they have a common base in memory and predict that overt production of both speech and CSGs would be preceded by neural activity related to memory processes. However, to date the neural correlates and timing of CSG production are still largely unknown. In the current study, we addressed these questions with magnetoencephalography and a semantic association paradigm in which participants overtly produced speech or gesture responses that were either meaningfully related to a stimulus or not. Using spectral and beamforming analyses to investigate the neural activity preceding the responses, we found a desynchronization in the beta band (15-25 Hz), which originated 900 ms prior to the onset of speech and was localized to motor and somatosensory regions in the cortex and cerebellum, as well as right inferior frontal gyrus. Beta desynchronization is often seen as an indicator of motor processing and thus reflects motor activity related to the hand movements that gestures add to speech. Furthermore, our results show oscillations in the high gamma band (50-90 Hz), which originated 400 ms prior to speech onset and were localized to the left medial temporal lobe. High gamma oscillations have previously been found to be involved in memory processes and we thus interpret them to be related to contextual association of semantic information in memory. The results of our study show that high gamma oscillations in medial temporal cortex play an important role in the binding of information in human memory during speech and CSG production.

Reduced CHRNA7 expression in C3H mice is associated with increases in hippocampal parvalbumin and glutamate decarboxylase-67 (GAD67) as well as altered levels of GABA(A) receptor subunits

Decreased expression of CHRNA7, the gene encoding the α7(∗) subtype of nicotinic receptor, may contribute to the cognitive dysfunction observed in schizophrenia by disrupting the inhibitory/excitatory balance in the hippocampus. C3H mice with reduced Chrna7 expression have significant reductions in hippocampal α7(∗) receptor density, deficits in hippocampal auditory gating, increased hippocampal activity as well as significant decreases in hippocampal glutamate decarboxylase-65 (GAD65) and γ-aminobutyric acid-A (GABAA) receptor levels. The current study investigated whether altered Chrna7 expression is associated with changes in the levels of parvalbumin, GAD67 and/or GABAA receptor subunits in the hippocampus from male and female C3H Chrna7 wildtype, C3H Chrna7 heterozygous and C3H Chrna7 knockout (KO) mice using quantitative Western immunoblotting. Reduced Chrna7 expression was associated with significant increases in hippocampal parvalbumin and GAD67 and with complex alterations in GABAA receptor subunits. A decrease in α3 subunit protein was seen in both female C3H Chrna7 Het and KO mice while a decrease in α4 subunit protein was also detected in C3H Chrna7 KO mice with no sex difference. In contrast, an increase in δ subunit protein was observed in C3H Chrna7 Het mice while a decrease in this subunit was observed in C3H Chrna7 KO mice, with δ subunit protein levels being greater in males than in females. Finally, an increase in γ2 subunit protein was found in C3H Chrna7 KO mice with the levels of this subunit again being greater in males than in females. The increases in hippocampal parvalbumin and GAD67 observed in C3H Chrna7 mice are contrary to reports of reductions in these proteins in the postmortem hippocampus from schizophrenic individuals. We hypothesize that the disparate results may occur because of the influence of factors other than CHRNA7 that have been found to be abnormal in schizophrenia.

The role of astrocytes in the regulation of synaptic plasticity and memory formation

Astrocytes regulate synaptic transmission and play a role in the formation of new memories, long-term potentiation (LTP), and functional synaptic plasticity. Specifically, astroglial release of glutamate, ATP, and cytokines likely alters the survivability and functioning of newly formed connections. Among these pathways, regulation of glutamate appears to be most directly related to the promotion of LTP, which is highly dependent on the synchronization of synaptic receptors through the regulation of excitatory postsynaptic potentials. Moreover, regulation of postsynaptic glutamate receptors, particularly AMPA receptors, is dependent on signaling by ATP synthesized in astrocytes. Finally, cytokine signaling is also implicated in regulating LTP, but is likely most important in plasticity following tissue damage. Despite the role of these signaling factors in regulating LTP and functional plasticity, an integrative model of these factors has not yet been elucidated. In this review, we seek to summarize the current body of evidence on astrocytic mechanisms for regulation of LTP and functional plasticity, and provide an integrative model of the processes.