Perirhinal-hippocampal connectivity during reactivation is a marker for object-based memory consolidation

Intracranial EEGs evidenced visual object processing in the human medial temporal lobe subregions

The perirhinal cortex (PRC) and parahippocampal cortex (PHC) are core regions along the visual dual-stream. The specific functional roles of the PRC and PHC and their interactions with the downstream hippocampus cortex (HPC) are crucial for understanding visual memory. Our research used human intracranial EEGs to study the neural mechanism of the PRC, PHC, and HPC in visual object encoding. Single-regional function analyses found evidence that the PRC, PHC, and HPC are activated ∼100 ms within the broad-gamma band and that the PRC was more strongly activated than either the PHC or the HPC after an object stimulus. Inter-regional analyses showed strong bidirectional interactions of the PRC with both the PHC and HPC in the low-frequency band, whereas the interactions between the PHC and HPC were not significant. These findings demonstrated the core role of the PRC in encoding visual object information and supported the hypothesis of PRC-HPC-ventral object pathway. The recruitment of the PHC and its interaction with the PRC in visual object encoding also provide new insights beyond the traditional dorsal-stream hypothesis.

Contributions of repeated learning to memory in humans: insights from single-neuron recordings in the hippocampus and amygdala

Despite the well-established phenomenon of improved memory performance through repeated learning, studies investigating the associated neural mechanisms have yielded complex and sometimes contradictory findings, and direct evidence from human neuronal recordings has been lacking. This study employs single-neuron recordings with exceptional spatial-temporal resolution, combined with representational similarity analysis, to explore the neural dynamics within the hippocampus and amygdala during repeated learning. Our results demonstrate that in the hippocampus, repetition enhances both representational specificity and fidelity, with these features predicting learning times. Conversely, the amygdala exhibits heightened representational specificity and fidelity during initial learning but does not show improvement with repetition, suggesting functional specialization of the hippocampus and amygdala during different stages of the learning repetition. Specifically, the hippocampus appears to contribute to sustained engagement necessary for benefiting from repeated learning, while the amygdala may play a role in the representation of novel items. These findings contribute to a comprehensive understanding of the intricate interplay between these brain regions in memory processes. Significance statement  For over a century, understanding how repetition contributes to memory enhancement has captivated researchers, yet direct neuronal evidence has been lacking, with a primary focus on the hippocampus and a neglect of the neighboring amygdala. Employing advanced single-neuron recordings and analytical techniques, this study unveils a nuanced functional specialization within the amygdala-hippocampal circuit during various learning repetition. The results highlight the hippocampus's role in sustaining engagement for improved memory with repetition, contrasting with the amygdala's superior ability in representing novel items. This exploration not only deepens our comprehension of memory enhancement intricacies but also sheds light on potential interventions to optimize learning and memory processes.

The effects of mnemonic variability and spacing on memory over multiple timescales

The memory benefit that arises from distributing learning over time rather than in consecutive sessions is one of the most robust effects in cognitive psychology. While prior work has mainly focused on repeated exposures to the same information, in the real world, mnemonic content is dynamic, with some pieces of information staying stable while others vary. Thus, open questions remain about the efficacy of the spacing effect in the face of variability in the mnemonic content. Here, in two experiments, we investigated the contributions of mnemonic variability and the timescale of spacing intervals, ranging from seconds to days, to long-term memory. For item memory, both mnemonic variability and spacing intervals were beneficial for memory; however, mnemonic variability was greater at shorter spacing intervals. In contrast, for associative memory, repetition rather than mnemonic variability was beneficial for memory, and spacing benefits only emerged in the absence of mnemonic variability. These results highlight a critical role for mnemonic variability and the timescale of spacing intervals in the spacing effect, bringing this classic memory paradigm into more ecologically valid contexts.

Human brain activity and functional connectivity associated with verbal long-term memory consolidation across 1 month

Introduction

Declarative memories are initially dependent on the hippocampus and become stabilized through the neural reorganization of connections between the medial temporal lobe and neocortex. The exact time-course of these neural changes is not well established, although time-dependent changes in retrieval-related brain function can be detected across relatively short time periods in humans (e.g., hours to months).

Methods

In a study involving older adults with normal cognition (N = 24), we investigated changes in brain activity and functional connectivity associated with the long-term memory consolidation of verbal material over one month. Participants studied fact-like, three-word sentences at 1-month, 1-week, 1-day, and 1-hour intervals before a recognition memory test inside an MRI scanner. Old/new recognition with confidence ratings and response times were recorded. We examined whole-brain changes in retrieval-related brain activity, as well as functional connectivity of the hippocampus and ventromedial prefrontal cortex (vmPFC), as memories aged from 1 hour to 1 month. Secondary analyses minimized the effect of confounding factors affected by memory age (i.e., changes in confidence and response time or re-encoding of targets).

Results

Memory accuracy, confidence ratings, and response times changed with memory age. A memory age network was identified where retrieval-related brain activity in cortical regions increased or decreased as a function of memory age. Hippocampal brain activity in an anatomical region of interest decreased with memory age. Importantly, these changes in retrieval-related activity were not confounded with changes in activity related to concomitant changes in behavior or encoding. Exploratory analyses of vmPFC functional connectivity as a function of memory age revealed increased connectivity with the posterior parietal cortex, as well as with the vmPFC itself. In contrast, hippocampal functional connectivity with the vmPFC and orbitofrontal cortex decreased with memory age.

Discussion

The observed changes in retrieval-related brain activity and functional connectivity align with the predictions of standard systems consolidation theory. These results suggest that processes consistent with long-term memory consolidation can be identified over short time periods using fMRI, particularly for verbal material.

Effects of schema on the relationship between post-encoding brain connectivity and subsequent durable memory

Schemas can facilitate memory consolidation. Studies have suggested that interactions between the hippocampus and the ventromedial prefrontal cortex (vmPFC) are important for schema-related memory consolidation. However, in humans, how schema accelerates the consolidation of new information and relates to durable memory remains unclear. To address these knowledge gaps, we used a human analogue of the rodent spatial schema task and resting-state fMRI to investigate how post-encoding brain networks can predict long-term memory performance in different schema conditions. After participants were trained to obtain schema-consistent or schema-inconsistent object-location associations, they learned new object-location associations. The new associations were tested after the post-encoding rest in the scanner and 24 h later outside the scanner. The Bayesian multilevel modelling was applied to analyse the post-encoding brain networks. The results showed that during the post-encoding, stronger vmPFC- anterior hippocampal connectivity was associated with durable memory in the schema-consistent condition, whereas stronger object-selective lateral occipital cortex (LOC)-ventromedial prefrontal connectivity and weaker connectivity inside the default mode network were associated with durable memory in the schema inconsistent condition. In addition, stronger LOC-anterior hippocampal connectivity was associated with memory in both schema conditions. These results shed light on how schemas reconfigure early brain networks, especially the prefrontal-hippocampal and stimuli-relevant cortical networks and influence long-term memory performance.

Mapping the long-term delayed recall-based cortex-hippocampus network constrained by the structural and functional connectome: a case-control multimodal MRI study

Background

Connectome mapping may reveal new treatment targets for patients with neurological and psychiatric diseases. However, the long-term delayed recall based-network with structural and functional connectome is still largely unknown. Our objectives were to (1) identify the long-term delayed recall-based cortex-hippocampus network with structural and functional connectome and (2) investigate its relationships with various cognitive functions, age, and activities of daily living.

Methods

This case-control study enrolled 131 subjects (73 amnestic mild cognitive impairment [aMCI] patients and 58 age- and education-matched healthy controls [HCs]). All subjects completed a neuropsychological battery, activities of daily living assessment, and multimodal magnetic resonance imaging. Nodes of the cortical-hippocampal network related to long-term delayed recall were identified by probabilistic fiber tracking and functional connectivity (FC) analysis. Then, the main and interaction effects of the network on cognitive functions were assessed by a generalized linear model. Finally, the moderating effects of the network on the relationships between long-term delayed recall and clinical features were analyzed by multiple regression and Hayes’ bootstrap method. All the effects of cortex-hippocampus network were analyzed at the connectivity and network levels.

Results

The result of a generalized linear model showed that the bilateral hippocampus, left dorsolateral superior frontal gyrus, right supplementary motor area, left lingual gyrus, left superior occipital gyrus, left superior parietal gyrus, left precuneus, and right temporal pole (superior temporal gyrus) are the left and right cortex-hippocampus network nodes related to long-term delayed recall (P < 0.05). Significant interaction effects were found between the Auditory Verbal Learning Test Part 5 (AVLT 5) scores and global properties of the left cortex-hippocampus network [hierarchy, clustering coefficient, characteristic path length, global efficiency, local efficiency, Sigma and synchronization (P < 0.05 Bonferroni corrected)]. Significant interaction effects were found between the general cognitive function/executive function/language and global properties of the left cortex-hippocampus network [Sigma and synchronization (P < 0.05 Bonferroni corrected)].

Conclusion

This study introduces a novel symptom-based network and describes relationships among cognitive functions, brain function, and age. The cortex–hippocampus network constrained by the structural and functional connectome is closely related to long-term delayed recall.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13195-023-01197-7.

The Relationship Between Default Mode and Dorsal Attention Networks Is Associated With Depressive Disorder Diagnosis and the Strength of Memory Representations Acquired Prior to the Resting State Scan

Previous research indicates that individuals with depressive disorders (DD) have aberrant resting state functional connectivity and may experience memory dysfunction. While resting state functional connectivity may be affected by experiences preceding the resting state scan, little is known about this relationship in individuals with DD. Our study examined this question in the context of object memory. 52 individuals with DD and 45 healthy controls (HC) completed clinical interviews, and a memory encoding task followed by a forced-choice recognition test. A 5-min resting state fMRI scan was administered immediately after the forced-choice task. Resting state networks were identified using group Independent Component Analysis across all participants. A network modeling analysis conducted on 22 networks using FSLNets examined the interaction effect of diagnostic status and memory accuracy on the between-network connectivity. We found that this interaction significantly affected the relationship between the network comprised of the medial prefrontal cortex, posterior cingulate cortex, and hippocampal formation and the network comprised of the inferior temporal, parietal, and prefrontal cortices. A stronger positive correlation between these two networks was observed in individuals with DD who showed higher memory accuracy, while a stronger negative correlation (i.e., anticorrelation) was observed in individuals with DD who showed lower memory accuracy prior to resting state. No such effect was observed for HC. The former network cross-correlated with the default mode network (DMN), and the latter cross-correlated with the dorsal attention network (DAN). Considering that the DMN and DAN typically anticorrelate, we hypothesize that our findings indicate aberrant reactivation and consolidation processes that occur after the task is completed. Such aberrant processes may lead to continuous "replay" of previously learned, but currently irrelevant, information and underlie rumination in depression.

Age-related impairments on the touchscreen paired associates learning (PAL) task in male rats

Discovery research in rodent models of cognitive aging is instrumental for identifying mechanisms of behavioral decline in old age that can be therapeutically targeted. Clinically relevant behavioral paradigms, however, have not been widely employed in aged rats. The current study aimed to bridge this translational gap by testing cognition in a cross-species touchscreen-based platform known as paired-associates learning (PAL) and then utilizing a trial-by-trial behavioral analysis approach. This study found age-related deficits in PAL task acquisition in male rats. Furthermore, trial-by-trial analyses and testing rats on a novel interference version of PAL suggested that age-related impairments were not due to differences in vulnerability to an irrelevant distractor, motivation, or to forgetting. Rather, impairment appeared to arise from vulnerability to accumulating, proactive interference, with aged animals performing worse than younger rats in later trial blocks within a single testing session. The detailed behavioral analysis employed in this study provides new insights into the etiology of age-associated cognitive deficits.

VTA and Anterior Hippocampus Target Dissociable Neocortical Networks for Post-Novelty Enhancements

The detection of novelty indicates changes in the environment and the need to update existing representations. In response to novelty, interactions across the VTA-hippocampal circuit support experience-dependent plasticity in the hippocampus. While theories have broadly suggested plasticity-related changes are also instantiated in the cortex, research has also shown evidence for functional heterogeneity in cortical networks. It therefore remains unclear how the hippocampal-VTA circuit engages cortical networks, and whether novelty targets specific cortical regions or diffuse, large-scale cortical networks. To adjudicate the role of the VTA and hippocampus in cortical network plasticity, we used fMRI to compare resting-state functional coupling before and following exposure to novel scene images in human subjects of both sexes. Functional coupling between right anterior hippocampus and VTA was enhanced following novelty exposure. However, we also found evidence for a double dissociation, with anterior hippocampus and VTA showing distinct patterns of post-novelty functional coupling enhancements, targeting task-relevant regions versus large-scale networks, respectively. Further, significant correlations between these networks and the novelty-related plasticity in the anterior hippocampal-VTA functional network suggest that the central hippocampal-VTA network may facilitate the interactions with the cortex. These findings support an extended model of novelty-induced plasticity, in which novelty elicits plasticity-related changes in both local and global cortical networks.SIGNIFICANCE STATEMENT Novelty detection is critical for adaptive behavior, signaling the need to update existing representations. By engaging the bidirectional hippocampal-VTA circuit, novelty has been shown to induce plasticity-related changes in the hippocampus. However, it remains an open question how novelty targets such plasticity-related changes in cortical networks. We show that anterior hippocampus and VTA target cortical networks at different spatial scales, with respective enhancements in post-novelty functional coupling with a task-relevant cortical region and a large-scale memory network. The results presented here support an extended model of novelty-related plasticity, in which engaging the anterior hippocampal-VTA circuit through novelty exposure propagates cortical plasticity through hippocampal and VTA functional pathways at distinct scales, targeting specific or diffuse cortical networks.

Memory consolidation as an adaptive process

We rely on our long-term memories to guide future behaviors, making it adaptive to prioritize the retention of goal-relevant, salient information in memory. In this review, we discuss findings from rodent and human research to demonstrate that active processes during post-encoding consolidation support the selective stabilization of recent experience into adaptive, long-term memories. Building upon literatures focused on dynamics at the cellular level, we highlight that consolidation also transforms memories at the systems level to support future goal-relevant behavior, resulting in more generalized memory traces in the brain and behavior. We synthesize previous literatures spanning animal research, human cognitive neuroscience, and cognitive psychology to propose an integrative framework for adaptive consolidation by which goal-relevant memoranda are "tagged" for subsequent consolidation, resulting in selective transformations to the structure of memories that support flexible, goal-relevant behaviors.

The interaction of relational encoding and unitization: Effects on medial temporal lobe processing during retrieval

In retrieval of typical episodic memories, recollection leads to retrieval of context details whereas familiarity is only diagnostic for item memory. Unitization is an encoding strategy that allows context details to be processed as item features and therefore increases the involvement of familiarity-based recognition in retrieval of these context details. Relational encoding is a hippocampally-dependent process that stores items and contexts independently. Our previous study Tu and Diana [1] concluded that mixing unitized and non-unitized context details in the same episode reduced the contribution of familiarity to retrieval of any one detail. In the current study, we modified the paradigm by removing visual cues to the context details and the condition-specific blocking during test. Surprisingly, the behavioral data diverged from our 2016 study and indicated that the two manipulated context details in the modified paradigm were processed independently of one another. Neuroimaging data further revealed anterior hippocampal activation was associated with unitization of source information as compared to relational encoding. We also found the predicted increase in bilateral perirhinal cortex activation and decrease in parahippocampal cortex activation during retrieval of unitized color information when compared to relationally-encoded color information. We did not find that same predicted pattern of differences due to unitization of size information.

Abnormalities of Cortical Thickness in Pediatric Mesial Temporal Lobe Epilepsy with Hippocampal Sclerosis

OBJECTIVE

Mesial temporal lobe epilepsy with hippocampal sclerosis (MTLE-HS) is the most common intractable seizure type of pediatric epilepsy, with alterations in the cortex across the whole brain. The aim of this study is to investigate the abnormalities of cortical thickness in pediatric MTLE-HS.

METHODS

Subjects were recruited from Shenzhen Children's Hospital between September 2015 and December 2016. MTLE was confirmed by the experienced neurological physician based on International League Against Epilepsy (ILAE) diagnosis criteria, and structural magnetic resonance imaging (MRI) was performed at 3T for quantitative assessment of cortical thickness. A general linear model with age and gender as covariates was used to examine the vertex-wise differences in cortical thickness between 1) left MTLE-HS (LMTLE-HS) and healthy controls (HC), and 2) right MTLE-HS (RMTLE-HS) and HC. The family-wise error corrected significance threshold was set at P < 0.05. Through a combination of probability and cluster-size thresholding, cluster-wise P values were obtained for the resulting clusters.

RESULTS

13 LMTLE-HS, 6 RMTLE-HS, and 20 age-matched HC were finally enrolled in the study. No significant difference in the mean age (LMTLE-HS vs. HC, p=0.57; RMTLE-HS vs. HC, p=0.39) and gender ratio (LMTLE-HS vs. HC, p=0.24; RMTLE-HS vs. HC, p=0.72) was found between MTLE-HS and HC. In LMTLE-HS, cortical thickness was found significantly decreased in the ipsilateral caudal middle frontal gyrus (p=0.012) and increased in the contralateral inferior temporal gyrus (p=0.020). In RMTLE-HS, cortical thickness significantly decreased in the ipsilateral posterior parietal lobe (superior, p<0.001 and inferior parietal gyrus, p=0.03), the anterior parietal lobe (postcentral gyrus, p=0.006), the posterior frontal lobe (precentral gyrus, p=0.04 and the lateral occipital gyrus, p<0.001), and the contralateral lateral occipital gyrus, middle frontal (p<0.0001) and superior frontal gyrus (p<0.001), and pericalcarine cortex (p=0.020).

CONCLUSION

We detected significant cortical abnormalities in pediatric MTLE-HS patients compared with HC. These cortical abnormalities could be explained by specific pathogenesis in MTLE-HS, and may finally contribute to understanding the intrinsic mechanism of MTLE-HS.

Object Recognition Memory: Distinct Yet Complementary Roles of the Mouse CA1 and Perirhinal Cortex

While the essential contribution of the hippocampus to spatial memory is well established, object recognition memory has been traditionally attributed to the perirhinal cortex (PRh). However, the results of several studies indicate that under specific procedural conditions, temporary or permanent lesions of the hippocampus affect object memory processes as measured in the Spontaneous Object Recognition (SOR) task. The PRh and hippocampus are considered to contribute distinctly to object recognition memory based on memory strength. Allowing mice more, or less, exploration of novel objects during the encoding phase of the task (i.e., sample session), yields stronger, or weaker, object memory, respectively. The current studies employed temporary local inactivation and immunohistochemistry to determine the differential contributions of neuronal activity in PRh and the CA1 region of the hippocampus to strong and weak object memory. Temporary inactivation of the CA1 immediately after the SOR sample session impaired strong object memory but spared weak object memory; while temporary inactivation of PRh post-sample impaired weak object memory but spared strong object memory. Furthermore, mRNA transcription and de novo protein synthesis are required for the consolidation of episodic memory, and activation patterns of immediate early genes (IEGs), such as c-Fos and Arc, are linked to behaviorally triggered neuronal activation and synaptic plasticity. Analyses of c-Fos and Arc protein expression in PRh and CA1 neurons by immunohistochemistry, and of Arc mRNA by qPCR after distinct stages of SOR, provide additional support that strong object memory is dependent on CA1 neuronal activity, while weak object memory is dependent on PRh neuronal activity. Taken together, the results support the view that both PRh and CA1 are required for object memory under distinct conditions. Specifically, our results are consistent with a model that as the mouse begins to explore a novel object, information about it accumulates within PRh, and a weak memory of the object is encoded. If object exploration continues beyond some threshold, strong memory for the event of object exploration is encoded; the consolidation of which is CA1-dependent. These data serve to reconcile the dissension in the literature by demonstrating functional and complementary roles for CA1 and PRh neurons in rodent object memory.

Molecular mechanisms within the dentate gyrus and the perirhinal cortex interact during discrimination of similar nonspatial memories

Differentiating between similar memories is a crucial cognitive function that enables correct episodic memory formation. The ability to separate the components of memories into distinct representations is thought to rely on a computational process known as pattern separation, by which differences are amplified to disambiguate similar events. Although pattern separation has been localized to the dentate gyrus (DG) of the hippocampus and shown to occur in a spatial domain, this cognitive function takes place also during processing of other types of information. In particular, there is some debate on whether the DG participates in pattern separation of nonspatial representations. Considering the classic role of the Prh in the acquisition and storage of object memories in general and tasks with similar features in particular, this cognitive function could rely more heavily on perirhinal regions when object-related information is processed. Here we show that two plasticity-related proteins, BDNF, and Arc, are required in the DG for nonspatial mnemonic differentiation. Moreover, we found that the crucial role of the DG is transient since activity of AMPAR is only required in the Prh but not the DG during differentiated object memory retrieval. Additionally, this memory is not modifiable by postacquisition rhBDNF infusions in the DG that are known to improve memory when given in the Prh. This highlights a differential role of Prh and DG during differentiated object memory consolidation. Additionally, we found that these molecular mechanisms actively interact in the DG and Prh for the formation of distinguishable memories, with infusions of rhBDNF in the Prh being able to rescue mnemonic deficits caused by reduced Arc expression in the DG. These results reveal a complex interaction between plasticity mechanisms in the Prh and DG for nonspatial pattern separation and posit the Prh as the key structure where unique object representations are stored.

Dynamic behavior of the locus coeruleus during arousal-related memory processing in a multi-modal 7T fMRI paradigm

A body of animal and human evidence points to the norepinephrine (NE) locus coeruleus (LC) system in modulating memory for arousing experiences, but whether the LC would recast its role along memory stages remains unknown. Sedation precluded examination of LC dynamics during memory processing in animals. Here, we addressed the contribution of the LC during arousal-associated memory processing through a unique combination of dedicated ultra-high-field LC-imaging methods, a well-established emotional memory task, online physiological and saliva alpha-amylase measurements in young adults. Arousal-related LC activation followed amygdala engagement during encoding. During consolidation and recollection, activation transitioned to hippocampal involvement, reflecting learning and model updating. NE-LC activation is dynamic, plays an arousal-controlling role, and is not sufficient but requires interactions with the amygdala to form adaptive memories of emotional experiences. These findings have implications for understanding contributions of LC dysregulation to disruptions in emotional memory formation, observed in psychiatric and neurocognitive disorders.

Role of the hippocampus in the spacing effect during memory retrieval

It is well known that distributed learning (DL) leads to improved memory performance compared with massed learning (ML) (i.e., spacing effect). However, the extent to which the hippocampus is involved in the spacing effect at shorter and longer retention intervals remains unclear. To address this issue, two groups of participants were asked to encode face-scene pairs at 20-min, 1-day, and 1-month intervals before they were scanned using fMRI during an associative recognition task. The pairs were repeated six times in either a massed (i.e., six times in 1 day) or a distributed (i.e., six times over 3 days, twice per day) manner. The results showed that compared with that in the ML group, the activation of the left hippocampus was stronger in the DL group when the participants retrieved old pairs correctly and rejected new pairs correctly at different retention intervals. In addition, the posterior hippocampus was more strongly activated when the new associations were rejected correctly after DL than ML, especially at the 1-month interval. Hence, our results provide evidence that the hippocampus is involved in better memory performance after DL compared to ML at both shorter and longer retention intervals.

Sleep Spindles Promote the Restructuring of Memory Representations in Ventromedial Prefrontal Cortex through Enhanced Hippocampal-Cortical Functional Connectivity

Memory consolidation is hypothesized to involve the distribution and restructuring of memory representations across hippocampal and cortical regions. Theories suggest that, through extended hippocampal-cortical interactions, cortical ensembles come to represent more integrated, or overlapping, memory traces that prioritize commonalities across related memories. Sleep processes, particularly fast sleep spindles, are thought to support consolidation, but evidence for this relationship has been mostly limited to memory retention benefits. Whether fast spindles provide a mechanism for neural changes hypothesized to support consolidation, including the strengthening of hippocampal-cortical networks and integration across memory representations, remains unclear, as does the specificity of regions involved. Using functional connectivity analyses of human fMRI data (both sexes), we show that fast spindle density during overnight sleep is related to enhanced hippocampal-cortical functional connectivity the next day, when restudying information learned before sleep. Spindle density modulated connectivity in distinct hippocampal-cortical networks depending on the category of the consolidated stimuli. Specifically, spindle density correlated with functional connectivity between anterior hippocampus and ventromedial prefrontal cortex (vmPFC) for object-word pairs, and posterior hippocampus and posteromedial cortex for scene-word pairs. Using multivariate pattern analyses, we also show that fast spindle density during postlearning sleep is associated with greater pattern similarity, or representational overlap, across individual object-word memories in vmPFC the next day. Further, the relationship between fast spindle density and representational overlap in vmPFC was mediated by the degree of anterior hippocampal-vmPFC functional connectivity. Together, these results suggest that fast spindles support the network distribution of memory traces, potentially restructuring memory representations in vmPFC.SIGNIFICANCE STATEMENT How new experiences are transformed into long-term memories remains a fundamental question for neuroscience research. Theories suggest that memories are stabilized as they are reorganized in the brain, a process thought to be supported by sleep oscillations, particularly sleep spindles. Although sleep spindles have been associated with benefits in memory retention, it is not well understood how spindles modify neural memory traces. This study found that spindles during overnight sleep correlate with changes in neural memory traces, including enhanced functional connectivity in distinct hippocampal-cortical networks and increased pattern similarity among memories in the cortex. The results provide critical evidence that spindles during overnight sleep may act as a physiological mechanism for the restructuring of neural memory traces.

Differential activation of the medial temporal lobe during item and associative memory across time

Studies have shown that the hippocampus plays a crucial role in associative memory. One central issue is whether the involvement of the hippocampus in associative memory remains stable or declines with the passage of time. In the majority of studies, memory performance declines with delay, confounding attempts at interpreting differences in hippocampal activation over time. To address this issue, we tried to equate behavioral performance as much as possible across time for memory of items and associations separately. After encoding words and word pairs, participants were tested for item and associative memories at four time intervals: 20-min, 1-day, 1-week, and 1-month. The results revealed that MTL activation differed over time for associative and item memories. For associative memory, the activation of the anterior hippocampus decreased from 20-min to 1-day then remained stable, whereas in the posterior hippocampus, the activation was comparable for different time intervals when old pairs were correctly retrieved. The hippocampal activation also remained stable when recombined pairs were correctly rejected. As this condition controls for familiarity of the individual items, correct performance depends only on associative memory. For item memory, hippocampal activation declined progressively from 20-min to 1-week and remained stable afterwards. By contrast, the activation in the perirhinal/entorhinal cortex increased over time irrespective of item and associative memories. Drawing on Tulving's distinction between recollection and familiarity, we interpret this pattern of results in accordance with Trace Transformation Theory, which states that as memories are transformed with time and experience, the neural structures mediating item and associative memories will vary according to the underlying representations to which the memories have been transformed.

Hippocampal Engagement during Recall Depends on Memory Content

The hippocampus is considered pivotal to recall, allowing retrieval of information not available in the immediate environment. In contrast, neocortex is thought to signal familiarity, contributing to recall only when called upon by the hippocampus. However, this view is not compatible with representational accounts of memory, which reject the mapping of cognitive processes onto brain regions. According to representational accounts, the hippocampus is not engaged by recall per se, rather it is engaged whenever hippocampal representations are required. To test whether hippocampus is engaged by recall when hippocampal representations are not required, we used functional imaging and a non-associative recall task, with images (objects, scenes) studied in isolation, and image patches as cues. As predicted by a representational account, hippocampal activation was modulated by the content of the recalled memory, increasing during recall of scenes-which are known to be processed by hippocampus-but not during recall of objects. Object recall instead engaged neocortical regions known to be involved in object-processing. Further supporting the representational account, effective connectivity analyses revealed that changes in functional activation during recall were driven by increased information flow from neocortical sites, rather than by the spreading of recall-related activation from hippocampus back to neocortex.

Understanding perirhinal contributions to perception and memory: Evidence through the lens of selective perirhinal damage

Although a memory systems view of the medial temporal lobe (MTL) has been widely influential in understanding how memory processes are implemented, a large body of work across humans and animals has converged on the idea that the MTL can support various other decisions, beyond those involving memory. Specifically, recent work suggests that perception of and memory for visual representations may interact in order to support ongoing cognition. However, given considerations involving lesion profiles in neuropsychological investigations and the correlational nature of fMRI, the precise nature of representations supported by the MTL are not well understood in humans. In the present investigation, three patients with highly specific lesions to MTL were administered a task that taxed perceptual and mnemonic judgments with highly similar face stimuli. A striking double dissociation was observed such that I.R., a patient with a cyst localized to right posterior PRc, displayed a significant impairment in perceptual discriminations, whereas patient A.N., an individual with a lesion in right posterior parahippocampal cortex and the tail of the right hippocampus, and S.D., an individual with bilateral hippocampal damage, did not display impaired performance on the perceptual task. A.N. and S.D. did, however, show impairments in memory performance, whereas patient I.R. did not. These results causally implicate right PRc in successful perceptual oddity judgments, however they suggest that representations supported by PRc are not necessary for correct mnemonic judgments, even in situations of high featural overlap.

Transcranial Current Stimulation During Sleep Facilitates Insight into Temporal Rules, but does not Consolidate Memories of Individual Sequential Experiences

Slow-wave sleep (SWS) is known to contribute to memory consolidation, likely through the reactivation of previously encoded waking experiences. Contemporary studies demonstrate that when auditory or olfactory stimulation is administered during memory encoding and then reapplied during SWS, memory consolidation can be enhanced, an effect that is believed to rely on targeted memory reactivation (TMR) induced by the sensory stimulation. Here, we show that transcranial current stimulations (tCS) during sleep can also be used to induce TMR, resulting in the facilitation of high-level cognitive processes. Participants were exposed to repeating sequences in a realistic 3D immersive environment while being stimulated with particular tCS patterns. A subset of these tCS patterns was then reapplied during sleep stages N2 and SWS coupled to slow oscillations in a closed-loop manner. We found that in contrast to our initial hypothesis, performance for the sequences corresponding to the reapplied tCS patterns was no better than for other sequences that received stimulations only during wake or not at all. In contrast, we found that the more stimulations participants received overnight, the more likely they were to detect temporal regularities governing the learned sequences the following morning, with tCS-induced beta power modulations during sleep mediating this effect.

Differentiation of Human Medial Prefrontal Cortex Activity Underlies Long-Term Resistance to Forgetting in Memory

It is well known that distributing study events over time leads to better memory over long time scales, compared with massing study events together. One explanation for such long-term resistance to forgetting is that distributed study leads to neural differentiation in memory, which supports retrieval of past experiences by disambiguating highly similar memory representations. Neuroanatomical models of episodic memory retrieval propose that the hippocampus and medial prefrontal cortex (MPFC) work together to enable retrieval of behaviorally appropriate memories. However, it is not known how representations in these regions jointly support resistance to forgetting long after initial learning. Using fMRI, we measured differentiation in retrieved memory representations following an extended delay in male and female human participants. After 1 week, word-object associations were better remembered if studied across 2 d (overnight), allowing associations to be learned in distinct temporal contexts, compared with learning within a single day (same day). MPFC retrieval patterns showed differentiation for overnight relative to same day memories, whereas hippocampal patterns reflected associative retrieval success. Overnight memory differentiation in MPFC was higher for associative than item memories and higher than differentiation assessed over a brain-wide set of retrieval-active voxels. The memory-related difference in MPFC pattern differentiation correlated with memory success for overnight learning and with hippocampal-MPFC functional connectivity. These results show that learning information across days leads to differentiated MPFC memory representations, reducing forgetting after 1 week, and suggest this arises from persistent interactions between MPFC and hippocampus.SIGNIFICANCE STATEMENT Neural activity in both the hippocampus and medial prefrontal cortex (MPFC) has been linked to memory-related representations, but prior work has not examined how these representations support episodic memory retrieval over extended time scales that are characteristic of everyday retrieval. We show that differentiation in MPFC activity 1 week after encoding is higher for retrieved information learned across 2 d compared with within a single day. In hippocampus, differentiation was greater for detailed memory retrieval but was not influenced by whether information had been learned over 1 or 2 d. Differentiation in MPFC predicted behavioral robustness to forgetting and was correlated with hippocampal-MPFC connectivity. The results suggest that context-based differentiation supports robust long-term memory via persistent MPFC-hippocampal interactions.

Hippocampal signatures of awake targeted memory reactivation

Dominant theories of episodic memory propose that a key mechanism of memory consolidation is replay-a process, whereby neural patterns of activation during learning are reinstated during offline post-learning periods. Here, we tested whether key signatures of replay defined by studies in rodents, such as recapitulation of specific memory traces, as well as sequences, are apparent in humans during post-encoding memory reactivation. Thirty participants underwent functional imaging that consisted of interleaved encoding and rest periods. During an offline period of wakeful rest, we biased reactivation towards some memories by presenting sound cues that had previously been associated with particular stimulus sequences. Results showed that targeted hippocampal reactivation was biased towards cued memory sequences and that reactivation signatures preserved the temporal order of particular sequences. Importantly, the biased reactivation was related to differences in subsequent memory, suggesting that preferential reactivation may be a mechanism by which specific memory traces can be strengthened for enhanced subsequent memory retrieval.

Pretraining hippocampal stimulation of melatonin type 2 receptors can improve memory acquisition in rats

BACKGROUND

Learning and memory are among the most important cognitive functions of the brain. Melatonin receptor type 2 (MT2R) is located in the hippocampus and participates in learning and memory processes. In the present study, we examined the role of hippocampal MT2R activation in the acquisition, consolidation, and retrieval of learning and memory in novel object recognition (NOR) and passive avoidance (PA) tasks.

METHODS

IIK7 (0.03, 0.3, and 3 μg/μl/side), as a selective MT2R agonist, or vehicle was injected bilaterally into the dentate gyrus (DG) region of the hippocampus in rats five minutes before training, immediately after training, and five minutes before the retrieval-behavioral tasks, respectively. The discrimination index (DI) was measured in the NOR task, while step-through latency in acquisition (STLa), number of trials to acquisition (NOT), step-through latency in the retention trial (STLr), and time spent in the dark compartment (TDC) were determined in the PA task.

RESULTS

The pretraining intrahippocampal injection of IIK7 at all doses significantly improved acquisition in the PA task. On the other hand, the posttraining intrahippocampal administration of IIK7 had no significant effects on consolidation. The preretrieval intrahippocampal injection of IIK7 at different doses attenuated the retrieval of memory. However, the NOR data showed that the intrahippocampal injection of IIK7 at different doses had no significant effects on the acquisition, consolidation, or retrieval in this task.

DISCUSSION

Based on the findings, stimulation of MT2R could improve acquisition, whereas it had no effects on consolidation. It could impair retrieval in the PA task, while it had no effects on object recognition in rats.

Ameliorating Episodic Memory Deficits in a Young Adult With Developmental (Congenital) Amnesia

OBJECTIVES

Although the spacing effect has been investigated extensively in a variety of populations, few studies have focused on individuals with hippocampal amnesia and none, to our knowledge, have investigated differences in performance as a function of spacing schedule in these cases. In the current study, we investigated the benefit of expanding and equal-interval, compared to massed, spacing schedules in a developmental amnesic person, H.C., who shows congenitally based abnormal development of the hippocampal memory system.

METHODS

Given the possibility of plasticity and reorganization in the developing brain, we investigated whether H.C. would benefit more from an expanding versus equal-interval schedule using a continuous recognition paradigm, even though this task has been shown to recruit structures within the medial temporal lobe, including the hippocampus.

RESULTS

H.C. and matched controls both showed a clear spacing effect, although neither group benefited more from an equal-interval or expanding spacing schedule.

CONCLUSIONS

The results of the current study show that the spacing effect is an effective and clinically meaningful memory intervention technique that may be applied to clinical conditions known to affect hippocampal function and episodic memory early in life. (JINS, 2018, 24, 1003-1012).

Functional connectivity in category-selective brain networks after encoding predicts subsequent memory

Activity in category selective regions of the temporal and parietal lobes during encoding has been associated with subsequent memory for face and scene stimuli. Reactivation theories of memory consolidation predict that after encoding connectivity between these category-selective regions and the hippocampus should be modulated and predict recognition memory. However, support for this proposal has been limited in humans. Here, participants completed a resting-state functional MRI (fMRI) scan, followed by face- and place-encoding tasks, followed by another resting-state fMRI scan during which they were asked to think about the stimuli they had previously encountered. Individual differences in face recognition memory were predicted by the degree to which connectivity between face-responsive regions of the fusiform gyrus and perirhinal cortex increased following the face-encoding task. In contrast, individual differences in scene recognition were predicted by connectivity between the hippocampus and a scene-selective region of the retrosplenial cortex before and after the place-encoding task. Our results provide novel evidence for category specificity in the neural mechanisms supporting memory consolidation.

Decision-making Increases Episodic Memory via Postencoding Consolidation

The ability for individuals to actively make decisions engages regions within the mesolimbic system and enhances memory for chosen items. In other behavioral contexts, mesolimbic engagement has been shown to enhance episodic memory by supporting consolidation. However, research has yet to investigate how consolidation may support interactions between decision-making and episodic memory. Across two studies, participants encoded items that were covered by occluder screens and could either actively decide which of two items to uncover or an item was preselected by the experimenter. In Study 1, we show that active decision-making reduces forgetting rates across an immediate and 24-hr memory test, a behavioral marker of consolidation. In Study 2, we use functional neuroimaging to characterize putative neural markers of memory consolidation by measuring interactions between the hippocampus and perirhinal cortex (PRC) during a postencoding period that reexposed participants to elements of the decision-making context without exposing them to memoranda. We show that choice-related striatal engagement is associated with increased postencoding hippocampal-PRC interactions. Finally, we show that a previous reported relationship between choice-related striatal engagement and long-term memory is accounted for by these postencoding hippocampal-PRC interactions. Together, these findings support a model by which actively deciding to encode information enhances memory consolidation to preserve episodic memory for outcomes, a process that may be facilitated by reexposure to the original decision-making context.

Effects of Repetition Learning on Associative Recognition Over Time: Role of the Hippocampus and Prefrontal Cortex

When stimuli are learned by repetition, they are remembered better and retained for a longer time. However, current findings are lacking as to whether the medial temporal lobe (MTL) and cortical regions are involved in the learning effect when subjects retrieve associative memory, and whether their activations differentially change over time due to learning experience. To address these issues, we designed an fMRI experiment in which face-scene pairs were learned once (L1) or six times (L6). Subjects learned the pairs at four retention intervals, 30-min, 1-day, 1-week and 1-month, after which they finished an associative recognition task in the scanner. The results showed that compared to learning once, learning six times led to stronger activation in the hippocampus, but weaker activation in the perirhinal cortex (PRC) as well as anterior ventrolateral prefrontal cortex (vLPFC). In addition, the hippocampal activation was positively correlated with that of the parahippocampal place area (PPA) and negatively correlated with that of the vLPFC when the L6 group was compared to the L1 group. The hippocampal activation decreased over time after L1 but remained stable after L6. These results clarified how the hippocampus and cortical regions interacted to support associative memory after different learning experiences.

Polygalae Radix Extract Prevents Axonal Degeneration and Memory Deficits in a Transgenic Mouse Model of Alzheimer's Disease

Memory impairments in Alzheimer's disease (AD) occur due to degenerated axons and disrupted neural networks. Since only limited recovery is possible after the destruction of neural networks, preventing axonal degeneration during the early stages of disease progression is necessary to prevent AD. Polygalae Radix (roots of Polygala tenuifolia; PR) is a traditional herbal medicine used for sedation and amnesia. In this study, we aimed to clarify and analyze the preventive effects of PR against memory deficits in a transgenic AD mouse model, 5XFAD. 5XFAD mice demonstrated memory deficits at the age of 5 months. Thus, the water extract of Polygalae Radix (PR extract) was orally administered to 4-month-old 5XFAD mice that did not show signs of memory impairment. After consecutive administrations for 56 days, the PR extract prevented cognitive deficit and axon degeneration associated with the accumulation of amyloid β (Aβ) plaques in the perirhinal cortex of the 5XFAD mice. PR extract did not influence the formation of Aβ plaques in the brain of the 5XFAD mice. In cultured neurons, the PR extract prevented axonal growth cone collapse and axonal atrophy induced by Aβ. Additionally, it prevented Aβ-induced endocytosis at the growth cone of cultured neurons. Our previous study reported that endocytosis inhibition was enough to prevent Aβ-induced growth cone collapse, axonal degeneration, and memory impairments. Therefore, the PR extract possibly prevented axonal degeneration and memory impairment by inhibiting endocytosis. PR is the first preventive drug candidate for AD that inhibits endocytosis in neurons.

Intact perceptual ability, but impaired familiarity judgment, after neonatal perirhinal lesions in rhesus macaques

The perirhinal cortex is known to support high-level perceptual abilities as well as familiarity judgments that may affect recognition memory. We tested whether poor perceptual abilities or a loss of familiarity judgment contributed to the recognition memory impairments reported earlier in monkeys with PRh lesions received in infancy (Neo-PRh) (Weiss and Bachevalier, 2016; Zeamer et al., 2015). Perceptual abilities were assessed using a version of the Visual Paired Comparison task with black&white (B&W) stimuli, and familiarity judgments were assessed using the Constant Negative task requiring repeated familiarization exposures. Adult monkeys with Neo-PRh lesions were able to recognize B&W stimuli after short delays, suggesting that their perceptual abilities were within the range of control animals. However, the same Neo-PRh monkeys were slower to acquire the Constant Negative task, requiring more exposures to objects before judging them as familiar compared to control animals. Taken together, the data help to account for the differential patterns of functional compensation on previously reported recognition tasks following neonatal versus adult-onset PRh lesions, and provide further support to the view that the PRh is involved in familiarity processes.

Potential avenues for exercise to activate episodic memory-related pathways: a narrative review

Memory function plays an important role in activities of daily living, and consequently, quality and quantity of life. In this narrative review, we discuss the anatomical components of episodic memory, including the structure of the hippocampus and the routes of communication to and from this structure. We also highlight cellular traces of memory, such as the engram cell and pathway. To provide etiological insight, the biological mechanisms of episodic memory are discussed, including factors subserving memory encoding (e.g., cognitive attention, neuroelectrical indices), consolidation (i.e., synaptic and brain systems level), and retrieval (e.g., availability of cues, context-dependent, state-dependent, and cognitive processing). Central to this manuscript, we highlight how exercise may influence each of these aforementioned parameters (e.g., exercise-induced hippocampal growth, synaptic plasticity, and cue retrieval) and then discuss the implications of these findings to enhance and preserve memory function. Collectively, this narrative review briefly summarizes potential mechanisms of episodic memory, and how exercise may activate these mechanistic pathways.

Spacing Repetitions Over Long Timescales: A Review and a Reconsolidation Explanation

Recent accounts of the spacing effect have proposed molecular explanations that explain spacing over short, but not long timescales. In the first half of this paper, we review research on the spacing effect that has employed spaces of 24 h or more across skill-related tasks, language-related tasks and generalization for adults and children. Throughout this review, we distinguish between learning and retention by defining learning (or acquisition) as performance at the end of training and retention as performance after a delay period. Using this distinction, we find age- and task-related differences in the manifestation of the spacing effect over long timescales. In the second half of this paper, we discuss a reconsolidation account of the spacing effect. In particular, we review the evidence that suggests the spacing of repetitions influences the subsequent consolidation and reconsolidation processes; we explain how a reconsolidation account may explain the findings for learning; the inverted-U curve for retention; and compare the reconsolidation account with previous consolidation accounts of the spacing effect.

Word and object recognition during reading acquisition: MEG evidence

Studies on adults suggest that reading-induced brain changes might not be limited to linguistic processes. It is still unclear whether these results can be generalized to reading development. The present study shows to which extent neural responses to verbal and nonverbal stimuli are reorganized while children learn to read. MEG data of thirty Basque children (4-8y) were collected while they were presented with written words, spoken words and visual objects. The evoked fields elicited by the experimental stimuli were compared to their scrambled counterparts. Visual words elicited left posterior (200-300ms) and temporal activations (400-800ms). The size of these effects increased as reading performance improved, suggesting a reorganization of children's visual word responses. Spoken words elicited greater left temporal responses relative to scrambles (300-700ms). No evidence for the influence of reading expertise was observed. Brain responses to objects were greater than to scrambles in bilateral posterior regions (200-500ms). There was a greater left hemisphere involvement as reading errors decreased, suggesting a strengthened verbal decoding of visual configurations with reading acquisition. The present results reveal that learning to read not only influences written word processing, but also affects visual object recognition, suggesting a non-language specific impact of reading on children's neural mechanisms.

Parahippocampal and Entorhinal Resection Extent Predicts Verbal Memory Decline in an Epilepsy Surgery Cohort

The differential contribution of medial-temporal lobe regions to verbal declarative memory is debated within the neuroscience, neuropsychology, and cognitive psychology communities. We evaluate whether the extent of surgical resection within medial-temporal regions predicts longitudinal verbal learning and memory outcomes. This single-center retrospective observational study involved patients with refractory temporal lobe epilepsy undergoing unilateral anterior temporal lobe resection from 2007 to 2015. Thirty-two participants with Engel Class 1 and 2 outcomes were included (14 left, 18 right) and followed for a mean of 2.3 years after surgery (±1.5 years). Participants had baseline and postsurgical neuropsychological testing and high-resolution T1-weighted MRI scans. Postsurgical lesions were manually traced and coregistered to presurgical scans to precisely quantify resection extent of medial-temporal regions. Verbal learning and memory change scores were regressed on hippocampal, entorhinal, and parahippocampal resection volume after accounting for baseline performance. Overall, there were no significant differences in learning and memory change between patients who received left and right anterior temporal lobe resection. After controlling for baseline performance, the extent of left parahippocampal resection accounted for 27% (p = .021) of the variance in verbal short delay free recall. The extent of left entorhinal resection accounted for 37% (p = .004) of the variance in verbal short delay free recall. Our findings highlight the critical role that the left parahippocampal and entorhinal regions play in recall for verbal material.

Age-related impairments in object-place associations are not due to hippocampal dysfunction

Age-associated cognitive decline can reduce an individual's quality of life. As no single neurobiological deficit can account for the wide spectrum of behavioral impairments observed in old age, it is critical to develop an understanding of how interactions between different brain regions change over the life span. The performance of young and aged animals on behaviors that require the hippocampus and cortical regions to interact, however, has not been well characterized. Specifically, the ability to link a spatial location with specific features of a stimulus, such as object identity, relies on the hippocampus, perirhinal and prefrontal cortices. Although aging is associated with dysfunction in each of these brain regions, behavioral measures of functional change within the hippocampus, perirhinal and prefrontal cortices in individual animals are often not correlated. Thus, how dysfunction of a single brain region within this circuit, such as the hippocampus, impacts behaviors that require communication with the perirhinal and prefrontal cortices remains unknown. To address this question, young and aged rats were tested on the interregion dependent object-place paired association task, as well as a hippocampal-dependent test of spatial reference memory. This particular cohort of aged rats did not show deficits on the hippocampal-dependent task, but were significantly impaired at acquiring object-place associations relative to young. These data suggest that behaviors requiring functional connectivity across different regions of the memory network may be particularly sensitive to aging, and can be used to develop models that will clarify the impact of systems-level dysfunction in the elderly.

Effects of learning experience on forgetting rates of item and associative memories

Are associative memories forgotten more quickly than item memories, and does the level of original learning differentially influence forgetting rates? In this study, we addressed these questions by having participants learn single words and word pairs once (Experiment 1), three times (Experiment 2), and six times (Experiment 3) in a massed learning (ML) or a distributed learning (DL) mode. Then they were tested for item and associative recognition separately after four retention intervals: 10 min, 1 d, 1 wk, and 1 mo. The contribution of recollection and familiarity processes were assessed by participants' remember/know judgments. The results showed that for both item and associative memories, across different degrees of learning, recollection decreased significantly and was the main source of forgetting over time, whereas familiarity remained relatively stable over time. Learning multiple times led to slower forgetting at shorter intervals, depending on recollection and familiarity processes. Compared with massed learning, distributed learning (six times) especially benefited associative memory by increasing recollection, leading to slower forgetting at longer intervals. This study highlighted the importance of process contribution and learning experiences in modulating the forgetting rates of item and associative memories. We interpret these results within the framework of a dual factor representational model of forgetting (as noted in a previous study) in which recollection is more prone to decay over time than familiarity.

The Effects of Changing Attention and Context in an Awake Offline Processing Period on Visual Long-Term Memory

There is accumulating evidence that sleep as well as awake offline processing is important for the transformation of new experiences into long-term memory (LTM). Yet much remains to be understood about how various cognitive factors influence the efficiency of awake offline processing. In the present study we investigated how changes in attention and context in the immediate period after exposure to new visual information influences LTM consolidation. After presentation of multiple naturalistic scenes within a working memory paradigm, recognition was assessed 30 min and 24 h later in three groups of subjects. One group of subjects engaged in a focused attention task [the Revised Attentional Network Task (R-ANT)] in the 30 min after exposure to the scenes. Another group of subjects remained in the testing room during the 30 min after scene exposure and engaged in no goal- or task-directed activities. A third group of subjects left the testing room and returned 30 min later. A signal detection analysis revealed no significant differences among the three groups in hits, false alarms, or sensitivity on the 30-min recognition task. At the 24-h recognition test, the group that performed the R-ANT made significantly fewer hits compared to the group that left the testing room and did not perform the attention ask. The group that performed the R-ANT and the group that remained in the testing room during the 30-min post-exposure interval made significantly fewer false alarms on the 24-h recognition test compared to the group that left the testing room. The group that stayed in the testing room and engaged in no goal- or task-directed activities exhibited significantly higher sensitivity (d′) compared to the group that left the testing room and the group that performed the R-ANT task. Staying in the same context after exposure to new information and resting quietly with minimal engagement of attention results in the best ability to distinguish old from novel visual stimuli after 24 h. These findings suggest that changes in attentional demands and context during an immediate post-exposure offline processing interval modulate visual memory consolidation in a subtle but significant manner.

The Functional and Structural Neuroanatomy of Systems Consolidation for Autobiographical and Semantic Memory

It is well established that patients with memory impairment have more difficulty retrieving memories from the recent past relative to the remote past and that damage to the medial temporal lobe (MTL) plays a key role in this pattern of impairment. The precise role of the MTL and how it may interact with other brain regions remains an area of active research. We investigated the role of structures in a memory network that supports remembering. Our chapter focuses on two types of memory: episodic memory and semantic memory. Findings from studies of patients with brain damage and neuroimaging studies in patients and healthy individuals were considered together to identify the functional and structural neuroanatomy of past remembrance.

Object and spatial memory after neonatal perirhinal lesions in monkeys

The contribution of the perirhinal cortex (PRh) to recognition memory is well characterized in adults, yet the same lesions have limited effect on recognition of spatial locations. Here, we assessed whether the same outcomes will follow when perirhinal lesions are performed in infancy. Monkeys with neonatal perirhinal (Neo-PRh) lesions and control animals were tested in three operant recognition tasks as they reached adulthood: Delayed Nonmatching-to-Sample (DNMS) and Object Memory Span (OMS), measuring object recognition, and Spatial Memory Span (SMS), measuring recognition of spatial locations. Although Neo-PRh lesions did not impact acquisition of the DNMS rule, they did impair performance when the delays were extended from 30s to 600s. In contrast, the same neonatal lesions had no impact on either the object or spatial memory span tasks, suggesting that the lesions impacted the maintenance of information across longer delays and not memory capacity. Finally, the magnitude of recognition memory impairment after the Neo-PRh lesions was similar to that previously observed after adult-onset perirhinal lesions, indicating minimal, or no, functional compensation after the early PRh lesions. Overall, the results indicate that the PRh is a cortical structure that is important for the normal development of mechanisms supporting object recognition memory. Its contribution may be relevant to the memory impairment observed with human cases of temporal lobe epilepsy without hippocampal sclerosis, but not to the memory impairment found in developmental amnesia cases.

Rapid memory stabilization by transient theta coherence in the human medial temporal lobe

Presenting stimuli again after presentation of intervening stimuli improves their retention, an effect known as the spacing effect. However, using event-related potentials (ERPs), we had observed that immediate, in comparison to spaced, repetition of pictures induced a positive frontal potential at 200-300 ms. This potential appeared to emanate from the left medial temporal lobe (MTL), a structure critical for memory consolidation. In this study, we tested the behavioral relevance of this signal and explored functional connectivity changes during picture repetition. We obtained high-density electroencephalographic recordings from 14 healthy subjects performing a continuous recognition task where pictures were either repeated immediately or after 9 intervening items. Conventional ERP analysis replicated the positive frontal potential emanating from the left MTL at 250-350 ms in response to immediately repeated stimuli. Connectivity analysis showed that this ERP was associated with increased coherence in the MTL region--left more that right--in the theta-band (3.5-7 Hz) 200-400 ms following immediate, but not spaced, repetition. This increase was stronger in subjects who better recognized immediately repeated stimuli after 30 min. These findings indicate that transient theta-band synchronization between the MTL and the rest of the brain at 200-400 ms reflects a memory stabilizing signal.

Episodic memory in aspects of large-scale brain networks

Understanding human episodic memory in aspects of large-scale brain networks has become one of the central themes in neuroscience over the last decade. Traditionally, episodic memory was regarded as mostly relying on medial temporal lobe (MTL) structures. However, recent studies have suggested involvement of more widely distributed cortical network and the importance of its interactive roles in the memory process. Both direct and indirect neuro-modulations of the memory network have been tried in experimental treatments of memory disorders. In this review, we focus on the functional organization of the MTL and other neocortical areas in episodic memory. Task-related neuroimaging studies together with lesion studies suggested that specific sub-regions of the MTL are responsible for specific components of memory. However, recent studies have emphasized that connectivity within MTL structures and even their network dynamics with other cortical areas are essential in the memory process. Resting-state functional network studies also have revealed that memory function is subserved by not only the MTL system but also a distributed network, particularly the default-mode network (DMN). Furthermore, researchers have begun to investigate memory networks throughout the entire brain not restricted to the specific resting-state network (RSN). Altered patterns of functional connectivity (FC) among distributed brain regions were observed in patients with memory impairments. Recently, studies have shown that brain stimulation may impact memory through modulating functional networks, carrying future implications of a novel interventional therapy for memory impairment.

Functional topography of the human entorhinal cortex

Despite extensive research on the role of the rodent medial and lateral entorhinal cortex (MEC/LEC) in spatial navigation, memory and related disease, their human homologues remain elusive. Here, we combine high-field functional magnetic resonance imaging at 7 T with novel data-driven and model-based analyses to identify corresponding subregions in humans based on the well-known global connectivity fingerprints in rodents and sensitivity to spatial and non-spatial information. We provide evidence for a functional division primarily along the anteroposterior axis. Localising the human homologue of the rodent MEC and LEC has important implications for translating studies on the hippocampo-entorhinal memory system from rodents to humans.

DOI:http://dx.doi.org/10.7554/eLife.06738.001

In the early 1950s, an American named Henry Molaison underwent an experimental type of brain surgery to treat his severe epilepsy. The surgeon removed a region of the brain known as the temporal lobe from both sides of his brain. After the surgery, Molaison's epilepsy was greatly improved, but he was also left with a profound amnesia, unable to form new memories of recent events.

Subsequent experiments, including many with Molaison himself as a subject, have attempted to identify the roles of the various structures within the temporal lobes. The hippocampus—which is involved in memory and spatial navigation—has received the most attention, but in recent years a region called the entorhinal cortex has also come to the fore. Known as the gateway to the hippocampus, the entorhinal cortex relays sensory information from the outer cortex of the brain to the hippocampus.

In rats and mice the entorhinal cortex can be divided into two subregions that have distinct connections to other parts of the temporal lobe and to the rest of the brain. These are the medial entorhinal cortex, which is the subregion nearest the centre of the brain, and the lateral entorhinal cortex, which is to the left or right of the centre.

For many years researchers had assumed that human entorhinal subregions were located simply to the center or to the sides of the brain. However, it was difficult to check this as the entorhinal cortex measures less than 1 cm across, which placed it beyond the reach of most brain-imaging techniques. Now, two independent groups of researchers have used a technique called functional magnetic resonance imaging to show a different picture. The fMRI data—which were collected in a magnetic field of 7 Tesla, rather than the 1.5 Tesla used in previous experiments—reveal that the entorhinal cortex is predominantly divided from front-to-back in humans.

One of the groups—Navarro Schröder, Haak et al.—used three different sets of functional MRI data to show that the human entorhinal cortex has anterior-lateral and posterior-medial subregions. In one of these experiments, functional MRI was used to measure activity across the whole brain as subjects performed a virtual reality task: this task included some components that involved spatial navigation and other components that did not. The other group—Maass, Berron et al.—used the imaging data to show that the pattern of connections between the anterior-lateral subregion and the hippocampus was different to that between the posterior-medial subregion and the hippocampus.

The discovery of these networks in the temporal lobe in humans will help to bridge the gap between studies of memory in rodents and in humans. Given that the lateral entorhinal cortex is one of the first regions to be affected in Alzheimer's disease, identifying the specific properties and roles of these networks could also provide insights into disease mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.06738.002

Brain Events Underlying Episodic Memory Changes in Aging: A Longitudinal Investigation of Structural and Functional Connectivity

Episodic memories are established and maintained by close interplay between hippocampus and other cortical regions, but degradation of a fronto-striatal network has been suggested to be a driving force of memory decline in aging. We wanted to directly address how changes in hippocampal-cortical versus striatal-cortical networks over time impact episodic memory with age. We followed 119 healthy participants (20-83 years) for 3.5 years with repeated tests of episodic verbal memory and magnetic resonance imaging for quantification of functional and structural connectivity and regional brain atrophy. While hippocampal-cortical functional connectivity predicted memory change in young, changes in cortico-striatal functional connectivity were related to change in recall in older adults. Within each age group, effects of functional and structural connectivity were anatomically closely aligned. Interestingly, the relationship between functional connectivity and memory was strongest in the age ranges where the rate of reduction of the relevant brain structure was lowest, implying selective impacts of the different brain events on memory. Together, these findings suggest a partly sequential and partly simultaneous model of brain events underlying cognitive changes in aging, where different functional and structural events are more or less important in various time windows, dismissing a simple uni-factorial view on neurocognitive aging.

Consolidation in older adults depends upon competition between resting-state networks

Memory encoding and retrieval problems are inherent to aging. To date, however, the effect of aging upon the neural correlates of forming memory traces remains poorly understood. Resting-state fMRI connectivity can be used to investigate initial consolidation. We compared within and between network connectivity differences between healthy young and older participants before encoding, after encoding and before retrieval by means of resting-state fMRI. Alterations over time in the between-network connectivity analyses correlated with retrieval performance, whereas within-network connectivity did not: a higher level of negative coupling or competition between the default mode and the executive networks during the after encoding condition was associated with increased retrieval performance in the older adults, but not in the young group. Data suggest that the effective formation of memory traces depends on an age-dependent, dynamic reorganization of the interaction between multiple, large-scale functional networks. Our findings demonstrate that a cross-network based approach can further the understanding of the neural underpinnings of aging-associated memory decline.

Virtual water maze learning in human increases functional connectivity between posterior hippocampus and dorsal caudate

Recent work has demonstrated that functional connectivity between remote brain regions can be modulated by task learning or the performance of an already well-learned task. Here, we investigated the extent to which initial learning and stable performance of a spatial navigation task modulates functional connectivity between subregions of hippocampus and striatum. Subjects actively navigated through a virtual water maze environment and used visual cues to learn the position of a fixed spatial location. Resting-state functional magnetic resonance imaging scans were collected before and after virtual water maze navigation in two scan sessions conducted 1 week apart, with a behavior-only training session in between. There was a large significant reduction in the time taken to intercept the target location during scan session 1 and a small significant reduction during the behavior-only training session. No further reduction was observed during scan session 2. This indicates that scan session 1 represented initial learning and scan session 2 represented stable performance. We observed an increase in functional connectivity between left posterior hippocampus and left dorsal caudate that was specific to scan session 1. Importantly, the magnitude of the increase in functional connectivity was correlated with offline gains in task performance. Our findings suggest cooperative interaction occurs between posterior hippocampus and dorsal caudate during awake rest following the initial phase of spatial navigation learning. Furthermore, we speculate that the increase in functional connectivity observed during awake rest after initial learning might reflect consolidation-related processing.