Hippocampal-neocortical networks differ during encoding and retrieval of relational memory: functional and effective connectivity analyses

Tau trajectory in Alzheimer's disease: Evidence from the connectome-based computational models

Alzheimer's disease (AD) is a progressive neurodegenerative disorder with an impairment of cognition and memory. Current research on connectomics have now related changes in the network organization in AD to the patterns of accumulation and spread of amyloid and tau, providing insights into the neurobiological mechanisms of the disease. In addition, network analysis and modeling focus on particular use of graphs to provide intuition into key organizational principles of brain structure, that stipulate how neural activity propagates along structural connections. The utility of connectome-based computational models aids in early predicting, tracking the progression of biomarker-directed AD neuropathology. In this article, we present a short review of tau trajectory, the connectome changes in tau pathology, and the dependent recent connectome-based computational modelling approaches for tau spreading, reproducing pragmatic findings, and developing significant novel tau targeted therapies.

Hippocampal-cortical functional connectivity during memory encoding and retrieval

Memory encoding and retrieval are critical sub-processes of episodic memory. While the hippocampus is involved in both, less is known about its connectivity with the neocortex during memory processing in humans. This is partially due to variations in demands in common memory tasks, which inevitably recruit cognitive processes other than episodic memory. Conjunctive analysis of data from different tasks with the same core elements of encoding and retrieval can reduce the intrusion of patterns related to subsidiary perceptual and cognitive processing. Leveraging data from two large-scale functional resonance imaging studies with different episodic memory tasks (514 and 237 participants), we identified hippocampal-cortical networks active during memory tasks. Whole-brain functional connectivity maps were similar during resting state, encoding, and retrieval. Anterior and posterior hippocampus had distinct connectivity profiles, which were also stable across resting state and memory tasks. When contrasting encoding and retrieval connectivity, conjunctive encoding-related connectivity was sparse. During retrieval hippocampal connectivity was increased with areas known to be active during recollection, including medial prefrontal, inferior parietal, and parahippocampal cortices. This indicates that the stable functional connectivity of the hippocampus along its longitudinal axis is superposed by increased functional connectivity with the recollection network during retrieval, while auxiliary encoding connectivity likely reflects contextual factors.

Sustained upregulation of widespread hippocampal-neocortical coupling following memory encoding

Systems consolidation of new experiences into lasting episodic memories involves hippocampal-neocortical interactions. Evidence of this process is already observed during early post-encoding rest periods, both as increased hippocampal coupling with task-relevant perceptual regions and reactivation of stimulus-specific patterns following intensive encoding tasks. We investigate the spatial and temporal characteristics of these hippocampally anchored post-encoding neocortical modulations. Eighty-nine adults participated in an experiment consisting of interleaved memory task- and resting-state periods. We observed increased post-encoding functional connectivity between hippocampus and individually localized neocortical regions responsive to stimuli encountered during memory encoding. Post-encoding modulations were manifested as a nearly system-wide upregulation in hippocampal coupling with all major functional networks. The configuration of these extensive modulations resembled hippocampal-neocortical interaction patterns estimated from active encoding operations, suggesting hippocampal post-encoding involvement exceeds perceptual aspects. Reinstatement of encoding patterns was not observed in resting-state scans collected 12 h later, nor when using other candidate seed regions. The similarity in hippocampal functional coupling between online memory encoding and offline post-encoding rest suggests reactivation in humans involves a spectrum of cognitive processes engaged during the experience of an event. There were no age effects, suggesting that upregulation of hippocampal-neocortical connectivity represents a general phenomenon seen across the adult lifespan.

Intrinsic connectivity reveals functionally distinct cortico-hippocampal networks in the human brain

Episodic memory depends on interactions between the hippocampus and interconnected neocortical regions. Here, using data-driven analyses of resting-state functional magnetic resonance imaging (fMRI) data, we identified the networks that interact with the hippocampus-the default mode network (DMN) and a "medial temporal network" (MTN) that included regions in the medial temporal lobe (MTL) and precuneus. We observed that the MTN plays a critical role in connecting the visual network to the DMN and hippocampus. The DMN could be further divided into 3 subnetworks: a "posterior medial" (PM) subnetwork comprised of posterior cingulate and lateral parietal cortices; an "anterior temporal" (AT) subnetwork comprised of regions in the temporopolar and dorsomedial prefrontal cortex; and a "medial prefrontal" (MP) subnetwork comprised of regions primarily in the medial prefrontal cortex (mPFC). These networks vary in their functional connectivity (FC) along the hippocampal long axis and represent different kinds of information during memory-guided decision-making. Finally, a Neurosynth meta-analysis of fMRI studies suggests new hypotheses regarding the functions of the MTN and DMN subnetworks, providing a framework to guide future research on the neural architecture of episodic memory.

Sleep Deprivation Impairs Binding of Information with Its Context

Binding information to its context in long-term memory is critical for many tasks, including memory tasks and decision making. Failure to associate information to its context could be an important aspect of sleep deprivation effects on cognition, but little is known about binding problems from being sleep-deprived at the time of encoding. We studied how sleep deprivation affects binding using a well-established paradigm testing the ability to remember auditorily presented words (items) and their speakers (source context). In a laboratory study, 68 healthy young adults were randomly assigned to total sleep deprivation or a well-rested control condition. Participants completed an affective item and source memory task twice: once after 7h awake during baseline and again 24h later, after nearly 31h awake in the total sleep deprivation condition or 7h awake in the control condition. Participants listened to negative, positive, and neutral words presented by a male or female speaker and were immediately tested for recognition of the words and their respective speakers. Recognition of items declined during sleep deprivation, but even when items were recognized accurately, recognition of their associated sources also declined. Negative items were less bound with their sources than positive or neutral items,but sleep deprivation did not significantly affect this pattern.Our findings indicate that learning while sleep-deprived disrupts the binding of information to its context independent of item valence. Such binding failures may contribute to sleep deprivation effects on tasks requiring the ability to bind new information together in memory.

Deactivation of default-mode network and early suppression of decision-making areas during retrieval period by high-arousing emotions improves performance in verbal working memory task

Emotions affect many aspects of cognition (attention, decision-making, problem solving, conflict resolution, task switching, social cognition, etc.), but the cortical areas or networks through which these effects are achieved are still debatable. In the present study, the effect of emotion on cognition was studied in healthy young individuals (n = 56). Emotions were induced using high-arousing negative, positive, and low-arousing neutral pictures from the International Affective Picture System (IAPS). Sternberg's verbal working memory task was administered at baseline and after each emotion exposure, while high-density EEG was recorded. Cortical sources were calculated using sLORETA in the 500-ms window (for every 100 ms bin) before the response and were compared with baseline. Though the number of correct responses were comparable, reaction times after emotion exposure reduced significantly. Source analysis revealed significant deactivation of default mode network (DMN) areas as well as early deactivation of decision-making areas during Sternberg's task performed after both the negative and positive emotions. This early deactivation, much before the response was made, when compared with baseline suggests that tasks performed under high-arousing emotional states may help in making decisions earlier or faster. We conclude that the exposure to high-arousing emotional stimuli improves verbal working memory by helping in directing the attentional resources toward the task, thus decreasing the decision-making time and further suppressing the DMN areas.

Functional networks underlying item and source memory: shared and distinct network components and age-related differences

Although the medial temporal lobes (MTLs) are critical for both item memory (IM) and source memory (SM), the lateral prefrontal cortex and posterior parietal cortex play a greater role during SM than IM. It is unclear, however, how these differences translate into shared and distinct IM versus SM network components and how these network components vary with age. Within a sample of younger adults (YAs; n = 15, M_age = 19.5 years) and older adults (OAs; n = 40, M_age = 68.6 years), we investigated the functional networks underlying IM and SM. Before functional MRI scanning, participants encoded nouns while making either pleasantness or size judgments. During functional MRI scanning, participants completed IM and SM retrieval tasks. We found that MTL nodes were similarly interconnected among each other during both IM and SM (shared network components) but maintained more intermodule connections during SM (distinct network components). Also, during SM, OAs (compared to YAs) had MTL nodes with more widespread connections. These findings provide a novel viewpoint on neural mechanism differences underlying IM versus SM in YAs and OAs.

Meta-Analysis of the Structural Equation Models' Parameters for the Estimation of Brain Connectivity with fMRI

Structural Equation Models (SEM) is among of the most extensively applied statistical techniques in the study of human behavior in the fields of Neuroscience and Cognitive Neuroscience. This paper reviews the application of SEM to estimate functional and effective connectivity models in work published since 2001. The articles analyzed were compiled from Journal Citation Reports, PsycInfo, Pubmed, and Scopus, after searching with the following keywords: fMRI, SEMs, and Connectivity.

Results: A 100 papers were found, of which 25 were rejected due to a lack of sufficient data on basic aspects of the construction of SEM. The other 75 were included and contained a total of 160 models to analyze, since most papers included more than one model. The analysis of the explained variance (R²) of each model yields an effect of the type of design used, the type of population studied, the type of study, the existence of recursive effects in the model, and the number of paths defined in the model. Along with these comments, a series of recommendations are included for the use of SEM to estimate of functional and effective connectivity models.

Hippocampal Contributions to the Large-Scale Episodic Memory Network Predict Vivid Visual Memories

A common approach in memory research is to isolate the function(s) of individual brain regions, such as the hippocampus, without addressing how those regions interact with the larger network. To investigate the properties of the hippocampus embedded within large-scale networks, we used functional magnetic resonance imaging and graph theory to characterize complex hippocampal interactions during the active retrieval of vivid versus dim visual memories. The study yielded 4 main findings. First, the right hippocampus displayed greater communication efficiency with the network (shorter path length) and became a more convergent structure for information integration (higher centrality measures) for vivid than dim memories. Second, vivid minus dim differences in our graph theory measures of interest were greater in magnitude for the right hippocampus than for any other region in the 90-region network. Moreover, the right hippocampus significantly reorganized its set of direct connections from dim to vivid memory retrieval. Finally, beyond the hippocampus, communication throughout the whole-brain network was more efficient (shorter global path length) for vivid than dim memories. In sum, our findings illustrate how multivariate network analyses can be used to investigate the roles of specific regions within the large-scale network, while also accounting for global network changes.

Mild Cognitive Impairment Is Not "Mild" at All in Altered Activation of Episodic Memory Brain Networks: Evidence from ALE Meta-Analysis

The present study conducted a quantitative meta-analysis aiming at assessing consensus across the functional neuroimaging studies of episodic memory in individuals with amnestic mild cognitive impairment (aMCI) and elucidating consistent activation patterns. An activation likelihood estimation (ALE) was conducted on the functional neuroimaging studies of episodic encoding and retrieval in aMCI individuals published up to March 31, 2015. Analyses covered 24 studies, which yielded 770 distinct foci. Compared to healthy controls, aMCI individuals showed statistically significant consistent activation differences in a widespread episodic memory network, not only in the bilateral medial temporal lobe and prefrontal cortex, but also in the angular gyrus, precunes, posterior cingulate cortex, and even certain more basic structures. The present ALE meta-analysis revealed that the abnormal patterns of widespread episodic memory network indicated that individuals with aMCI may not be completely "mild" in nature.

Old/New Effect of Digital Memory Retrieval in Chinese Dyscalculia

This study reports the neurophysiological and behavioral correlates of digital memory retrieval features in Chinese individuals with and without dyscalculia. A total of 18 children with dyscalculia (ages 11.5-13.5) and 18 controls were tested, and their event-related potentials were digitally recorded simultaneously with behavior measurement. Behavioral data showed that the dyscalculia group had lower hit rates and higher false rates than the control group. The electroencephalography results showed that both groups had a significant old/new effect and that this effect was greater in the control group. In the 300 to 400 ms processing stages, both groups showed significant differences in digital memory retrieval in the frontal regions. In the 400 to 500 and 500 to 600 ms epochs, the old/new effect in the control group was significantly greater than it was in the dyscalculia group at the frontal, central, and parietal regions. In the 600 to 700 ms processing stages, both groups showed significant differences in digital memory retrieval in the frontal, central, parietal, and occipital regions. These results suggest that individuals with dyscalculia exhibit impaired digital memory retrieval. Extraction failure may be an important cause of calculation difficulties.

Using fMRI to understand event construction in developmental amnesia

Recently, neuroimaging and patient-lesion methods have been combined to explain anomalies such as patients' intact performance on tasks on which they would be predicted to perform poorly. In some cases, preserved performance has been attributed to activation of residual tissue within the damaged region. However, activation of remnant tissue can also occur in relation to impaired performance and, thus, may not necessarily correspond to successful recruitment. To constrain these neuroimaging interpretations, what is needed is a paradigm with closely matched conditions that yields intact and impaired performance in the same patient. We investigated this in H.C., an amnesic person with congenital abnormalities of the hippocampus and its connections, who was scanned during remembering and imagining, abilities known to depend on the hippocampus. Specifically, we examined whether differences in activation and/or functional connectivity would explain H.C.'s compromised ability to construct events relating to herself in autobiographical memory (SELF condition) and events relating to personally familiar others (FAMILIAR condition) versus her intact ability to construct events relating to unknown others (UNFAMILIAR condition). Despite behavioral dissociations in H.C., the pattern of activation and functional connectivity supporting her performance was strikingly similar to that of controls across conditions. Most notably, like controls, H.C. showed robust hippocampal activation and functional connectivity to the hippocampus, both when her performance was intact and impaired. Across all conditions, H.C. activated several extra-hippocampal regions to a greater extent than did controls, and modest differences were observed in functional connectivity between extra-hippocampal regions. Taken together, these findings urge caution when drawing conclusions about the functional integrity of a structurally compromised brain region even when it is activated and/or co-activated with other regions.

Patterns of effective connectivity during memory encoding and retrieval differ between patients with mild cognitive impairment and healthy older adults

Previous research has shown that there is considerable overlap in the neural networks mediating successful memory encoding and retrieval. However, little is known about how the relevant human brain regions interact during these distinct phases of memory or how such interactions are affected by memory deficits that characterize mild cognitive impairment (MCI), a condition that often precedes dementia due to Alzheimer's disease. Here we employed multivariate Granger causality analysis using autoregressive modeling of inferred neuronal time series obtained by deconvolving the hemodynamic response function from measured blood oxygenation level-dependent (BOLD) time series data, in order to examine the effective connectivity between brain regions during successful encoding and/or retrieval of object location associations in MCI patients and comparable healthy older adults. During encoding, healthy older adults demonstrated a left hemisphere dominant pattern where the inferior frontal junction, anterior intraparietal sulcus (likely involving the parietal eye fields), and posterior cingulate cortex drove activation in most left hemisphere regions and virtually every right hemisphere region tested. These regions are part of a frontoparietal network that mediates top-down cognitive control and is implicated in successful memory formation. In contrast, in the MCI patients, the right frontal eye field drove activation in every left hemisphere region examined, suggesting reliance on more basic visual search processes. Retrieval in the healthy older adults was primarily driven by the right hippocampus with lesser contributions of the right anterior thalamic nuclei and right inferior frontal sulcus, consistent with theoretical models holding the hippocampus as critical for the successful retrieval of memories. The pattern differed in MCI patients, in whom the right inferior frontal junction and right anterior thalamus drove successful memory retrieval, reflecting the characteristic hippocampal dysfunction of these patients. These findings demonstrate that neural network interactions differ markedly between MCI patients and healthy older adults. Future efforts will investigate the impact of cognitive rehabilitation of memory on these connectivity patterns.

O Procedimento Selective Reminding na Avaliação da Memória e Aprendizagem: Um Levantamento de Estudos Brasileiros

RESUMO Os pesquisadores têm à sua disposição uma variedade de métodos que auxiliam a compreensão dos processos de memória e aprendizagem. O presente artigo tem como objetivo revisitar o procedimento de recordação seletiva, apresentando suas características e variações, e realizar um levantamento de estudos brasileiros que fizeram uso do mesmo. São apresentadas as diferenças entre a recordação seletiva e o procedimento padrão em tarefas de aprendizagem por recordação livre. Constatou-se que, no Brasil, além de diferentes tarefas, são utilizadas formas de aplicações distintas do procedimento. A utilização da recordação seletiva apresenta relevância no estudo da memória e da aprendizagem, em especial na busca pela diferenciação entre as dificuldades normais de memórias e os déficits patológicos dessa função cognitiva.

Multivariate fMRI and Eye Tracking Reveal Differential Effects of Visual Interference on Recognition Memory Judgments for Objects and Scenes

Recent work has demonstrated that the perirhinal cortex (PRC) supports conjunctive object representations that aid object recognition memory following visual object interference. It is unclear, however, how these representations interact with other brain regions implicated in mnemonic retrieval and how congruent and incongruent interference influences the processing of targets and foils during object recognition. To address this, multivariate partial least squares was applied to fMRI data acquired during an interference match-to-sample task, in which participants made object or scene recognition judgments after object or scene interference. This revealed a pattern of activity sensitive to object recognition following congruent (i.e., object) interference that included PRC, prefrontal, and parietal regions. Moreover, functional connectivity analysis revealed a common pattern of PRC connectivity across interference and recognition conditions. Examination of eye movements during the same task in a separate study revealed that participants gazed more at targets than foils during correct object recognition decisions, regardless of interference congruency. By contrast, participants viewed foils more than targets for incorrect object memory judgments, but only after congruent interference. Our findings suggest that congruent interference makes object foils appear familiar and that a network of regions, including PRC, is recruited to overcome the effects of interference.

Functional and effective hippocampal-neocortical connectivity during construction and elaboration of autobiographical memory retrieval

Autobiographical memory (AM) provides the opportunity to study interactions among brain areas that support the search for a specific episodic memory (construction), and the later experience of mentally reliving it (elaboration). While the hippocampus supports both construction and elaboration, it is unclear how hippocampal-neocortical connectivity differs between these stages, and how this connectivity involves the anterior and posterior segments of the hippocampus, as these have been considered to support the retrieval of general concepts and recollection processes, respectively. We acquired fMRI data in 18 healthy participants during an AM retrieval task in which participants were asked to access a specific AM (construction) and then to recollect it by recovering as many episodic details as possible (elaboration). Using multivariate analytic techniques, we examined changes in functional and effective connectivity of hippocampal-neocortical interactions during these phases of AM retrieval. We found that the left anterior hippocampus interacted with frontal areas during construction and bilateral posterior hippocampi with visual perceptual areas during elaboration, indicating key roles for both hippocampi in coordinating transient neocortical networks at both AM stages. Our findings demonstrate the importance of direct interrogation of hippocampal-neocortical interactions to better illuminate the neural dynamics underlying complex cognitive tasks such as AM retrieval.

Categorization = decision making + generalization

We rarely, if ever, repeatedly encounter exactly the same situation. This makes generalization crucial for real world decision making. We argue that categorization, the study of generalizable representations, is a type of decision making, and that categorization learning research would benefit from approaches developed to study the neuroscience of decision making. Similarly, methods developed to examine generalization and learning within the field of categorization may enhance decision making research. We first discuss perceptual information processing and integration, with an emphasis on accumulator models. We then examine learning the value of different decision making choices via experience, emphasizing reinforcement learning modeling approaches. Next we discuss how value is combined with other factors in decision making, emphasizing the effects of uncertainty. Finally, we describe how a final decision is selected via thresholding processes implemented by the basal ganglia and related regions. We also consider how memory related functions in the hippocampus may be integrated with decision making mechanisms and contribute to categorization.

The parahippocampal gyrus links the default-mode cortical network with the medial temporal lobe memory system

The default-mode network (DMN) is a distributed functional-anatomic network implicated in supporting memory. Current resting-state functional connectivity studies in humans remain divided on the exact involvement of medial temporal lobe (MTL) in this network at rest. Notably, it is unclear to what extent the MTL regions involved in successful memory encoding are connected to the cortical nodes of the DMN during resting state. Our findings using functional connectivity MRI analyses of resting-state data indicate that the parahippocampal gyrus (PHG) is the primary hub of the DMN in the MTL during resting state. Also, connectivity of the PHG is distinct from connectivity of hippocampal regions identified by an associative memory-encoding task. We confirmed that several hippocampal encoding regions lack significant functional connectivity with cortical DMN nodes during resting state. Additionally, a mediation analysis showed that resting-state connectivity between the hippocampus and posterior cingulate cortex--a major hub of the DMN--is indirect and mediated by the PHG. Our findings support the hypothesis that the MTL memory system represents a functional subnetwork that relates to the cortical nodes of the DMN through parahippocampal functional connections.

Brain networks underlying episodic memory retrieval

The importance of the medial temporal lobe to episodic memory has been recognized for decades. Recent human fMRI findings have begun to delineate the functional roles of different MTL regions, most notably the hippocampus, for the retrieval of episodic memories. Importantly, these studies have also identified a network of cortical regions--each interconnected with the MTL--that are also consistently engaged during successful episodic retrieval. Along with the MTL these regions appear to constitute a content-independent network that acts in concert with cortical regions representing the contents of retrieval to support consciously accessible representations of prior experiences.

Posterior parietal cortex and long-term memory: some data from laboratory animals

The posterior parietal cortex (PPC) was long viewed as just involved in the perception of spatial relationships between the body and its surroundings and of movements related to them. In recent years the PPC has been shown to participate in many other cognitive processes, among which working memory and the consolidation and retrieval of episodic memory. The neurotransmitter and other molecular processes involved have been determined to a degree in rodents. More research will no doubt determine the extent to which these findings can be extrapolated to primates, including humans. In these there appears to be a paradox: imaging studies strongly suggest an important participation of the PPC in episodic memory, whereas lesion studies are much less suggestive, let alone conclusive. The data on the participation of the PPC in episodic memory so far do not permit any conclusion as to what aspect of consolidation and retrieval it handles in addition to those dealt with by the hippocampus and basolateral amygdala, if any.

Distinct patterns of functional and effective connectivity between perirhinal cortex and other cortical regions in recognition memory and perceptual discrimination

Traditionally, the medial temporal lobe (MTL) is thought to be dedicated to declarative memory. Recent evidence challenges this view, suggesting that perirhinal cortex (PrC), which interfaces the MTL with the ventral visual pathway, supports highly integrated object representations in recognition memory and perceptual discrimination. Even with comparable representational demands, perceptual and memory tasks differ in numerous task demands and the subjective experience they evoke. Here, we tested whether such differences are reflected in distinct patterns of connectivity between PrC and other cortical regions, including differential involvement of prefrontal control processes. We examined functional magnetic resonance imaging data for closely matched perceptual and recognition memory tasks for faces that engaged right PrC equivalently. Multivariate seed analyses revealed distinct patterns of interactions: Right ventrolateral prefrontal and posterior cingulate cortices exhibited stronger functional connectivity with PrC in recognition memory; fusiform regions were part of the pattern that displayed stronger functional connectivity with PrC in perceptual discrimination. Structural equation modeling revealed distinct patterns of effective connectivity that allowed us to constrain interpretation of these findings. Overall, they demonstrate that, even when MTL structures show similar involvement in recognition memory and perceptual discrimination, differential neural mechanisms are reflected in the interplay between the MTL and other cortical regions.

The medial temporal lobe and the attributes of memory

Neuroimaging and lesion studies have seemed to converge on the idea that the hippocampus selectively supports recollection. However, these studies usually involve a comparison between strong recollection-based memories and weak familiarity-based memories. Studies that avoid confounding memory strength with recollection and familiarity almost always find that the hippocampus supports both recollection and familiarity. We argue that the functional organization of the medial temporal lobe (MTL) is unlikely to be illuminated by the psychological distinction between recollection and familiarity and will be better informed by findings from neuroanatomy and neurophysiology. These findings indicate that the different structures of the MTL process different attributes of experience. By representing the widest array of attributes, the hippocampus supports recollection-based and familiarity-based memory of multiattribute stimuli.