The Longitudinal Trajectory of Default Mode Network Connectivity in Healthy Older Adults Varies As a Function of Age and Is Associated with Changes in Episodic Memory and Processing Speed

Sex-related differences in the relationship between β-amyloid and cognitive trajectories in older adults

Objective: We aimed to test the hypothesis that elevated neocortical β-amyloid (Aβ), a hallmark feature of Alzheimer's disease (AD), predicts sex-specific cognitive trajectories in clinically normal older adults, with women showing greater risk of decline than men. Method: Florbetapir Aβ positron emission tomography (PET) was acquired in 149 clinically normal older adults (52% female, Mage = 74). Participants underwent cognitive testing at baseline and during annual follow-up visits over a timespan of up to 5.14 years. Mixed-effects regression models evaluated whether relations between baseline neocortical Standardized Uptake Value Ratio (SUVR) and composite scores of episodic memory, executive functioning, and processing speed were moderated by sex (male/female) and apolipoprotein E (APOE) status (ε4 carrier/noncarrier). Results: Higher baseline SUVR was associated with longitudinal decline in episodic memory in women (b = -1.32, p < .001) but not men (b = -0.30, p = .28). Female APOE ε4 carriers with elevated SUVR showed particularly precipitous declines in episodic memory (b = -4.33, p < .001) whereas other cognitive domains were spared. SUVR did not predict changes in executive functioning or processing speed, regardless of sex (ps >.63), though there was a main effect of SUVR on processing speed (b = 2.50, p = .003). Conclusions: Clinically normal women with elevated Aβ are more vulnerable to episodic memory decline than men. Understanding sex-related differences in AD, particularly in preclinical stages, is crucial for guiding precision medicine approaches to early detection and intervention. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Vocal music enhances memory and language recovery after stroke: pooled results from two RCTs

Objective

Previous studies suggest that daily music listening can aid stroke recovery, but little is known about the stimulus‐dependent and neural mechanisms driving this effect. Building on neuroimaging evidence that vocal music engages extensive and bilateral networks in the brain, we sought to determine if it would be more effective for enhancing cognitive and language recovery and neuroplasticity than instrumental music or speech after stroke.

Methods

Using data pooled from two single‐blind randomized controlled trials in stroke patients (N = 83), we compared the effects of daily listening to self‐selected vocal music, instrumental music, and audiobooks during the first 3 poststroke months. Outcome measures comprised neuropsychological tests of verbal memory (primary outcome), language, and attention and a mood questionnaire performed at acute, 3‐month, and 6‐month stages and structural and functional MRI at acute and 6‐month stages.

Results

Listening to vocal music enhanced verbal memory recovery more than instrumental music or audiobooks and language recovery more than audiobooks, especially in aphasic patients. Voxel‐based morphometry and resting‐state and task‐based fMRI results showed that vocal music listening selectively increased gray matter volume in left temporal areas and functional connectivity in the default mode network.

Interpretation

Vocal music listening is an effective and easily applicable tool to support cognitive recovery after stroke as well as to enhance early language recovery in aphasia. The rehabilitative effects of vocal music are driven by both structural and functional plasticity changes in temporoparietal networks crucial for emotional processing, language, and memory.

Alteration in topological properties of brain functional network after 2-year high altitude exposure: A panel study

INTRODUCTION

High altitude (HA) exposure leads to cognitive impairment while the underlying mechanism is still unclear. Brain functional network is crucial for advanced functions, and its alteration is implicated in cognitive decline in multiple diseases. The aim of current study was to investigate the topological changes in HA-exposed brain functional network.

METHODS

Based on Shaanxi-Tibet immigrant cohort, neuropsychological tests and resting-state functional MRI were applied to evaluate the participants' cognitive function and functional connection (FC) changes, respectively. GRETNA toolbox was used to construct the brain functional network. The gray matter was parcellated into 116 anatomically defined regions according to Automated Anatomical Labeling atlas. Subsequently, the mean time series for each of the 116 regions were extracted and computed for Pearson's correlation coefficients. The relation matrix was further processed and seen as brain functional network. Correlation between functional network changes and neuropsychological results was also examined.

RESULTS

The cognitive performance was impaired by HA exposure as indicated by neuropsychological test. HA exposure led to alterations of degree centrality and nodal efficiency in multiple brain regions. Moreover, two subnetworks were extracted in which the FCs significantly decreased after exposure. In addition, the alterations in FCs within above two subnetworks were significantly correlated with changes of memory and reaction time.

CONCLUSIONS

Our results suggest that HA exposure modulates the topological property of functional network and FCs of some important regions, which may impair the attention, perception, memory, motion ignition, and modulation processes, finally decreasing cognitive performance in neuropsychological tests.

Resting-state functional MRI of the default mode network in epilepsy

The default mode network (DMN) is a major neuronal network that deactivates during goal-directed tasks. Recent advances in neuroimaging have shed light on its structure and function. Alterations in the DMN are increasingly recognized in a range of neurological and psychiatric conditions including epilepsy. This review first describes the current understanding of the DMN in health, normal aging, and disease as it is acquired via resting-state functional magnetic resonance imaging (MRI), before focusing on how it is affected in various types of focal and generalized epilepsy. These findings support the potential use of DMN parameters as future biomarkers in epilepsy research, diagnosis, and management.

Salience and default-mode network connectivity during threat and safety processing in older adults

The appropriate assessment of threat and safety is important for decision‐making but might be altered in old age due to neurobiological changes. The literature on threat and safety processing in older adults is sparse and it is unclear how healthy ageing affects the brain's functional networks associated with affective processing. We measured skin conductance responses as an indicator of sympathetic arousal and used functional magnetic resonance imaging and independent component analysis to compare young and older adults' functional connectivity in the default mode (DMN) and salience networks (SN) during a threat conditioning and extinction task. While our results provided evidence for differential threat processing in both groups, they also showed that functional connectivity within the SN – but not the DMN – was weaker during threat processing in older compared to young adults. This reduction of within‐network connectivity was accompanied by an age‐related decrease in low frequency spectral power in the SN and a reduction in inter‐network connectivity between the SN and DMN during threat and safety processing. Similarly, we found that skin conductance responses were generally lower in older compared to young adults. Our results are the first to demonstrate age‐related changes in brain activation during aversive conditioning and suggest that the ability to adaptively filter affective information is reduced in older adults.

Parsing inter- and intra-individual variability in key nervous system mechanisms of stress responsivity and across functional domains

Exposure to stressful events is omnipresent in modern human life, yet people show considerable heterogeneity in the impact of stress exposure(s) on their functionality and overall health. Encounter with stressor(s) is counteracted by an intricate repertoire of nervous-system responses. This narrative review starts with a brief summary of the vast evidence that supports heart rate variability, cortisol secretion, and large-scale cortical network interactions as kay physiological, endocrinological, and neural mechanisms of stress responsivity, respectively. The second section highlights potential sources for inter-individual variability in these mechanisms, by focusing on biological, environmental, social, habitual, and psychological factors that may influence stress responsivity patterns and thus contribute to heterogeneity in the impact of stress exposure on functionality and health. The third section introduces intra-individually variability in stress responsivity across functional domains as a novel putative source for heterogeneity in the impact of stress exposure. Challenges and future directions are further discussed. Parsing inter- and intra-individual variability in nervous-system mechanisms of stress responsivity and across functional domains is critical towards potential clinical translation.

A Longitudinal Study of Changes in Resting-State Functional Magnetic Resonance Imaging Functional Connectivity Networks During Healthy Aging

Background: Vast increases in life expectancy over the last century have led to shifts in population demographics and the emergence of a largely aged population, globally. This has led to a need to understand neurobiological changes associated with healthy aging. Studies on age-related changes in functional connectivity networks have largely been cross-sectional and focused on the default mode network (DMN). The current study investigated longitudinal changes in functional connectivity in multiple resting-state networks over 4 years of aging in cognitively normal older adults. Methods: Resting-state functional magnetic resonance imaging scans from older adults (n = 16) who maintained "cognitive normal" status over 4 years were retrieved at baseline and follow-up from the Alzheimer's Disease Neuroimaging Initiative database. A seed-based approach was executed in Functional MRI of the Brain Software Library (FSL) to examine significant changes in functional connectivity within the DMN, frontoparietal network (FPN), and salience network (SN) within subjects over time. Results: Results indicated significantly (p < 0.05, corrected) reduced functional connectivity in the FPN and SN, but not in the DMN at year 4 compared with baseline in older adults who were cognitively stable. Conclusions: The current study highlights the importance of a longitudinal approach for understanding changes in functional connectivity. The findings also underscore the need to examine multiple networks within the same participants, given that changes were apparent in the FPN and SN but not in the DMN. Future studies should also examine changes in internetwork connectivity as well as shifts in structural connectivity over time. Impact statement Investigations of age-related changes in functional connectivity have largely been cross-sectional and focused on the default mode network (DMN). The current study examined the DMN as well as the frontoparietal network (FN) and salience network (SN), in a group of healthy aging adults over four years. The results revealed decreased functional connectivity over time, in the FN and SN, but not the DMN. These findings provide insights about the healthy aging brain. They also underscore the need to broaden the scope of functional connectivity analyses beyond the DMN and highlight the use of longitudinal methods.

Aging alters neural activity at event boundaries in the hippocampus and Posterior Medial network

Recent research has highlighted a role for the hippocampus and a Posterior Medial cortical network in signaling event boundaries. However, little is known about whether or how these neural processes change over the course of healthy aging. Here, 546 cognitively normal participants 18-88 years old viewed a short movie while brain activity was measured using fMRI. The hippocampus and regions of the Posterior Medial network show increased activity at event boundaries, but these boundary-evoked responses decrease with age. Boundary-evoked activity in the posterior hippocampus predicts performance on a separate test of memory for stories, suggesting that hippocampal activity during event segmentation may be a broad indicator of individual differences in episodic memory ability. In contrast, boundary-evoked responses in the medial prefrontal cortex and middle temporal gyrus increase across the age range. These findings suggest that aging may alter neural processes for segmenting and remembering continuous real-world experiences.

Age-related change in episodic memory: role of functional and structural connectivity between the ventral posterior cingulate and the parietal cortex

While the neural correlates of age-related episodic memory decline have been extensively studied, the precise involvement of the Posterior Cingulate Cortex (PCC) and posterior parietal cortex (the precuneus and the angular gyrus), remains unclear. The present study examined functional and structural neural correlates of age-related episodic memory change assessed over 12 years in 120 older adults (range 76-90 years). Episodic memory performance was measured using the Free and Cued Selective Reminding Test (FCSRT); functional connectivity metrics were computed from resting-state fMRI images and structural connectivity metrics were assessed through microstructural properties of reconstructed tract using a native space pipeline. We found that FCSRT change was significantly associated with the functional connectivity between the ventral PCC and three parietal regions, the ventral superior, the inferior part of the precuneus, and the rostro dorsal part of the angular gyrus. This association was independent of hippocampal volume. In addition, we found the that change in FCSRT scores was associated with fractional anisotropy of the tract connecting the ventral PCC and the ventral superior part of the precuneus. Change in episodic memory in aging was therefore related to a combination of high functional connectivity and low structural connectivity between the ventral PCC and the ventral superior part of the precuneus.

Systemic Tumor Necrosis Factor-Alpha Trajectories Relate to Brain Health in Typically Aging Older Adults

BACKGROUND

Central nervous system levels of tumor necrosis factor-alpha (TNF-α), a pro-inflammatory cytokine, regulate the neuroinflammatory response and may play a role in age-related neurodegenerative diseases. The longitudinal relation between peripheral levels of TNF-α and typical brain aging is understudied. We hypothesized that within-person increases in systemic TNF-α would track with poorer brain health outcomes in functionally normal adults.

METHODS

Plasma-based TNF-α concentrations (pg/mL; fasting morning draws) and magnetic resonance imaging were acquired in 424 functionally intact adults (mean age = 71) followed annually for up to 8.4 years (mean follow-up = 2.2 years). Brain outcomes included total gray matter volume and white matter hyperintensities. Cognitive outcomes included composites of memory, executive functioning, and processing speed, as well as Mini-Mental State Examination total scores. Longitudinal mixed-effects models were used, controlling for age, sex, education, and total intracranial volume, as appropriate.

RESULTS

TNF-α concentrations significantly increased over time (p < .001). Linear increases in within-person TNF-α were longitudinally associated with declines in gray matter volume (p < .001) and increases in white matter hyperintensities (p = .003). Exploratory analyses suggested that the relation between TNF-α and gray matter volume was curvilinear (TNF-α 2p = .002), such that initial increases in inflammation were associated with more precipitous atrophy. There was a negative linear relationship of within-person changes in TNF-α to Mini-Mental State Examination scores over time (p = .036) but not the cognitive composites (all ps >.05).

CONCLUSION

Systemic inflammation, as indexed by plasma TNF-α, holds potential as a biomarker for age-related declines in brain health.

Functional Connectivity Disruption in Frail Older Adults Without Global Cognitive Deficits

Frailty is a common representation of cumulative age-related decline that may precede disability in older adults. In our study, we used magnetoencephalography (MEG) to explore the existence of abnormalities in the synchronization patterns of frail individuals without global cognitive impairment. Fifty-four older (≥70 years) and cognitively healthy (Mini-Mental State Examination ≥24) adults, 34 robust (not a single positive Fried criterion) and 20 frail (≥3 positive Fried criteria) underwent a resting-state MEG recording and a T1-weighted magnetic resonance imaging scan. Seed-based functional connectivity (FC) analyses were used to explore group differences in the synchronization of fronto-parietal areas relevant to motor function. Additionally, we performed group comparisons of intra-network FC for key resting-state networks such as the sensorimotor, fronto-parietal, default mode, and attentional (dorsal and ventral) networks. Frail participants exhibited reduced FC between posterior regions of the parietal cortex (bilateral supramarginal gyrus, right superior parietal lobe, and right angular gyrus) and widespread clusters spanning mainly fronto-parietal regions. Frail participants also demonstrated reduced intra-network FC within the fronto-parietal, ventral attentional, and posterior default mode networks. All the FC results concerned the upper beta band, a frequency range classically linked to motor function. Overall, our findings reveal the existence of abnormalities in the synchronization patterns of frail individuals within central structures important for accurate motor control. This study suggests that alterations in brain connectivity might contribute to some motor impairments associated with frailty.

Default Mode Network Analysis of APOE Genotype in Cognitively Unimpaired Subjects Based on Persistent Homology

Current researches on default mode network (DMN) in normal elderly have mainly focused on finding some dysfunctional areas with decreased or increased connectivity. The global network dynamics of apolipoprotein E (APOE) e4 allele group is rarely studied. In our previous brain network study, we have demonstrated the advantage of persistent homology. It can distinguish robust and noisy topological features over multiscale nested networks, and the derived properties are more stable. In this study, for the first time we applied persistent homology to analyze APOE-related effects on whole-brain functional network. In our experiments, the risk allele group exhibited lower network radius and modularity in whole brain DMN based on graph theory, suggesting the abnormal organization structure. Moreover, two suggested measures from persistent homology detected significant differences between groups within the left hemisphere and in the whole brain in two datasets. They were more statistically sensitive to APOE genotypic differences than standard graph-based measures. In summary, we provide evidence that the e4 genotype leads to distinct DMN functional alterations in the early phases of Alzheimer's disease using persistent homology approach. Our study offers a novel insight to explore potential biomarkers in healthy elderly populations carrying APOE e4 allele.

BHA-CS: A novel cognitive composite for Alzheimer's disease and related disorders

INTRODUCTION

Composite scores based on psychometrically rigorous cognitive assessments are well suited for early diagnosis and disease monitoring.

METHODS

We developed and cross-validated the Brain Health Assessment-Cognitive Score (BHA-CS), based on a brief computerized battery, in 451 cognitively normal (CN) and 399 cognitively impaired (mild cognitive impairment [MCI] or dementia) older adults. We investigated its long-term reliability and reliable change indices at longitudinal follow-up (N = 340), and the association with amyloid beta (Aβ) burden in the CN subgroup with Aβ positron emission tomography (N = 119).

RESULTS

The BHA-CS was accurate at detecting cognitive impairment and exhibited excellent long-term stability. Reliable decline over one year was detected in 75% of participants with dementia, 44% with MCI, and 3% of CN. Among CN, the Aβ-positive group showed worse longitudinal performance on the BHA-CS compared to the Aβ-negative group.

DISCUSSION

The BHA-CS is sensitive to cognitive decline in preclinical and prodromal neurodegenerative disease.

Functional brain connectivity changes across the human life span: From fetal development to old age

The dynamic of the temporal correlations between brain areas, called functional connectivity (FC), undergoes complex transformations through the life span. In this review, we aim to provide an overview of these changes in the nonpathological brain from fetal life to advanced age. After a brief description of the main methods, we propose that FC development can be divided into four main phases: first, before birth, a strong change in FC leads to the emergence of functional proto-networks, involving mainly within network short-range connections. Then, during the first years of life, there is a strong widespread organization of networks which starts with segregation processes followed by a continuous increase in integration. Thereafter, from adolescence to early adulthood, a refinement of existing networks in the brain occurs, characterized by an increase in integrative processes until about 40 years. Middle age constitutes a pivotal period associated with an inversion of the functional brain trajectories with a decrease in segregation process in conjunction to a large-scale reorganization of between network connections. Studies suggest that these processes are in line with the development of cognitive and sensory functions throughout life as well as their deterioration. During aging, results support the notion of dedifferentiation processes, which refer to the decrease in functional selectivity of the brain regions, resulting in more diffuse and less specialized FC, associated with the disruption of cognitive functions with age. The inversion of developmental processes during aging is in accordance with the developmental models of neuroanatomy for which the latest matured regions are the first to deteriorate.

Get Moving! Increases in Physical Activity Are Associated With Increasing Functional Connectivity Trajectories in Typically Aging Adults

Background: Physical activity closely relates to cognition and brain structure as we age. However, the neural mechanisms underlying this relationship in humans remain less clear. Functional connectivity (FC), measured by task-free functional MRI (tf-fMRI) is a dynamic marker of network activity and may be a sensitive indicator of the brain's response to exercise over time. We aimed to test the longitudinal relationship between physical activity and FC trajectories in functionally normal older adults. Methods: Two hundred and twelve functionally normal, longitudinally-followed older adults completed the Physical Activity Scale for the Elderly (PASE) and tf-fMRI scans at each visit [mean = 1.5 visits (range:1-3)]. We studied FC of the default mode network (DMN), frontal-parietal (FP), subcortical networks (SubCort), and frontal-subcortical inter-network connectivity (FS), given that previous studies implicate these regions in age-related changes. Linear mixed-effects models examined the relationship between within-person changes in PASE and FC (in SD units), covarying for age, sex, education and systemic cardiovascular risk factors (heart rate, BMI and systolic blood pressure). We additionally examined models covarying for DTI fractional anisotropy (FA) and mean diffusivity (MD) of tracts underlying networks of interest, as a marker of cerebrovascular disease. Furthermore, we examined the longitudinal relationship between PASE and neuropsychological trajectories. Results: In our first model, within-subject increases in physical activity tracked with increasing SubCort (β = 0.33, p = 0.007) and FS inter-network (β = 0.27, p = 0.03) synchrony, while between-subject parameters did not reach significance (β = -0.042 to -0.07, ps > 0.37). No significant longitudinal associations were observed between PASE and DMN (β = -0.02 p = 0.89) or FP networks (β = 0.15, p = 0.23). Adjusting for markers of cerebrovascular health (FA/MD) did not change estimated effects (SubCort: β = 0.31, p = 0.01, FS inter-network: β = 0.28, p = 0.03). Associations between changes in physical activity and neuropsychological trajectories were small (β = -0.14 to 0.002) and did not reach statistical significance (p-values >0.42). Conclusions: Our findings suggest that changes in exercise over time are specifically associated with frontal-subcortical processes in older adults. This relationship appears to be independent of cardio- or cerebrovascular disease, possibly driven by a more direct neural response to exercise.

Medial temporal lobe connectivity and its associations with cognition in early Alzheimer's disease

Human episodic memory critically depends on subregions of the medial temporal lobe, which are part of functional brain systems such as the anterior-temporal and the posterior-medial system. Here we analysed how Alzheimer's pathology affects functional connectivity within these systems. Data from 256 amyloid-β-negative cognitively unimpaired, 103 amyloid-β-positive cognitively unimpaired, and 83 amyloid-β-positive individuals with mild cognitive impairment were analysed. Amyloid-β and tau pathology were measured using the CSF amyloid-β42/40 ratio and phosphorylated tau, respectively. We found that amyloid-β-positive cognitively unimpaired individuals were mainly characterized by decreased functional connectivity between the medial temporal lobe and regions in the anterior-temporal system, most prominently between left perirhinal/entorhinal cortices and medial prefrontal cortex. Furthermore, correlation analysis in this group revealed decreasing functional connectivity between bilateral perirhinal/entorhinal cortices, anterior hippocampus and posterior-medial regions with increasing levels of phosphorylated tau. The amyloid-β-positive individuals with mild cognitive impairment mostly exhibited reduced connectivity between the medial temporal lobe and posterior-medial regions, predominantly between the anterior hippocampus and posterior cingulate cortex. In addition, they showed hyperconnectivity within the medial temporal lobe and its immediate proximity. Lower medial temporal-cortical functional connectivity networks resulting from the group comparisons of cognitively unimpaired individuals were associated with reduced memory performance and more rapid longitudinal memory decline as shown by linear mixed-effects regression analysis. Finally, we found that reduced medial temporal-cortical connectivity in mildly cognitively impaired individuals was related to reduced entorhinal thickness and white matter integrity of the parahippocampal cingulum and the fornix. No such relationships were found in cognitively unimpaired individuals. In conclusion, our findings show that the earliest changes in preclinical Alzheimer's disease might involve decreased connectivity within the anterior-temporal system, and early changes in connectivity might be related to memory impairment, but not to structural changes. With disease progression and increased tau pathology, medial temporal functional connectivity with posterior-medial regions seems to be increasingly impaired. In individuals with mild cognitive impairment, reduced functional connectivity is associated with structural brain changes as well as the emergence of locally increased connectivity patterns. Thus, functional connectivity between the medial temporal lobe and the anterior-temporal and posterior-medial system could serve as stage-specific functional markers in early Alzheimer's disease.

Multi-scale network regression for brain-phenotype associations

Brain networks are increasingly characterized at different scales, including summary statistics, community connectivity, and individual edges. While research relating brain networks to behavioral measurements has yielded many insights into brain‐phenotype relationships, common analytical approaches only consider network information at a single scale. Here, we designed, implemented, and deployed Multi‐Scale Network Regression (MSNR), a penalized multivariate approach for modeling brain networks that explicitly respects both edge‐ and community‐level information by assuming a low rank and sparse structure, both encouraging less complex and more interpretable modeling. Capitalizing on a large neuroimaging cohort (n = 1, 051), we demonstrate that MSNR recapitulates interpretable and statistically significant connectivity patterns associated with brain development, sex differences, and motion‐related artifacts. Compared to single‐scale methods, MSNR achieves a balance between prediction performance and model complexity, with improved interpretability. Together, by jointly exploiting both edge‐ and community‐level information, MSNR has the potential to yield novel insights into brain‐behavior relationships.

Longitudinal Changes in Whole-Brain Functional Connectivity Strength Patterns and the Relationship With the Global Cognitive Decline in Older Adults

Aging is associated with changes in brain functional patterns as well as cognition. The present research sought to investigate longitudinal changes in whole brain functional connectivity strength (FCS) and cognitive performance scores in very old cognitively unimpaired individuals. We studied 34 cognitively normal elderly individuals at both baseline and 4-year follow-up (baseline age = 78 ± 3.14 years) with resting-state functional magnetic resonance imaging (r-fMRI), structural MRI scans, and neuropsychological assessments conducted. Voxel-based whole brain FCS was calculated and we found that bilateral superior parietal and medial frontal regions showed decreased FCS, while the supplementary motor area (SMA) and insula showed increased FCS with age, along with a decrease in bilateral prefrontal cortical thickness. The changes of FCS in left precuneus were associated with an aging-related decline in global cognition. Taken together, our results suggest changes in FCS with aging with the precuneus as a hub and this may underlie changes in global cognition that accompany aging. These findings help better understand the normal aging mechanism.

APOE-ε4 Shapes the Cerebral Organization in Cognitively Intact Individuals as Reflected by Structural Gray Matter Networks

Gray matter networks (GMn) provide essential information on the intrinsic organization of the brain and appear to be disrupted in Alzheimer’s disease (AD). Apolipoprotein E (APOE)-ε4 represents the major genetic risk factor for AD, yet the association between APOE-ε4 and GMn has remained unexplored. Here, we determine the impact of APOE-ε4 on GMn in a large sample of cognitively unimpaired individuals, which was enriched for the genetic risk of AD. We used independent component analysis to retrieve sources of structural covariance and analyzed APOE group differences within and between networks. Analyses were repeated in a subsample of amyloid-negative subjects. Compared with noncarriers and heterozygotes, APOE-ε4 homozygotes showed increased covariance in one network including primarily right-lateralized, parietal, inferior frontal, as well as inferior and middle temporal regions, which mirrored the formerly described AD-signature. This result was confirmed in a subsample of amyloid-negative individuals. APOE-ε4 carriers showed reduced covariance between two networks encompassing frontal and temporal regions, which constitute preferential target of amyloid deposition. Our data indicate that, in asymptomatic individuals, APOE-ε4 shapes the cerebral organization in a way that recapitulates focal morphometric alterations observed in AD patients, even in absence of amyloid pathology. This suggests that structural vulnerability in neuronal networks associated with APOE-ε4 may be an early event in AD pathogenesis, possibly upstream of amyloid deposition.

Genetic influence on ageing-related changes in resting-state brain functional networks in healthy adults: A systematic review

This systematic review examines the genetic and epigenetic factors associated with resting-state functional connectivity (RSFC) in healthy human adult brains across the lifespan, with a focus on genes associated with Alzheimer's disease (AD). There were 58 studies included. The key findings are: (i) genetic factors have a low to moderate contribution; (ii) the apolipoprotein E ε2/3/4 polymorphism was the most studied genetic variant, with the APOE-ε4 allele most consistently associated with deficits of the default mode network, but there were insufficient studies to determine the relationships with other AD candidate risk genes; (iii) a single genome-wide association study identified several variants related to RSFC; (iv) two epigenetic independent studies showed a positive relationship between blood DNA methylation of the SLC6A4 promoter and RSFC measures. Thus, there is emerging evidence that genetic and epigenetic variation influence the brain's functional organisation and connectivity over the adult lifespan. However, more studies are required to elucidate the roles genetic and epigenetic factors play in RSFC measures across the adult lifespan.

Functional dedifferentiation of associative resting state networks in older adults - A longitudinal study

Healthy aging is associated with weaker functional connectivity within resting state brain networks and stronger functional interaction between these networks. This phenomenon has been characterized as reduced functional segregation and has been investigated mainly in cross-sectional studies. Here, we used a longitudinal dataset which consisted of four occasions of resting state fMRI and psychometric cognitive ability data, collected from a sample of healthy older adults (baseline N = 232, age range: 64-87 y, age M = 70.8 y), to investigate the functional segregation of several well-defined resting state networks encompassing the whole brain. We characterized the ratio of within-network and between-network correlations via the well-established segregation index. Our findings showed a decrease over a 4-year interval in the functional segregation of the default mode, frontoparietal control and salience ventral attention networks. In contrast, we showed an increase in the segregation of the limbic network over the same interval. More importantly, the rate of change in functional segregation of the frontoparietal control network was associated with the rate of change in processing speed. These findings support the hypothesis of functional dedifferentiation in healthy aging as well as its role in cognitive function in elderly.

Longitudinal Characterization and Biomarkers of Age and Sex Differences in the Decline of Spatial Memory

The current longitudinal study examined factors (sex, physical function, response to novelty, ability to adapt to a shift in light/dark cycle, brain connectivity), which might predict the emergence of impaired memory during aging. Male and female Fisher 344 rats were tested at 6, 12, and 18 months of age. Impaired spatial memory developed in middle-age (12 months), particularly in males, and the propensity for impairment increased with advanced age. A reduced response to novelty was observed over the course of aging, which is inconsistent with cross-sectional studies. This divergence likely resulted from differences in the history of environmental enrichment/impoverishment for cross-sectional and longitudinal studies. Animals that exhibited lower level exploration of the inner region on the open field test exhibited better memory at 12 months. Furthermore, males that exhibited a longer latency to enter a novel environment at 6 months, exhibited better memory at 12 months. For females, memory at 12 months was correlated with the ability to behaviorally adapt to a shift in light/dark cycle. Functional magnetic resonance imaging of the brain, conducted at 12 months, indicated that the decline in memory was associated with altered functional connectivity within different memory systems, most notably between the hippocampus and multiple regions such as the retrosplenial cortex, thalamus, striatum, and amygdala. Overall, some factors, specifically response to novelty at an early age and the capacity to adapt to shifts in light cycle, predicted spatial memory in middle-age, and spatial memory is associated with corresponding changes in brain connectivity. We discuss similarities and differences related to previous longitudinal and cross-sectional studies, as well as the role of sex differences in providing a theoretical framework to guide future longitudinal research on the trajectory of cognitive decline. In addition to demonstrating the power of longitudinal studies, these data highlight the importance of middle-age for identifying potential predictive indicators of sexual dimorphism in the trajectory in brain and cognitive aging.

Language and spatial dysfunction in Alzheimer disease with white matter thorn-shaped astrocytes

OBJECTIVES

Alzheimer disease (AD) shows a broad array of clinical presentations, but the mechanisms underlying these phenotypic variants remain elusive. Aging-related astrogliopathy (ARTAG) is a relatively recent term encompassing a broad array of tau deposition in astroglia outside the range of traditional tauopathies. White matter thorn-shaped astrocyte (WM-TSA) clusters, a specific ARTAG subtype, has been associated with atypical language presentation of AD in a small study lacking replication. To interrogate the impact of WM-TSA in modifying clinical phenotype in AD, we investigated a clinicopathologic sample of 83 persons with pure cortical AD pathology and heterogeneous clinical presentations.

METHODS

We mapped WM-TSA presence and density throughout cortical areas and interrogated whether WM-TSA correlated with atypical AD presentation or worse performance in neuropsychological testing.

RESULTS

WM-TSA was present in nearly half of the cases and equally distributed in typical and atypical AD presentations. Worsening language and visuospatial functions were correlated with higher WM-TSA density in language-related and visuospatial-related regions, respectively. These findings were unrelated to regional neurofibrillary tangle burden. Next, unsupervised clustering divided the participants into 2 groups: a high-WM-TSA (n = 9) and low-WM-TSA (n = 74) pathology signature. The high-WM-TSA group scored significantly worse in language but not in other cognitive domains.

CONCLUSIONS

The negative impact of WM-TSA pathology to language and possibly visuospatial networks suggests that WM-TSA is not as benign as other ARTAG types and may be explored as a framework to understand the mechanisms and impact of astrocytic tau deposition in AD in humans.

Differential tDCS and tACS Effects on Working Memory-Related Neural Activity and Resting-State Connectivity

Transcranial direct and alternating current stimulation (tDCS and tACS, respectively) entail capability to modulate human brain dynamics and cognition. However, the comparability of these approaches at the level of large-scale functional networks has not been thoroughly investigated. In this study, 44 subjects were randomly assigned to receive sham (N = 15), tDCS (N = 15), or tACS (N = 14). The first electrode (anode in tDCS) was positioned over the left dorsolateral prefrontal cortex, the target area, and the second electrode (cathode in tDCS) was placed over the right supraorbital region. tDCS was delivered with a constant current of 2 mA. tACS was fixed to 2 mA peak-to-peak with 6 Hz frequency. Stimulation was applied concurrently with functional magnetic resonance imaging (fMRI) acquisitions, both at rest and during the performance of a verbal working memory (WM) task. After stimulation, subjects repeated the fMRI WM task. Our results indicated that at rest, tDCS increased functional connectivity particularly within the default-mode network (DMN), while tACS decreased it. When comparing both fMRI WM tasks, it was observed that tDCS displayed decreased brain activity post-stimulation as compared to online. Conversely, tACS effects were driven by neural increases online as compared to post-stimulation. Interestingly, both effects primarily occurred within DMN-related areas. Regarding the differences in each fMRI WM task, during the online fMRI WM task, tACS engaged distributed neural resources which did not overlap with the WM-dependent activity pattern, but with some posterior DMN regions. In contrast, during the post-stimulation fMRI WM task, tDCS strengthened prefrontal DMN deactivations, being these activity reductions associated with faster responses. Furthermore, it was observed that tDCS neural responses presented certain consistency across distinct fMRI modalities, while tACS did not. In sum, tDCS and tACS modulate fMRI-derived network dynamics differently. However, both effects seem to focus on DMN regions and the WM network-DMN shift, which are highly affected in aging and disease. Thus, albeit exploratory and needing further replication with larger samples, our results might provide a refined understanding of how the DMN functioning can be externally modulated through commonly used non-invasive brain stimulation techniques, which may be of eventual clinical relevance.

Resting State Functional Connectivity Signature Differentiates Cognitively Normal from Individuals Who Convert to Symptomatic Alzheimer's Disease

BACKGROUND

Changes in resting state functional connectivity (rs-fc) occur in Alzheimer's disease (AD), but few longitudinal rs-fc studies have been performed. Most studies focus on single networks and not a global measure of rs-fc. Although the amyloid tau neurodegeneration (AT(N)) framework is increasingly utilized by the AD community, few studies investigated when global rs-fc signature changes occur within this model.

OBJECTIVE

1) Identify a global rs-fc signature that differentiates cognitively normal (CN) individuals from symptomatic AD. 2) Assess when longitudinal changes in rs-fc occur relative to conversion to symptomatic AD. 3) Compare rs-fc with amyloid, tau, and neurodegeneration biomarkers.

METHODS

A global rs-fc signature composed of intra-network connections was longitudinally evaluated in a cohort of cognitively normal participants at baseline (n = 335). Biomarkers, including cerebrospinal fluid (Aβ42 and tau), structural magnetic resonance imaging, and positron emission tomography were obtained.

RESULTS

Global rs-fc signature distinguished CN individuals from individuals who developed symptomatic AD. Changes occurred nearly four years before conversion to symptomatic AD. The global rs-fc signature most strongly correlated with markers of neurodegeneration.

CONCLUSION

The global rs-fc signature changes near symptomatic onset and is likely a neurodegenerative biomarker. Rs-fc changes could serve as a biomarker for evaluating potential therapies for symptomatic conversion to AD.

Longitudinal stability of the brain functional connectome is associated with episodic memory performance in aging

The brain functional connectome forms a relatively stable and idiosyncratic backbone that can be used for identification or “fingerprinting” of individuals with a high level of accuracy. While previous cross‐sectional evidence has demonstrated increased stability and distinctiveness of the brain connectome during the course of childhood and adolescence, less is known regarding the longitudinal stability in middle and older age. Here, we collected structural and resting‐state functional MRI data at two time points separated by 2–3 years in 75 middle‐aged and older adults (age 49–80, SD = 6.91 years) which allowed us to assess the long‐term stability of the functional connectome. We show that the connectome backbone generally remains stable over a 2–3 years period in middle and older age. Independent of age, cortical volume was associated with the connectome stability of several canonical resting‐state networks, suggesting that the connectome backbone relates to structural properties of the cortex. Moreover, the individual longitudinal stability of subcortical and default mode networks was associated with individual differences in cross‐sectional and longitudinal measures of episodic memory performance, providing new evidence for the importance of these networks in maintaining mnemonic processing in middle and old age. Together, the findings encourage the use of within‐subject connectome stability analyses for understanding individual differences in brain function and cognition in aging.

Longitudinal degradation of the default/salience network axis in symptomatic individuals with elevated amyloid burden

Resting-state functional connectivity MRI (rs-fcMRI) is a non-invasive imaging technique that has come into increasing use to understand disrupted neural network function in neuropsychiatric disease. However, despite extensive study over the past 15 years, the development of rs-fcMRI as a biomarker has been impeded by a lack of reliable longitudinal rs-fcMRI measures. Here we focus on longitudinal change along the Alzheimer's disease (AD) trajectory and demonstrate the utility of Template Based Rotation (TBR) in detecting differential longitudinal rs-fcMRI change between higher and lower amyloid burden individuals with mildly impaired cognition. Specifically, we examine a small (N = 24), but densely sampled (~5 observations over ~3 years), cohort of symptomatic individuals with serial rs-fcMRI imaging and PiB-PET imaging for β-amyloid pathology. We observed longitudinal decline of the Default Mode and Salience network axis (DMN/SAL) among impaired individuals with high amyloid burden. No other networks showed differential change in high vs. low amyloid individuals over time. The standardized effect size of AD related DMN/SAL change is comparable to the standardized effect size of amyloid-related change on the mini-mental state exam (MMSE) and hippocampal volume (HV). Last, we show that the AD-related change in DMN/SAL connectivity is almost completely independent of change on MMSE or HV, suggesting that rs-fcMRI is sensitive to an aspect of AD progression that is not captured by these other measures. Together these analyses demonstrate that longitudinal rs-fcMRI using TBR can capture disease-relevant network disruption in a clinical population.

The gut microbiota-inflammation-brain axis in end-stage renal disease: perspectives from default mode network

The gut-brain axis in end-stage renal disease (ESRD) is attracting more and more attention. However, the mechanism of gut-brain axis based cognitive disorders in ESRD patients remains unclear. The purpose of this study was to investigate the linkages between the gut microbiota, inflammatory cytokines, brain default mode network (DMN) and cognitive function in ESRD patients. Methods: This prospective study enrolled 28 ESRD patients (13 males and 15 females, mean age of 44 ± 14 years) and 19 healthy controls (HCs) (12 males and 7 females, mean age of 44 ± 10 years). All subjects underwent stool microbiota analysis, blood inflammatory cytokines examination, brain MRI scans and cognitive assessments. Resting state functional MRI (rs-fMRI) data were used to construct DMN and graph theory was applied to characterize network topological properties. Two samples t-test was applied for the comparisons between ESRD and HCs. Correlation analysis and mediation analysis were conducted among factors with significant group differences. Results: ESRD patients displayed gut microbiota alterations, increased systemic inflammation and worse cognitive performance compared to HCs (all p < 0.05). Graph analysis revealed disrupted DMN topological organization, aberrant nodal centralities and functional connectivities (FCs) in ESRD patients relative to HCs (all p < 0.05, FDR corrected). Significant correlations were found between gut microbiota, inflammatory cytokines, DMN network measures and cognitive assessments. Mediation analysis found that gut microbiota alteration impaired DMN connectivity by increasing systemic inflammation. Conclusion: The present study first revealed gut microbiota alterations, systemic inflammation, DMN dissociation and cognitive dysfunction in ESRD patients simultaneously and further illuminated their inner relationship.

Tau PET and multimodal brain imaging in patients at risk for chronic traumatic encephalopathy

OBJECTIVE

To characterize individual and group-level neuroimaging findings in patients at risk for Chronic Traumatic Encephalopathy (CTE).

METHODS

Eleven male patients meeting criteria for Traumatic Encephalopathy Syndrome (TES, median age: 64) underwent neurologic evaluation, 3-Tesla MRI, and PET with [18F]-Flortaucipir (FTP, tau-PET) and [11C]-Pittsburgh compound B (PIB, amyloid-PET). Six patients underwent [18F]-Fluorodeoxyglucose-PET (FDG, glucose metabolism). We assessed imaging findings at the individual patient level, and in group-level comparisons with modality-specific groups of cognitively normal older adults (CN). Tau-PET findings in patients with TES were also compared to a matched group of patients with mild cognitive impairment or dementia due to Alzheimer's disease (AD).

RESULTS

All patients with TES sustained repetitive head injury participating in impact sports, ten in American football. Three patients met criteria for dementia and eight had mild cognitive impairment. Two patients were amyloid-PET positive and harbored the most severe MRI atrophy, FDG hypometabolism, and FTP-tau PET binding. Among the nine amyloid-negative patients, tau-PET showed either mildly elevated frontotemporal binding, a "dot-like" pattern, or no elevated binding. Medial temporal FTP was mildly elevated in a subset of amyloid-negative patients, but values were considerably lower than in AD. Voxelwise analyses revealed a convergence of imaging abnormalities (higher FTP binding, lower FDG, lower gray matter volumes) in frontotemporal areas in TES compared to controls.

CONCLUSIONS

Mildly elevated tau-PET binding was observed in a subset of amyloid-negative patients at risk for CTE, in a distribution consistent with CTE pathology stages III-IV. FTP-PET may be useful as a biomarker of tau pathology in CTE but is unlikely to be sensitive to early disease stages.

Alzheimer's disease clinical variants show distinct regional patterns of neurofibrillary tangle accumulation

The clinical spectrum of Alzheimer's disease (AD) extends well beyond the classic amnestic-predominant syndrome. The previous studies have suggested differential neurofibrillary tangle (NFT) burden between amnestic and logopenic primary progressive aphasia presentations of AD. In this study, we explored the regional distribution of NFT pathology and its relationship to AD presentation across five different clinical syndromes. We assessed NFT density throughout six selected neocortical and hippocampal regions using thioflavin-S fluorescent microscopy in a well-characterized clinicopathological cohort of pure AD cases enriched for atypical clinical presentations. Subjects underwent apolipoprotein E genotyping and neuropsychological testing. Main cognitive domains (executive, visuospatial, language, and memory function) were assessed using an established composite z score. Our results showed that NFT regional burden aligns with the clinical presentation and region-specific cognitive scores. Cortical, but not hippocampal, NFT burden was higher among atypical clinical variants relative to the amnestic syndrome. In analyses of specific clinical variants, logopenic primary progressive aphasia showed higher NFT density in the superior temporal gyrus (p = 0.0091), and corticobasal syndrome showed higher NFT density in the primary motor cortex (p = 0.0205) relative to the amnestic syndrome. Higher NFT burden in the angular gyrus and CA1 sector of the hippocampus were independently associated with worsening visuospatial dysfunction. In addition, unbiased hierarchical clustering based on regional NFT densities identified three groups characterized by a low overall NFT burden, high overall burden, and cortical-predominant burden, respectively, which were found to differ in sex ratio, age, disease duration, and clinical presentation. In comparison, the typical, hippocampal sparing, and limbic-predominant subtypes derived from a previously proposed algorithm did not reproduce the same degree of clinical relevance in this sample. Overall, our results suggest domain-specific functional consequences of regional NFT accumulation. Mapping these consequences presents an opportunity to increase understanding of the neuropathological framework underlying atypical clinical manifestations.

Aberrant brain network connectivity in presymptomatic and manifest Huntington's disease: A systematic review

Resting‐state functional magnetic resonance imaging (rs‐fMRI) has the potential to shed light on the pathophysiological mechanisms of Huntington's disease (HD), paving the way to new therapeutic interventions. A systematic literature review was conducted in three online databases according to PRISMA guidelines, using keywords for HD, functional connectivity, and rs‐fMRI. We included studies investigating connectivity in presymptomatic (pre‐HD) and manifest HD gene carriers compared to healthy controls, implementing seed‐based connectivity, independent component analysis, regional property, and graph analysis approaches. Visual network showed reduced connectivity in manifest HD, while network/areas underpinning motor functions were consistently altered in both manifest HD and pre‐HD, showing disease stage‐dependent changes. Cognitive networks underlying executive and attentional functions showed divergent anterior–posterior alterations, possibly reflecting compensatory mechanisms. The involvement of these networks in pre‐HD is still unclear. In conclusion, aberrant connectivity of the sensory‐motor network is observed in the early stage of HD while, as pathology spreads, other networks might be affected, such as the visual and executive/attentional networks. Moreover, sensory‐motor and executive networks exhibit hyper‐ and hypo‐connectivity patterns following different spatiotemporal trajectories. These findings could potentially help to implement future huntingtin‐lowering interventions.

Sex Moderates Amyloid and Apolipoprotein ε4 Effects on Default Mode Network Connectivity at Rest

Women are more likely to have Alzheimer's disease (AD) and decline more rapidly once diagnosed despite greater verbal memory early in the disease compared to men—an advantage that has been termed “memory reserve.” Resting state functional MRI (fMRI) investigations demonstrate interactions between sex and AD risk factors in default mode network (DMN) connectivity, a network of brain regions showing progressive dysfunction in AD. Separate work suggests connectivity of left prefrontal cortex (PFC) may correlate with more general cognitive reserve in healthy aging. It is unknown whether left prefrontal functional connectivity with anterior and posterior default mode network (aDMN, pDMN) might differ by sex in AD. This study employed group independent component analysis (ICA) to analyze resting state fMRI data from 158 participants from the Alzheimer's Disease Neuroimaging Initiative (ADNI) with baseline diagnoses of normal cognition or early mild cognitive impairment (eMCI). pDMN and aDMN were defined on a subject-specific basis; prefrontal areas were selected from the Brodmann atlas (BA 6, 44, 8, and 9). Moderation regression analyses examined whether sex and amyloid PET positivity (A+/–) moderated effects of apolipoprotein ε4 (APOE ε4) on connectivity between left PFC, aDMN, and pDMN; and between aDMN and pDMN. Significant analyses were followed up with partial correlations assessing relationship of connectivity to verbal memory on the Rey Auditory Verbal Learning Test (RAVLT), and with preliminary analyses within NC and eMCI groups separately. Results showed no sex moderation of effects of A+ and APOE ε4 on left prefrontal/DMN connectivity in the full sample. However, sex significantly moderated impact of A+ and APOE ε4 on connectivity between aDMN and pDMN (p < 0.01). Women with an APOE allele (ε4+) and A+ showed greater aDMN/pDMN connectivity than their ε4- counterparts. No significant results were observed in men. Subgroup analyses suggested the aDMN/pDMN finding was true for those with NC, not eMCI. Partial correlations controlling for age and education showed increased aDMN/pDMN connectivity related to better verbal learning in women (p < 0.01) and not men (p = 0.18). In women at risk for AD or in early symptomatic stages who also have evidence of amyloid burden, stronger aDMN/pDMN connectivity may support verbal learning.

Combining non-invasive brain stimulation with functional magnetic resonance imaging to investigate the neural substrates of cognitive aging

As aging population is increasing, new methodologies to apprehend and enhance the mechanisms related to optimal brain function in advancing age become urgent. This review describes how the combined use of non-invasive brain stimulation (NIBS) with functional magnetic resonance imaging (fMRI) provides novel experimental data on the putative neurophysiological mechanisms underlying inter-individual differences in cognitive status among older adults, also further illuminating our understanding of theoretical models proposed within the cognitive neuroscience of aging literature. In addition, it explores published evidence of how this combined procedure entails the capacity to modify the activity and connectivity of specific brain networks in older adults, potentially leading to improvements in cognitive function and other measures reflecting mental health status. Although additional research is needed, combining NIBS with fMRI might provide innovative understanding of how fundamental brain plasticity mechanisms operate in advancing age, a knowledge that may be eventually used to refine more individually tailored approaches to promote brain health in aged populations.

A longitudinal characterization of perfusion in the aging brain and associations with cognition and neural structure

Cerebral perfusion declines across the lifespan and is altered in the early stages of several age-related neuropathologies. Little is known, however, about the longitudinal evolution of perfusion in healthy older adults, particularly when perfusion is quantified using magnetic resonance imaging with arterial spin labeling (ASL). The objective was to characterize longitudinal perfusion in typically aging adults and elucidate associations with cognition and brain structure. Adults who were functionally intact at baseline (n = 161, ages 47-89) underwent ASL imaging to quantify whole-brain gray matter perfusion; a subset (n = 136) had repeated imaging (average follow-up: 2.3 years). Neuropsychological testing at each visit was summarized into executive function, memory, and processing speed composites. Global gray matter volume, white matter microstructure (mean diffusivity), and white matter hyperintensities were also quantified. We assessed baseline associations among perfusion, cognition, and brain structure using linear regression, and longitudinal relationships using linear mixed effects models. Greater baseline perfusion, particularly in the left dorsolateral prefrontal cortex and right thalamus, was associated with better executive functions. Greater whole-brain perfusion loss was associated with worsening brain structure and declining processing speed. This study helps validate noninvasive MRI-based perfusion imaging and underscores the importance of cerebral blood flow in cognitive aging.

EEG Resting-State Networks Responsible for Gait Disturbance Features in Idiopathic Normal Pressure Hydrocephalus

Idiopathic normal pressure hydrocephalus (iNPH) is a neuropsychiatric disease characterized by gait disturbance, cognitive dysfunction, and urinary incontinence that affects a large population of elderly people. These symptoms, especially gait disturbance, can potentially be improved by cerebrospinal fluid (CSF) drainage, which is more effective if performed at an early stage of the disease. However, the neurophysiological mechanisms of these symptoms and their recovery by CSF drainage are poorly understood. In this study, using exact low-resolution brain electromagnetic tomography-independent component analysis (eLORETA-ICA) with electroencephalography (EEG) data, we assessed activities of five EEG resting-state networks (EEG-RSNs) in 58 iNPH patients before and after drainage of CSF by lumbar puncture (CSF tapping). In addition, we assessed correlations of changes in these five EEG-RSNs activities with CSF tapping-induced changes in iNPH symptoms. The results reveal that compared with 80 healthy controls, iNPH patients had significantly decreased activities in the occipital alpha rhythm, visual perception network, and self-referential network before CSF tapping. Furthermore, CSF tapping-induced changes in occipital alpha activity correlated with changes in postural sway and frontal lobe function. Changes in visual perception network activity correlated with changes in gait speed. In addition, changes in memory perception network activity correlated with changes in Parkinsonian gait features. These results indicate a recruitment of cognitive networks in gait control, and involvement of the occipital alpha activity in cognitive dysfunction in iNPH patients. Based on these findings, eLORETA-ICA with EEG data can be considered a noninvasive, useful tool for detection of EEG-RSN activities and for understanding the neurophysiological mechanisms underlying this disease.

EEG Resting-State Networks in Dementia with Lewy Bodies Associated with Clinical Symptoms

BACKGROUND

Dementia with Lewy bodies (DLB) is characterized by progressive cognitive decline, fluctuating cognition, visual hallucinations, rapid eye movement sleep behavior disorder, and parkinsonism. DLB is the second most common type of degenerative dementia of all dementia cases. However, DLB, particularly in the early stage, is underdiagnosed and sometimes misdiagnosed with other types of dementia. Thus, it is of great interest investigating neurophysiological markers of DLB.

METHOD

We introduced exact low-resolution brain electromagnetic tomography (eLORETA)-independent component analysis (ICA) to assess activities of 5 electroencephalography (EEG) resting-state networks (RSNs) in 41 drug-free DLB patients.

RESULTS

Compared to 80 healthy controls, DLB patients had significantly decreased activities in occipital visual and sensorimotor networks, where DLB patients and healthy controls showed no age dependences in all EEG-RSN activities. Also, we found correlations between all EEG-RSN activities and DLB symptoms. Specifically, decreased occipital α activity showed correlations with worse brain functions related to attention/concentration, visuospatial discrimination, and global cognition. Enhanced visual perception network activity correlated with milder levels of depression and anxiety. Enhanced self-referential network activity correlated with milder levels of depression. Enhanced memory perception network activity correlated with better semantic memory, visuospatial discrimination function, and global cognitive function as well as with severer visual hallucination. In addition, decreased sensorimotor network activity correlated with a better semantic memory.

CONCLUSION

These results indicate that eLORETA-ICA can detect EEG-RSN activity alterations in DLB related to symptoms. Therefore, eLORETA-ICA with EEG data can be a useful noninvasive tool for sensitive detection of EEG-RSN activity changes characteristic of DLB and for understanding the neurophysiological mechanisms underlying this disease.

Connecting Mind, Brain, and Seizures by Default Mode

Effective Connectivity Within the Default Mode Network in Left Temporal Lobe Epilepsy: Findings From the Epilepsy Connectome Project

Cook CJ, Hwang G, Mathis J, et al. Brain Connect. 2018. doi:10.1089/brain.2018.0600. [Epub ahead of print]; PMID: 30398367

The Epilepsy Connectome Project examines the differences in connectomes between patients with temporal lobe epilepsy (TLE) and healthy controls. Using these data, the effective connectivity of the default mode network (DMN) in patients with left TLE compared to healthy controls was investigated using spectral dynamic causal modeling of resting state functional magnetic resonance imaging data. Group comparisons were made using 2 parametric empirical Bayes (PEB) models. The first level of each PEB model consisted of each participant’s spectral dynamic causal modeling. Two different second-level models were constructed: the first comparing effective connectivity of the groups directly and the second using the Rey Auditory Verbal Learning Test (RAVLT) delayed free recall index as a covariate at the second level in order to assess effective connectivity controlling for the poor memory performance of patients with left TLE. After an automated search over the nested parameter space and thresholding parameters at 95% posterior probability, both models revealed numerous connections in the DMN that lead to inhibition of the left hippocampal formation. Left hippocampal formation inhibition may be an inherent result of the left temporal epileptogenic focus as memory differences were controlled for in one model and the same connections remained. An excitatory connection from the posterior cingulate cortex to the medial prefrontal cortex was found to be concomitant with left hippocampal formation inhibition in patients with TLE when including RAVLT delayed free recall at the second level.

Intrinsic neurocognitive network connectivity differences between normal aging and mild cognitive impairment are associated with cognitive status and age

Mild cognitive impairment (MCI) of the amnestic type is considered to be a transitionary stage between healthy aging and Alzheimer's disease (AD). Previous studies have demonstrated that intrinsic functional connectivity of the default network (DN) is altered in normal aging and AD and impacts both within- and between-network connectivity. Although changes within the DN have been reported in MCI, it remains uncertain how interactions with other large-scale brain networks are altered in this prodromal stage of AD. We investigated within- and between-network connectivity in healthy older adults (HOAs) and older adults with MCI across 3 canonical brain networks: DN, dorsal attention network, and frontoparietal control network. We also assessed how patterns of functional connectivity among the 3 networks predicted cognitive status and age using multivariate partial least squares. A total of 91 MCI and 71 HOA resting-state scans were analyzed from the Alzheimer's Disease Neuroimaging Initiative. There were 3 key findings. First, a circumscribed pattern of greater between-network and interhemispheric connectivity was associated with higher cognitive status in HOAs. Second, for individuals with MCI, cognitive status was positively associated with a more distributed, less-differentiated pattern of intrinsic functional connectivity across the 3 networks. Finally, greater within-network functional connectivity was positively associated with cognitive status for HOAs irrespective of age; however, this compensation-like effect diminished with increasing age for participants with MCI. Although reliable differences between healthy aging and MCI in the intrinsic network architecture of the brain are apparent, these differences emerge as shifting associations between network interactivity, cognitive functioning, and age.

Functional network interactions at rest underlie individual differences in memory ability

Intrinsic network interactions may underlie individual differences in the ability to remember. The default mode network (DMN) comprises subnetworks implicated in memory, and interactions between the DMN and frontoparietal network (FPN) were shown to support mnemonic processing. However, it is unclear if such interactions during resting-state predict episodic memory ability. We investigated whether intrinsic network interactions within and between the DMN and FPN are related to individual differences in memory performance. Resting-state activity was measured using functional MRI in healthy young adults followed by a memory test for object–location associations that were studied 3 d earlier. We identified two subnetworks within the DMN, the main-DMN and the medial temporal lobe, retrosplenial cortex (MTL_RSC)-DMN. Further, we found regions forming the FPN. Memory performance was associated with lower connectivity within the MTL_RSC-DMN, and stronger connectivity between the main-DMN and FPN. Exploratory whole-brain analysis revealed stronger MTL connectivity with the left posterior parietal cortex that was related to better memory performance. Furthermore, we found increased task-evoked activation during successful retrieval within the main-DMN and FPN, but not within the MTL_RSC-DMN. In sum, lower intrinsic connectivity within the MTL_RSC-DMN, combined with stronger connectivity between the main-DMN and FPN, explain individual differences in memory ability.

Age-related differences in default-mode network connectivity in response to intermittent theta-burst stimulation and its relationships with maintained cognition and brain integrity in healthy aging

The default-mode network (DMN) is affected by advancing age, where particularly long-range connectivity has been consistently reported to be reduced as compared to young individuals. We examined whether there were any differences in the effects of intermittent theta-burst stimulation (iTBS) in DMN connectivity between younger and older adults, its associations with cognition and brain integrity, as well as with long-term cognitive status. Twenty-four younger and 27 cognitively normal older adults were randomly assigned to receive real or sham iTBS over the left inferior parietal lobule between two resting-state functional magnetic resonance imaging (rs-fMRI) acquisitions. Three years later, those older adults who had received real iTBS underwent a cognitive follow-up assessment. Among the younger adults, functional connectivity increased following iTBS in distal DMN areas from the stimulation site. In contrast, older adults exhibited increases in connectivity following iTBS in proximal DMN regions. Moreover, older adults with functional responses to iTBS resembling those of the younger participants exhibited greater brain integrity and higher cognitive performance at baseline and at the 3-year follow-up, along with less cognitive decline. Finally, we observed that 'young-like' functional responses to iTBS were also related to the educational background attained amongst older adults. The present study reveals that functional responses of the DMN to iTBS are modulated by age. Furthermore, combining iTBS and rs-fMRI in older adults may allow characterizing distinctive cognitive profiles in aging and its progression, probably reflecting network plasticity systems that may entail a neurobiological substrate of cognitive reserve.

Bilateral Transcranial Magnetic Stimulation on DLPFC Changes Resting State Networks and Cognitive Function in Patients With Bipolar Depression

Introduction: Bipolar patients have abnormalities in cognitive functions and emotional processing. Two resting state networks (RSNs), the default mode network (DMN) and the sensorimotor network (SMN), play a decisive role in these two functions. Dorsolateral prefrontal cortex (DLPFC) is one of the main areas in the central executive network (CEN), which is linked to the activities of each of the two networks. Studies have found DLPFC abnormalities in both hemispheres of patients with bipolar depression. We hypothesized that the bilateral repetitive transcranial magnetic stimulation (rTMS) of DLPFC would produce changes in the activity of both the SMN and DMN as well as relevant cognitive function in patients with bipolar depression that responded to treatment.

Methods: 20 patients with bipolar depression underwent 10 sessions of 1 Hz rTMS on right DLPFC with subsequent 10 Hz rTMS on left DLPFC. Changes in electroencephalography resting networks between pre and post rTMS were evaluated utilizing low-resolution electromagnetic tomography (eLORETA). Depression symptom was assessed using the Beck Depression Inventory (BDI-II) and cognitive function was assessed by Verbal Fluency Test (VFT), Rey Auditory Verbal Learning Test (RAVLT), Stroop Test, and Wisconsin Card Sorting Test (WCST).

Results: Responders to rTMS showed significantly lower DMN activity at baseline and a significant decrease in SMN connectivity after treatment. Non-responders did not significantly differ from the control group at the baseline and they showed higher activity in the SMN, visual network, and visual perception network compared to control group following treatment. Bilateral rTMS resulted in significant changes in the executive functions, verbal memory, and depression symptoms. No significant changes were observed in selective attention and verbal fluency.

Conclusion: Bilateral stimulation of DLPFC, as the main node of CEN, results in changes in the activity of the SMN and consequently improves verbal memory and executive functions in patients with bipolar depression.

A longitudinal model for functional connectivity networks using resting-state fMRI

Many neuroimaging studies collect functional magnetic resonance imaging (fMRI) data in a longitudinal manner. However, the current fMRI literature lacks a general framework for analyzing functional connectivity (FC) networks in fMRI data obtained from a longitudinal study. In this work, we build a novel longitudinal FC model using a variance components approach. First, for all subjects' visits, we account for the autocorrelation inherent in the fMRI time series data using a non-parametric technique. Second, we use a generalized least squares approach to estimate 1) the within-subject variance component shared across the population, 2) the baseline FC strength, and 3) the FC's longitudinal trend. Our novel method for longitudinal FC networks seeks to account for the within-subject dependence across multiple visits, the variability due to the subjects being sampled from a population, and the autocorrelation present in fMRI time series data, while restricting the number of parameters in order to make the method computationally feasible and stable. We develop a permutation testing procedure to draw valid inference on group differences in the baseline FC network and change in FC over longitudinal time between a set of patients and a comparable set of controls. To examine performance, we run a series of simulations and apply the model to longitudinal fMRI data collected from the Alzheimer's Disease Neuroimaging Initiative (ADNI) database. Overall, we found no difference in the global FC network between Alzheimer's disease patients and healthy controls, but did find differing local aging patterns in the FC between the left hippocampus and the posterior cingulate cortex.