Compensatory recruitment of neural resources during overt rehearsal of word lists in Alzheimer's disease

Disrupted Dynamic Functional Network Connectivity Among Cognitive Control Networks in the Progression of Alzheimer's Disease

Background: Alzheimer's disease (AD) is the most common age-related dementia that promotes a decline in memory, thinking, and social skills. The initial stages of dementia can be associated with mild symptoms, and symptom progression to a more severe state is heterogeneous across patients. Recent work has demonstrated the potential for functional network mapping to assist in the prediction of symptomatic progression. However, this work has primarily used static functional connectivity (sFC) from resting-state functional magnetic resonance imaging. Recently, dynamic functional connectivity (dFC) has been recognized as a powerful advance in functional connectivity methodology to differentiate brain network dynamics between healthy and diseased populations. Methods: Group independent component analysis was applied to extract 17 components within the cognitive control network (CCN) from 1385 individuals across varying stages of AD symptomology. We estimated dFC among 17 components within the CCN, followed by clustering the dFCs into 3 recurring brain states, and then estimated a hidden Markov model and the occupancy rate for each subject. Then, we investigated the link between CCN dFC features and AD progression. Also, we investigated the link between sFC and AD progression and compared its results with dFC results. Results: Progression of AD symptoms was associated with increases in connectivity within the middle frontal gyrus. Also, the very mild AD (vmAD) showed less connectivity within the inferior parietal lobule (in both sFC and dFC) and between this region and the rest of CCN (in dFC analysis). Also, we found that within-middle frontal gyrus connectivity increases with AD progression in both sFC and dFC results. Finally, comparing with vmAD, we found that the normal brain spends significantly more time in a state with lower within-middle frontal gyrus connectivity and higher connectivity between the hippocampus and the rest of CCN, highlighting the importance of assessing the dynamics of brain connectivity in this disease. Conclusion: Our results suggest that AD progress not only alters the CCN connectivity strength but also changes the temporal properties in this brain network. This suggests the temporal and spatial pattern of CCN as a biomarker that differentiates different stages of AD.

The Effects of Total Sleep Deprivation on Attention Capture Processes in Young and Older Adults: An ERP Study

BACKGROUND

The present study investigated whether sleep deprivation affects attention capture in young and older adults using event-related potentials (ERPs).

METHODS

Eleven young adults (20-30 y) and nine older adults (60-70 y) were tested following both normal sleep (NS) and total sleep deprivation (TSD). ERPs were recorded during an auditory discrimination task consisting of standard and deviant stimuli.

RESULTS

Deviant stimuli elicited the MMN, P3a, and RON ERPs. TSD attenuated the differences in reaction times between standards and deviants in young adults but not older adults. The P3a was attenuated in older adults compared to young adults. Older adults had a larger RON amplitude compared to young adults following NS, but not TSD.

CONCLUSIONS

The reduced P3a and the absence of behavioral performance alteration in the older group suggests that older adults may utilize different neural processing strategies compared to younger adults to compensate for age-related declines in neural resources for attention capture. Sleep loss influenced age-related differences on the RON, suggesting that older adults may have reduced access to compensatory strategies following sleep loss.

Retrospective metamemory monitoring of semantic memory in community-dwelling older adults with subjective cognitive decline and mild cognitive impairment

In neurodegenerative conditions, better memory/cognitive awareness, indexed by greater "metamemory monitoring accuracy", is linked to stronger cognitive remediation outcomes. Differences in metamemory monitoring accuracy in predementia conditions, which could inform treatment effectiveness, have not been systematically investigated. We utilized a retrospective confidence judgment (RCJ) task for general knowledge recognition in community-dwelling older adults: 106 cognitively healthy (HC), 68 subjective cognitive decline (SCD) despite intact neuropsychological function, 14 amnestic mild cognitive impairment (aMCI), and 31 non-amnestic mild cognitive impairment (naMCI). Participants gave confidence ratings after making recognition responses to general knowledge questions. Recognition accuracy, confidence levels, and absolute and relative RCJ accuracy (i.e., metamemory monitoring accuracy) were analysed. Compared to HC and SCD, absolute RCJ accuracy was significantly poorer in both MCI groups but relative RCJ accuracy was significantly poorer in naMCI, but not aMCI. This novel result may be driven by lower confidence for correct recognition responses in naMCI and suggests that poorer RCJ accuracy in naMCI may be attributable to poorer performance monitoring. We discuss results in relation to the possibility that individuals in distinct preclinical dementia conditions, who have different levels of memory/cognitive awareness, may differentially benefit from cognitive remediation strategies tailored to their levels of memory/cognitive awareness.

Phosphodiesterase 4D Gene Modifies the Functional Network of Patients With Mild Cognitive Impairment and Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that is affected by several genetic variants. It has been demonstrated that genetic variants affect brain organization and function. In this study, using whole genome-wide association studies (GWAS), we analyzed the functional magnetic resonance imaging and genetic data from the Alzheimer’s Disease Neuroimaging Initiative dataset (ADNI) dataset and identified genetic variants associated with the topology of the functional brain network http://www.adni-info.org. We found three novel loci (rs2409627, rs9647533, and rs11955845) in an intron of the phosphodiesterase 4D (PDE4D) gene that contribute to abnormalities in the topological organization of the functional network. In particular, compared to the wild-type genotype, the subjects carrying the PDE4D variants had altered network properties, including a significantly reduced clustering coefficient, small-worldness, global and local efficiency, a significantly enhanced path length and a normalized path length. In addition, we found that all global brain network attributes were affected by PDE4D variants to different extents as the disease progressed. Additionally, brain regions with alterations in nodal efficiency due to the variations in PDE4D were predominant in the limbic lobe, temporal lobe and frontal lobes. PDE4D has a great effect on memory consolidation and cognition through long-term potentiation (LTP) effects and/or the promotion of inflammatory effects. PDE4D variants might be a main reasons underlyling for the abnormal topological properties and cognitive impairment. Furthermore, we speculated that PDE4D is a risk factor for neural degenerative diseases and provided important clues for the earlier detection and therapeutic intervention for AD.

Brain's compensatory response to drug-induced cognitive impairment

INTRODUCTION

Topiramate (TPM), a frequently prescribed antiseizure medication, can cause severe cognitive side-effects. Though these side-effects have been studied behaviorally, the underlying neural mechanisms are unknown. In a double-blind, randomized, placebo-controlled, crossover study of TPM's impact on cognition, nine healthy volunteers completed three study sessions: a no-drug baseline session and two sessions during which they received either TPM or placebo. Electroencephalogram was recorded during each session while subjects performed a working-memory task with three memory-loads.

RESULTS

Comparing TPM with baseline we found the following results. (a) TPM administration led to declines in behavioral performance. (b) Fronto-central event-related potentials (ERP) elicited by probe stimuli, representing the primary task network activity, showed strong memory-load modulations at baseline, but the magnitude of these load-dependent modulations was significantly reduced during TPM session, suggesting drug-induced impairments of the primary task network. (c) ERP responses over bilateral fronto-temporal electrodes, which were not load sensitive at baseline, showed significant memory-load modulations after TPM administration, suggesting the drug-related recruitment of additional neural resources. (d) At fronto-central scalp sites, there was significant increase in response amplitude for low memory-load during TPM session compared to baseline, and the amplitude increase was dependent on TPM plasma concentration, suggesting that the primary task network became less efficient under TPM impact. (e) At bilateral fronto-temporal electrodes, there were no ERP differences when comparing low memory-load trials, but TPM administration led to an increase in ERP responses to high load, the magnitude of which was positively correlated with task performance, suggesting that the recruited neural resources were beneficial for task performance. Placebo-TPM comparison yielded similar effects albeit with generally reduced significance and effect sizes.

CONCLUSION

Our findings support the hypothesis that TPM impairs the primary task network by reducing its efficiency, which triggers compensatory recruitment of additional resources to maintain task performance.

Regional homogeneity changes in amnestic mild cognitive impairment patients

OBJECTIVE

Regional Homogeneity (ReHo) measures the local coherence of spontaneous brain activity, and it is sensitive to detect aberrant local functional connectivity of brain region. We tried to explore the activity of brain network by ReHo method in amnestic mild cognitive impairment (aMCI) patients and examine the impact of regional brain atrophy on the functional results.

METHODS

Data of both structural magnetic resonance images (MRI) and resting-state functional MRI scans were collected from 36 aMCI patients and 46 age-matched healthy controls.

RESULTS

Compared with the HC subjects, the aMCI patients showed significant decreased ReHo areas in the right inferior parietal lobule (IPL), left posterior cingulate cortex/precuneus (PCC/PCu), left inferior temporal gyrus (ITG), right supramarginal gyrus (SMG), right fusiform gyrus (FG), bilateral lentiform nucleus (LN) and right cerebellum posterior lobe, with the right IPL being the most significant area. In addition, the aMCI group also had some significant increased ReHo areas in the right medial frontal gyrus (MFG), bilateral postcentral gyrus (PoCG), left cuneus and right lingual gyrus (LG), possibly reflective of some underlining compensatory mechanism. Furthermore, in the aMCI patients, we found the ReHo index of the left PCC was positively correlated with the AVLT-Immediate Recall scores, while the ReHo index of the left cuneus was negatively correlated with the MMSE scores. In addition, we found that after regressing out the identified regional brain atrophy, the significant correlations between fitted ReHo index and clinical variables still remained.

CONCLUSIONS

Our study indicated that aMCI patients showed significant abnormal local coherence of biological activity in resting state and ReHo could serve as a sensitive biomarker in functional imaging studies of aMCI.

Topological Properties of Large-Scale Cortical Networks Based on Multiple Morphological Features in Amnestic Mild Cognitive Impairment

Previous studies have demonstrated that amnestic mild cognitive impairment (aMCI) has disrupted properties of large-scale cortical networks based on cortical thickness and gray matter volume. However, it is largely unknown whether the topological properties of cortical networks based on geometric measures (i.e., sulcal depth, curvature, and metric distortion) change in aMCI patients compared with normal controls because these geometric features of cerebral cortex may be related to its intrinsic connectivity. Here, we compare properties in cortical networks constructed by six different morphological features in 36 aMCI participants and 36 normal controls. Six cortical features (3 volumetric and 3 geometric features) were extracted for each participant, and brain abnormities in aMCI were identified by cortical network based on graph theory method. All the cortical networks showed small-world properties. Regions showing significant differences mainly located in the medial temporal lobe and supramarginal and right inferior parietal lobe. In addition, we also found that the cortical networks constructed by cortical thickness and sulcal depth showed significant differences between the two groups. Our results indicated that geometric measure (i.e., sulcal depth) can be used to construct network to discriminate individuals with aMCI from controls besides volumetric measures.

Aberrant intra- and inter-network connectivity architectures in Alzheimer's disease and mild cognitive impairment

Alzheimer’s disease (AD) patients and those with high-risk mild cognitive impairment are increasingly considered to have dysfunction syndromes. Large-scale network studies based on neuroimaging techniques may provide additional insight into AD pathophysiology. The aim of the present study is to evaluate the impaired network functional connectivity with the disease progression. For this purpose, we explored altered functional connectivities based on previously well-defined brain areas that comprise the five key functional systems [the default mode network (DMN), dorsal attention network (DAN), control network (CON), salience network (SAL), sensorimotor network (SMN)] in 35 with AD and 27 with mild cognitive impairment (MCI) subjects, compared with 27 normal cognitive subjects. Based on three levels of analysis, we found that intra- and inter-network connectivity were impaired in AD. Importantly, the interaction between the sensorimotor and attention functions was first attacked at the MCI stage and then extended to the key functional systems in the AD individuals. Lower cognitive ability (lower MMSE scores) was significantly associated with greater reductions in intra- and inter-network connectivity across all patient groups. These profiles indicate that aberrant intra- and inter-network dysfunctions might be potential biomarkers or predictors of AD progression and provide new insight into AD pathophysiology.

The effects of sleep deprivation on brain functioning in older adults

Few studies have examined the effects of total sleep deprivation (TSD) on cognitive performance and brain activation using functional MRI (fMRI) in older adults. The current study examines blood oxygen level-dependent (BOLD) activation in older adults and younger adults during the sustained attention (GO) and response inhibition (NOGO) portions of a GO-NOGO cognitive task following 36 hr of total sleep deprivation. No significant performance differences were observed between the groups on the behavioral outcome measures of total hits and false alarms. Neuroimaging results, however, revealed a significant interaction between age-group and sleep-deprivation status. Specifically, older adults showed greater BOLD activation as compared to younger adults after 36 hours total sleep deprivation in brain regions typically associated with attention and inhibitory processes. These results suggest in order for older adults to perform the GO-NOGO task effectively after sleep deprivation, they rely on compensatory recruitment of brain regions that aide in the maintenance of cognitive performance.

Progressive changes in hippocampal resting-state connectivity across cognitive impairment: a cross-sectional study from normal to Alzheimer disease

We investigate the changes in functional connectivity of the left and right hippocampus by comparing the resting-state low-frequency fluctuations in the blood oxygen level-dependent signal from these regions with relation to Alzheimer disease (AD) progression. AD patients were divided into subgroups based on the clinical dementia rating (CDR) scores. Patients with amnestic mild cognitive impairment (aMCI) were also analyzed as an intermediate stage between normal controls and AD. We found that the total functional connectivity of both the right and left hippocampus was maintained during aMCI and the early stages of AD and that it decreased in the later stages of AD. However, when total functional connectivity was broken down into specific regions of the brain, we observed increased or decreased connectivity to specific regions beginning with aMCI. Direct correlation analysis in seeding the left hippocampus revealed a significant decrease in the functional connectivity with the posterior cingulate cortex region and lateral parietal areas, and an increase in functional connectivity in and the anterior cingulate cortex beginning with aMCI. In this study, we were able to quantify the deterioration of resting-state hippocampal connectivity with disease severity and formation of compensatory recruitment in the early stages of AD.

Mild cognitive impairment and asymptomatic Alzheimer disease subjects: equivalent β-amyloid and tau loads with divergent cognitive outcomes

Older adults with intact cognition before death and substantial Alzheimer disease (AD) lesions at autopsy have been termed "asymptomatic AD subjects" (ASYMAD). We previously reported hypertrophy of neuronal cell bodies, nuclei, and nucleoli in the CA1 of the hippocampus (CA1), anterior cingulate gyrus, posterior cingulate gyrus, and primary visual cortex of ASYMAD versus age-matched Control and mild cognitive impairment (MCI) subjects. However, it was unclear whether the neuronal hypertrophy could be attributed to differences in the severity of AD pathology. Here, we performed quantitative analyses of the severity of β-amyloid (Aβ) and phosphorylated tau (tau) loads in the brains of ASYMAD, Control, MCI, and AD subjects (n = 15 per group) from the Baltimore Longitudinal Study of Aging. Tissue sections from CA1, anterior cingulate gyrus, posterior cingulate gyrus, and primary visual cortex were immunostained for Aβ and tau; the respective loads were assessed using unbiased stereology by measuring the fractional areas of immunoreactivity for each protein in each region. The ASYMAD and MCI groups did not differ in Aβ and tau loads. These data confirm that ASYMAD and MCI subjects have comparable loads of insoluble Aβ and tau in regions vulnerable to AD pathology despite divergent cognitive outcomes. These findings imply that cognitive impairment in AD may be caused or modulated by factors other than insoluble forms of Aβ and tau.

Anodal tDCS during face-name associations memory training in Alzheimer's patients

Objective: Given the limited effectiveness of pharmacological treatments, non-pharmacological interventions to treat Alzheimer's disease (AD) have gained attention in recent years. The aim of the present study is to investigate the effects of anodal tDCS (AtDCS) combined with memory training on face-name associations in an AD patient sample.

Methods: Thirty six AD patients were randomly assigned to one of three study groups: Group 1, AtDCS plus individualized computerized memory training; Group 2, placebo tDCS plus individualized computerized memory training; Group 3, AtDCS plus motor training.

Results: A general improvement in performance was observed after 2 weeks of memory training. Both the anodal tDCS plus individualized computerized memory training and the placebo tDCS plus individualized computerized memory training groups had significantly improved performances at 2 weeks compared with the AtDCS plus motor training group.

Conclusion: Our findings suggest a beneficial effect of individualized memory rehabilitation in AD patients.

Electrophysiological indices of response inhibition in human polydrug users

Previous research in ecstasy users suggests impairment of various executive functions. In general, the executive function of response inhibition appears unaffected by ecstasy use. Nonetheless, it remains a possibility that cognitive tasks alone are not sensitive enough to pick up subtle changes in function. The current study sought to investigate behavioural measures of response inhibition and their electrophysiological correlates in drug users. Twenty ecstasy polydrug users, 20 non-ecstasy polydrug users and 20 drug naïve controls were recruited. Participants completed questionnaires about their background drug use, sleep quality, fluid intelligence and mood state. Each individual also completed a Go/NoGo response inhibition task whilst electroencephalography (EEG) measures were recorded. Analysis of variance (ANOVA) revealed that there were no between-group differences on the behavioural measure of response inhibition. Multivariate analysis of variance (MANOVA) revealed no main effect of group across midline electrodes for the P3, N2 and P2 components. Univariate ANOVA revealed significant between-group differences in the P2 component with the ecstasy user group having a significantly higher mean amplitude than drug naïve controls at two midline frontal electrodes: at Fz and significantly higher mean amplitude than both control groups at FCz. The present study provides evidence of atypical early processing in ecstasy users that is suggestive of compensatory mechanisms ameliorating any behavioural differences.

Impact of chronic hypercortisolemia on affective processing

Cushing syndrome (CS) is the classic condition of cortisol dysregulation, and cortisol dysregulation is the prototypic finding in Major Depressive Disorder (MDD). We hypothesized that subjects with active CS would show dysfunction in frontal and limbic structures relevant to affective networks, and also manifest poorer facial affect identification accuracy, a finding reported in MDD. Twenty-one patients with confirmed CS (20 ACTH-dependent and 1 ACTH-independent) were compared to 21 healthy control subjects. Identification of affective facial expressions (Facial Emotion Perception Test) was conducted in a 3 Tesla GE fMRI scanner using BOLD fMRI signal. The impact of disease (illness duration, current hormone elevation and degree of disruption of circadian rhythm), performance, and comorbid conditions secondary to hypercortisolemia were evaluated. CS patients made more errors in categorizing facial expressions and had less activation in left anterior superior temporal gyrus, a region important in emotion processing. CS patients showed higher activation in frontal, medial, and subcortical regions relative to controls. Two regions of elevated activation in CS, left middle frontal and lateral posterior/pulvinar areas, were positively correlated with accuracy in emotion identification in the CS group, reflecting compensatory recruitment. In addition, within the CS group, greater activation in left dorsal anterior cingulate was related to greater severity of hormone dysregulation. In conclusion, cortisol dysregulation in CS patients is associated with problems in accuracy of affective discrimination and altered activation of brain structures relevant to emotion perception, processing and regulation, similar to the performance decrements and brain regions shown to be dysfunctional in MDD. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Abnormal cortical networks in mild cognitive impairment and Alzheimer's disease

Recently, many researchers have used graph theory to study the aberrant brain structures in Alzheimer's disease (AD) and have made great progress. However, the characteristics of the cortical network in Mild Cognitive Impairment (MCI) are still largely unexplored. In this study, the gray matter volumes obtained from magnetic resonance imaging (MRI) for all brain regions except the cerebellum were parcellated into 90 areas using the automated anatomical labeling (AAL) template to construct cortical networks for 98 normal controls (NCs), 113 MCIs and 91 ADs. The measurements of the network properties were calculated for each of the three groups respectively. We found that all three cortical networks exhibited small-world properties and those strong interhemispheric correlations existed between bilaterally homologous regions. Among the three cortical networks, we found the greatest clustering coefficient and the longest absolute path length in AD, which might indicate that the organization of the cortical network was the least optimal in AD. The small-world measures of the MCI network exhibited intermediate values. This finding is logical given that MCI is considered to be the transitional stage between normal aging and AD. Out of all the between-group differences in the clustering coefficient and absolute path length, only the differences between the AD and normal control groups were statistically significant. Compared with the normal controls, the MCI and AD groups retained their hub regions in the frontal lobe but showed a loss of hub regions in the temporal lobe. In addition, altered interregional correlations were detected in the parahippocampus gyrus, medial temporal lobe, cingulum, fusiform, medial frontal lobe, and orbital frontal gyrus in groups with MCI and AD. Similar to previous studies of functional connectivity, we also revealed increased interregional correlations within the local brain lobes and disrupted long distance interregional correlations in groups with MCI and AD.

Understanding the progression of Alzheimer's disease (AD) is essential. We investigated networks of cortical connectivity along a continuum from normal to AD. Mild Cognitive Impairment (MCI) has been implicated as transitional between normal aging and AD. By investigating the characteristics of cortical networks in these three stages (normal, MCI and AD), we found that all three networks exhibited small-world properties. These properties indicate efficient information transfer in the human brain. We also found that the small-world measures of the MCI network were intermediate to those of the normal controls and the patients with AD. This supports the opinion that MCI is a transitional stage between normal aging and AD. Additionally, we found altered interregional correlations in patients with MCI and AD, which may indicate that a compensatory system interacts with cerebral atrophy. The presence of compensatory mechanisms in patients with MCI and AD may enable them to use additional cognitive resources to function on a more nearly normal level. In future, we need to integrate the multi-level network features obtained with various functional and anatomical brain imaging technologies on different scales to understand the pathophysiological mechanism of MCI and AD. We propose brainnetome to represent such integration framework.

Altered functional adaptation to attention and working memory tasks with increasing complexity in relapsing-remitting multiple sclerosis patients

As attention, processing speed, and working memory seem to be fundamental for a broad range of cognitive performance, the present study on patients with mild forms of relapsing-remitting multiple sclerosis (RR-MS) focused on these domains. To explore subtle neuropsychological changes in either the clinical or fMRI domain, we applied a multistep experimental design with increasing task complexity to investigate global brain activity, functional adaptation, and behavioral responses to typical cognitive processes related to attention and working memory. Fifteen patients with RR-MS (mean age 38 years, 22-49 years, 9 females, mean disease duration 5.9 years (SD = 3.6 years), mean Expanded Disability Status Scale score, 2.3 (SD = 1.3) but without reported cognitive impairment), and 15 age-matched healthy controls (HC; mean age, 34 years, 23-50 years, 6 women) participated. After a comprehensive neuropsychological assessment, participants performed different fMRI experiments testing attention and working memory. In the neuropsychological assessment, patients showed only subtle reduction in learning and memory abilities. In the fMRI experiments, both groups activated the brain areas typically involved in attention and working memory. HC showed a linear in- or decrease in activation paralleling the changing task complexity. Patients showed stronger activation change at the level of the simple tasks and a subsequent saturation effect of (de-)activation at the highest task load. These group/task interaction differences were found in the right parahippocampal cortex and in the middle and medial frontal regions. Our results indicate that, in MS, functional adaptation patterns can be found which precede clinical evidence of apparent cognitive decline.

Loss of 'small-world' networks in Alzheimer's disease: graph analysis of FMRI resting-state functional connectivity

Background

Local network connectivity disruptions in Alzheimer's disease patients have been found using graph analysis in BOLD fMRI. Other studies using MEG and cortical thickness measures, however, show more global long distance connectivity changes, both in functional and structural imaging data. The form and role of functional connectivity changes thus remains ambiguous. The current study shows more conclusive data on connectivity changes in early AD using graph analysis on resting-state condition fMRI data.

Methodology/Principal Findings

18 mild AD patients and 21 healthy age-matched control subjects without memory complaints were investigated in resting-state condition with MRI at 1.5 Tesla. Functional coupling between brain regions was calculated on the basis of pair-wise synchronizations between regional time-series. Local (cluster coefficient) and global (path length) network measures were quantitatively defined. Compared to controls, the characteristic path length of AD functional networks is closer to the theoretical values of random networks, while no significant differences were found in cluster coefficient. The whole-brain average synchronization does not differ between Alzheimer and healthy control groups. Post-hoc analysis of the regional synchronization reveals increased AD synchronization involving the frontal cortices and generalized decreases located at the parietal and occipital regions. This effectively translates in a global reduction of functional long-distance links between frontal and caudal brain regions.

Conclusions/Significance

We present evidence of AD-induced changes in global brain functional connectivity specifically affecting long-distance connectivity. This finding is highly relevant for it supports the anterior-posterior disconnection theory and its role in AD. Our results can be interpreted as reflecting the randomization of the brain functional networks in AD, further suggesting a loss of global information integration in disease.

CSF Biomarkers for Alzheimer's Disease Diagnosis

Alzheimer's disease (AD) is the most common form of dementia that affects several million people worldwide. The major neuropathological hallmarks of AD are the presence of extracellular amyloid plaques that are composed of Aβ40 and Aβ42 and intracellular neurofibrillary tangles (NFT), which is composed of hyperphosphorylated protein Tau. While the amyloid plaques and NFT could define the disease progression involving neuronal loss and dysfunction, significant cognitive decline occurs before their appearance. Although significant advances in neuroimaging techniques provide the structure and physiology of brain of AD cases, the biomarker studies based on cerebrospinal fluid (CSF) and plasma represent the most direct and convenient means to study the disease progression. Biomarkers are useful in detecting the preclinical as well as symptomatic stages of AD. In this paper, we discuss the recent advancements of various biomarkers with particular emphasis on CSF biomarkers for monitoring the early development of AD before significant cognitive dysfunction.

Learning brain connectivity of Alzheimer's disease by sparse inverse covariance estimation

Rapid advances in neuroimaging techniques provide great potentials for study of Alzheimer's disease (AD). Existing findings have shown that AD is closely related to alteration in the functional brain network, i.e., the functional connectivity between different brain regions. In this paper, we propose a method based on sparse inverse covariance estimation (SICE) to identify functional brain connectivity networks from PET data. Our method is able to identify both the connectivity network structure and strength for a large number of brain regions with small sample sizes. We apply the proposed method to the PET data of AD, mild cognitive impairment (MCI), and normal control (NC) subjects. Compared with NC, AD shows decrease in the amount of inter-region functional connectivity within the temporal lobe especially between the area around hippocampus and other regions and increase in the amount of connectivity within the frontal lobe as well as between the parietal and occipital lobes. Also, AD shows weaker between-lobe connectivity than within-lobe connectivity and weaker between-hemisphere connectivity, compared with NC. In addition to being a method for knowledge discovery about AD, the proposed SICE method can also be used for classifying new subjects, which makes it a suitable approach for novel connectivity-based AD biomarker identification. Our experiments show that the best sensitivity and specificity our method can achieve in AD vs. NC classification are 88% and 88%, respectively.

The Nun study: clinically silent AD, neuronal hypertrophy, and linguistic skills in early life

BACKGROUND

It is common to find substantial Alzheimer disease (AD) lesions, i.e., neuritic beta-amyloid plaques and neurofibrillary tangles, in the autopsied brains of elderly subjects with normal cognition assessed shortly before death. We have termed this status asymptomatic AD (ASYMAD). We assessed the morphologic substrate of ASYMAD compared to mild cognitive impairment (MCI) in subjects from the Nun Study. In addition, possible correlations between linguistic abilities in early life and the presence of AD pathology with and without clinical manifestations in late life were considered.

METHODS

Design-based stereology was used to measure the volumes of neuronal cell bodies, nuclei, and nucleoli in the CA1 region of hippocampus (CA1). Four groups of subjects were compared: ASYMAD (n = 10), MCI (n = 5), AD (n = 10), and age-matched controls (n = 13). Linguistic ability assessed in early life was compared among all groups.

RESULTS

A significant hypertrophy of the cell bodies (+44.9%), nuclei (+59.7%), and nucleoli (+80.2%) in the CA1 neurons was found in ASYMAD compared with MCI. Similar differences were observed with controls. Furthermore, significant higher idea density scores in early life were observed in controls and ASYMAD group compared to MCI and AD groups.

CONCLUSIONS

1) Neuronal hypertrophy may constitute an early cellular response to Alzheimer disease (AD) pathology or reflect compensatory mechanisms that prevent cognitive impairment despite substantial AD lesions; 2) higher idea density scores in early life are associated with intact cognition in late life despite the presence of AD lesions.

Semantic memory activation in amnestic mild cognitive impairment

Cognitively intact older individuals at risk for developing Alzheimer's disease frequently show increased functional magnetic resonance imaging (fMRI) brain activation presumably associated with compensatory recruitment, whereas mild cognitive impairment (MCI) patients tend not to show increased activation presumably due to reduced neural reserve. Previous studies, however, have typically used episodic memory activation tasks, placing MCI participants at a performance disadvantage relative to healthy elders. In this event-related fMRI study, we employed a low effort, high accuracy semantic memory task to determine if increased activation of memory circuits is preserved in amnestic MCI when task performance is controlled. Fifty-seven participants, aged 65-85 years, comprised three groups (n = 19 each): amnestic MCI patients; cognitively intact older participants at risk for developing Alzheimer's disease based on having at least one ApoE epsilon4 allele and a positive family history of Alzheimer's disease (At Risk); and cognitively intact participants without Alzheimer's disease risk factors (Control). fMRI was conducted on a 3T MR scanner while participants performed a famous name discrimination task. Participants also underwent neuropsychological testing outside the scanner; whole brain and hippocampal atrophy were assessed from anatomical MRI scans. The three groups did not differ on demographic variables or on fame discrimination performance (>87% correct for all groups). As expected, the amnestic MCI participants demonstrated reduced episodic memory performance. Spatial extent of activation (Fame--Unfamiliar subtraction) differentiated the three groups (Control = 0 ml, At Risk = 9.7 ml, MCI = 34.7 ml). The MCI and At Risk groups showed significantly greater per cent signal change than Control participants in 8 of 14 functionally defined regions, including the medial temporal lobe, temporoparietal junction, and posterior cingulate/precuneus. MCI participants also showed greater activation than Controls in two frontal regions. At Risk, but not MCI, participants showed increased activity in the left hippocampal complex; MCI participants, however, evidenced increased activity in this region when hippocampal atrophy was controlled. When performance is equated, MCI patients demonstrate functional compensation in brain regions subserving semantic memory systems that generally equals or exceeds that observed in cognitively intact individuals at risk for Alzheimer's disease. This hyperactivation profile in MCI is even observed in the left hippocampal complex, but only when the extent of hippocampal atrophy is taken into consideration.

Biomarkers of Alzheimer's disease

Although a battery of neuropsychological tests is often used in making a clinical diagnosis of Alzheimer's disease (AD), definitive diagnosis still relies on pathological evaluation at autopsy. The identification of AD biomarkers may allow for a less invasive and more accurate diagnosis as well as serve as a predictor of future disease progression and treatment response. Importantly, biomarkers may also allow for the identification of individuals who are already developing the underlying pathology of AD such as plaques and tangles yet who are not yet demented, i.e. "preclinical" AD. Attempts to identify biomarkers have included fluid and imaging studies, with a number of candidate markers showing significant potential. More recently, better reagent availability and novel methods of assessment have further spurred the search for biomarkers of AD. This review will discuss promising fluid and imaging markers to date.

The predictive brain state: timing deficiency in traumatic brain injury?

Attention and memory deficits observed in traumatic brain injury (TBI) are postulated to result from the shearing of white matter connections between the prefrontal cortex, parietal lobe, and cerebellum that are critical in the generation, maintenance, and precise timing of anticipatory neural activity. These fiber tracts are part of a neural network that generates predictions of future states and events, processes that are required for optimal performance on attention and working memory tasks. The authors discuss the role of this anticipatory neural system for understanding the varied symptoms and potential rehabilitation interventions for TBI. Preparatory neural activity normally allows the efficient integration of sensory information with goal-based representations. It is postulated that an impairment in the generation of this activity in traumatic brain injury (TBI) leads to performance variability as the brain shifts from a predictive to reactive mode. This dysfunction may constitute a fundamental defect in TBI as well as other attention disorders, causing working memory deficits, distractibility, a loss of goal-oriented behavior, and decreased awareness.

Neuronal hypertrophy in asymptomatic Alzheimer disease

The pathologic changes of Alzheimer disease (AD) evolve very gradually over decades before the disease becomes clinically manifest. Thus, it is not uncommon to find substantial numbers of Abeta plaques and neurofibrillary tangles in autopsy brains of older subjects with documented normal cognition, a state that we define as asymptomatic AD (ASYMAD). The goal of this study is to understand the morphometric substrate of ASYMAD subjects compared with mild cognitive impairment and definite AD cases. We used designed-based stereology to measure the volumes of neuronal cell bodies, nuclei, and nucleoli in 4 cerebral regions: anterior cingulate gyrus, posterior cingulate gyrus, primary visual cortex, and CA1 of hippocampus. We examined and compared autopsy brains from 4 groups (n = 15 each) of participants in the Baltimore Longitudinal Study of Aging: ASYMAD, mild cognitive impairment, AD, and age-matched controls. We found significant hypertrophy of the neuronal cell bodies, nuclei, and nucleoli of CA1 of hippocampus and anterior cingulate gyrus neurons in ASYMAD subjects compared with control and mild cognitive impairment cases. In the posterior cingulate gyrus and primary visual cortex, the hypertrophy was limited to the nuclei and nucleoli. The hypertrophy of cortical neurons and their nuclei and nucleoli in ASYMAD may represent an early reaction to the presence of neurotoxic Abeta or tau, or a compensatory mechanism that prevents the progression of the disease into dementia.

Concept of functional imaging of memory decline in Alzheimer's disease

Functional imaging methods such as Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI) have contributed inestimably to the understanding of physiological cognitive processes in the brain in the recent decades. These techniques for the first time allowed the in vivo assessment of different features of brain function in the living human subject. It was therefore obvious to apply these methods to evaluate pathomechanisms of cognitive dysfunction in disorders such as Alzheimer's disease (AD) as well. One of the most dominant symptoms of AD is the impairment of memory. In this context, the term "memory" represents a simplification and summarizes a set of complex cognitive functions associated with encoding and retrieval of different types of information. A number of imaging studies assessed the functional changes of neuronal activity in the brain at rest and also during performance of cognitive work, with regard to specific characteristics of memory decline in AD. In the current article, basic principles of common functional imaging procedures will be explained and it will be discussed how they can be reasonably applied for the assessment of memory decline in AD. Furthermore, it will be illustrated how these imaging procedures have been employed to improve early and specific diagnosis of the disease, to understand specific pathomechanisms of memory dysfunction and associated compensatory mechanisms, and to draw reverse conclusions on physiological function of memory.

Discrepancy between subjective symptomatology and objective neuropsychological performance in insomnia

STUDY OBJECTIVES

While daytime impairment is a defining feature of primary insomnia (PI), prior research using objective measures has not yielded clear and reliable evidence of global or specific deficits. In this investigation subjective and neuropsychological measures of daytime impairment were concurrently evaluated in subjects with primary insomnia (PIs) and in healthy good sleeper subjects (GSs). The goals for the study were to assess (1) whether PIs differ from GSs on subjective and/or objective measures and (2) the extent to which subjective and objective measures provide discordant information.

DESIGN

Subjects were evaluated on multiple self-report measures of sleep and daytime performance and were administered a comprehensive set of neuropsychological tests.

SETTING

The University of Rochester Sleep and Neurophysiology Research Laboratory (Rochester, NY).

PATIENTS OR PARTICIPANTS

Forty-nine subjects (32 PIs and 17 GSs). Seventy-one percent of the sample was female; average age 39 +/- 11 yrs.

RESULTS

Overall, PIs reported worse sleep, diminished activity levels, and a greater number and severity of daytime complaints. However, PIs did not show deficits on neuropsychological tests. Additionally, neuropsychological measures were not associated with severity of daytime complaints. Objectively measured sleep was found to be associated with performance (motor speed), while prospective and objective sleep measures were associated with level of daytime complaint.

CONCLUSIONS

The discrepancy between subjective daytime complaints and objective performance in individuals with insomnia is common, but poorly understood. This discordance may suggest that daytime impairment corresponds less to "output" and more to attentional bias or to the realistic appraisal that "effort" is required to maintain normal performance.

Altered functional connectivity in early Alzheimer's disease: a resting-state fMRI study

Previous studies have led to the proposal that patients with Alzheimer's disease (AD) may have disturbed functional connectivity between different brain regions. Furthermore, recent resting-state functional magnetic resonance imaging (fMRI) studies have also shown that low-frequency (<0.08 Hz) fluctuations (LFF) of the blood oxygenation level-dependent signals were abnormal in several brain areas of AD patients. However, few studies have investigated disturbed LFF connectivity in AD patients. By using resting-state fMRI, this study sought to investigate the abnormal functional connectivities throughout the entire brain of early AD patients, and analyze the global distribution of these abnormalities. For this purpose, the authors divided the whole brain into 116 regions and identified abnormal connectivities by comparing the correlation coefficients of each pair. Compared with healthy controls, AD patients had decreased positive correlations between the prefrontal and parietal lobes, but increased positive correlations within the prefrontal lobe, parietal lobe, and occipital lobe. The AD patients also had decreased negative correlations (closer to zero) between two intrinsically anti-correlated networks that had previously been found in the resting brain. By using resting-state fMRI, our results supported previous studies that have reported an anterior-posterior disconnection phenomenon and increased within-lobe functional connectivity in AD patients. In addition, the results also suggest that AD may disturb the correlation/anti-correlation effect in the two intrinsically anti-correlated networks.

rCBF pathology in Alzheimer's disease is associated with slow processing speed

Decreased information processing speed (mental slowing) is a known sequelae of many brain disorders, and can be assessed by continuous naming tasks. Functional imaging studies have shown that pause and articulation times in continuous speech are normally associated with different brain regions, but knowledge about such association in dementia is lacking. We therefore tested the hypothesis that perfusion deficits in Alzheimer's disease (AD) are not only associated with slower processing, but also with these speech measures. Using regional cerebral blood flow (rCBF) measurements during the performance of a continuous colour and form-naming task, we found that naming speed was substantially slower in AD patients than in controls. This slower naming was exclusively determined by an increase in mean pause time, and only to a limited extent by articulation time. The increased pause time was uniquely associated with temporo-parietal rCBF reductions of the patients, while articulation was not. By contrast, the rCBF of healthy elderly control subjects was consistently accompanied by substantially shorter articulation and pause times, although the naming measures were not statistically associated with rCBF. These findings suggest that pause time (in contrast to articulation time) may serve as a sensitive measure in the assessment of information processing speed deficits in dementia, by virtue of its close association with brain pathology.

Use of functional magnetic resonance imaging in the early identification of Alzheimer's disease

A growing body of evidence suggests that a preclinical phase of Alzheimer's disease (AD) exists several years or more prior to the overt manifestation of clinical symptoms and is characterized by subtle neuropsychological and brain changes. Identification of individuals prior to the development of significant clinical symptoms is imperative in order to have the greatest treatment impact by maintaining cognitive abilities and preserving quality of life. Functional magnetic resonance imaging (fMRI) offers considerable promise as a non-invasive tool for detecting early functional brain changes in asymptomatic adults. In fact, evidence to date indicates that functional brain decline precedes structural decline in preclinical samples. Therefore, fMRI may offer the unique ability to capture the dynamic state of change in the degenerating brain. This review examines the clinical utility of blood oxygen level dependent (BOLD) fMRI in those at risk for AD as well as in early AD. We provide an overview of fMRI findings in at-risk groups by virtue of genetic susceptibility or mild cognitive decline followed by an appraisal of the methodological issues concerning the diagnostic usefulness of fMRI in early AD. We conclude with a discussion of future directions and propose that BOLD-fMRI in combination with cerebral blood flow or diffusion techniques will provide a more complete accounting of the neurovascular changes that occur in preclinical AD and thus improve our ability to reliably detect early brain changes prior to disease onset.

In vivo characterization of traumatic brain injury neuropathology with structural and functional neuroimaging

Quantitative neuroimaging is increasingly used to study the effects of traumatic brain injury (TBI) on brain structure and function. This paper reviews quantitative structural and functional neuroimaging studies of patients with TBI, with an emphasis on the effects of diffuse axonal injury (DAI), the primary neuropathology in TBI. Quantitative structural neuroimaging has evolved from simple planometric measurements through targeted region-of-interest analyses to whole-brain analysis of quantified tissue compartments. Recent studies converge to indicate widespread volume loss of both gray and white matter in patients with moderate-to-severe TBI. These changes can be documented even when patients with focal lesions are excluded. Broadly speaking, performance on standard neuropsychological tests of speeded information processing are related to these changes, but demonstration of specific brain-behavior relationships requires more refined experimental behavioral measures. The functional consequences of these structural changes can be imaged with activation functional neuroimaging. Although this line of research is at an early stage, results indicate that TBI causes a more widely dispersed activation in frontal and posterior cortices. Further progress in analysis of the consequences of TBI on neural structure and function will require control of variability in neuropathology and behavior.

A high-resolution single photon emission computed tomography study of verbal recognition memory in Alzheimer's disease

BACKGROUND

In view of the recent technological advances and its ease of availability, we used single photon emission computed tomography (SPECT) to examine the performance of early Alzheimer's disease (AD) subjects on a verbal recognition memory task.

METHODS

Eighteen early AD and 10 matched healthy control subjects underwent split-dose (99m)Tc-HMPAO (Ceretec) SPECT using a verbal recognition memory and control task. SPECT images co-registered with MRI scans were used to determine relative regional cerebral blood flow (rCBF) changes in regions of interest.

RESULTS

In healthy control subjects, verbal recognition increased rCBF in the right occipital region, thalamus, left prefrontal pole, posterior parietal region and cerebellum, and decreased rCBF in the right hippocampus. AD subjects showed bilateral prefrontal, posterior parietal and occipital increases, unilateral increase in the left posterior temporal region, and bilateral reductions in the hippocampus. Although activation was significantly different between the two groups in the right thalamus and left medial prefrontal region, the verbal recognition task did not enhance discrimination between groups.

CONCLUSIONS

Compared with controls, AD subjects activate a similar but more extensive bilateral network during verbal recognition, possibly reflecting an attempt to compensate for impaired processing.

Age-related changes in brain activation during a delayed item recognition task

To test competing models of age-related changes in brain functioning (capacity limitation, neural efficiency, compensatory reorganization, and dedifferentiation), young (n=40; mean age=25.1 years) and elderly (n=18; mean age=74.4 years) subjects performed a delayed item recognition task for visually presented letters with three set sizes (1, 3, or 6 letters) while being scanned with BOLD fMRI. Spatial patterns of brain activity corresponding to either the slope or y-intercept of fMRI signal with respect to set size during memory set encoding, retention delay, or probe stimulus presentation trial phases were compared between elder and young populations. Age effects on fMRI slope during encoding and on fMRI y-intercept during retention delay were consistent with neural inefficiency; age effects on fMRI slope during retention delay were consistent with dedifferentiation. None of the other fMRI signal components showed any detectable age effects. These results suggest that, even within the same task, the nature of brain activation changes with aging can vary based on cognitive process engaged.

Reduced responsiveness is an essential feature of chronic fatigue syndrome: a fMRI study

Background

Although the neural mechanism of chronic fatigue syndrome has been investigated by a number of researchers, it remains poorly understood.

Methods

Using functional magnetic resonance imaging, we studied brain responsiveness in 6 male chronic fatigue syndrome patients and in 7 age-matched male healthy volunteers. Responsiveness of auditory cortices to transient, short-lived, noise reduction was measured while subjects performed a fatigue-inducing continual visual search task.

Results

Responsiveness of the task-dependent brain regions was decreased after the fatigue-inducing task in the normal and chronic fatigue syndrome subjects and the decrement of the responsiveness was equivalent between the 2 groups. In contrast, during the fatigue-inducing period, although responsiveness of auditory cortices remained constant in the normal subjects, it was attenuated in the chronic fatigue syndrome patients. In addition, the rate of this attenuation was positively correlated with the subjective sensation of fatigue as measured using a fatigue visual analogue scale, immediately before the magnetic resonance imaging session.

Conclusion

Chronic fatigue syndrome may be characterised by attenuation of the responsiveness to stimuli not directly related to the fatigue-inducing task.

Discriminative analysis of early Alzheimer's disease based on two intrinsically anti-correlated networks with resting-state fMRI

In this work, we proposed a discriminative model of Alzheimer's disease (AD) on the basis of multivariate pattern classification and functional magnetic resonance imaging (fMRI). This model used the correlation/anti-correlation coefficients of two intrinsically anti-correlated networks in resting brains, which have been suggested by two recent studies, as the feature of classification. Pseudo-Fisher Linear Discriminative Analysis (pFLDA) was then performed on the feature space and a linear classifier was generated. Using leave-one-out (LOO) cross validation, our results showed a correct classification rate of 83%. We also compared the proposed model with another one based on the whole brain functional connectivity. Our proposed model outperformed the other one significantly, and this implied that the two intrinsically anti-correlated networks may be a more susceptible part of the whole brain network in the early stage of AD.

Compensatory recruitment after sleep deprivation and the relationship with performance

This study examined the effects of total sleep deprivation (TSD) on cerebral responses to a verbal learning task with two levels of word difficulty. A total of 32 subjects were studied with functional magnetic resonance imaging (FMRI) after normal sleep and following 36 h of TSD. Cerebral responses to EASY words were identical on both nights, but several brain regions showed increased activation to HARD words following TSD compared with following a normal night of sleep (NORM). These regions included bilateral inferior frontal gyrus, bilateral dorsolateral prefrontal cortex, and bilateral inferior parietal lobe. Better free recall performance on the HARD words after TSD was related to increased cerebral responses within the left inferior and superior parietal lobes and left inferior frontal gyrus. Recall was negatively related to activation within the right inferior frontal gyrus. Overall, the findings support the predictions of the compensatory recruitment hypothesis that task demands influence both the likelihood and location of increased cerebral activation during task performance following TSD, and refine that hypothesis by identifying a specific task demand that plays a role. The performance relationships suggest increased activation may be both beneficial (compensatory) and interfere with task performance, depending on the brain regions involved.

Increased hippocampal activation in mild cognitive impairment compared to normal aging and AD

OBJECTIVE

To use fMRI to investigate whether hippocampal and entorhinal activation during learning is altered in the earliest phase of mild cognitive impairment (MCI).

METHODS

Three groups of older individuals were studied: 10 cognitively intact controls, 9 individuals at the mild end of the spectrum of MCI, and 10 patients with probable Alzheimer disease (AD). Subjects performed a face-name associative encoding task during fMRI scanning, and were tested for recognition of stimuli afterward. Data were analyzed using a functional-anatomic method in which medial temporal lobe (MTL) regions of interest were identified from each individual's structural MRI, and fMRI activation was quantified within each region.

RESULTS

Significantly greater hippocampal activation was present in the MCI group compared to controls; there were no differences between these two groups in hippocampal or entorhinal volumes. In contrast, the AD group showed hippocampal and entorhinal hypoactivation and atrophy in comparison to controls. The subjects with MCI performed similarly to controls on the fMRI recognition memory task; patients with AD exhibited poorer performance. Across all 29 subjects, greater mean entorhinal activation was found in the subgroup of 13 carriers of the APOE epsilon4 allele than in the 16 noncarriers.

CONCLUSIONS

The authors hypothesize that there is a phase of increased medial temporal lobe activation early in the course of prodromal Alzheimer disease followed by a subsequent decrease as the disease progresses.

Functional activation imaging in aging and dementia

With life expectancy increasing continuously, the effects of neurodegeneration on brain function are a topic of ever increasing importance. Thus there is a need for tools and models that probe both the functional consequences of neurodegenerative processes and compensatory mechanisms that might occur. As neurodegenerative burden and compensatory mechanisms may change over time, these tools will ideally be applied multiple times over the lifespan. Specifically, in order to elucidate whether brain-activation patterns in Alzheimer's disease (AD) and in healthy aging follow general rules in the context of degeneration and compensation, it is necessary to compare functional brain-activation patterns during different states of neurodegeneration. This article integrates the findings of functional activation studies at different stages of neurodegeneration: in healthy aging, in subjects at high risk of developing dementia, in subjects with mild cognitive impairment (MCI), and in patients suffering from AD. We review existing theoretical models that aim to explain the underlying mechanisms of functional activation changes in aging and dementia, and we propose an integrative account, which allows for different neural response patterns depending on the amount of neuronal damage and the recruitment of compensatory pathways.

Neuroimaging as a marker of the onset and progression of Alzheimer's disease

Several neuroimaging techniques are promising tools as early markers of brain pathology in Alzheimer's disease (AD). On structural MRI, atrophy of the entorhinal cortex is present already in mild cognitive impairment (MCI). In the autosomal dominant forms of AD, the rate of atrophy of medial temporal structures separates affected from control persons even 3 years before the clinical onset of cognitive impairment. The elevated annual rate of brain atrophy offers a surrogate tool for the evaluation of newer therapies using smaller samples, thereby saving time and resources. On functional MRI, activation paradigms activate a larger area of parieto-temporal association cortex in persons at higher risk for AD, whereas the entorhinal cortex activation is lesser in MCI. Similar findings have been detected with activation procedures and water (H(2)(15)O) PET. Regional metabolism in the entorhinal cortex, studied with FDG PET, seems to predict normal elderly who will deteriorate to MCI or AD. SPECT shows decreased regional perfusion in limbic areas, both in MCI and AD, but with a lower likelihood ratio than PET. Newer PET compounds allow for the determination in AD of microglial activation, regional deposition of amyloid and the evaluation of enzymatic activity in the brain of AD patients.

Brain bases of error-related ERPs as influenced by age and task

Age effects in the error negativity (Ne) and error positivity (Pe) were examined in a standard letter flanker task and an age-sensitive source memory exclusion task. Older adults made more errors and produced Ne and Pe components of lower amplitude in both tasks. The Ne was insensitive to task and error rate. The Pe, however, was reduced in the source memory relative to the flanker task and was correlated with error rate in both tasks. Ne and Pe dipoles were generally localized to anterior cingulate cortex, but dipoles associated with the Pe were more frontal for flanker errors and, for young adults, more posterior for source errors. These data suggest that the Ne reflects an automatic response to error as it occurs whereas the Pe, being more sensitive to age and task demands, and more closely linked to accuracy, reflects the allocation of attention to an error that has been made.

Structure–Function Correlates of Cognitive Decline in Aging

To explore neural correlates of cognitive decline in aging, we used longitudinal behavioral data to identify two groups of older adults (n = 40) that differed with regard to whether their performance on tests of episodic memory remained stable or declined over a decade. Analysis of structural and diffusion tensor imaging (DTI) revealed a heterogeneous set of differences associated with cognitive decline. Manual tracing of hippocampal volume showed significant reduction in those older adults with a declining memory performance as did DTI-measured fractional anisotropy in the anterior corpus callosum. Functional magnetic resonance imaging during incidental episodic encoding revealed increased activation in left prefrontal cortex for both groups and additional right prefrontal activation for the elderly subjects with the greatest decline in memory performance. Moreover, mean DTI measures in the anterior corpus callosum correlated negatively with activation in right prefrontal cortex. These results demonstrate that cognitive decline is associated with differences in the structure as well as function of the aging brain, and suggest that increased activation is either caused by structural disruption or is a compensatory response to such disruption.

Age-dependent change in executive function and gamma 40 Hz phase synchrony

Decline in cognitive function is well recognized, yet few neurophysiological correlates of age-related cognitive decline have been identified. In this study we examined the impact of age on neurocognitive function and Gamma phase synchrony among 550 normal subjects (aged 11-70). Gamma phase synchrony was acquired to targets in the auditory oddball paradigm. The two tasks of executive function were switching of attention and an electronic maze. Subjects were divided into four age groups, which were balanced for sex. We hypothesized that reduced cognitive performance among older healthy individuals would be associated with age-related changes in gamma phase synchrony. Results showed a significant decrease in executive function in the oldest (51-70 years) age group. ANOVAs of age-by-frontal Gamma synchrony also showed a significant effect of age on Gamma phase synchrony in the left frontal region that corresponded modestly to the age effect found on executive task performance, with reduced performance associated with increased gamma synchrony. The results indicate that age-related changes in cognitive function evident among elderly individuals may in part be related to decreased ability to integrate information and this may be reflected as a compensatory increase in gamma synchrony in frontal regions of the brain.

Alpha rhythms in mild dements during visual delayed choice reaction time tasks: A MEG study

Can simple delayed response tasks affect latency and amplitude of magnetoencephalographic midline alpha rhythms (6-12 Hz) in early dementia? We recruited 15 mild Alzheimer's disease (AD) and 10 vascular dementia (VaD) patients (paired mini mental state exam of 17-24). The control groups comprised 18 young and 22 elderly normal subjects. In the first task, a simple "cue" stimulus (one bit) was memorized along a brief delay period (3.5-5.5s) up to a "go" stimulus triggering (right or left) button press. In the second task, the "cue" stimulus remained available along the delay period. Event-related reduction in power of the alpha rhythms indexed the cortical activation (event-related desynchronization, ERD) for the trials associated with correct behavioral responses. Behavioral performances to both tasks were lower in the AD and VaD patients than in the normal subjects. In particular, just four AD and five VaD patients executed a sufficient amount of correct responses for the alpha ERD analysis, so they were included in a unique group. In both tasks, the alpha ERD peak was later in latency in the demented and normal elderly subjects than in the normal young subjects. Furthermore, the alpha ERD peak was stronger in amplitude in the demented patients than in the normal subjects. These results suggest that simple delayed response tasks during physiological recordings are quite difficult for patients even at an early dementia stage. Such difficulty may induce the abnormal amount of the related cortical activation in dementia as revealed by the alpha ERD.

Event-related functional magnetic resonance imaging investigation of executive control in very old individuals with mild cognitive impairment

BACKGROUND

Attentional control of executive cognitive function (ECF) decreases in older individuals with Alzheimer Disease (AD). In order to examine early AD-related changes in the neural substrates of ECF attentional control, we measured activation dorsolateral prefrontal (dLPFC), posterior parietal (PPC), and anterior cingulate cortex (ACC) in adults with mild cognitively impairment (MCI) and in cognitively normal (CN) adults.

METHODS

Functional magnetic resonance imaging analysis of brain activation in MCI (n = 8, mean age 79.5) and CN (n = 8 mean age 81.5) during increasing loads of attentional demands.

RESULTS

MCI and CN older adults performed with similar accuracy and reaction time. MCI had greater activation than CN in PPC (right p = .03 and left p = .05) and dlPFC areas (right p = .002 and left p = .004), while activation in ACC was similar in the two groups. Response to increasing loads of the task differed by group: MCI selectively engaged bilateral PPC (right p = .03, left p = .04), while CN subjects increased bilateral dlPFC activation (right p = .005 and left p = .02) and ACC activation (p = .04). Among MCI, greater load-related changes in PPC activity were associated with smaller load-related changes in accuracy rates (r = -.85, p = .07) and greater increases in reaction times (r = .97, p = .01). In CN subjects, load-related change in PPC activation was associated with load-related change in reaction time (r = .76, p = .02) but not with changes in accuracy rates.

CONCLUSIONS

PPC and dlPFC may show early functional changes associated with MCI.