Use of high-dimensional spectral data to evaluate organic matter, reflectance relationships in soils

Recent breakthroughs in remote sensing technology have led to the development of a spaceborne high spectral resolution imaging sensor, HIRIS, to be launched in the mid-1990s for observation of earth surface features. The effects of organic carbon content on soil reflectance over the spectral range of HIRIS, and to examine the contributions of humic and fulvic acid fractions to soil reflectance was evaluated. Organic matter from four Indiana agricultural soils was extracted, fractionated, and purified, and six individual components of each soil were isolated and prepared for spectral analysis. The four soils, ranging in organic carbon content from 0.99 percent, represented various combinations of genetic parameters such as parent material, age, drainage, and native vegetation. An experimental procedure was developed to measure reflectance of very small soil and organic component samples in the laboratory, simulating the spectral coverage and resolution of the HIRIS sensor. Reflectance in 210 narrow (10 nm) bands was measured using the CARY 17D spectrophotometer over the 400 to 2500 nm wavelength range. Reflectance data were analyzed statistically to determine the regions of the reflective spectrum which provided useful information about soil organic matter content and composition. Wavebands providing significant information about soil organic carbon content were located in all three major regions of the reflective spectrum: visible, near infrared, and middle infrared. The purified humic acid fractions of the four soils were separable in six bands in the 1600 to 2400 nm range, suggesting that longwave middle infrared reflectance may be useful as a non-destructive laboratory technique for humic acid characterization

Resting-State Connectivity of the Left Frontal Cortex to the Default Mode and Dorsal Attention Network Supports Reserve in Mild Cognitive Impairment

Reserve refers to the phenomenon of relatively preserved cognition in disproportion to the extent of neuropathology, e.g., in Alzheimer’s disease. A putative functional neural substrate underlying reserve is global functional connectivity of the left lateral frontal cortex (LFC, Brodmann Area 6/44). Resting-state fMRI-assessed global LFC-connectivity is associated with protective factors (education) and better maintenance of memory in mild cognitive impairment (MCI). Since the LFC is a hub of the fronto-parietal control network that regulates the activity of other networks, the question arises whether LFC-connectivity to specific networks rather than the whole-brain may underlie reserve. We assessed resting-state fMRI in 24 MCI and 16 healthy controls (HC) and in an independent validation sample (23 MCI/32 HC). Seed-based LFC-connectivity to seven major resting-state networks (i.e., fronto-parietal, limbic, dorsal-attention, somatomotor, default-mode, ventral-attention, visual) was computed, reserve was quantified as residualized memory performance after accounting for age and hippocampal atrophy. In both samples of MCI, LFC-activity was anti-correlated with the default-mode network (DMN), but positively correlated with the dorsal-attention network (DAN). Greater education predicted stronger LFC-DMN-connectivity (anti-correlation) and LFC-DAN-connectivity. Stronger LFC-DMN and LFC-DAN-connectivity each predicted higher reserve, consistently in both MCI samples. No associations were detected for LFC-connectivity to other networks. These novel results extend our previous findings on global functional connectivity of the LFC, showing that LFC-connectivity specifically to the DAN and DMN, two core memory networks, enhances reserve in the memory domain in MCI

Neural mechanisms of cognitive reserve in Alzheimer's disease

Alzheimer’s disease (AD) is the most common cause of age-related dementia, where neuropathological changes develop gradually over years before the onset of dementia symptoms. Yet, despite the progression of AD pathology, the decline in cognitive abilities such as episodic memory can be relatively slow. A slower decline of cognition and delayed onset of dementia relative to the progression of neuropathology has been associated with particular intellectual and lifestyle factors such as more years of education and IQ. Thus education and IQ are seen as protective factors that are associated with an increased ability to cope with brain pathology, i.e. cognitive reserve. While numerous studies showed that education, IQ and other lifestyle factors are associated with relatively high cognitive abilities in AD, little is known about the underlying brain mechanisms of reserve. Most previous studies tested the association between protective factors such as education or IQ and differences in brain structure and function in order to identify brain mechanisms underlying reserve. Since such protective factors are global in nature and unspecific with regard to reserve, the results were highly variable. So far, there is a lack of knowledge of brain features that are associated with a higher ability to maintain cognition in the face of AD pathology. The overall aim of this dissertation was to test a priori selected functional network features that may underlie cognitive reserve. We focused on resting-state functional networks, and in particular the fronto-parietal control network as correlate of cognitive reserve. Such functional networks are thought to be composed of brain regions that are co-activated during a particular task, where the interaction between brain regions may be critical to support cognitive function. During task-free resting-state periods, the different and often distant brain regions of such network show correlated activity, i.e. functional connectivity. For the fronto-parietal control network, and in particular its globally connected hub in the left frontal cortex (LFC), higher resting-state connectivity has been previously shown to be associated with higher cognitive abilities as well as higher education and IQ, i.e. protective factors associated with reserve. Since that network and its LFC hub are relatively spared in AD, in contrast to more posterior parietal networks, we investigated whether higher connectivity of the fronto-parietal control network is associated with higher reserve in AD. We argued that the fronto-parietal control network is relatively stable during the initial stages of AD and may thus be well posited to subserve reserve in AD. In contrast, networks like the default mode network (DMN) that cover midline brain structures including the medial frontal lobe and the posterior cingulate may be highly vulnerable to AD pathology, given the previous observations of altered DMN connectivity and posterior parietal FDG-PET hypometabolism in AD. In particular, the resting-state connectivity between the DMN and the dorsal attention network (DAN) may be predictive of lower episodic memory in AD. Both networks interact in a competitive (i.e. anti-correlated) way during task and resting-state, which is critical for cognitive processes such as episodic memory. In a first step, we tested whether the resting-state connectivity between the DMN and theDAN (i.e. anti-correlated activity) is associated with lower episodic memory in subjects with amnestic mild cognitive impairment (MCI), i.e. subjects at increased risk to convertto AD dementia. Furthermore, we tested whether protective factors such as higher education moderate the association between the DMN-DAN anti-correlation andcognition. Here, the DMN-DAN anti-correlation was a measure of AD relatedpathological change rather than a substrate of reserve.We could show in two independent samples of patients at risk of AD dementia that a weaker DMN-DAN anti-correlation was associated with lower episodic memory, where the decrements in episodic memory were however weaker in subjects with higher education or IQ (interaction DMN-DAN x education/IQ). These results suggest that MCI subjects with higher protective factors (education, IQ) maintain episodic memory relatively well at a given level of AD-related brain changes. In the second step, we sought to identify those network differences that support cognitive reserve, i.e. that may explain the association between higher education and milder cognitive impairment in AD. Here, we could show that greater resting-state fMRI assessed global connectivity of the LFC, i.e. a key hub of the fronto-parietal control network, was associated with greater education and attenuated effects of neurodegeneration (measured by parietal FDG-PET hypometabolism) on memory in prodromal AD. Together, these results support the idea that global connectivity of a fronto-parietal control network hub supports cognitive reserve in AD. Based on this finding, we developed a novel restingstate fMRI index of fronto-parietal control network connectivity as a functional imaging marker of cognitive reserve. This marker is highly correlated with education and may thus be used as an imaging-based index of cognitive reserve. Together, our results provide for the first time evidence that cognitive reserve in AD is supported by higher functional connectivity of the fronto-parietal control network, in particular its LFC hub

Neural mechanisms of cognitive reserve in Alzheimer's disease

Alzheimer’s disease (AD) is the most common cause of age-related dementia, where neuropathological changes develop gradually over years before the onset of dementia symptoms. Yet, despite the progression of AD pathology, the decline in cognitive abilities such as episodic memory can be relatively slow. A slower decline of cognition and delayed onset of dementia relative to the progression of neuropathology has been associated with particular intellectual and lifestyle factors such as more years of education and IQ. Thus education and IQ are seen as protective factors that are associated with an increased ability to cope with brain pathology, i.e. cognitive reserve. While numerous studies showed that education, IQ and other lifestyle factors are associated with relatively high cognitive abilities in AD, little is known about the underlying brain mechanisms of reserve. Most previous studies tested the association between protective factors such as education or IQ and differences in brain structure and function in order to identify brain mechanisms underlying reserve. Since such protective factors are global in nature and unspecific with regard to reserve, the results were highly variable. So far, there is a lack of knowledge of brain features that are associated with a higher ability to maintain cognition in the face of AD pathology. The overall aim of this dissertation was to test a priori selected functional network features that may underlie cognitive reserve. We focused on resting-state functional networks, and in particular the fronto-parietal control network as correlate of cognitive reserve. Such functional networks are thought to be composed of brain regions that are co-activated during a particular task, where the interaction between brain regions may be critical to support cognitive function. During task-free resting-state periods, the different and often distant brain regions of such network show correlated activity, i.e. functional connectivity. For the fronto-parietal control network, and in particular its globally connected hub in the left frontal cortex (LFC), higher resting-state connectivity has been previously shown to be associated with higher cognitive abilities as well as higher education and IQ, i.e. protective factors associated with reserve. Since that network and its LFC hub are relatively spared in AD, in contrast to more posterior parietal networks, we investigated whether higher connectivity of the fronto-parietal control network is associated with higher reserve in AD. We argued that the fronto-parietal control network is relatively stable during the initial stages of AD and may thus be well posited to subserve reserve in AD. In contrast, networks like the default mode network (DMN) that cover midline brain structures including the medial frontal lobe and the posterior cingulate may be highly vulnerable to AD pathology, given the previous observations of altered DMN connectivity and posterior parietal FDG-PET hypometabolism in AD. In particular, the resting-state connectivity between the DMN and the dorsal attention network (DAN) may be predictive of lower episodic memory in AD. Both networks interact in a competitive (i.e. anti-correlated) way during task and resting-state, which is critical for cognitive processes such as episodic memory. In a first step, we tested whether the resting-state connectivity between the DMN and theDAN (i.e. anti-correlated activity) is associated with lower episodic memory in subjects with amnestic mild cognitive impairment (MCI), i.e. subjects at increased risk to convertto AD dementia. Furthermore, we tested whether protective factors such as higher education moderate the association between the DMN-DAN anti-correlation andcognition. Here, the DMN-DAN anti-correlation was a measure of AD relatedpathological change rather than a substrate of reserve.We could show in two independent samples of patients at risk of AD dementia that a weaker DMN-DAN anti-correlation was associated with lower episodic memory, where the decrements in episodic memory were however weaker in subjects with higher education or IQ (interaction DMN-DAN x education/IQ). These results suggest that MCI subjects with higher protective factors (education, IQ) maintain episodic memory relatively well at a given level of AD-related brain changes. In the second step, we sought to identify those network differences that support cognitive reserve, i.e. that may explain the association between higher education and milder cognitive impairment in AD. Here, we could show that greater resting-state fMRI assessed global connectivity of the LFC, i.e. a key hub of the fronto-parietal control network, was associated with greater education and attenuated effects of neurodegeneration (measured by parietal FDG-PET hypometabolism) on memory in prodromal AD. Together, these results support the idea that global connectivity of a fronto-parietal control network hub supports cognitive reserve in AD. Based on this finding, we developed a novel restingstate fMRI index of fronto-parietal control network connectivity as a functional imaging marker of cognitive reserve. This marker is highly correlated with education and may thus be used as an imaging-based index of cognitive reserve. Together, our results provide for the first time evidence that cognitive reserve in AD is supported by higher functional connectivity of the fronto-parietal control network, in particular its LFC hub

Left Frontal Hub Connectivity during Memory Performance Supports Reserve in Aging and Mild Cognitive Impairment

Reserve in aging and Alzheimer's disease (AD) is defined as maintaining cognition at a relatively high level in the presence of neurodegeneration, an ability often associated with higher education among other life factors. Recent evidence suggests that higher resting-state functional connectivity within the frontoparietal control network, specifically the left frontal cortex (LFC) hub, contributes to higher reserve. Following up these previous resting-state fMRI findings, we probed memory-task related functional connectivity of the LFC hub as a neural substrate of reserve. In elderly controls (CN, n = 37) and patients with mild cognitive impairment (MCI, n = 17), we assessed global connectivity of the LFC hub during successful face-name association learning, using generalized psychophysiological interaction analyses. Reserve was quantified as residualized memory performance, accounted for gender and proxies of neurodegeneration (age, hippocampus atrophy, and APOE genotype). We found that greater education was associated with higher LFC-connectivity in both CN and MCI during successful memory. Furthermore, higher LFC-connectivity predicted higher residualized memory (i.e., reserve). These results suggest that higher LFC-connectivity contributes to reserve in both healthy and pathological aging

Abstracts

Abstracts included:E. A. Matney and R. V. Ruhe - Clay Mineral Study of Soils from the Savannah River Plant South CarolinaAllan K. Nickell and Stanley M. Totten - Parent Material — Landscape — Soil Interrelationships in Jefferson County, IndianaLawrence A. Schaal - The Varying Length of the Growing Season inIndian

The left frontal cortex supports reserve in aging by enhancing functional network efficiency

Background: Recent evidence from fMRI studies suggests that functional hubs, i.e. highly connected brain regions, are important for mental health. We found recently that global connectivity of a hub in the left frontal cortex (LFC-connectivity) is associated with relatively preserved memory abilities and higher levels of protective factors (education, IQ) in normal aging and Alzheimer’s disease. These results suggest that LFC-connectivity supports reserve capacity alleviating memory decline. An open question is, however, why LFC-connectivity is beneficial and supports memory function in the face of neurodegeneration. We hypothesized that higher LFCconnectivity is associated with enhanced efficiency in connected major networks involved in episodic memory. We further hypothesized that higher LFC-related network efficiency predicts higher memory abilities. Methods: We assessed fMRI during a face-name association learning task in 26 healthy cognitively normal elderly participants. Using beta-series correlation analysis, we computed task-related LFC-connectivity to key memory networks including the default-mode network (DMN) and dorsal attention network (DAN). Network efficiency within the DMN and DAN was estimated by the graph theoretical small-worldness statistic. We applied linear regression analyses in order to test the association between LFC-connectivity to the DMN/DAN and small-worldness of these networks. Mediation analysis was applied to test LFC-connectivity to the DMN and DAN as a mediator of the association between education and higher DMN and DAN smallworldness. Lastly, we tested network small-worldness as a predictor of memory performance. Results: We found that higher LFC-connectivity to the DMN and DAN during successful memory encoding and recognition was associated with higher small-worldness of those networks. Higher task-related LFC-connectivity mediated the association between education and higher small-worldness in the DMN and DAN. Further, higher small-worldness of these networks predicted better performance in the memory task. Conclusions: The current results suggest that higher education-related LFC-connectivity to key memory networks during a memory task is associated with higher network efficiency and thus enhanced reserve of memory abilities in aging