Life-Span Changes of the Human Brain White Matter: Diffusion Tensor Imaging (DTI) and Volumetry

Sex differences and structural brain maturation from childhood to early adulthood

Recent advances in structural brain imaging have demonstrated that brain development continues through childhood and adolescence. In the present cross-sectional study, structural MRI data from 442 typically developing individuals (range 8-30) were analyzed to examine and replicate the relationship between age, sex, brain volumes, cortical thickness and surface area. Our findings show differential patterns for subcortical and cortical areas. Analysis of subcortical volumes showed that putamen volume decreased with age and thalamus volume increased with age. Independent of age, males demonstrated larger amygdala and thalamus volumes compared to females. Cerebral white matter increased linearly with age, at a faster pace for females than males. Gray matter showed nonlinear decreases with age. Sex-by-age interactions were primarily found in lobar surface area measurements, with males demonstrating a larger cortical surface up to age 15, while cortical surface in females remained relatively stable with increasing age. The current findings replicate some, but not all prior reports on structural brain development, which calls for more studies with large samples, replications, and specific tests for brain structural changes. In addition, the results point toward an important role for sex differences in brain development, specifically during the heterogeneous developmental phase of puberty.

The relationship between cortical blood flow and sub-cortical white-matter health across the adult age span

Degeneration of cerebral white matter is commonly observed in aging, and the associated degradation in neural connectivity contributes to cognitive decline in older adults. Vascular dysfunction has been implicated as a potential mechanism for general age-related neural tissue deterioration; however, no prior study has examined the direct relationship between cortical vascular health and subcortical white-matter integrity. In this work, we aimed to determine whether blood supply to the brain is associated with microstructural integrity of connective tissue, and whether such associations are regionally specific and mainly accounted for by aging. We examined the association between cerebral blood flow (CBF) in the cortical mantle, measured using arterial spin labeling (ASL), and subcortical white-matter integrity, measured using diffusion tensor imaging (DTI), in a group of healthy adults spanning early to late adulthood. We found cortical CBF to be significantly associated with white-matter integrity throughout the brain. In addition, these associations were only partially tied to aging, as they remained even when statistically controlling for age, and when restricting the analyses to a young subset of the sample. Furthermore, vascular risk was not a prominent determinant of these effects. These findings suggest that the overall blood supply to the brain is an important indicator of white-matter health in the normal range of variations amongst adults, and that the decline in CBF with advancing age may potentially exacerbate deterioration of the connective anatomy of the brain.

Onset of progressive phase is an age-dependent clinical milestone in multiple sclerosis

BACKGROUND

It is unclear if all patients with relapsing-remitting multiple sclerosis (RRMS) ultimately develop progressive MS. Onset of progressive disease course seems to be age- rather than disease duration-dependent. Some forms of progressive MS (e.g. primary progressive MS (PPMS)) are uncommon in population-based studies. Ascertainment of patients with PPMS from clinic-based populations can facilitate a powerful comparison of age at progression onset between secondary progressive MS (SPMS) and PPMS but may introduce unclear biases.

OBJECTIVE

Our aim is to confirm that onset of progressive disease course is more relevant to the patient's age than the presence or duration of a pre-progression relapsing disease course in MS.

METHODS

We studied a population-based MS cohort (n=210, RRMS n=109, progressive MS n=101) and a clinic-based progressive MS cohort (n=754). Progressive course was classified as primary (PPMS; n=322), single attack (SAPMS; n=112) and secondary progressive (SPMS; n=421). We studied demographics (chi(2) or t-test), age-of-progression-onset (t-test) and time to Expanded Disability Status Scale of 6 (EDSS6) (Kaplan-Meier analyses).

RESULTS

Sex ratio (p=0.58), age at progression onset (p=0.37) and time to EDSS6 (p=0.16) did not differ between the cohorts. Progression had developed before age 75 in 99% of patients with known progressive disease course; 38% with RRMS did not develop progression by age 75. Age at progression onset did not differ between SPMS (44.9±9.6), SAPMS (45.5±9.6) and PPMS (45.7±10.8). In either cohort, only 2% of patients had reached EDSS6 before onset of progression.

CONCLUSIONS

Patients with RRMS do not inevitably develop a progressive disease course. Onset of progression is more dependent on age than the presence or duration of a pre-progression symptomatic disease course. Moderate disability is sustained predominantly after the onset of a progressive disease course in MS.

Clinical applications of diffusion tensor imaging

The potential utility of diffusion tensor (DT) imaging in clinical practice is broad, and new applications continue to evolve as technology advances. Clinical applications of DT imaging and tractography include tissue characterization, lesion localization, and mapping of white matter tracts. DT imaging metrics are sensitive to microstructural changes associated with central nervous system disease; however, further research is needed to enhance specificity so as to facilitate more widespread clinical application. Preoperative tract mapping, with either directionally encoded color maps or tractography, provides useful information to the neurosurgeon and has been shown to improve clinical outcomes.

Tracking cerebral white matter changes across the lifespan: insights from diffusion tensor imaging studies

Delineating the normal development of brain white matter (WM) over the human lifespan is crucial to improved understanding of underlying WM pathology in neuropsychiatric and neurological conditions. We review the extant literature concerning diffusion tensor imaging studies of brain WM development in healthy individuals available until October 2012, summarise trends of normal development of human brain WM and suggest possible future research directions. Temporally, brain WM maturation follows a curvilinear pattern with an increase in fractional anisotropy (FA) from newborn to adolescence, decelerating in adulthood till a plateau around mid-adulthood, and a more rapid decrease of FA from old age onwards. Spatially, brain WM tracts develop from central to peripheral regions, with evidence of anterior-to-posterior maturation in commissural and projection fibres. The corpus callosum and fornix develop first and decline earlier, whilst fronto-temporal WM tracts like cingulum and uncinate fasciculus have protracted maturation and decline later. Prefrontal WM is most vulnerable with greater age-related FA reduction compared with posterior WM. Future large scale studies adopting longitudinal design will better clarify human brain WM changes over time.

Brain volumes and neuropsychological performance are related to current smoking and alcoholism history

BACKGROUND

Dual dependence on alcohol and nicotine is common, with many reports suggesting that more than 80% of alcoholics also smoke cigarettes. Even after cessation of alcohol consumption, many recovering alcoholics continue to smoke. In this exploratory study, we examined how current smoking and a history of alcoholism interacted in relation to brain volumes and neuropsychological performance.

METHODS

Participants were 14 abstinent long-term alcoholics (seven current smokers and seven nonsmokers), and 13 nonalcoholics (six current smokers and seven nonsmokers). The groups were equivalent in age, gender, education, and intelligence quotient. Two multiecho magnetization-prepared rapid acquisition with gradient echo (MP-RAGE) scans were collected for all participants using a 3T magnetic resonance imaging scanner with a 32 channel head coil. Brain volumes for each gray and white matter region of interest were derived using FreeSurfer. Participants completed a battery of neuropsychological tests measuring intelligence quotient, memory, executive functions, personality variables, and affect.

RESULTS

COMPARED TO NONSMOKING NONALCOHOLICS, ALCOHOLICS WHO SMOKE (THE COMORBID GROUP) HAD VOLUMETRIC ABNORMALITIES IN: pre- and para-central frontal cortical areas and rostral middle frontal white matter; parahippocampal and temporal pole regions; the amygdala; the pallidum; the ventral diencephalic region; and the lateral ventricle. The comorbid group performed worse than nonsmoking nonalcoholics on tests of executive functioning and on visually-based memory tests. History of alcoholism was associated with higher neuroticism scores among smokers, and current smoking was associated with higher sensation seeking scores and lower extraversion scores among nonalcoholics.

CONCLUSION

Results from this exploratory study support and extend prior reports showing that alcoholism and smoking, alone and in combination, are associated with structural brain abnormalities and poorer performance on neuropsychological tests. Therefore, it is important to consider smoking status in alcoholism studies and vice versa.

Early gray-matter and white-matter concentration in infancy predict later language skills: a whole brain voxel-based morphometry study

Magnetic resonance imaging (MRI) brain scans were obtained from 19 infants at 7 months. Expressive and receptive language performance was assessed at 12 months. Voxel-based morphometry (VBM) identified brain regions where gray-matter and white-matter concentrations at 7 months correlated significantly with children's language scores at 12 months. Early gray-matter concentration in the right cerebellum, early white-matter concentration in the right cerebellum, and early white-matter concentration in the left posterior limb of the internal capsule (PLIC)/cerebral peduncle were positively and strongly associated with infants' receptive language ability at 12 months. Early gray-matter concentration in the right hippocampus was positively and strongly correlated with infants' expressive language ability at 12 months. Our results suggest that the cerebellum, PLIC/cerebral peduncle, and the hippocampus may be associated with early language development. Potential links between these structural predictors and infants' linguistic functions are discussed.

Lithium and GSK3-β promoter gene variants influence white matter microstructure in bipolar disorder

Lithium is the mainstay for the treatment of bipolar disorder (BD) and inhibits glycogen synthase kinase 3-β (GSK3-β). The less active GSK3-β promoter gene variants have been associated with less detrimental clinical features of BD. GSK3-β gene variants and lithium can influence brain gray matter structure in psychiatric conditions. Diffusion tensor imaging (DTI) measures of white matter (WM) integrity showed widespred disruption of WM structure in BD. In a sample of 70 patients affected by a major depressive episode in course of BD, we investigated the effect of ongoing long-term lithium treatment and GSK3-β promoter rs334558 polymorphism on WM microstructure, using DTI and tract-based spatial statistics with threshold-free cluster enhancement. We report that the less active GSK3-β rs334558*C gene-promoter variants, and the long-term administration of the GSK3-β inhibitor lithium, were associated with increases of DTI measures of axial diffusivity (AD) in several WM fiber tracts, including corpus callosum, forceps major, anterior and posterior cingulum bundle (bilaterally including its hippocampal part), left superior and inferior longitudinal fasciculus, left inferior fronto-occipital fasciculus, left posterior thalamic radiation, bilateral superior and posterior corona radiata, and bilateral corticospinal tract. AD reflects the integrity of axons and myelin sheaths. We suggest that GSK3-β inhibition and lithium could counteract the detrimental influences of BD on WM structure, with specific benefits resulting from effects on specific WM tracts contributing to the functional integrity of the brain and involving interhemispheric, limbic, and large frontal, parietal, and fronto-occipital connections.

Developmental trajectories of the fronto-temporal lobes from infancy to early adulthood in healthy individuals

Brain development during early life in healthy individuals is rapid and dynamic, indicating that this period plays a very important role in neural and functional development. The frontal and temporal lobes are known to play a particularly important role in cognition. The study of healthy frontal and temporal lobe development in children is therefore of considerable importance. A better understanding of how these brain regions develop could also aid in the diagnosis and treatment of neurodevelopmental disorders. Some developmental studies have used magnetic resonance imaging (MRI) to examine infant brains, but it remains the case that relatively little is known about cortical brain development in the first few years of life. In the present study we examined whole brain, temporal lobe and frontal lobe developmental trajectories from infancy to early adulthood in healthy individuals, considering gender and brain hemisphere differences. We performed a cross-sectional, longitudinal morphometric MRI study of 114 healthy individuals (54 females and 60 males) aged 1 month to 25 years old (mean age ± SD 8.8 ± 6.9). We measured whole brain, temporal and frontal lobe gray matter (GM)/white matter (WM) volumes, following previously used protocols. There were significant non-linear age-related volume changes in all regions. Peak ages of whole brain, temporal lobe and frontal lobe development occurred around pre-adolescence (9-12 years old). GM volumes for all regions increased significantly as a function of age. Peak age was nevertheless lobe specific, with a pattern of earlier peak ages for females in both temporal and frontal lobes. Growth change in whole brain GM volume was larger in males than in females. However, GM volume growth changes for the temporal and frontal lobes showed a somewhat different pattern. GM volume for both temporal and frontal lobes showed a greater increase in females until around 5-6 years old, at which point this tendency reversed (GM volume changes in males became greater), with male GM volume increasing for a longer time than that of females. WM volume growth changes were similar across regions, all increasing rapidly until early childhood but slowing down thereafter. All regions displayed significant rightward volumetric asymmetry regardless of sex. Furthermore, the right temporal and frontal lobes showed a greater volumetric increase than the left for the first several years, with this tendency reversing at around 6 years of age. In addition, the left frontal and temporal lobes increased in volume for a longer period of time. Taken together, these findings indicated that brain developmental trajectories differ depending on brain region, sex and brain hemisphere. Gender-related factors such as sex hormones and functional laterality may affect brain development.

Differential diagnosis of normal pressure hydrocephalus by MRI mean diffusivity histogram analysis

BACKGROUND AND PURPOSE

Accurate diagnosis of normal pressure hydrocephalus is challenging because the clinical symptoms and radiographic appearance of NPH often overlap those of other conditions, including age-related neurodegenerative disorders such as Alzheimer and Parkinson diseases. We hypothesized that radiologic differences between NPH and AD/PD can be characterized by a robust and objective MR imaging DTI technique that does not require intersubject image registration or operator-defined regions of interest, thus avoiding many pitfalls common in DTI methods.

MATERIALS AND METHODS

We collected 3T DTI data from 15 patients with probable NPH and 25 controls with AD, PD, or dementia with Lewy bodies. We developed a parametric model for the shape of intracranial mean diffusivity histograms that separates brain and ventricular components from a third component composed mostly of partial volume voxels. To accurately fit the shape of the third component, we constructed a parametric function named the generalized Voss-Dyke function. We then examined the use of the fitting parameters for the differential diagnosis of NPH from AD, PD, and DLB.

RESULTS

Using parameters for the MD histogram shape, we distinguished clinically probable NPH from the 3 other disorders with 86% sensitivity and 96% specificity. The technique yielded 86% sensitivity and 88% specificity when differentiating NPH from AD only.

CONCLUSIONS

An adequate parametric model for the shape of intracranial MD histograms can distinguish NPH from AD, PD, or DLB with high sensitivity and specificity.

Developmental patterns of chimpanzee cerebral tissues provide important clues for understanding the remarkable enlargement of the human brain

Developmental prolongation is thought to contribute to the remarkable brain enlargement observed in modern humans (Homo sapiens). However, the developmental trajectories of cerebral tissues have not been explored in chimpanzees (Pan troglodytes), even though they are our closest living relatives. To address this lack of information, the development of cerebral tissues was tracked in growing chimpanzees during infancy and the juvenile stage, using three-dimensional magnetic resonance imaging and compared with that of humans and rhesus macaques (Macaca mulatta). Overall, cerebral development in chimpanzees demonstrated less maturity and a more protracted course during prepuberty, as observed in humans but not in macaques. However, the rapid increase in cerebral total volume and proportional dynamic change in the cerebral tissue in humans during early infancy, when white matter volume increases dramatically, did not occur in chimpanzees. A dynamic reorganization of cerebral tissues of the brain during early infancy, driven mainly by enhancement of neuronal connectivity, is likely to have emerged in the human lineage after the split between humans and chimpanzees and to have promoted the increase in brain volume in humans. Our findings may lead to powerful insights into the ontogenetic mechanism underlying human brain enlargement.

Brain development and aging: overlapping and unique patterns of change

Early-life development is characterized by dramatic changes, impacting lifespan function more than changes in any other period. Developmental origins of neurocognitive late-life functions are acknowledged, but detailed longitudinal magnetic resonance imaging studies of brain maturation and direct comparisons with aging are lacking. To these aims, a novel method was used to measure longitudinal volume changes in development (n=85, 8-22 years) and aging (n=142, 60-91 years). Developmental reductions exceeded 1% annually in much of the cortex, more than double to that seen in aging, with a posterior-to-anterior gradient. Cortical reductions were greater than the subcortical during development, while the opposite held in aging. The pattern of lateral cortical changes was similar across development and aging, but the pronounced medial temporal reduction in aging was not precast in development. Converging patterns of change in adolescents and elderly, particularly in the medial prefrontal areas, suggest that late developed cortices are especially vulnerable to atrophy in aging. A key question in future research will be to disentangle the neurobiological underpinnings for the differences and the similarities between brain changes in development and aging.

Accelerating cortical thinning: unique to dementia or universal in aging?

Does accelerated cortical atrophy in aging, especially in areas vulnerable to early Alzheimer's disease (AD), unequivocally signify neurodegenerative disease or can it be part of normal aging? We addressed this in 3 ways. First, age trajectories of cortical thickness were delineated cross-sectionally (n = 1100) and longitudinally (n = 207). Second, effects of undetected AD on the age trajectories were simulated by mixing the sample with a sample of patients with very mild to moderate AD. Third, atrophy in AD-vulnerable regions was examined in older adults with very low probability of incipient AD based on 2-year neuropsychological stability, CSF Aβ(1-42) levels, and apolipoprotein ε4 negativity. Steady decline was seen in most regions, but accelerated cortical thinning in entorhinal cortex was observed across groups. Very low-risk older adults had longitudinal entorhinal atrophy rates similar to other healthy older adults, and this atrophy was predictive of memory change. While steady decline in cortical thickness is the norm in aging, acceleration in AD-prone regions does not uniquely signify neurodegenerative illness but can be part of healthy aging. The relationship between the entorhinal changes and changes in memory performance suggests that non-AD mechanisms in AD-prone areas may still be causative for cognitive reductions.

Whole-brain voxel-based analysis of diffusion tensor MRI parameters in patients with primary open angle glaucoma and correlation with clinical glaucoma stage

INTRODUCTION

Glaucoma is the second leading cause of blindness worldwide. The purpose of this study is to identify areas of neurodegeneration in glaucoma utilizing 3 T magnetic resonance (MR) diffusion tensor imaging (DTI) parameters with whole-brain voxel-based analysis (VBA) and determine whether these parameters correlate with disease severity.

METHODS

Twenty-five glaucoma patients and 25 age-matched healthy volunteers were prospectively examined. Clinical glaucoma severity was assessed utilizing static threshold visual field parameters. All subjects underwent 3 T MRI utilizing a DTI sequence (repetition time/echo time 13,000/68.9 ms, maximal b value 800 s/mm(2) along 30 directions) and an anatomic sequence to provide structural information. All data sets were processed by VBA. Brain fractional anisotropy, mean diffusivity (MD), radial diffusivity (RD), and axial diffusivity (AD) were compared in the two groups. Correlation between DTI parameters and glaucoma stage were determined.

RESULTS

The bilateral optic radiations and chiasma of glaucoma patients demonstrated statistically significantly lower fractional anisotropy (p < 0.05). Optic radiation RD was similarly decreased in glaucoma patients (p < 0.05). There were no statistically significant differences noted in MD or AD between the two groups (p > 0.05). Optic chiasm fractional anisotropy values were negatively correlated with glaucoma stage (r = -0.53, p < 0.05) and optic radiation RD values positively correlated (left r = 0.45, p < 0.05; right = 0.38, p = 0.06).

CONCLUSION

DTI parameters fractional anisotropy and RD are altered in the optic chiasm and radiations of glaucoma patients. As fractional anisotropy and RD also correlate with glaucoma stage, these values could serve as potential noninvasive markers of disease severity.

Multimodal imaging of the self-regulating developing brain

Self-regulation refers to the ability to control behavior, cognition, and emotions, and self-regulation failure is related to a range of neuropsychiatric problems. It is poorly understood how structural maturation of the brain brings about the gradual improvement in self-regulation during childhood. In a large-scale multicenter effort, 735 children (4-21 y) underwent structural MRI for quantification of cortical thickness and surface area and diffusion tensor imaging for quantification of the quality of major fiber connections. Brain development was related to a standardized measure of cognitive control (the flanker task from the National Institutes of Health Toolbox), a critical component of self-regulation. Ability to inhibit responses and impose cognitive control increased rapidly during preteen years. Surface area of the anterior cingulate cortex accounted for a significant proportion of the variance in cognitive performance. This finding is intriguing, because characteristics of the anterior cingulum are shown to be related to impulse, attention, and executive problems in neurodevelopmental disorders, indicating a neural foundation for self-regulation abilities along a continuum from normality to pathology. The relationship was strongest in the younger children. Properties of large-fiber connections added to the picture by explaining additional variance in cognitive control. Although cognitive control was related to surface area of the anterior cingulate independently of basic processes of mental speed, the relationship between white matter quality and cognitive control could be fully accounted for by speed. The results underscore the need for integration of different aspects of brain maturation to understand the foundations of cognitive development.

Effects of systolic blood pressure on white-matter integrity in young adults in the Framingham Heart Study: a cross-sectional study

BACKGROUND

Previous studies have identified effects of age and vascular risk factors on brain injury in elderly individuals. We aimed to establish whether the effects of high blood pressure in the brain are evident as early as the fifth decade of life.

METHODS

In an investigation of the third generation of the Framingham Heart Study, we approached all participants in 2009 to ask whether they would be willing to undergo MRI. Consenting patients underwent clinical assessment and cerebral MRI that included T1-weighted and diffusion tensor imaging to obtain estimates of fractional anisotropy, mean diffusivity, and grey-matter volumes. All images were coregistered to a common minimum deformation template for voxel-based linear regressions relating fractional anisotropy, mean diffusivity, and grey-matter volumes to age and systolic blood pressure, with adjustment for potential confounders.

FINDINGS

579 (14·1%) of 4095 participants in the third-generation cohort (mean age 39·2 years, SD 8·4) underwent brain MRI between June, 2009 and June, 2010. Age was associated with decreased fractional anisotropy and increased mean diffusivity in almost all cerebral white-matter voxels. Age was also independently associated with reduced grey-matter volumes. Increased systolic blood pressure was linearly associated with decreased regional fractional anisotropy and increased mean diffusivity, especially in the anterior corpus callosum, the inferior fronto-occipital fasciculi, and the fibres that project from the thalamus to the superior frontal gyrus. It was also strongly associated with reduced grey-matter volumes, particularly in Brodmann's area 48 on the medial surface of the temporal lobe and Brodmann's area 21 of the middle temporal gyrus.

INTERPRETATION

Our results suggest that subtle vascular brain injury develops insidiously during life, with discernible effects even in young adults. These findings emphasise the need for early and optimum control of blood pressure.

FUNDING

National Institutes of Health and National Heart, Lung, and Blood Institute; National Institute on Aging; and National Institute of Neurological Disorders and Stroke.

Age-related changes in human and non-human primate white matter: from myelination disturbances to cognitive decline

The cognitive decline associated with normal aging was long believed to be due primarily to decreased synaptic density and neuron loss. Recent studies in both humans and non-human primates have challenged this idea, pointing instead to disturbances in white matter (WM) including myelin damage. Here, we review both cross-sectional and longitudinal studies in humans and non-human primates that collectively support the hypothesis that WM disturbances increase with age starting at middle age in humans, that these disturbances contribute to age-related cognitive decline, and that age-related WM changes may occur as a result of free radical damage, degenerative changes in cells in the oligodendrocyte lineage, and changes in microenvironments within WM.

Regional changes of cortical mean diffusivities with aging after correction of partial volume effects

Accurately measuring the cortical mean diffusivity (MD) derived from diffusion tensor imaging (DTI) at the comprehensive lobe, gyral and voxel level of young, elderly healthy brains and those with Alzheimer's disease (AD) may provide insights on heterogeneous cortical microstructural changes caused by aging and AD. Due to partial volume effects (PVE), the measurement of cortical MD is overestimated with contamination of cerebrospinal fluid (CSF). The bias is especially severe for aging and AD brains because of significant cortical thinning of these brains. In this study, we aimed to quantitatively characterize the unbiased regional cortical MD changes due to aging and AD and delineate the effects of cortical thinning of elderly healthy and AD groups on MD measurements. DTI and T1-weighted images of 14 young, 15 elderly healthy subjects and 17 AD patients were acquired. With the parcellated cortical gyri and lobes from T1 weighted image transformed to DTI, regional cortical MD of all subjects before and after PVE correction were measured. CSF contamination model was used to correct bias of MD caused by PVE. Compared to cortical MD of young group, significant increases of corrected MD for elderly healthy and AD groups were found only in frontal and limbic regions, respectively, while there were significant increases of uncorrected MD all over the cortex. Uncorrected MD are significantly higher in limbic and temporal gyri in AD group, compared to those in elderly healthy group but higher MD only remained in limbic gyri after PVE correction. Cortical thickness was also measured for all groups. The correlation slopes between cortical MD and thickness for elderly healthy and AD groups were significantly decreased after PVE correction compared to before correction while no significant change of correlation slope was detected for young group. It suggests that the cortical thinning in elderly healthy and AD groups is a significant contributor to the bias of uncorrected cortical MD measurement. The established comprehensive unbiased cortical MD profiles of young, elderly healthy subjects and AD patients at the lobe, gyral and voxel level may serve as clinical references for cortical microstructure.

Decoding lifespan changes of the human brain using resting-state functional connectivity MRI

The development of large-scale functional brain networks is a complex, lifelong process that can be investigated using resting-state functional connectivity MRI (rs-fcMRI). In this study, we aimed to decode the developmental dynamics of the whole-brain functional network in seven decades (8–79 years) of the human lifespan. We first used parametric curve fitting to examine linear and nonlinear age effect on the resting human brain, and then combined manifold learning and support vector machine methods to predict individuals' “brain ages” from rs-fcMRI data. We found that age-related changes in interregional functional connectivity exhibited spatially and temporally specific patterns. During brain development from childhood to senescence, functional connections tended to linearly increase in the emotion system and decrease in the sensorimotor system; while quadratic trajectories were observed in functional connections related to higher-order cognitive functions. The complex patterns of age effect on the whole-brain functional network could be effectively represented by a low-dimensional, nonlinear manifold embedded in the functional connectivity space, which uncovered the inherent structure of brain maturation and aging. Regression of manifold coordinates with age further showed that the manifold representation extracted sufficient information from rs-fcMRI data to make prediction about individual brains' functional development levels. Our study not only gives insights into the neural substrates that underlie behavioral and cognitive changes over age, but also provides a possible way to quantitatively describe the typical and atypical developmental progression of human brain function using rs-fcMRI.

Neuroanatomical assessment of biological maturity

Structural MRI allows unparalleled in vivo study of the anatomy of the developing human brain. For more than two decades, MRI research has revealed many new aspects of this multifaceted maturation process, significantly augmenting scientific knowledge gathered from postmortem studies. Postnatal brain development is notably protracted and involves considerable changes in cerebral cortical, subcortical, and cerebellar structures, as well as significant architectural changes in white matter fiber tracts (see [12]). Although much work has described isolated features of neuroanatomical development, it remains a critical challenge to characterize the multidimensional nature of brain anatomy, capturing different phases of development among individuals. Capitalizing on key advances in multisite, multimodal MRI, and using cross-validated nonlinear modeling, we demonstrate that developmental brain phase can be assessed with much greater precision than has been possible using other biological measures, accounting for more than 92% of the variance in age. Further, our composite metric of morphology, diffusivity, and signal intensity shows that the average difference in phase among children of the same age is only about 1 year, revealing for the first time a latent phenotype in the human brain for which maturation timing is tightly controlled.

The structure of the cerebral cortex across adult life: age-related patterns of surface area, thickness, and gyrification

Older adults exhibit global reductions in cortical surface area, but little is known about the regional patterns of reductions or how these relate to other measures of brain structure. This knowledge is critical to understanding the dynamic relationship between different macrostructural properties of the cortex throughout adult life. Here, cortical arealization, local gyrification index (LGI), and cortical thickness were measured vertex wise across the brain surface in 322 healthy adults (20-85 years), with the aims of 1) characterizing age patterns of the three separate cortical measures and 2) testing the age-independent relationships among cortical surface area, gyrification, and thickness. Surface area showed strong age-related decreases, particularly pronounced in dorsomedial prefrontal, lateral temporal, and fusiform cortices, independently of total white matter volume. LGI decreased with age independently of regional surface area, with strongest effects laterally, extending from the angular gyrus in all directions. As expected, regional surface area and LGI were positively related. However, both measures correlated negatively with thickness, indicating increasing local arealization and gyrification with decreasing cortical thickness. We suggest that this pattern of regional "cortical stretching" reflects the well-established phylogenetic principle of maximizing surface area and gyrification rather than increase thickness to facilitate brain connectivity and functional development.

Age-dependent visuomotor performance and white matter structure: a DTI study

The Trail Making Test (TMT), which assesses motor performance, selective attention, working memory and cognitive flexibility is highly sensitive to age-related performance differences. However, the structural basis of this age-performance association is largely unknown. This DTI study examined the influence of white matter characteristics on the association between TMT performance (i.e., speed of processing) and age in a sample of 86 healthy, middle-aged subjects (mean age 27.9 years, range 18-55). Voxel-wise correlation yielded a significant negative association between FA in the body of the corpus callosum (CC) and TMT-A performance (i.e., time taken to complete the test). There was also a significant association between age and TMT-A performance. However, this association between age and TMT-A performance was neither mediated nor moderated by FA in the CC. Results suggest that fast motor performance is strongly dependent on individual white matter characteristics of the CC. This indicates that interindividual variations in white matter of the CC known to be relevant for interhemispheric motor signal transduction critically influence speed of motor processing. However, these interindividual variations do not explain the observed association between age and TMT performance.

Testing the white matter retrogenesis hypothesis of cognitive aging

The retrogenesis hypothesis postulates that late-myelinated white matter fibers are most vulnerable to age- and disease-related degeneration, which in turn mediate cognitive decline. While recent evidence supports this hypothesis in the context of Alzheimer's disease, it has not been tested systematically in normal cognitive aging. In the current study, we examined the retrogenesis hypothesis in a group (n = 282) of cognitively normal individuals, ranging in age from 7 to 87 years, from the Brain Resource International Database. Participants were evaluated with a comprehensive neuropsychological battery and were imaged with diffusion tensor imaging. Fractional anisotropy (FA), radial diffusivity (RD), and axial diffusivity (DA), measures of white matter coherence, were computed in 2 prototypical early-myelinated fiber tracts (posterior limb of the internal capsule, cerebral peduncles) and 2 prototypical late-myelinated fiber tracts (superior longitudinal fasciculus, inferior longitudinal fasciculus) chosen to parallel previous studies; mean summary values were also computed for other early- and late-myelinated fiber tracts. We examined age-associated differences in FA, RD, and DA in the developmental trajectory (ages 7-30 years) and degenerative trajectory (ages 31-87 years), and tested whether the measures of white matter coherence mediated age-related cognitive decline in the older group. FA and DA values were greater for early-myelinated fibers than for late-myelinated fibers, and RD values were lower for early-myelinated than late-myelinated fibers. There were age-associated differences in FA, RD, and DA across early- and late-myelinated fiber tracts in the younger group, but the magnitude of differences did not vary as a function of early or late myelinating status. FA and RD in most fiber tracts showed reliable age-associated differences in the older age group, but the magnitudes were greatest for the late-myelinated tract summary measure, inferior longitudinal fasciculus (late fiber tract), and cerebral peduncles (early fiber tract). Finally, FA in the inferior longitudinal fasciculus and cerebral peduncles and RD in the cerebral peduncles mediated age-associated differences in an executive functioning factor. Taken together, the findings highlight the importance of white matter coherence in cognitive aging and provide some, but not complete, support for the white matter retrogenesis hypothesis in normal cognitive aging.

Benefits of multi-modal fusion analysis on a large-scale dataset: life-span patterns of inter-subject variability in cortical morphometry and white matter microstructure

Neuroimaging studies have become increasingly multimodal in recent years, with researchers typically acquiring several different types of MRI data and processing them along separate pipelines that provide a set of complementary windows into each subject's brain. However, few attempts have been made to integrate the various modalities in the same analysis. Linked ICA is a robust data fusion model that takes multi-modal data and characterizes inter-subject variability in terms of a set of multi-modal components. This paper examines the types of components found when running Linked ICA on a large magnetic resonance imaging (MRI) morphometric and diffusion tensor imaging (DTI) data set comprising 484 healthy subjects ranging from 8 to 85 years of age. We find several strong global features related to age, sex, and intracranial volume; in particular, one component predicts age to a high accuracy (r=0.95). Most of the remaining components describe spatially localized modes of variability in white or gray matter, with many components including both tissue types. The multimodal components tend to be located in anatomically-related brain areas, suggesting a morphological and possibly functional relationship. The local components show relationships between surface-based cortical thickness and arealization, voxel-based morphometry (VBM), and between three different DTI measures. Further, we report components related to artifacts (e.g. scanner software upgrades) which would be expected in a dataset of this size. Most of the 100 extracted components showed interpretable spatial patterns and were found to be reliable using split-half validation. This work provides novel information about normal inter-subject variability in brain structure, and demonstrates the potential of Linked ICA as a feature-extracting data fusion approach across modalities. This exploratory approach automatically generates models to explain structure in the data, and may prove especially powerful for large-scale studies, where the population variability can be explored in increased detail.

Drinking history associations with regional white matter volumes in alcoholic men and women

BACKGROUND

Alcoholism has been repeatedly associated with gray and white matter pathology. Although neuroimaging has shown alcoholism-related brain volume reductions and axonal compromise, the integrity of white matter volumes in chronic alcoholism has been challenging to measure on a regional level.

METHODS

We first examined the effects of alcoholism on cerebral white matter volumes by lobar and gyral subdivisions in 42 abstinent alcoholics and 42 control participants (split evenly by gender). We also examined cerebellar white matter and regions of the corpus callosum, as well as ventricular volumes. Next, relationships between white matter and ventricular volumes with measures of drinking patterns were assessed. Finally, an examination of early versus late abstinence was conducted. Within each examination, gender effects were explored.

RESULTS

Differences in regional white matter volumes between alcoholics and controls were observed primarily in the corpus callosum, with a stronger group difference among men than women. Years of heavy drinking had a strong negative impact on frontal and temporal white matter among alcoholic women, and on the corpus callosum among alcoholic men. Quantity of alcohol consumption was associated with smaller corpus callosum and larger ventricular volumes among alcoholic women, whereas abstinence duration was associated with larger corpus callosum volume among alcoholic men. Preliminary data indicated that alcoholic women showed stronger positive associations between sobriety duration and white matter volume than men within the first year of abstinence, whereas men showed this association more so than women after 1 year of abstinence.

CONCLUSIONS

Effects of drinking history on white matter and ventricular volumes vary by gender, with alcoholic women showing greatest sensitivity in frontal, temporal, ventricular, and corpus callosum regions, and alcoholic men showing effects mainly in the corpus callosum. Preliminary results indicate that recovery of white matter volume may occur sooner for women than for men.

Extent of bilateral neuronal network reorganization and functional recovery in relation to stroke severity

Remodeling of neuronal structures and networks is believed to significantly contribute to (partial) restoration of functions after stroke. However, it has been unclear to what extent the brain reorganizes and how this correlates with functional recovery in relation to stroke severity. We applied serial resting-state functional MRI and diffusion tensor imaging together with behavioral testing to relate longitudinal modifications in functional and structural connectivity of the sensorimotor neuronal network to changes in sensorimotor function after unilateral stroke in rats. We found that gradual improvement of functions is associated with wide-ranging changes in functional and structural connectivity within bilateral neuronal networks, particularly after large stroke. Both after medium and large stroke, brain reorganization eventually leads to (partial) normalization of neuronal signal synchronization within the affected sensorimotor cortical network (intraregional signal coherence), as well as between the affected and unaffected sensorimotor cortices (interhemispheric functional connectivity). Furthermore, the bilateral network configuration shifts from subacutely increased "small-worldness," possibly reflective of initial excessive neuronal clustering and wiring, toward a baseline small-world topology, optimal for global information transfer and local processing, at chronic stages. Cortical network remodeling was accompanied by recovery of initially disrupted structural integrity in corticospinal tract regions, which correlated positively with retrieval of sensorimotor functions. Our study demonstrates that the degree of functional recovery after stroke is associated with the extent of preservation or restoration of ipsilesional corticospinal tracts in combination with reinstatement of interhemispheric neuronal signal synchronization and normalization of small-world cortical network organization.

The brain matures with stronger functional connectivity and decreased randomness of its network

We investigated the development of the brain's functional connectivity throughout the life span (ages 5 through 71 years) by measuring EEG activity in a large population-based sample. Connectivity was established with Synchronization Likelihood. Relative randomness of the connectivity patterns was established with Watts and Strogatz' (1998) graph parameters C (local clustering) and L (global path length) for alpha (~10 Hz), beta (~20 Hz), and theta (~4 Hz) oscillation networks. From childhood to adolescence large increases in connectivity in alpha, theta and beta frequency bands were found that continued at a slower pace into adulthood (peaking at ~50 yrs). Connectivity changes were accompanied by increases in L and C reflecting decreases in network randomness or increased order (peak levels reached at ~18 yrs). Older age (55+) was associated with weakened connectivity. Semi-automatically segmented T1 weighted MRI images of 104 young adults revealed that connectivity was significantly correlated to cerebral white matter volume (alpha oscillations: r = 33, p<01; theta: r = 22, p<05), while path length was related to both white matter (alpha: max. r = 38, p<001) and gray matter (alpha: max. r = 36, p<001; theta: max. r = 36, p<001) volumes. In conclusion, EEG connectivity and graph theoretical network analysis may be used to trace structural and functional development of the brain.

A multi-modal investigation of behavioral adjustment: post-error slowing is associated with white matter characteristics

When people make mistakes in speeded cognitive tasks, their response time on the next trial will typically be slower. This is referred to as post-error slowing (PES), and is important for optimization of performance, but its exact function remains to be decided. However, although PES is relatively stable over time, we have almost no knowledge about how PES is affected by structural brain characteristics. The aim of this study was to test to what extent white matter (WM) macro- and microstructure can account for individual differences in PES. PES was calculated for 255 healthy participants who performed a modified version of the Eriksen flanker task and underwent structural magnetic resonance imaging and diffusion tensor imaging (DTI). PES was positively related to WM volume in the caudal and rostral middle and superior frontal, medial orbitofrontal gyri and pars orbitalis. DTI analyses with tract-based spatial statistics (TBSS) showed that mean diffusivity in the superior longitudinal fasciculus, inferior fronto-occipital fasciculus and anterior thalamic radiation, as well as axial diffusivity in the corpus callosum, was negatively related to PES. Path analysis demonstrated that WM micro- and macrostructure were complementary in accounting for PES. It is concluded that individual differences in WM characteristics can partly explain why some people are better at adjusting their behavior in response to poor performance than others.

Becoming consistent: developmental reductions in intraindividual variability in reaction time are related to white matter integrity

Cognitive development is known to involve improvements in accuracy, capacity, and processing speed. Less is known about the role of performance consistency, and there has been virtually no empirical examination of the neural underpinnings of within-person variability in development. In a sample of 92 healthy children and adolescents aged 8-19 years, we aimed to characterize age-related changes in trial-to-trial intraindividual variability (IIV) of reaction time (RT) and to test whether IIV is related to white matter (WM) integrity as indexed by diffusion tensor imaging. IIV was quantified as the SD of correct RTs in a speeded arrow flanker task, and Tract-Based Spatial Statistics was used to test relationships with diffusion characteristics. Large age-related reductions in IIV in both simple congruent trials and more complex incongruent trials were found. Independently of sex, age, and median RT (mRT), lower IIV was associated with higher fractional anisotropy and lower overall diffusivity. Effects were seen for IIV in one or both trial types in the corticospinal tract, the left superior longitudinal fasciculus, the uncinate fasciculus, the forceps minor, and in the genu and splenium of the corpus callosum. There were no significant associations between mRT and any of the diffusion indices. The findings support the proposition that developmental reductions in IIV reflect maturation of WM connectivity and highlight the importance of considering within-person variability in theories of cognitive development and its neurobiological foundation.

The dimensionality of between-person differences in white matter microstructure in old age

Between-person differences in white matter microstructure may partly generalize across the brain and partly play out differently for distinct tracts. We used diffusion-tensor imaging and structural equation modeling to investigate this issue in a sample of 260 adults aged 60-87 years. Mean fractional anisotropy and mean diffusivity of seven white matter tracts in each hemisphere were quantified. Results showed good fit of a model positing that individual differences in white matter microstructure are structured according to tracts. A general factor, although accounting for variance in the measures, did not adequately represent the individual differences. This indicates the presence of a substantial amount of tract-specific individual differences in white matter microstructure. In addition, individual differences are to a varying degree shared between tracts, indicating that general factors also affect white matter microstructure. Age-related differences in white matter microstructure were present for all tracts. Correlations among tract factors did not generally increase as a function of age, suggesting that aging is not a process with homogenous effects on white matter microstructure across the brain. These findings highlight the need for future research to examine whether relations between white matter microstructure and diverse outcomes are specific or general.

The effects of the serotonin transporter polymorphism and age on frontal white matter integrity in healthy adult women

Studies of populations at genetic risk have the potential to explore the underlying structural and functional mechanisms in the development of psychological disorders. The polymorphic region (5-HTTLPR) in the serotonin transporter gene (SLC6A4) has been associated with major depression (MDD) (Caspi et al., 2003). In healthy women, variation in the human brain white matter microstructure integrity in the uncinate fascicule (UF) has been suggested as an endophenotypes in the development of MDD. Pacheco et al. (2009) found a unique effect of age and 5-HTTLPR within the left frontal UF. The present study examined whether these associations persist along the adult life span. Thirty-seven right-handed healthy women between 21 and 61 years of age were invited for a diffusion MRI study. The functional polymorphism 5-HTTLPR located in the promoter region of the SLC6A4 gene was genotyped using polymerase chain reaction (PCR). Fractional anisotropy (FA) was generated for the UF based on Tract-Based Spatial Statistics (TBSS). Models of emotion regulation circuitry suggest that working memory is important in conscious emotion regulation (Price and Drevets, 2010). To explore if 5-HTTLPR is related to this aspects of emotion processing, a working memory pathway, the superior longitudinal fascicule (SLF) was included. The results demonstrate that age may explain the hypothesized association between 5-HTTLPR and frontal UF white matter integrity in healthy adult women. Both white matter changes associated with the aging process and those associated with growth and development may explain why the earlier reported unique effects of genotype in frontal UF FA do not persist into adulthood.

Reduced white matter integrity is related to cognitive instability

Increased performance variability has been demonstrated in several groups and conditions, including aging and cognitive decline. Structural brain characteristics underlying this phenomenon have so far been elusive. However, there is reason to expect that disconnectivity in associative pathways, whether caused by immature or degraded white matter (WM) tracts, will increase performance variability by neural noise. The aim of this study was to test whether the quality of WM, measured by diffusion tensor imaging, is related to performance variability in healthy adults. Intraindividual standard deviation of the reaction time (sdRT) across trials and median reaction time (mRT) from 270 participants were obtained from a speeded continuous performance task (Eriksen flanker task) with two conditions (congruent, incongruent). Tract-based spatial statistics was used to test the relationship with diffusion characteristics [fractional anisotropy (FA), mean diffusion (MD), radial diffusion (RD), axial diffusion (AD)]. Robust relationships between sdRT and all diffusion measures were found in most WM areas, independently of mRT, age, and sex. The effects were anatomically more widespread in the congruent than the incongruent condition, covering almost 50% of the voxels for RD and MD, and >25% of the voxels for FA and AD. Partial betas were in the range 0.45-0.55, and the strength of the relationships increased significantly with age. For mRT, the effects were smaller and unstable across condition. We concluded that performance variability is a likely consequence of individual differences in WM integrity, and that it is a promising behavioral correlate of individual differences in WM microstructure.

Cortisol awakening response and negative emotionality linked to asymmetry in major limbic fibre bundle architecture

The limbic system plays an important role in regulating the hypothalamic-pituitary-adrenal (HPA) axis as well as aspects of emotion, and both neuroendocrine disturbance and increased negative emotionality are associated with risk for developing affective disorders. However, the extent to which the architecture of connections between limbic structures may be linked to individual differences in basal HPA-axis reactivity and negative emotionality is unknown. Here we tested the hypotheses that microstructural asymmetry of the major limbic fibre bundles would be associated with cortisol awakening response (CAR) and neuroticism, a personality trait associated with the tendency to experience negative emotions. Sixty-nine healthy adults were studied with diffusion-weighted imaging, and fractional anisotropy (FA) was extracted from the cingulum and uncinate fasciculus. Higher neuroticism scores, which were associated with higher CAR, were also correlated with higher right relative to left cingulum FA. Elevated CAR was associated with the degree of FA asymmetry within both the cingulum and the uncinate fasciculus, but in opposing directions. These results suggest that the balance between left- and right-sided limbic circuits may bear an important relationship to hypothalamic-pituitary-adrenal axis reactivity, and to the tendency to experience negative emotions, and they raise important questions about the significance of limbic system architecture.

Neuroanatomical substrates of age-related cognitive decline

There are many reports of relations between age and cognitive variables and of relations between age and variables representing different aspects of brain structure and a few reports of relations between brain structure variables and cognitive variables. These findings have sometimes led to inferences that the age-related brain changes cause the age-related cognitive changes. Although this conclusion may well be true, it is widely recognized that simple correlations are not sufficient to warrant causal conclusions, and other types of correlational information, such as mediation and correlations between longitudinal brain changes and longitudinal cognitive changes, also have limitations with respect to causal inferences. These issues are discussed, and the existing results on relations of regional volume, white matter hyperintensities, and diffusion tensor imaging measures of white matter integrity to age and to measures of cognitive functioning are reviewed. It is concluded that at the current time the evidence that these aspects of brain structure are neuroanatomical substrates of age-related cognitive decline is weak. The final section contains several suggestions concerning measurement and methodology that may lead to stronger conclusions in the future.

All data collection and analysis methods have limitations: reply to Rabbitt (2011) and Raz and Lindenberger (2011)

The commentaries on my article contain a number of points with which I disagree but also several with which I agree. For example, I continue to believe that the existence of many cases in which between-person variability does not increase with age indicates that greater variance with increased age is not inevitable among healthy individuals up to about 80 years of age. I also do not believe that problems of causal inferences from correlational information are more severe in the cognitive neuroscience of aging than in other research areas; I contend instead that neglect of these problems has led to confusion about neurobiological underpinnings of cognitive aging. I agree that researchers need to be cautious in extrapolating from cross-sectional to longitudinal relations, but I also note that even longitudinal data are limited with respect to their ability to support causal inferences.

The effect of gradient sampling schemes on diffusion metrics derived from probabilistic analysis and tract-based spatial statistics

PURPOSE

The purpose was to systematically evaluate the effect of diffusion gradient encoding scheme on estimated fractional anisotropy (FA), mean diffusivity (MD) and the voxel-wise probability of identifying crossing fibers in the brain.

MATERIALS AND METHODS

Eight healthy volunteers (mean age 26.5±1.3 years, 5 males, 3 females) were imaged using a Spin-Echo Echo-Planar-Imaging sequence acquired with two signal averages [number of signals averaged (NSA)], 127 diffusion directions, and b-values of 750 s/mm(2) and 1500 s/mm(2). The number of diffusion gradient directions (N(d)) was reduced from the original value whilst maintaining a homogeneous gradient distribution enabling direct comparison of subsampled data sets with N(d)=15, 28, 43, 84, 112 and 127. FA and MD maps were generated and analyzed using tract-based spatial statistics. Effect of N(d) on estimated FA and MD was tested with voxel-wise statistics in 13 regions of interest. The number of voxels supporting two fiber populations (NV(2)) at different N(d) values was estimated using Bayesian estimation of diffusion parameters.

RESULTS

Low FA values decreased significantly with increasing N(d) and with increasing NSA. MD was only marginally sensitive to N(d) and NSA. NV(2) increased significantly with N(d) but not with NSA. Thus, we conclude that accurate estimation of standard diffusion metrics FA and MD is mainly dependent on the signal-to-noise ratio (SNR), whereas the ability to differentiate multiple fiber populations requires a high diffusion sampling density.

The relationship of developmental changes in white matter to the onset of psychosis

Schizophrenia is a disorder with a pronounced developmental component. Accordingly, there is a growing interest in characterizing developmental changes in the period leading up to disease onset, in an effort to develop effective preventative interventions. One of the ongoing neurodevelopmental changes known to occur in the late adolescent period that often overlaps with the prodromal phase and time of onset is white matter development and myelination. In this critical review, a disruption in the normal trajectory of white matter development could potentially play an important role in the onset of psychosis. We seek to summarize the existing state of research on white matter development in prodromal subjects, with a particular focus on diffusion tensor imaging (DTI) measures. First, we describe the physiological basis of developmental white matter changes and myelination. Next, we characterize the pattern of white matter changes associated with typical development across adolescence as measured with DTI. Then, we discuss white matter changes observed in adult patients with schizophrenia and in individuals seen in genetic and clinical high risk states. Finally, we discuss the implications of these findings for future research directions and for potential therapeutic interventions.

Diffusion tensor imaging of white matter tract evolution over the lifespan

Diffusion tensor imaging (DTI) has been used widely to show structural brain changes during both development and aging. Lifespan studies are valuable because they connect these two processes, yet few DTI studies have been conducted that include both children and elderly subjects. This study used DTI tractography to investigate 12 major white matter connections in 403 healthy subjects aged 5-83 years. Poisson fits were used to model changes of fractional anisotropy (FA) and mean diffusivity (MD) across the age span, and were highly significant for all tracts. FA increased during childhood and adolescence, reached a peak between 20 and 42 years of age, and then decreased. MD showed an opposite trend, decreasing first, reaching a minimum at 18-41 years, and then increasing later in life. These trajectories demonstrate rates and timing of development and degradation that vary regionally in the brain. The corpus callosum and fornix showed early reversals of development trends, while frontal-temporal connections (cingulum, uncinate, superior longitudinal) showed more prolonged maturation and delayed declines. FA changes were driven by perpendicular diffusivity, suggesting changes of myelination and/or axonal density. Tract volume changed significantly with age for most tracts, but did not greatly influence the FA and MD trajectories. This study demonstrates clear age-related microstructural changes throughout the brain white matter, and provides normative data that will be useful for studying white matter development in a variety of diseases and abnormal conditions.

Social reward dependence and brain white matter microstructure

People show consistent differences in their cognitive and emotional responses to environmental cues, manifesting, for example, as variability in social reward processing and novelty-seeking behavior. However, the neurobiological foundation of human temperament and personality is poorly understood. A likely hypothesis is that personality traits rely on the integrity and function of distributed neurocircuitry. In this diffusion tensor imaging (DTI) study, this hypothesis was tested by examining the associations between reward dependence (RD) and novelty seeking (NS), as measured by Cloninger's Temperament and Character Inventory, and fractional anisotropy (FA) and mean diffusivity (MD) as DTI-derived indices of white matter (WM) microstructure across the brain. The results supported the hypothesis. RD was associated with WM architecture coherence as indicated by a negative correlation between RD and FA in frontally distributed areas including pathways connecting important constituents of reward-related neurocircuitry. The associations between RD and FA could not be explained by age, sex, alcohol consumption, or trait anxiety. In contrast, no effects were observed for NS. These findings support the theory that WM fiber tract properties modulate individual differences in social reward processing.

Are individual differences in rates of aging greater at older ages?

Although differences among people are frequently assumed to increase with age, cross-sectional comparisons of measures of brain structure and measures of cognitive functioning often reveal similar magnitudes of between-person variability across most of adulthood. The phenomenon of nearly constant variability despite systematically lower means with increased age suggests that individual differences in rates of aging may be relatively small, particularly compared with the individual differences apparent at any age. The current study examined between-person variability in cross-sectional means and in short-term longitudinal changes in 5 cognitive abilities at different ages in adulthood. The variability in both level and change in cognitive performance was found to be similar among healthy adults from 25 to 75 years of age in all 5 cognitive abilities. Furthermore, the correlations between scores at the first and second occasions were very high, and nearly the same magnitude at all ages. The results indicate that between-person differences in short-term cognitive changes are not inevitably greater among healthy older adults than among young adults.

Allostasis and the developing human brain: Explicit consideration of implicit models

We previously used the theory of allostasis as the foundation for a model of the current stress process. This work highlighted the core emotional systems of the brain as the central mediator of the relationship between stress and health. In this paper, we extend this theoretical approach to consider the role of developmental timing. In doing so, we note that there are strong implicit models that underlie current developmental stress research in the social and life sciences. We endeavor to illustrate these modelsexplicitlyas we review the evidence behind each one and discuss their implications. We then extend these models to reflect recent findings from research in life span human neuroscience. The result is a new set of developmental allostatic models that provide fodder for future empirical research, as well as novel perspectives on intervention.

Longitudinal development of human brain wiring continues from childhood into adulthood

Healthy human brain development is a complex process that continues during childhood and adolescence, as demonstrated by many cross-sectional and several longitudinal studies. However, whether these changes end in adolescence is not clear. We examined longitudinal white matter maturation using diffusion tensor tractography in 103 healthy subjects aged 5-32 years; each volunteer was scanned at least twice, with 221 total scans. Fractional anisotropy (FA) and mean diffusivity (MD), parameters indicative of factors including myelination and axon density, were assessed in 10 major white matter tracts. All tracts showed significant nonlinear development trajectories for FA and MD. Significant within-subject changes occurred in the vast majority of children and early adolescents, and these changes were mostly complete by late adolescence for projection and commissural tracts. However, association tracts demonstrated postadolescent within-subject maturation of both FA and MD. Diffusion parameter changes were due primarily to decreasing perpendicular diffusivity, although increasing parallel diffusivity contributed to the prolonged increases of FA in association tracts. Volume increased significantly with age for most tracts, and longitudinal measures also demonstrated postadolescent volume increases in several association tracts. As volume increases were not directly associated with either elevated FA or reduced MD between scans, the observed diffusion parameter changes likely reflect microstructural maturation of brain white matter tracts rather than just gross anatomy.

Morphometry and connectivity of the fronto-parietal verbal working memory network in development

Two distinctly different maturational processes - cortical thinning and white matter maturation - take place in the brain as we mature from late childhood to adulthood. To what extent does each contribute to the development of complex cognitive functions like working memory? The independent and joint contributions of cortical thickness of regions of the left fronto-parietal network and the diffusion characteristics of the connecting pathway of the left superior longitudinal fasciculus (SLF) in accounting for verbal working memory performance were investigated, using a predefined regions of interest-approach. 108 healthy participants aged 8-19 years underwent MRI, including anatomical and diffusion tensor imaging (DTI), as well as cognitive testing using a digit span task. Radial diffusivity of the SLF, as well as cortical thickness of supramarginal gyrus and rostral middle frontal cortex, were negatively related to digit span forwards performance, independently of age. Radial diffusivity of the SLF was also negatively related to digit span backwards. A multi-modal analysis showed that cortical thickness and SLF microstructure were complementary in explaining working memory span. Furthermore, SLF microstructure and cortical thickness had different impact on working memory performance during the developmental period, suggesting a complex developmental interplay. The results indicate that cortical and white matter maturation each play unique roles in the development of working memory.

Normative development of white matter tracts: similarities and differences in relation to age, gender, and intelligence

The white matter of the brain undergoes a range of structural changes throughout development; from conception to birth, in infancy, and onwards through childhood and adolescence. Several studies have used diffusion magnetic resonance imaging (dMRI) to investigate these changes, but a consensus has not yet emerged on which white matter tracts undergo changes in the later stages of development or what the most important driving factors are behind these changes. In this study of typically developing 8- to 16-year-old children, we use a comprehensive data-driven approach based on principal components analysis to identify effects of age, gender, and brain volume on dMRI parameters, as well as their relative importance. We also show that secondary components of these parameters predict full-scale IQ, independently of the age- and gender-related effects. This overarching assessment of the common factors and gender differences in normal white matter tract development will help to advance understanding of this process in late childhood and adolescence.

The brain dynamics of intellectual development: waxing and waning white and gray matter

Distributed brain areas support intellectual abilities in adults. How structural maturation of these areas in childhood enables development of intelligence is not established. Neuroimaging can be used to monitor brain development, but studies to date have typically considered single imaging modalities. To explore the impact of structural brain maturation on the development of intelligence, we used a combination of cortical thickness, white matter (WM) volume and WM microstructure in 168 healthy participants aged 8-30 years. Principal component analyses (PCAs) were conducted separately for cortical thickness, WM volume, fractional anisotropy (FA) and mean diffusivity (MD) in 64 different brain regions. For all four parameters, the PCAs revealed a general factor explaining between 40% and 53% of the variance across regions. When tested separately, negative age-independent relationships were found between intellectual abilities and cortical thickness and MD, respectively, while WM volume and FA were positively associated with intellectual abilities. The relationships between intellectual abilities and brain structure varied with age, with stronger relationships seen in children and adolescents than in young adults. Multiple regression analysis with the different imaging measures as simultaneous predictors, showed that cortical thickness, WM volume and MD all yielded unique information in explaining intellectual abilities in development. The present study demonstrates that different imaging modalities and measures give complementary information about the neural substrates of intellectual abilities in development, emphasizing the importance of multimodal imaging in investigations of neurocognitive development.