MRI studies, including recent diffusion tensor imaging (DTI) studies, have shown corpus callosum abnormalities in children prenatally exposed to alcohol, especially in the posterior regions. These abnormalities appear across the range of Fetal Alcohol Spectrum Disorders (FASD). Several studies have demonstrated cognitive correlates of callosal abnormalities in FASD including deficits in visual-motor skill, verbal learning, and executive functioning. The goal of this study was to determine if inter-hemispheric structural connectivity abnormalities in FASD are associated with disrupted inter-hemispheric functional connectivity and disrupted cognition.
Twenty-one children with FASD and 23 matched controls underwent a six minute resting-state functional MRI scan as well as anatomical imaging and DTI. Using a semiautomated method, we parsed the corpus callosum and delineated seven inter-hemispheric white matter tracts with DTI tractography. Cortical regions of interest (ROIs) at the distal ends of these tracts were identified. Right-left correlations in resting fMRI signal were computed for these sets of ROIs and group comparisons were done. Correlations with facial dysmorphology, cognition, and DTI measures were computed.
A significant group difference in inter-hemispheric functional connectivity was seen in a posterior set of ROIs, the para-central region. Children with FASD had functional connectivity that was 12% lower than controls in this region. Sub-group analyses were not possible due to small sample size, but the data suggest that there were effects across the FASD spectrum. No significant association with facial dysmorphology was found. Para-central functional connectivity was significantly correlated with DTI mean diffusivity, a measure of microstructural integrity, in posterior callosal tracts in controls but not in FASD. Significant correlations were seen between these structural and functional measures and Wechsler perceptual reasoning ability.
Inter-hemispheric functional connectivity disturbances were observed in children with FASD relative to controls. The disruption was measured in medial parietal regions (para-central) that are connected by posterior callosal fiber projections. We have previously shown microstructural abnormalities in these same posterior callosal regions and the current study suggests a possible relationship between the two. These measures have clinical relevance as they are associated with cognitive functioning.
Fetal alcohol (FAS, FASD); Brain; functional MRI (fMRI); resting-state, connectivity; neuropsychological
The corpus callosum is the largest white matter tract, important for interhemispheric communication. The aim of this study was to investigate and compare corpus callosum size, shape and diffusion characteristics in 106 very preterm infants and 22 full-term infants. Structural and diffusion magnetic resonance images were obtained at term equivalent. The corpus callosum was segmented, cross-sectional areas were calculated, and shape was analyzed. Fractional anisotropy, mean, axial and radial diffusivity measures were obtained from within the corpus callosum, with additional probabilistic tractography analysis. Very preterm infants had significantly reduced callosal cross sectional area compared with term infants (p=0.004), particularly for the mid-body and posterior sub-regions. Very preterm callosi were more circular (p=0.01). Fractional anisotropy was lower (p=0.007) and mean (p=0.006) and radial (p=0.001) diffusivity values were higher in very preterm infants’ callosi, particularly at the anterior and posterior ends. The volume of tracts originating from the corpus callosum was reduced in very preterm infants (p=0.001), particularly for anterior mid-body (p=0.01) and isthmus tracts (p=0.04). This study characterizes callosal size, shape and diffusion in typically developing infants at term equivalent age, and reports macro- and micro-structural abnormalities as a result of prematurity.
Brain; Corpus callosum; Prematurity; Neonate; Infant; Magnetic resonance imaging; Diffusion tensor imaging
In an effort to understand the role of interhemispheric transfer in numerical development, we investigated the relationship between children’s developing knowledge of numbers and the integrity of their white matter connections between the cerebral hemispheres (the corpus callosum). We used diffusion tensor imaging (DTI) tractography analyses to test the link between the development of the corpus callosum and performance on symbolic and non-symbolic numerical judgment tasks. We were especially interested in the interhemispheric connections of parietal cortex in 6-year-old children, because regions of parietal cortex have been implicated in the development of numerical skills by several prior studies. Our results revealed significant structural differences between children and adults in the fibers of the corpus callosum connecting the left and right parietal lobes. Importantly, these structural differences were predictive of individual differences among children in performance on numerical judgment tasks: children with poor numerical performance relative to their peers exhibited reduced white matter coherence in the fibers passing through the isthmus of the corpus callosum, which connects the parietal hemispheres.
White matter tracts are like the “highways” of the brain, allowing for fast and efficient communication among diverse brain regions. The purpose of this paper is to review the results of autism studies that have used Diffusion Tensor Imaging (DTI), which is a neuroimaging method that allows us to examine the structure and integrity of these white matter tracts. From the 48 studies we reviewed, persons with ASD tended to have decreased white matter integrity spanning across many regions of the brain but most consistently in regions such as the corpus callosum (connecting the left and right hemispheres and associated with motor skill and complex information processing), the cingulum bundles (connecting regions along the middle-line of the brain with important frontal projections and associated with executive function), and white matter tracts that pass through the temporal lobe (connecting temporal lobe regions with other brain regions and associated with social functioning). The pattern of results in these studies suggests that the white matter tracts may be atypical in persons with ASD. Additionally, the review suggests that people with ASD may not exhibit the typical left-greater-than-right-brain asymmetry in white matter integrity compared to people with typical development. White matter alterations in persons with ASD are a target of emerging interventions and may help identify the brain basis of individual differences in this population.
White matter tracts of the brain allow neurons and neuronal networks to communicate and function with high efficiency. The aim of this review is to briefly introduce Diffusion Tensor Imaging (DTI) methods that examine white matter tracts and then to give an overview of the studies that have investigated white matter integrity in the brains of individuals with Autism Spectrum Disorder (ASD). From the 48 studies we reviewed, persons with ASD tended to have decreased fractional anisotropy and increased mean diffusivity in white matter tracts spanning many regions of the brain but most consistently in regions such as the corpus callosum, cingulum, and aspects of the temporal lobe. This decrease in fractional anisotropy was often accompanied by increased radial diffusivity. Additionally, the review suggests possible atypical lateralization in some white matter tracts of the brain and a possible atypical developmental trajectory of white matter microstructure in persons with ASD. Clinical implications and future research directions are discussed.
Diffusion Tensor Imaging; Neuroimaging; Autism; White Matter
The corpus callosum, the largest white matter tract in the brain, is a midline structure associated with the formation of the hippocampus, septum pellucidum, and cingulate cortex, which have been implicated in the pathogenesis of schizophrenia. Corpus callosum shape deformation, therefore, may reflect a midline neurodevelopmental abnormality.
Corpus callosum area and shape were analyzed in 14 first-episode psychotic patients with schizophrenia, 19 first-episode psychotic patients with affective disorder, and 18 normal comparison subjects.
No statistically significant corpus callosum area differences between groups were found, but there were differences in the structure’s shape between the patients with schizophrenia and the comparison subjects. A correlation between width and angle of the corpus callosum was found in patients with affective disorder.
Corpus callosum shape abnormalities in first-episode psychotic patients with schizophrenia may reflect a midline neurodevelopmental abnormality.
Patients with Parkinson’s disease have great difficulty in performing bimanual movements; this problem is more obvious when they perform bimanual anti-phase movements. The underlying mechanism of this problem remains unclear. In the current study, we used functional magnetic resonance imaging to study the bimanual coordination associated changes of brain activity and inter-regional interactions in Parkinson’s disease. Subjects were asked to perform right-handed, bimanual in-phase and bimanual anti-phase movements. After practice, normal subjects performed all tasks correctly. Patients with Parkinson’s disease performed in-phase movements correctly. However, some patients still made infrequent errors during anti-phase movements; they tended to revert to in-phase movement. Functional magnetic resonance imaging results showed that the supplementary motor area was more activated during anti-phase movement than in-phase movement in controls, but not in patients. In performing anti-phase movements, patients with Parkinson’s disease showed less activity in the basal ganglia and supplementary motor area, and had more activation in the primary motor cortex, premotor cortex, inferior frontal gyrus, precuneus and cerebellum compared with normal subjects. The basal ganglia and dorsolateral prefrontal cortex were less connected with the supplementary motor area, whereas the primary motor cortex, parietal cortex, precuneus and cerebellum were more strongly connected with the supplementary motor area in patients with Parkinson’s disease than in controls. Our findings suggest that dysfunction of the supplementary motor area and basal ganglia, abnormal interactions of brain networks and disrupted attentional networks are probably important reasons contributing to the difficulty of the patients in performing bimanual anti-phase movements. The patients require more brain activity and stronger connectivity in some brain regions to compensate for dysfunction of the supplementary motor area and basal ganglia in order to perform bimanual movements correctly.
Parkinson’s disease; bimanual movements; fMRI; brain activity; effective connectivity
The aim of this study was to relate altered corpus callosum (CC) integrity in 106 very preterm (VPT) infants (<30 weeks’ gestational age or <1250 g birth weight) at term equivalent to perinatal predictors and neurodevelopmental outcomes at two years. T1 and diffusion magnetic resonance images were obtained. The CC was traced, and divided into six sub-regions for cross-sectional area and shape analyses. Fractional anisotropy, mean, axial and radial diffusivity was sampled within the CC, and probabilistic tractography performed. Perinatal predictors were explored. The Bayley Scales of Infant Development (BSID-II) was administered at two years. Intraventricular hemorrhage was associated with a smaller genu and altered diffusion values within the anterior and posterior CC of VPT infants. White matter injury was associated with widespread alterations to callosal diffusion values, especially posteriorly, and radial diffusivity was particularly elevated, indicating altered myelination. Reduced CC tract volume related to lower gestational age, particularly posteriorly. Reduced posterior callosal skew was associated with postnatal corticosteroid exposure. This more circular CC was associated with delayed cognitive development. Higher diffusivity, particularly in splenium tracts, was associated with impaired motor development. This study elucidates perinatal predictors and adverse neurodevelopmental outcomes associated with altered callosal integrity in VPT infants.
Brain; Prematurity; Neonate; Magnetic resonance imaging; Diffusion tensor imaging; Tractography
The corpus callosum, which is the largest white matter structure in the human brain, connects the 2 cerebral hemispheres. It plays a crucial role in maintaining the independent processing of the hemispheres and in integrating information between both hemispheres. The functional integrity of interhemispheric interactions can be tested electrophysiologically in humans by using transcranial magnetic stimulation, electroencephalography, and functional magnetic resonance imaging. As a brain structural imaging, diffusion tensor imaging has revealed the microstructural connectivity underlying interhemispheric interactions. Sex, age, and motor training in addition to the size of the corpus callosum influence interhemispheric interactions. Several neurological disorders change hemispheric asymmetry directly by impairing the corpus callosum. Moreover, stroke lesions and unilateral peripheral impairments such as amputation alter interhemispheric interactions indirectly. Noninvasive brain stimulation changes the interhemispheric interactions between both motor cortices. Recently, these brain stimulation techniques were applied in the clinical rehabilitation of patients with stroke by ameliorating the deteriorated modulation of interhemispheric interactions. Here, we review the interhemispheric interactions and mechanisms underlying the pathogenesis of these interactions and propose rehabilitative approaches for appropriate cortical reorganization.
Sex differences have been reported in both overall corpus callosum area and its regional subdivisions in humans. Some have suggested this reflects a unique adaptation in humans, as similar sex differences in corpus callosum morphology have not been reported in any other species of primate examined to date. Furthermore, an association between various measurements of corpus callosum morphology and handedness has been found in humans and chimpanzees. In the current study, we report measurements of corpus callosum cross-sectional area from midsagittal MR images collected in vivo from 14 adult capuchin monkeys, 9 of which were also characterized for hand preference on a coordinated bimanual task. Adult females were found to have a significantly larger corpus callosum: brain volume ratio, rostral body, posterior midbody, isthmus, and splenium than adult males. Left-handed individuals had a larger relative overall corpus callosum area than did right-handed individuals. Additionally, a significant sex and handedness interaction was found for anterior midbody, with right-handed males having a significantly smaller area than right-handed females. These results suggest that sex and handedness influences on corpus callosum morphology are not restricted to Homo sapiens.
Bipolar disorder is characterized by extreme mood swings, including both manic and depressive episodes commonly accompanied by psychosis. Many imaging studies have investigated white matter changes in bipolar illness, and the results have suggested abnormal intra- and inter-hemispheric white matter structures, particularly in the fronto-limbic and callosal systems. However, some inconsistency remains in the literature, and no study to-date has utilized brain network analysis using graph theory. Here, we acquired 64-direction diffusion weighted imaging (DWI) on 25 euthymic bipolar I subjects and 25 gender/age matched healthy subjects. White matter integrity measures were computed and compared in 50 white matter ROIs. The results indicated impaired integrity in the corpus callosum. Guided by this, we constructed whole brain structural connectivity networks using graph theory. We devised brain network metrics (inter-hemispheric path length and efficiency) to further probe inter-hemispheric integration, and demonstrated relatively preserved intra-hemispheric but significantly impaired inter-hemispheric integration in our bipolar subjects.
bipolar disorder; diffusion imaging; tractography; brain network analysis
Quantitative tractography may provide insights into regional heterogeneity of changes in white matter structure in normal ageing. Here we examine how brain atrophy and white matter lesions affect correlations between tract shape, tract integrity and age in a range of frontal and non-frontal tracts in 90 non-demented subjects aged over 65 years using an enhanced version of probabilistic neighbourhood tractography. This novel method for automatic single seed point placement employs unsupervised learning and streamline selection to provide reliable and accurate tract segmentation, whilst also indicating how the shape of an individual tract compares to that of a predefined reference tract. There were significant negative correlations between tract shape similarity to reference tracts derived from a young brain white matter atlas and age in genu and splenium of corpus callosum. Controlling for intracranial and lateral ventricle volume, the latter of which increased significantly with age, attenuated these correlations by 40 and 84 % respectively, indicating that this age-related change in callosal tract topology is significantly mediated by global atrophy and ventricular enlargement. In accordance with the ‘frontal ageing’ hypothesis, there was a significant positive correlation between mean diffusivity (〈D〉) and age, and a significant negative correlation between fractional anisotropy (FA) and age in corpus callosum genu; correlations not seen in splenium. Significant positive correlations were also observed between 〈D〉 and age in bilateral cingulum cingulate gyri, uncinate fasciculi and right corticospinal tract. This pattern of correlations was not, however, reproduced when those subjects with significant white matter lesion load were analyzed separately from those without. These data therefore suggest that brain atrophy and white matter lesions play a significant role in driving regional patterns of age-related changes in white matter tract shape and integrity.
Ageing; white matter; magnetic resonance imaging; water diffusion tensor; tractography
There is a fundamental gap in understanding how brain structural and functional network connectivity are interrelated, how they change with age, and how such changes contribute to older adults’ sensorimotor deficits. Recent neuroimaging approaches including resting state functional connectivity MRI (fcMRI) and diffusion tensor imaging (DTI) have been used to assess brain functional (fcMRI) and structural (DTI) network connectivity, allowing for more integrative assessments of distributed neural systems than in the past. Declines in corpus callosum size and microstructure with advancing age have been well documented, but their contributions to age deficits in unimanual and bimanual function are not well defined. Our recent work implicates age-related declines in callosal size and integrity as a key contributor to unimanual and bimanual control deficits. Moreover, our data provide evidence for a fundamental shift in the balance of excitatory and inhibitory interhemispheric processes that occurs with age, resulting in age differences in the relationship between functional and structural network connectivity. Training studies suggest that the balance of interhemispheric interactions can be shifted with experience, making this a viable target for future interventions.
interhemispheric; inhibition; aging; motor control
Age-related decline in microstructural integrity of certain white matter tracts may explain cognitive decline associated with normal aging. Whole brain tractography and a clustering segmentation in 48 healthy individuals across the adult lifespan were used to examine: interhemispheric (corpus callosum), intrahemispheric association (cingulum, uncinate, arcuate, inferior longitudinal, inferior occipitofrontal), and projection (corticospinal) fibers. Principal components analysis reduced cognitive tests into 6 meaningful factors: (1) memory and executive function; (2) visuomotor dexterity; (3) motor speed; (4) attention and working memory; (5) set-shifting/flexibility; and (6) visuospatial construction. Using theory-based structural equation modeling, relationships among age, white matter tract integrity, and cognitive performance were investigated. Parsimonious model fit demonstrated relationships where decline in white matter integrity may explain age-related decline in cognitive performance: inferior longitudinal fasciculus (ILF) with visuomotor dexterity; the inferior occipitofrontal fasciculus with visuospatial construction; and posterior fibers (i.e., splenium) of the corpus callosum with memory and executive function. Our findings suggest that decline in the microstructural integrity of white matter fibers can account for cognitive decline in normal aging.
Aging; Anisotropy; Cognition; Diffusion tensor imaging; White matter
Recent studies have implicated anatomical differences in speech-relevant brain regions of adults who stutter (AWS) compared to normally fluent adults (NFA). The present study focused on the region of the corpus callosum (CC) which is involved in interhemispheric processing between the left and right cerebral hemispheres. Two-dimensional segmentation of area and voxel based morphometry were used to evaluate the corpus callosum. Results revealed that the rostrum and anterior midbody of the CC was larger in AWS than NFA. In addition, the overall callosa area was larger in AWS than NFA. The group comparison of white matter volume showed a cluster of increased white matter volume predominantly encompassing the rostrum across the midline portion in AWS. These results potentially reflect anatomical changes associated with differences in the hemispheric distribution of language processes that has been reported previously in AWS.
After reading this article, the reader will be able to: (1) summarize research findings on functional and anatomical differences between AWS and NFA; (2) summarize research findings on anatomical anomalies observed in AWS; (3) discuss the possible relationships between functional and anatomical aberrations in AWS; and (4) discuss how the findings of the present study may support results of previous behavioral investigations (e.g. dichotic listening) in AWS.
Stuttering; Corpus callosum; Voxel based morphometry; White matter; Cerebral dominance; Interhemispheric communication
OBJECTIVES: To establish a normative database for magnetisation transfer ratio (MTR) measurements in the white matter of healthy adult brains. Such MTR values were evaluated for regional variation and evidence of differences associated with aging, sex, and handedness. METHODS: Forty one healthy volunteers, ranging in age from 16 to 55 years, underwent axial brain magnetisation transfer (MT) imaging on a 1.5 Tesla magnetic resonance scanner. Calculated MT images allowed evaluation of MTR from specific regions within the corpus callosum, cerebral hemispheres, and pons. RESULTS: Highest values were noted in the corpus callosum. No significant sex differences were seen for any region studied. Small but significant age related reductions in MTR were noted in the corpus callosum and other cerebral white matter regions studied. Comparing MTR values between young (16-35 years) and older (36-55 years) age groups, this was most apparent in the corpus callosum (40.82% units in the young group v 40.28% units in the older group, P < 0.05) and frontal white matter (39.65% units in the young group v 39.18% units in the older group, P < 0.005). In addition, values for MTR were analysed for evidence of hemispheric asymmetry. MTR values were higher in the left hemisphere for all regions studied, reaching significance in the centrum semiovale (37.75% units v 37.57% units, P < 0.05) and parieto-occipital white matter (37.67% units v 37.43% units, P < 0.05). No relation between such interhemispheric MTR differences and handedness was noted. CONCLUSIONS: Magnetisation transfer imaging shows significant age related changes in normal brain white matter. In addition to regional variations in MTR in the normal brain, there seem to be small but significant variations in MTR between the cerebral hemispheres. It is important to consider such normal variations when evaluating MTR in pathological states.
Relative to other primates, Cebus monkeys display unusually fast postnatal brain growth and motor skill development. The neonatal capuchin brain, at approximately 29–34 g, is a smaller proportion of the adult brain weight (c. 50%) than is the brain of other primates except humans and great apes. Here we describe, from a cross-sectional sample, brain development in 29 brown capuchin monkeys (Cebus apella) using high-resolution structural magnetic resonance images, focusing on growth patterns in total brain volume, cortical gray and white matter volume, frontal lobe gray and white matter volume, and corpus callosum area. Non-linear age-related changes in total brain volume, cortical white matter volume and frontal white matter volume were detected from birth – 5 years. Sex differences in corpus callosum:brain ratio were also found, with males having a 10% smaller corpus callosum:brain ratio than females regardless of age. Female corpus callosum:brain ratio showed significant age-related related changes, whereas males did not display any significant changes across age. Sex differences were also found in cortical gray and frontal lobe gray matter volumes, with males having larger volumes than females. These findings support the conclusion that capuchins undergo rapid neurological change during the first few years of life.
We examined white matter abnormalities in patients with a distinctive extrapyramidal syndrome due to intravenous methcathinone (ephedrone) abuse. We performed diffusion tensor imaging in ten patients and fifteen age-matched controls to assess white matter structure across the whole brain. Diffuse significant decreases in white matter fractional anisotropy, a diffusion tensor imaging metric which reflects microstructural integrity, occurred in the patients compared with controls. In addition, we identified two foci of severe white matter abnormality underlying the right ventral premotor cortex and the medial frontal cortex, two cortical regions involved in higher-level executive control of motor function. Paths connecting different cortical regions with the globus pallidus, the nucleus previously shown to be abnormal on structural imaging in these patients, were generated using probabilistic tractography. The fractional anisotropy within all these tracts was lower in the patient group than controls. Finally, we tested for a relationship between white matter integrity and clinical outcome. We identified a region within the left corticospinal tract in which lower fractional anisotropy was associated with greater functional deficit but this region did not show reduced fractional anisotropy in the overall patient group compared to controls. These patients have widespread white matter damage with greatest severity of damage underlying executive motor areas.
Extrapyramidal syndrome; Methcathinone; Manganese toxicity; diffusion imaging; white matter tracts
There is increasing recognition that many of the core behavioral impairments that characterize autism potentially emerge from poor neural synchronization across nodes comprising dispersed cortical networks. A likely candidate for the source of this atypical functional connectivity in autism is an alteration in the structural integrity of intra- and inter-hemispheric white matter tracts that form large-scale cortical networks. To test this hypothesis, in a group of adults with high functioning autism (HFA) and matched control participants, we used diffusion tensor tractography to compare the structural integrity of three intra-hemispheric visual-association white matter tracts, the inferior longitudinal fasciculus (ILF), the inferior fronto-occipito fasciculus (IFOF) and the uncinate fasciculus (UF), with the integrity of three sub-portions of the major inter-hemispheric fiber tract, the corpus callosum. Compared with the control group, the HFA group evinced an increase in the volume of the intra-hemispheric fibers, particularly in the left hemisphere, and a reduction in the volume of the forceps minor and body of the corpus callosum. The reduction in the volume of the forceps minor also correlated with an increase in repetitive and stereotypical behavior as measured by the Autism Diagnostic Interview. These findings suggest that the abnormalities in the integrity of key inter-and intra-hemispheric white matter tracts may underlie the atypical information processing observed in these individuals.
Autism; White matter; Inferior fronto-occipito fasciculus; Inferior longitudinal fasciculus; Diffusion tensor tractography
The corpus callosum (CC) is the largest commissural white matter tract in mammalian brains, connecting homotopic and heterotopic regions of the cerebral cortex. Knowledge of the distribution of callosal fibers projecting into specific cortical regions has important implications for understanding the evolution of lateralized structures and functions of the cerebral cortex. No comparisons of CC topography in humans and great apes have yet been conducted. We investigated the topography of the CC in 21 chimpanzees using high-resolution magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI). Tractography was conducted based on fiber assignment by continuous tracking (FACT) algorithm. We expected chimpanzees to display topographical organization similar to humans, especially concerning projections into the frontal cortical regions. Similar to recent studies in humans, tractography identified five clusters of CC fibers projecting into defined cortical regions: prefrontal; premotor and supplementary motor; motor; sensory; parietal, temporal and occipital. Significant differences in fractional anisotropy (FA) were found in callosal regions, with highest FA values in regions projecting to higher-association areas of posterior cortical (including parietal, temporal and occipital cortices) and prefrontal cortical regions (p<0.001). The lowest FA values were seen in regions projecting into motor and sensory cortical areas. Our results indicate chimpanzees display similar topography of the CC as humans, in terms of distribution of callosal projections and microstructure of fibers as determined by anisotropy measures.
A reduction in interhemispheric connectivity is thought to contribute to the etiology of schizophrenia. Diffusion Tensor Imaging (DTI) measures the diffusion of water and can be used to describe the integrity of the corpus callosum white matter tracts, thereby providing information concerning possible interhemispheric connectivity abnormalities. Previous DTI studies in schizophrenia are inconsistent in reporting decreased Fractional Anisotropy (FA), a measure of anisotropic diffusion, within different portions of the corpus callosum. Moreover, none of these studies has investigated corpus callosum systematically, using anatomical subdivisions.
DTI and structural MRI scans were obtained from 32 chronic schizophrenic subjects and 42 controls. Corpus callosum cross sectional area and its probabilistic subdivisions were determined automatically from structural MRI scans using a model based deformable contour segmentation. These subdivisions employ a previously generated probabilistic subdivision atlas, based on fiber tractography and anatomical lobe subdivision. The structural scan was then co-registered with the DTI scan and the anatomical corpus callosum subdivisions were propagated to the associated FA map.
Results revealed decreased FA within parts of the corpus interconnecting frontal regions in schizophrenia compared with controls, but no significant changes for callosal fibers interconnecting parietal and temporo-occipital brain regions. In addition, integrity of the anterior corpus was statistically significantly correlated with negative as well as positive symptoms, while posterior measures correlated with positive symptoms only.
This study provides quantitative evidence for a reduction of interhemispheric brain connectivity in schizophrenia, involving corpus callosum, and further points to frontal connections as possibly disrupted in schizophrenia.
The coordination of movement between the upper limbs is a function highly distributed across the animal kingdom. How the central nervous system generates such bilateral, synchronous movements, and how this differs from the generation of unilateral movements, remains uncertain. Electrophysiologic and functional imaging studies support that the activity of many brain regions during bimanual and unimanual movement are quite similar. Thus, the same brain regions (and indeed the same neurons) respond similarly during unimanual and bimanual movements as measured by electrophysiological responses. How then are different motor behaviors generated?
To address this question, we studied unimanual and bimanual movements using fMRI and constructed networks of activation using Structural Equation Modeling (SEM). Our results suggest that (1) the dominant hemisphere appears to initiate activity responsible for bimanual movement; (2) activation during bimanual movement does not reflect the sum of right and left unimanual activation; (3) production of unimanual movement involves a network that is distinct from, and not a mirror of, the network for contralateral unimanual movement; and (4) using SEM, it is possible to obtain robust group networks representative of a population and to identify individual networks which can be used to detect subtle differences both between subjects as well as within a single subject over time. In summary, these results highlight a differential role for the dominant and non-dominant hemispheres during bimanual movements, further elaborating the concept of handedness and dominance. This knowledge increases our understanding of cortical motor physiology in health and after neurological damage.
Contrary to expectations derived from preclinical studies of the effects of stress, and imaging studies of adults with PTSD, there is no evidence of hippocampus atrophy in children with PTSD. Multiple pediatric studies have reported reductions in the corpus callosum – the primary white matter tract in the brain. Consequently, in the present study, Diffusion Tensor Imaging was used to assess corpus callosum white matter integrity in 17 maltreated children with PTSD and 15 demographically matched normal controls. Children with PTSD had reduced fractional anisotropy in the medial and posterior corpus, a region which contains interhemispheric projections from brain structures involved in circuits that mediate the processing of emotional stimuli and various memory functions --- core disturbances associated with a history of trauma. Further exploration of the effects of stress on corpus callosum and white matter development appears a promising strategy to better understanding the pathophysiology of PTSD in children.
Posttraumatic Stress Disorder; Imaging; DTI; children
Results from recent studies suggest that chronic cigarette smoking is associated with increased white matter volume in the brain as determined by in vivo neuroimaging. We used diffusion tensor imaging to examine the microstructural integrity of the white matter in 10 chronic smokers and 10 nonsmokers. All individuals were healthy, without histories of medical or psychiatric illness. Fractional anisotropy (FA) and trace were measured in the genu, body, and splenium of the corpus callosum. FA provides a measure of directional versus nondirectional water diffusion, whereas trace provides a measure of nondirectional water diffusion. Lower FA and higher trace values are considered to reflect less brain integrity. Voxel-based morphometry was used to define volumes in each of these regions of the corpus callosum. Chronic smokers exhibited significantly higher FA in the body and whole corpus callosum and a strong trend for higher FA in the splenium compared with nonsmokers. FA did not differ between groups in the genu, and neither trace nor white matter volumes differed between groups in any of the regions of interest. When subdivided by Fagerström score (low vs. high), the low Fagerström group exhibited significantly higher FA in the body of the corpus callosum compared with the high Fagerström group and the nonsmokers. These results suggest that, among healthy adults, lower exposure to cigarette smoking is associated with increased microstructural integrity of the white matter compared with either no exposure or higher exposure. Additional studies are needed to further explore differences in white matter integrity between smokers and nonsmokers.
Degradation of white matter fibers can affect the transmission of signals in brain circuits that normally enable integration of highly lateralized visual and motor processes. Here, we used diffusion tensor imaging (DTI) tractography in combination with functional magnetic resonance imaging (fMRI) to examine the specific contributions of interhemispheric and intrahemispheric white matter fibers to functional measures of hemispheric transfer and parallel information processing using bilateral and unilateral left and right visual field stimulation in normal and compromised systems. In healthy adults, a greater degree of bilateral processing advantage with the left (nondominant) hand correlated with higher integrity of callosal fibers connecting occipital cortices, whereas less unilateral processing advantage with the right hand correlated with higher integrity of left-hemispheric posterior cingulate fibers. By contrast, alcoholics who have compromised callosal integrity showed less bilateral processing advantage than controls when responding with the left hand and greater unilateral processing advantage when responding with the right hand. We also found degraded left posterior cingulate and posterior callosal fibers in chronic alcoholics, which is consistent with functional imaging results of less left posterior cingulate and extrastriate cortex activation in alcoholics than controls when processing bilateral compared with unilateral visual field stimulation. Together, our results demonstrated that inter- and intrahemispheric white matter fiber pathways mediate visuomotor integration asymmetrically, and that subtle white matter fiber degradation in alcoholism attenuated the normal pattern of hemispheric asymmetry, which may have ramifications for the efficiency of visual information processing and fast response execution.
Interhemispheric communication; corpus callosum; hemispheres; alcohol; quantitative fiber tracking; functional neuroimaging
The evidence that speech is the marker of hemisphere of action is overwhelming. Thus, contrary to the commonly accepted belief, the evidence indicates that both sides of the body are under the same command (major hemisphere) and that the nondominant side of the body is a callosum-width farther from the major hemisphere. Substantial controversy exists, however, as to the best method for determining the laterality of motor control in an individual case. According to the new understanding, ie, the one-way callosal traffic circuitry underpinning laterality of motor control, the larger excursions of effectors located opposite (contralateral to) the command center while performing bimanual simultaneous drawing tasks provides the best noninvasive and inexpensive approach for demonstrating the laterality of the major hemisphere of a person (who is able to perform such tasks). Here, it is documented pictorially that bimanual simultaneous drawing of geometrical designs or straight lines, as well as moving the arms simultaneously from side to side (or up and down) while noting the difference of speed of the two arms (represented by the distance between the two index fingers), both provide a reliable indication of the laterality of a person’s major hemisphere. In all these maneuvers the nondominant side of the body (even the diaphragms) lags behind the dominant side by an interval equal to the interhemispheric transfer time (IHTT). This lagging behind of the nondominant side of the body in bimanual simultaneous movements is the footprint of directionality of callosal traffic underpinning the laterality of motor control evidenced by worsening of the delay of the nondominant side following callosotomy (uncoupling). Here, the historical precedence of a novel understanding in motor control together with its neurological implications in daily life as well as in laterality of seizure onset are briefly addressed, pointing out the deleterious effects of Sir Isaac Newton’s influence in neurological research on interhemispheric connectivity by suggesting symmetrical representation of visual sense of space in the human brain.
speech; brain; hemisphere; motor; control; callosotomy