Gender effects on callosal thickness in scaled and unscaled space

Sex differences in brain structure in auditory and cingulate regions

We applied a new method to visualize the three-dimensional profile of sex differences in brain structure based on MRI scans of 100 young adults. We compared 50 men with 50 women, matched for age and other relevant demographics. As predicted, left hemisphere auditory and language-related regions were proportionally expanded in women versus men, suggesting a possible structural basis for the widely replicated sex differences in language processing. In men, primary visual, and visuo-spatial association areas of the parietal lobes were proportionally expanded, in line with prior reports of relative strengths in visuo-spatial processing in men. We relate these three-dimensional patterns to prior functional and structural studies, and to theoretical predictions based on nonlinear scaling of brain morphometry.

Total brain volume and corpus callosum size in medication-naïve adolescents and young adults with autism spectrum disorder

BACKGROUND

Increased total brain volume (TBV) has been reported for children with autism spectrum disorder (ASD) but studies in older ASD subjects have been contradictory. Similarly, studies of corpus callosum (CC) area in ASD differ with regard to inclusion criteria, age, and IQ.

METHODS

In the present study, TBV, gray matter (GM), and white matter (WM) volume as well as midsagittal CC area were compared between 15 medication-naïve, high-functioning adolescent and young adult ASD subjects and 15 healthy control individuals, and correlations with visuomotor coordination and imitation abilities were explored. In addition, computational surface-based methods were implemented to encode callosal thickness at high spatial resolution.

RESULTS

Total brain volume, GM, and WM were increased and CC area was decreased in ASD subjects, a finding that was predominantly due to ASD subjects with lower IQ. Positive correlations of IQ with volume measures were observed only in control subjects. Autism spectrum disorder subjects showed reduced thickness in the posterior part of the CC. White matter volume showed a trend for negative correlation with dynamic balance and imitation abilities across groups.

CONCLUSIONS

This study replicates previous structural magnetic resonance imaging (MRI) findings in ASD, emphasizes the role of IQ differences, and adds some evidence for functional implications of structural findings.

The relation between connection length and degree of connectivity in young adults: a DTI analysis

Using diffusion tensor imaging and tractography to detail the patterns of interhemispheric connectivity and to determine the length of the connections, and formulae based on histological results to estimate degree of connectivity, we show that connection length is negatively correlated with degree of connectivity in the normal adult brain. The degree of interhemispheric connectivity--the ratio of interhemispheric connections to total corticocortical projection neurons--was estimated for each of 5 subregions of the corpus callosum in 22 normal males between 20 and 45 years of age (mean 31.68; standard deviation 8.75), and the average length of the longest tracts passing through each point of each subregion was calculated. Regression analyses were used to assess the relation between connection length and the degree of connectivity. Connection length was negatively correlated with degree of connectivity in all 5 subregions, and the regression was significant in 4 of the 5, with an average r(2) of 0.255. This is contrasted with previous analyses of the relation between brain size and connectivity, and connection length is shown to be a superior predictor. The results support the hypothesis that cortical networks are optimized to reduce conduction delays and cellular costs.

Mapping callosal morphology in early- and late-onset elderly depression: an index of distinct changes in cortical connectivity

There is some evidence of corpus callosum abnormalities in elderly depression, but it is not known whether these deficits are region-specific or differ based on age at onset of depression. Twenty-four patients with early-onset depression (mean age = 68.00, SD+/-5.83), 22 patients with late-onset depression (mean age = 74.50, SD+/-8.09) and 34 elderly control subjects (mean age = 72.38; SD+/-6.93) were studied. Using 3D MRI data, novel mesh-based geometrical modeling methods were applied to compare the midsagittal thickness of the corpus callosum at high spatial resolution between groups. Neuropsychological correlates of midsagittal callosal area differences were additionally investigated in a subsample of subjects. Depressed patients exhibited significant callosal thinning in the genu and splenium compared to controls. Significant callosal thinning was restricted to the genu in early-onset patients, but patients with late-onset depression exhibited significant callosal thinning in both the genu and splenium relative to controls. The splenium of the corpus callosum was also significantly thinner in subjects with late- vs early-onset depression. Genu and splenium midsagittal areas significantly correlated with memory and attention functioning among late-onset depressed patients, but not early-onset depressed patients or controls. Circumscribed structural alterations in callosal morphology may distinguish late- from early-onset depression in the elderly. These findings suggest distinct abnormalities of cortical connectivity in late- and early-onset elderly depression with possible influence on the course of illness. Patients with a late onset of depression may be at higher risk of illness progression and eventually dementia conversion than early-onset depression, with potentially important implications for research and therapy.

Brain size and cortical structure in the adult human brain

We investigated the scale relationship between size and cortical structure of human brains in a large sample of magnetic resonance imaging data. Cortical structure was estimated with several measures (cortical volume, surface area, and thickness, sulcal depth, and absolute mean curvature in sulcal regions and sulcal walls) using three-dimensional surface-based methods in 148 normal subjects (n [men/women]: 83/65, age [mean +/- standard deviation]: 25.0 +/- 4.9 years). We found significantly larger scaling exponents than geometrically predicted for cortical surface area, absolute mean curvature in sulcal regions and in sulcal walls, and smaller ones for cortical volume and thickness. As brain size increases, the cortex thickens only slightly, but the degree of sulcal convolution increases dramatically, indicating that human cortices are not simply scaled versions of one another. Our results are consistent with previous hypotheses that greater local clustering of interneuronal connections would be required in a larger brain, and fiber tension between local cortical areas would induce cortical folds. We suggest that sex effects are explained by brain size effects in cortical structure at a macroscopic and lobar regional level, and that it is necessary to consider true relationships between cortical measures and brain size due to the limitations of linear stereotaxic normalization.

Morphologic alterations in the corpus callosum in abuse-related posttraumatic stress disorder: a preliminary study

Magnetic resonance imaging (MRI) studies in children with maltreatment-related posttraumatic stress disorder (PTSD) have demonstrated smaller corpus callosum area, with the greatest magnitude of change in posterior portions of the corpus callosum. The purpose of this study was to measure corpus callosum area in adult female patients with childhood abuse-related PTSD and comparison subjects. MRI was used to measure the midsagittal area of the corpus callosum as well as subregions of the corpus callosum in 9 female subjects with abuse-related PTSD and 9 healthy female subjects. No differences were found in total area of the corpus callosum or in individual subregions, but the subregion/total area ratio was significantly smaller in posterior midbody in PTSD compared with the healthy subjects. These results suggest that relatively smaller areas of the posterior midbody of the corpus callosum are associated with childhood abuse related PTSD in adults; these findings are consistent with findings in children with abuse-related PTSD.

Positive correlations between corpus callosum thickness and intelligence

Callosal morphology is thought to reflect the capacity for inter-hemispheric communication and thus, in addition to other cerebral characteristics, may serve as a neuroanatomical substrate of general intellectual capacity. We applied novel computational mesh-based methods to establish the presence and direction of correlations between intelligence and callosal thickness at high spatial resolution while removing the variance associated with overall brain size. Within healthy subjects (n=62), and within males (n=28) and females (n=34) separately, we observed significant positive correlations between callosal morphology and intelligence measures (full-scale, performance, and verbal). These relationships were pronounced in posterior callosal sections and were confirmed by permutation testing. Significant negative correlations were absent. Positive associations between intelligence and posterior callosal thickness may reflect a more efficient inter-hemispheric information transfer, positively affecting information processing and integration, and thus intellectual performance. At the same time, regional variations in callosal size might also partly reflect the underlying architecture of topographically connected cortical regions relevant for processing higher-order cognitive information. Our findings emphasize the importance of incorporating posterior (callosal) regions into the theories and models proposed to explain the anatomical substrates of intelligence.

Sex differences in cortical and subcortical recruitment during simple and complex motor control: an fMRI study

In this study, we compared brain activation patterns in men and women during performance of a fine motor task, in order to investigate the influence of motor task complexity upon asymmetries of hemispheric recruitment. Thirty-three right-handed participants (17 males, 16 females) performed a self-paced finger-tapping task comprising three conditions of increasing complexity with both the dominant and the non-dominant hand. Imaging results demonstrated significant sex differences in brain activation patterns. While women showed significantly larger activation of ipsi- and contralateral task-related cortical areas than men, men exhibited significantly stronger subcortical activation in striatal regions. The observed activation differences may reflect sex differences in control of voluntary motor skills related to differential emphasis upon cortical and subcortical correlates of motor sequence processing, as well as differences in hemispheric recruitment, by means of which men and women can nevertheless achieve comparable motor performance.

Interhemispheric callosal transfer in adults with attention-deficit/hyperactivity disorder: an event-related potential study

Although subtypes of attention-deficit/hyperactivity disorder have been recognized for some time, few studies have investigated neurophysiological differences that may reflect these. Here, adults with attention-deficit/hyperactivity disorder-inattentive type, attention-deficit/hyperactivity disorder-combined type and controls performed a simple reaction time laterality task (the Poffenberger) during 128-channel electroencephalography recording. Visual-evoked potentials were used to calculate interhemispheric transfer times. As expected for controls, mean right-to-left transfer time was faster than mean left-to-right transfer. No significant asymmetry was, however, found for clinical groups. Compared with controls, attention-deficit/hyperactivity disorder-combined participants demonstrated significantly faster left-to-right transfer, whereas attention-deficit/hyperactivity disorder-inattentive participants had significantly slower right-to-left transfer. These findings support the existence of functional differences between attention-deficit/hyperactivity disorder subtypes, and suggest that the right hemisphere dysfunction theory of attention-deficit/hyperactivity disorder may only be applicable to those without hyperactive symptoms.

Callosal morphology in Williams syndrome: a new evaluation of shape and thickness

We applied novel mesh-based geometrical modeling methods to calculate and compare the thickness of the corpus callosum at high spatial resolution and to create profiles of average callosal shape in a well-matched sample (n=24) of individuals with Williams syndrome and controls. In close agreement with previous observations, superimposed surface maps indicate that the corpus callosum in Williams syndrome individuals is shorter and less curved. Moreover, we observed significantly thinner callosal regions in Williams syndrome individuals across the posterior surface, where group effects were less pronounced and spatially restricted in brain-size-adjusted data compared with native data. Circumscribed structural alterations in callosal morphology might be candidate anatomic substrates for the unique cognitive and behavioral profile associated with Williams syndrome.