Biomedical subjects

K L Narr

Publications and source records attributed to K L Narr.

11 recordsLinked to original sources

PubMed2006

Gender effects on cortical thickness and the influence of scaling.

Using magnetic resonance imaging and well-validated computational cortical pattern matching methods in a large and well-matched sample of healthy subjects (n = 60), we analyzed the regional specificity of gender-related cortical thickness differences across the lateral and medial cortices at submillimeter resolution. To establish the influences of brain size correction on gender effects, comparisons were performed with and without applying affine transformations to scale each image volume to a template. We revealed significantly greater cortical thickness in women compared to men, after correcting for individual differences in brain size, while no significant regional thickness increases were observed in males. The pattern and direction of the results were similar without brain size correction, although effects were less pronounced and a small cortical region in the lateral temporal lobes showed greater thickness in males. Our gender-specific findings support a dimorphic organization in male and female brains that appears to involve the architecture of the cortical mantle and that manifests as increased thickness in female brains. This sexual dimorphism favoring women, even without correcting for brain size, may have functional significance and possibly account for gender-specific abilities and/or behavioral differences between sexes.

Adult↗

PubMed2005

Hemispheric asymmetries in cortical thickness.

Using magnetic resonance imaging and computational cortical pattern matching methods, we analyzed hemispheric differences in regional gray matter thickness across the lateral and medial cortices in young, healthy adults (n = 60). In addition, we investigated the influence of gender on the degree of thickness asymmetry. Results revealed global and regionally specific differences between the two hemispheres, with generally thicker cortex in the left hemisphere. Regions with significant leftward asymmetry were identified in the precentral gyrus, middle frontal, anterior temporal and superior parietal lobes, while rightward asymmetry was prominent in the inferior posterior temporal lobe and inferior frontal lobe. On the medial surface, significant rightward asymmetries were observed in posterior regions, while significant leftward asymmetries were evident from the vicinity of the paracentral gyrus extending anteriorly. Asymmetry profiles were similar in both sexes, but hemispheric differences appeared slightly pronounced in males compared with females, albeit a few regions also indicated greater asymmetry in females compared with males. Hemispheric differences in the thickness of the cortex might be related to hemisphere-specific functional specializations that are possibly related to behavioral asymmetries.

Adult↗

PubMed2005

A curvature-based approach to estimate local gyrification on the cortical surface.

Using magnetic resonance imaging and a new method to analyze local surface shape, we examined the effects of gender on gyrification in a large and well-matched sample of healthy subjects. Unlike traditional 2D methods that produce whole-brain measurements of cortical complexity or more sophisticated 3D parametric mesh-based techniques that allow only different sections (lobes) of the cortex to be investigated, we employed a novel approach with increased spatial resolution. Although our method is sensitive to similar cortical features like the classic whole-brain gyrification index (depths of sulci and heights of gyri), we are now able to provide detailed and regionally specific estimates of cortical convolution at thousands of points across the cortical surface without introducing any bias through the rater or the selected orientation of the slices. We revealed pronounced gender differences, showing increased gyrification in frontal and parietal regions in females compared to males that agree with recent regions-of-interest findings. In addition, we detected higher female gyrification in temporal and occipital cortices that was not previously identified in studies using more global measures. No cortical area was significantly more convoluted in males compared to females. Our results demonstrate the sensitivity of this automated approach for identifying very local changes in gyrification. This technique may serve to isolate regionally specific changes in fissuration/gyrification in neurodevelopmental or neuropsychiatric disorders.

Adult↗

PubMed2005

Parasagittal asymmetries of the corpus callosum.

Significant relationships have been reported between midsagittal areas of the corpus callosum and the degree of interhemispheric transfer, functional lateralization and structural brain asymmetries. No study, however, has examined whether parasagittal callosal asymmetries (i.e. those close to the midline of the brain), which may be of specific functional consequence, are present in the human brain. Thus, we applied magnetic resonance imaging and novel computational surface-based methods to encode hemispheric differences in callosal thickness at a very high resolution. Discrete callosal areas were also compared between the hemispheres. Furthermore, acknowledging the frequently reported sex differences in callosal morphology, parasagittal callosal asymmetries were examined within each gender. Results showed significant rightward asymmetries of callosal thickness predominantly in the anterior body and anterior third of the callosum, suggesting a more diffuse functional organization of callosal projections in the right hemisphere. Asymmetries were increased in men, supporting the assumption of a sexually dimorphic organization of male and female brains that involves hemispheric relations and is reflected in the organization and distribution of callosal fibers.

Adult↗

PubMed2005

Mapping cortical gray matter in the young adult brain: effects of gender.

Using magnetic resonance imaging and well-validated computational cortical pattern matching methods in a large and well-matched sample of healthy subjects, we analyzed the effects of gender on regional gray matter (GM) concentration across the cortex. To clarify discrepancies in previous reports, we also examined sexual dimorphisms for whole-brain tissue volumes with and without controlling for brain size differences. In addition, we generated spatially detailed maps of average GM distributions and variability across the entire cortex given that these descriptors are not well characterized in the normative literature. After brain size correction, we detected numerous cortical regions showing significantly increased GM concentration in females compared to males, but no regionally increased GM concentration in males. Permutation testing confirmed the statistical significance of these findings. Locally increased concentration of cortical GM in females corroborates findings of larger global GM volumes in females after correcting for individual brain sizes. Larger global volumes of GM, white matter and CSF, however, are observed in males when individual brain volumes are not taken into account. Our results show that gender is a major contributor to regional and global GM differences between individuals, although the nature of these effects depend on whether brain size is taken into account.

Adult↗

PubMed2003

Relationships between sulcal asymmetries and corpus callosum size: gender and handedness effects.

Magnetic resonance imaging was used to establish the presence and nature of relationships between sulcal asymmetries and mid-sagittal callosal size in neurologically intact subjects, and to determine the influences of sex and handedness. Against a background of long-standing disputes, effects of gender and handedness on callosal size, shape, and variability were additionally examined. Both positive and negative correlations between sulcal asymmetry and callosal size were observed, with effects influenced by sex and handedness. The direction of relationships, however, were dependent on the regional asymmetry measured and on whether real or absolute values were used to quantify sulcal asymmetries. Callosal measurements showed no significant effects of sex or handedness, although subtle differences in callosal shape were observed in anterior and posterior regions between males and females and surface variability was increased in males. Individual variations in callosal size appear to outrange any detectable divergences in size between groups. Relationships between sulcal asymmetries and callosal size, however, are influenced by both sex and handedness. Whether magnitudes of asymmetry are related to increases or decreases in callosal size appears dependent on the chosen indicators of asymmetry. It is an oversimplification, therefore, to assume a single relationship exists between cerebral asymmetries and callosal connections.

Adult↗

PubMed2001

Three-dimensional mapping of temporo-limbic regions and the lateral ventricles in schizophrenia: gender effects.

BACKGROUND: Local alterations in morphological parameters are poorly characterized in several brain regions widely implicated in schizophrenia neuropathology. METHODS: Surface-based anatomical modeling was applied to magnetic resonance data to obtain three-dimensional (3D) average anatomical maps and measures of location, shape, asymmetry, and volume for the lateral ventricles, hippocampus, amygdala, and superior temporal gyrus in schizophrenic (n = 25; 15 male) and normal subjects (n = 28; 15 male) matched for demographic variables. For all regions, intra-group variability was visualized and group differences assessed statistically to discriminate local alterations in anatomy across sex and diagnosis. RESULTS: Posterior hippocampal volumes, lengths, and widths were reduced in patients. The right amygdala showed volume increases in schizophrenia patients versus controls. Ventricular enlargements, pronounced in the left hemisphere, occurred in the superior and lateral dimensions in patients, and these effects interacted with gender. Superior horn anterior extremes, inferior horn volumes, and hippocampal asymmetries exhibited gender effects. Significant group differences were absent in superior temporal gyrus parameters. Finally, regional variability profiles differed across groups. CONCLUSIONS: Clear morphometric differences of the lateral ventricles, hippocampus, and amygdala indicate regional displacements and shape distortions in several functional systems in schizophrenia. Alterations in these structures as mapped in 3D may provide the foundation for establishing brain abnormalities not previously defined at such a local level.

Adult↗

PubMed2001

Mapping cortical asymmetry and complexity patterns in normal children.

This study reports the first comprehensive three-dimensional (3D) maps of cortical patterns in children. Using a novel parametric mesh-based analytic technique applied to high-resolution T1-weighted MRI scans, we examined age (6-16 years) and gender differences in cortical complexity (the fractal dimension or complexity of sulcal/gyral convolutions) and asymmetry of 24 primary cortical sulci in normally developing children (N=24). Three-dimensional models of the cerebral cortex were extracted and major sulci mapped in stereotaxic space. Given the documented age-related changes in frontal lobe functions and several neuroimaging studies that have reported accompanying volumetric changes in these regions, we hypothesized that, with age, we would find continued modifications of the cerebrum in frontal cortex. We also predicted that phylogenetically older regions of the cerebrum, such as olfactory cortex, would be less variable in anatomic location across subjects and with age. Age-related increases in cortical complexity were found in both left and right inferior frontal and left superior frontal regions, possibly indicating an increase in secondary branching with age in these regions. Moreover, a significant increase in the length of the left inferior frontal sulcus and a posterior shifting of the left pre-central sulcus was associated with age. Three-dimensional asymmetry and anatomic variability maps revealed a significant left-greater-than-right asymmetry of the Sylvian fissures and superior temporal sulci, and increased variance in dorsolateral frontal and perisylvian areas relative to ventral regions of the cortex. These results suggest increases in cortical complexity and subtle modifications of sulcal topography of frontal lobe regions, likely reflecting ongoing processes such as myelination and synaptic remodeling that continue into the second decade of life. More studies in a larger sample set and/or longitudinal design are needed to address the issues of normal individual variation and sulcal development.

Adolescent↗

PubMed2001

Probabilistic approaches for atlasing normal and disease-specific brain variability.

The extreme variability in the structural conformation of the human brain poses significant challenges for the creation of population-based atlases. The ability to statistically and visually compare and contrast brain image data from multiple individuals is essential to understanding normal variability within a particular population as well as differentiating normal from diseased populations. This paper introduces the application of probabilistic atlases that describe specific subpopulations, measures their variability and characterizes the structural differences between them. Utilizing data from structural MRI, we have built atlases with defined coordinate systems creating a framework for mapping data from functional, histological and other studies of the same population. This paper describes the basic approach and a brief description of the underlying mathematical constructs that enable the calculation of probabilistic atlases and examples of their results from several different normal and diseased populations.

Brain↗

PubMed2001

Genetic influences on brain structure.

Here we report on detailed three-dimensional maps revealing how brain structure is influenced by individual genetic differences. A genetic continuum was detected in which brain structure was increasingly similar in subjects with increasing genetic affinity. Genetic factors significantly influenced cortical structure in Broca's and Wernicke's language areas, as well as frontal brain regions (r2(MZ) > 0.8, p < 0.05). Preliminary correlations were performed suggesting that frontal gray matter differences may be linked to Spearman's g, which measures successful test performance across multiple cognitive domains (p < 0.05). These genetic brain maps reveal how genes determine individual differences, and may shed light on the heritability of cognitive and linguistic skills, as well as genetic liability for diseases that affect the human cortex.

Adult↗

PubMed2000

Mapping morphology of the corpus callosum in schizophrenia.

The nature and extent of callosal morphological alterations in schizophrenia remain unresolved. A parametric surface modeling approach using magnetic resonance (MR) images was employed. This provided spatially accurate representations of midsagittal callosal surfaces in schizophrenic patients (n = 25; 15 males) and normal controls (n = 28; 15 males). Areas of functionally relevant callosal channels and measures reflecting callosal shape were visualized and compared across groups. To register neuroanatomical landmarks surrounding the corpus callosum, each three-dimensional MR volume was scaled according to Talairach AC-PC distance, and raw distances included as covariates in multivariate analyses. Results revealed: (i) a marked vertical displacement of the corpus callosum in patients (P < 0.01); (ii) increases in curvature of superior and inferior callosal surfaces (P < 0.001); and (iii) significant increases in maximum widths in anterior and posterior regions in male patients compared to male controls; as well as (iv) increased patterns of callosal variability in female patients but no effects of diagnosis between female groups. These findings demonstrate a clear index of structural neuropathology in male schizophrenic patients. Displacement and curvature increases were highly correlated with structural differences in surrounding neuroanatomical regions, including increased volume of the lateral ventricles (P < 0.01).

Adult↗