Sexual dimorphism in the corpus callosum: methodological considerations in MRI morphometry.

Studies of sexual dimorphism in the corpus callosum (CC) have employed a variety of methodologies for measurement and normalization but have yielded disparate results. The present work demonstrates how in some cases different manipulations of the same raw data, corresponding to different commonly used methodologies, produce discordant results. Midsagittal CC area was measured from magnetic resonance images (MRIs) of 137 young normal volunteers. Three strategies intended to normalize for average differences in brain size between the sexes, as well as five different normalization variables, were contrasted and evaluated. The stereotaxic method normalizes for intersubject differences in overall brain size by scaling MRIs into a standardized space. The ratio method uses one of five different indices of brain size and divides it into CC area. The covariate method uses one of the indices as a covariate in statistical analyses. Male subjects show significantly larger absolute total area, as well as anterior third and posterior midbody. However, in two of three normalization strategies, namely the stereotaxic and ratio methods, females show relatively larger total area, anterior midbody, and splenium. The covariate method did not show any significant differences at the 0.05 level. Results suggest that different approaches to normalization and analysis are not necessarily equivalent and interchangeable.