A critical re-examination of sexual dimorphism in the corpus callosum microstructure.

Recent diffusion-tensor imaging (DTI) studies suggest sexual dimorphism in the micro-structural architecture of the corpus callosum. However, the corpus callosum is also found to be larger in males than in females, a fact that might introduce a systematic bias to the analysis of DTI parameters. Diffusion parameters obtained in the larger male corpus callosum could be less affected by partial-volume averaging with surrounding non-callosal tissue than respective parameters obtained in the smaller female corpus callosum, i.e. the sex of the subject and partial-volume effects would be confounded. The objective of the present DTI study was to re-examine microstructural sex differences in the corpus callosum, while controlling for corpus callosum size differences between sexes. We compared 41 female and 34 male participants using regional tract-based spatial statistics (TBSS) analysis. Clusters of significantly higher fractional anisotropy (FA) and lower diffusion strength in males compared to females were detected in the genu and truncus of the corpus callosum. However, only the sex difference located in the anterior genu subregions could be unequivocally interpreted. This was the only cluster where the diffusion parameters did not correlate with regional callosal size. The present results indicate a stronger inter-hemispheric connectivity between the frontal lobes in males than females, which might be related to sex differences in hemispheric asymmetry and brain size.