UCLA

corpus callosum axons into functionally distinct subregions. Witelson’s (1989) scheme was based largely on data obtained from rhesus macaques. In contrast, Hofer and Frahm’s (2006) scheme was based on data from humans using Diffusion Tensor Imaging (DTI) and as such should more accurately partition the human corpus callosum into functional units. Sex differences in the human isthmus have been reported using the Witelson scheme. Examining Hofer and Frahm’s scheme may reveal sexual dimorphisms in axon bundles that are allegedly more functionally homogeneous. In this study, we employed both schemes independently on human corpora callosa and compared results. Analysis of data from the Witelson’s scheme revealed that the isthmus area was larger in males than in females.This difference was also observed when corrected for overall brain size. No sex differences were foundin the corpus callosum subregions as defined by the Hofer and Frahm scheme. Witelson’s scheme designated some axons as part of the isthmus that the Hofer and Frahm scheme designated as part of splenium. However, an examination of this disputed region revealed no sex difference, so we were unable to attribute the differential outcomes to this set of axons. We also examined the possibility that there was proportionately more variance in measurements of the isthmus when using the Hofer and Frahm scheme relative to the Witelson scheme, but the two schemes generated similar coefficients of variation. Re-examination of both schemes revealed that neither consistently partitioned the corpus callosum into functionally homogeneous regions as defined by individual DTI data. Partitioning the corpus callosum may be best accomplished using an individual’s unique DTI data rather than using any general scheme for all individuals.