In part, this discrepancy might be related to age-related WM volume increases and age-related MTR 3-Methyladenine purchase (magnetization-transfer ration, indirect index of myelination) decreases during adolescence that was especially observed in boys but not in girls (Perrin et al., 2009). A limitation of this DTI study is that we are not able to directly image the degree of myelination in white matter (Alexander et al., 2011). Due to the effect of noise, the shape of the calculated diffusion ellipsoid and the pathology on the measured direction and magnitude of the eigenvalues and eigenvectors it is difficult to distinguish components of the

microstructural pathology based on DTI indices alone. The major difficulty occurs in areas of low anisotropy such as gray matter, voxels affected by partial volume,

areas of crossing fibers, or areas where the diffusion ellipsoid is oblate (cf. Wheeler-Kingshott & Cercignani, 2009). As morphological confounds affect primarily areas of low anisotropy, intelligence-related differences in the corpus callosum (high anisotropy) likely reflect true effects of intelligence on the white matter microstructure of men. Nevertheless, a replication of the present finding using click here complementary methods such as susceptibility tensor imaging (STI) or a longitudinal study comparing bundle-volume and configuration over time to uncouple true microstructural changes from morphological confounds (cf. Vos, Jones, Viergever, & Leemans, 2011), could be of particular interest. Also, future studies should try to match intelligence groups for age (rather than control effects of age statistically) and ensure equal sample sizes in all experimental groups. In this study fewer men were tested, thus the male

group was slightly underpowered and the power to detect a two-way interaction when looking at sex and intelligence group is rather low. Finally, although our results are only partially consistent with prior findings, it should be acknowledged that this study, compared find more to previous relevant studies, used a comparably large sample as well as a more conservative threshold criterion (FWE corrected) which typically ensures robust findings. The results provide evidence that white matter microstructure-correlates of intelligence are moderated by sex. By means of DTI-TBSS analyzes, the present study demonstrated that more intelligent men have higher FA accompanied by lower RD in the corpus callosum as compared to less intelligent men. According to this result and the given interpretation of FA and RD, intelligence might be associated with higher myelination and/or a higher axonal density in the tract connecting the right and left hemispheres and connecting areas within each hemisphere in men.