Voxelwise statistics on the skeleton space FA and MD data were carried out in AFNI using independent sample t-tests. A combination of individual voxel probability and cluster size thresholding using Monte Carlo simulation (Ward, 2000) for multiple comparison correction was employed for Type I error control. Under this criteria, clusters ≥ 54 μl (54 contiguous 1×1×1 voxels) with an individual voxel probability threshold of P<0.05, yielding a brain-wise P<0.01 of finding such a cluster under the null hypothesis, were interpreted. Cohen's d effect sizes (Cohen, 1988) were computed from the average t-value within each significant cluster. In addition to being subjected to a whole-brain analysis, MD values were examined in regions of significant between-group FA differences, to assess the contribution of water diffusion to structural differences. Anatomical identification of tract structures was confirmed using a white matter atlas (Wakana et al., 2004).
