All DTI analysis was performed with the 3D-Slicer software package (http://www.slicer.org, Surgical Planning Laboratory, Brigham and Women’s Hospital, Boston, Massachusetts). The splenium was manually defined on the midsagittal slice and the two neighboring sagittal slices on each participant’s FA image (Fig. 3a). The voxels defined by this Region-of-Interest (ROI) were then used as seed voxels for deterministic (streamline) tractography. Streamline tractography followed the direction defined by the principal eigenvector, based on a Runge-Kutta second-order protocol, with a fixed step size of 1.5 mm. Tractography terminated upon reaching a voxel of FA<0.25 (the stopping criterion), and a length criterion was also employed whereby fibers were excluded if they were shorter than 20 mm.
