In this processing step, all FA maps were aligned to a common space template using the nonlinear registration IRTK (65; www.doc.ic.ac.uk/~dr/software). The template was an average FA volume constructed from 72 participants drawn from our full research sample, aligned to 2 mm isotropic MNI voxel space. The 72 diffusion volumes were chosen to sample the adolescent age range evenly and with balanced sex ratios, while selecting only participants who had high quality diffusion scans from the perspective of intersubject registration (symmetrical anatomy, normal ventricular size, no cysts, minimal scanning artifacts, good signal-to-noise ratios, etc.). See reference (66) for further details. MD maps were aligned to common space by applying the transformation matrices previously computed during the FA map alignments. FA maps were masked at a threshold of FA = 0.2 to reduce partial volume effects, and the same voxel masks were applied to MD maps.
