After extraction of the visual callosal fibers, a binary label-map was generated for each participant by labeling those voxels through which any fibers passed. FA was calculated for each voxel as per the algorithm of Basser and Pierpaoli (1996), and mean FA was calculated by averaging the FA values of the constituent voxels of the label-map. Mode was calculated for each voxel as per the algorithm of Ennis and Kindlmann (2006), and average Mode was calculated in the same way as average FA. Mode is an index of the three-dimensional shape of the diffusion ellipsoid. Specifically, Mode describes whether the diffusion ellipsoid is prolate (i.e., shaped like a cigar, and associated with a Mode ~ 1) or oblate (i.e., shaped like a pancake, and associated with a Mode ~ −1). FA and Mode are mathematically orthogonal, and two tensors with identical FA (i.e., equally aspherical) can have very different three-dimensional shapes (Ennis and Kindlmann, 2006).
