The second aim was to use fiber tractography in conjunction with DTI to a) precisely extract the visual fibers of the corpus callosum, and b) quantify their structural integrity on the basis of two complementary diffusion indices, namely Fractional Anisotropy (FA) and Mode. While the majority of previous DTI studies have quantified fiber integrity solely on the basis of FA (which is an index of the asphericity of diffusion), FA cannot, as noted by Hasan (2006), determine the morphological underpinnings of this asphericity. FA, for example, is unable to distinguish between prolate diffusion (i.e., shaped like a cigar) and oblate diffusion (i.e., shaped like a pancake), despite their presumably different physiological bases. To distinguish between these scenarios, a diffusion index such as Mode is required (Ennis and Kindlmann, 2006), and hence the present study used both FA and Mode to quantify the structural integrity of the visual fibers of the corpus callosum.
