COMMENTARY Comments on the paper by Horowitz et al. (2014)

We have been impressed by the previous work of Assaf and collaborators, in particular that reporting differences of axon diameter in different sectors of the corpus callosum with applications of water diffusion methods (Barazany et al. 2009). That work returned the differences that have been well-documented histologically in macaque, chim-panzee, and humans by several groups including ourselves. Using histological as well as diffusion tractography (DT), two of us have unequivocally demonstrated that axon diameter differences in the corpus callosum and elsewhere relate both to the area of origin and to the termination of the projections. We have also computed the conduction delays that axon diameters and tract lengths generate in the brain and the predictions fit well the available, albeit scarce, electrophysiological evidence (Caminiti et al. 2009