1Radiology, Martinos Center/ MGH, Charlestown, MA, USA; 2Anatomy & Neurobiology, Boston University Medical, Boston, MA, USA; 3Cell & Developmental Biology, Vanderbilt University, Nashville, TN, USA; 4Center for Optoelectronic Biomedicine, National Taiwan University College of Medicine, Taipei, Taiwan
To investigate the 3D structure of the fiber pathways of the brain, we obtained diffusion spectrum MRI in fixed whole-brain specimens of 11 mammalian species including 4 primates and analyzed their tractography. Defining the neighborhood of a pathway to be the set of all pathways that approach within 1 voxel, we find such neighborhoods astonishingly well-organized, as parallel sheets of orthogonal pathways forming a 3D grid. This grid structure encompasses continuously the cerebral white matter in all species. Thus, the cerebral pathways form a single 3D curved coordinate grid continuous with the 3 axes of the bilaterian body plan.