Van Wedeen
Speaker of Workshop 3
Will talk about: Cerebral connectivity at multiple scales: A network and a geometry
Development of MRI methods and contrast mechanisms to address biological problems. Past inventions include • phase contrast flow imaging (1984) • magnetic resonance angiography with Rheto Meuli (CHUV) (1983-6) • tensor MRI of myocardial strain (1990) • MRI of myocardial fiber structure and fiber shortening with WYI Tseng (NTUH) (1993-8) • diffusion tractography (1995-6) • high angular resolution diffusion MRI, including Q-ball with Dave Tuch and DSI (1999) (see color icon this page top right) Other work includes • MRI dynamic range compression by means of a nonlinear gradient pulse (3D chirp) • fast 3D MRA by under sampling of k-space • discovery of displacement encoding with a stimulated echo (DENSE) with Tim Reese (1995) • balanced diffusion encoding for suppression of eddy current • co-developed TrackVis environment with Ruopeng Wang (Martinos Ctr) Present work is focused on discovery with diffusion MRI of new aspects of geometric order in the CNS, including complex path coherence within the cerebral cortex, and organization of cerebral white matter and connectivity across multiple scales.
The structural organization of the cerebral connectome – the fiber pathways of the cerebral white matter – remains unclear. While studies of the 3D structure of cerebral fiber pathways indicate a geometric plan governed by morphogenesis, studies of point-to-point connectivity, or connectional topology, often suggest a complex network. To investigate the topological and geometric organization of cerebral fiber pathways simultaneously, we imaged the fiber pathways of the brain with diffusion spectrum MRI (DSI) in ten mammalian species including four non-human primates ex-vivo and a human subject in vivo, and analyzed their patterns of path adjacency and intersection. Here we show the fiber pathways of the forebrain are organized substantially as a 3D grid derived from the three principal axes of the body. Cortico-cortical connections are derived from transverse and the longitudinal principal axes and organized as parallel sheets of orthogonal paths like the warp and weft of a fabric, while extrinsic projection pathways of the cortex chiefly follow the 3rd, or dorso-ventral, orthogonal axis. Thus the pathways of the cerebral connectome are highly correlated across all resolved scales and adhere to a tri-axial scheme like the spinal cord and brainstem. This structure is parsimoniously explained by, and mathematically equivalent to, axonal guidance by three mutually orthogonal chemotactic gradients. We hypothesize that the observed geometric organization of cerebral pathways provides a default structure for cerebral connectivity.
