1Human Motor Control Section, National Institute of Neurological Disorders and Stroke, NIH, Bethesda, MD, United States, 2Office of the Clinical Director, National Institute of Neurological Disorders and Stroke, NIH, Bethesda, MD, United States, 3Scientific and Statistical Computing Core, National Institutes of Health, Bethesda, MD, United States, 4Department of Human Biology, Faculty of Health Sciences, University of Cape Town, MRC/UCT Medical Imaging Research Unit, Cape Town, South Africa, 5African Institute for Mathematical Sciences, Muizenberg, South Africa, 6Surgical Neurology Branch, National Institute of Neurological Disorders and Stroke, NIH, Bethesda, MD, United States