1 Department of Neurology, David Geffen School of Medicine, Los Angeles, CA, USA; Parkinson's Disease Research, Education, and Clinical Center (PADRECC), Veterans Administration Greater Los Angeles Health Care System, Los Angeles, CA, USA.
2 Department of Rehabilitation and Health Services, University of North Texas, Denton, TX, USA.
3 Unshakable MD, Toronto, Ontario, Canada.
4 Radboud University Medical Center, Donders Institute for Brain, Cognition and Behavior, Department of Neurology, Center of Expertise for Parkinson & Movement Disorders, Nijmegen, the Netherlands.
5 UCL Queen Square Institute of Neurology, University College London, London, UK. Electronic address: a.schrag@ucl.ac.uk.