1. Sobell Department of Motor Neuroscience, Institute of Neurology, University College London, London, United Kingdom
2. Department of Neurosciences, Ophthalmology and Genetics, University of Genoa, Italy
3. Department of Neurologic and Psychiatric Sciences, University of Bari, Italy
4. Department of Neurology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands
5. Royal Free and University College Medical School, University College London, London, United Kingdom