[1] UCL Royal Free & Univ Coll Med Sch, UNiv Dept Clin Neurosci, London NW3 2PF, England
[2] Derriford Hosp, Peninsula Med Sch, Plymouth PL6 8DH, Devon, England
[3] Inst Neurol, Neurol Outcome Measures Unit, London WC1N 3BG, England
[4] Inst Neurol, Sobell Dept Motor Neurosci & Movement Disorders, London WC1N 3BG, England