[1] UCL, Inst Neurol, Unit Funct Neurosurg, Sobell Dept Motor Neurosci & Movement Disorders, London WC1N 3BG, England
[2] Natl Hosp Neurol & Neurosurg, Victor Horsley Dept Neurosurg, London WC1N 3BG, England
[3] Univ Hosp, Dept Neurosurg, Umea, Sweden