[1] UCL, UCL Inst Neurol, Sobell Dept Motor Neurosci & Movement Disorders, Unit Funct Neurosurg, London WC1E 6BT, England
[2] Natl Hosp Neurol & Neurosurg, Victor Horsley Dept Neurosurg, Queen Sq, London WC1N 3BG, England
[3] Inst Neurol, Headache Grp, Queen Sq, London WC1N 3BG, England
[4] Univ Hosp, Dept Neurosurg, Umea, Sweden