[1] MRC, Cognit & Brain Sci Unit, Cambridge CB2 2EF, England
[2] Addenbrookes Hosp, Div Acad Neurosurg, Cambridge CB2 2QQ, England
[3] Univ Cambridge, Wolfson Brain Imaging Ctr, Cambridge CB2 2QQ, England
[4] Univ Liege, Cyclotron Res Ctr, B-4000 Liege, Belgium
[5] Univ Liege, Dept Neurol, B-4000 Liege, Belgium