[1] Univ Cambridge, Addenbrookes Hosp, Neurol Unit, Cambridge CB2 2QQ, England
[2] Univ Cambridge, Addenbrookes Hosp, Dept Psychiat, Cambridge CB2 2QQ, England
[3] MRC, Cognit & Brain Sci Unit, Cambridge CB2 2EF, England
[4] Univ Cambridge, Addenbrookes Hosp, Dept Nucl Med, Cambridge CB2 2QQ, England