[1] Inst Psychiat, Dept Psychol Med, London SE5 8AF, England
[2] Inst Psychiat, Social Genet & Dev Psychiat Res Ctr, London SE5 8AF, England
[3] Kent & Canterbury Hosp, Dept Paediat, Canterbury, Kent, England
[4] Kings Coll London Hosp, Dept Neurol, London SE5 8RX, England