[1] Natl Hosp Neurol & Neurosurg, Epilepsy Res Grp, London WC1N 3BG, England
[2] Natl Hosp Neurol & Neurosurg, Dept Neurophysiol, London WC1N 3BG, England
[3] Natl Soc Epilepsy, Chalfont Ctr Epilepsy, Gerrards Cross, Bucks, England
[4] Kings Coll Hosp London, London, England
[5] Kent & Canterbury Hosp, Canterbury CT1 3NG, Kent, England