On the sensitivity of the human brain to magnetic fields: evocation of epileptiform activity.

Evocation of epileptiform activity by DC magnetic fields of between 0.9 and 1.8 millitesla (mT) has been demonstrated in 6 epileptic patients undergoing presurgical evaluation. The activity was monitored by electroencephalography (EEG) recording from both electrodes attached to the scalp as well as from intracranial electrodes inserted via the foramen ovale. Epileptiform activity evoked by the magnetic field application was distinguished from background levels by comparing the number of epileptiform discharges in the 10-s intervals before and after field applications. In nearly all cases, a delay of up to several seconds was observed between the application of the magnetic field and the onset of epileptiform firing. Removal of the field also appeared to cause firing in some instances, but this has not yet been investigated systematically. In all 6 patients, subsequent seizures confirmed that the epileptiform activity monitored during the experiments was originating from the primary epileptogenic zones of the patients.