The neurophysiological basis of epileptiform magnetic fields and localization of neocortical sources

In this review, we present animal studies that evaluate the appropriateness of the dipole model as a physical representation of neocortical sources produced by evoked and spontaneous epileptiform activity in neocortex. Three-dimensional electrical measurements of cellular currents in rat sensory neocortex are compared to associated extracranial magnetic fields. Comparisons are performed for the direct cortical response evoked by electrical stimulation of the cortical surface, and for evoked and spontaneous interictal and ictal discharge of the penicillin focus in the same animal preparation. These data support the hypothesis that evoked and epileptiform magnetic fields result from intradendritic currents oriented perpendicular to the cortical surface. Furthermore, magnetic fields can be detected from epileptic foci smaller than 3 x 3 mm2. Results from animal studies are then used as an empirical foundation for physical models with which to interpret noninvasive neuromagnetic recordings of epileptic discharge in human focal seizure disorders. Examples are presented in which the spatial and temporal distribution of extracranial magnetic fields produced by interictal events in humans may be adequately represented by multiple dipolar sources.