In cases of intractable partial seizure disorders, surgery may be the only treatment course to provide a significant reduction in seizures. A major factor in determining the successfulness of surgical approach is the ability to localize the primary or dominant focus of epileptic activity, particularly when multiple foci are present. The success of the neurosurgical approach has increased dramatically over the previous two decades because of the contributions of standard presurgical mapping techniques, such as EEG and ECoG, as well as newer advanced techniques such as MEG.

Invasive ECoG offer both high temporal and fair spatial resolution and thus may be indicated when precise localization of the epileptogenic zone with surface EEG and other noninvasive methods is inconclusive. Indeed invasive recordings have several advantages including access to deep brain structures, and the ability to record highly localized, low amplitude, high frequency discharges. Successful localization using subdural or depth electrodes is reported in several series to be in the range of 60-80%. However, invasive recordings require chronic intracranial implantation of surface electrode grids or depth electrodes. Thus these methods need additional surgeries, which result in increased expenses, time, and morbidity, including the risks of intracerebral hemorrhage and infection.

The temporal and spatial resolution of MEG is comparable to ECoG, raising the possibility that it can be used for localization of epileptic foci as a non-invasive procedure. In fact, MEG has already been demonstrated to be the superior technique for presurgical mapping in patients undergoing resection of tumors by allowing for better presurgical planning. MEG has also already proven to be a highly useful technique for investigating epileptic activity, and a significant source of information for guiding surgical decisions (Rezai et al., 1996; Hund et al., 1997). With the use of new whole head imaging MEG systems having increased spatio-temporal resolution, especially for deep structures, and with the capability for analyzing multiple sources, significantly more precise localization of epileptic sources should be possible.