Volume Conductor Principles: Their Application to the Surface and Depth Electroencephalogram

The success of neurosurgical treatment of drug-resistant partial epilepsy depends upon the eradication by surgical excision of the area of gray matter responsible for initiating the patient’s seizures. Accurate identification of this area is therefore a prerequisite for guiding the surgical act. Among the data contributing to such a localizing diagnosis, epileptiform abnormalities recorded in the EEG are of paramount importance, because they reflect most directly the presence of epileptogenic neuronal hyperex-citability in the form of interictal or ictal epileptiform discharges (Gloor 1975). In spite of this, EEG data present the interpreter with a number of problems: (a) extracranial recordings fail to provide access to some areas known to be involved in seizure precipitation; (b) epileptiform abnormalities, even if localized, may reflect abnormal activity originating from a distant site, not directly accessible to EEG recording, a factor which presents a problem in both extracranial and intracranial EEG recordings; and (c) EEG potentials are not narrowly confined to the area that generates them. They can be recorded over a considerable distance by virtue of volume conduction. The relationship between the topography of the potential distribution, either extra- or intracranially, and the location, extent, and configuration of the generator is anything but simple and straightforward (Nunez 1981; Gloor 1985). The importance of this aspect of the problem is often not sufficiently appreciated. This chapter will therefore focus on this particular issue.