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.
KeywordsSolid Angle Generator Surface Electrode Contact Volume Conductor Plane Angle
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