The localization of equivalent dipoles of EEG sources by the application of electrical field theory

A technique is described for finding the position, magnitude and orientation of the equivalent electrical dipole of EEG activity given the pattern of potential differences recorded at the scalp. The technique is based on an iterative computer program implementing equations describing the electrical field of a dipole in a spherical conductor. The computer program was tested in vitro against data obtained from an inert spherical conductor (a bowl containing physiological saline, fitted with recording electrodes and a movable dipole) and an anisotropic conductor (a similarly equipped human skill including a simulated scalp). In both practical models, the computer program accurately located the dipole, the mean differences between observed and computed loci being of the order of 1 cm at widely different locations. This accuracy was maintained in the anisotropic model even though potentials transmitted through the skill were attenuated by 80%. In vivo, the program successfully located the equivalent generator of blink artefacts within the remaining eye of a one-eyed subject and, in a normal subject, localized the dipole of corresponding potentials to a midline inter-ocular position. In further investigation of normal subjects, the distribution of amplitudes at latencies within Wave V of the visual evoked response confirmed the loci of equivalent generators within posterior cerebral regions. The technique was also applied to the alpha rhythm indicating a posterior locus compatible with the view that alpha rhythm is generated chiefly by posterior cerebral cortex. Factors affecting the accuracy of the technique and the limitations of one-dipole models are discussed.