A multistage process for computing virtual dipolar sources of EEG discharges from surface information.

A mathematical model consisting of a homogeneous spherical conductive medium was used to simulate the human head. A current dipole situated in the interior of the sphere produces, on the surface, a theoretical distribution of potential similar to certain EEG activities recorded on the human scalp. We propose a multi-stage method for computing the six parameters of a such virtual dipolar source by using only measurements on the surface of the scalp. Epileptic discharges observed on a young patient were thus analyzed. The computed dipole depends on the placement of pairs of electrodes used for the record. We compare the results of four ``assemblies'' using different pairings of electrodes. The center of the dipole varies within a volume of 2 cm 3 but its direction is stable. We discuss the importance and the influence of the approximations introduced by the model and the method. Although these approximations cannot be neglected, they do not change the signification of our results. The question is now raised: Has the virtual source a physical interpretation? Only intracerebral investigations can bring about a clear answer. However, whatever the answer may be, it appears that the method presented here can be helpful in the analysis of EEG surface situations.