Do optimal dipoles obtained by the dipole tracing method always suggest true source locations?
Scalp potentials generated by a concentrated electric source in the brain are very similar to potentials generated by an electric dipole at the source position. In this sense a concentrated source in the brain is modelled as an electric dipole. When the source is diffuse such a dipole which best approximates the scalp potential is called an optimal dipole. Its position is calculated by the Dipole Tracing Method based on a realistic head model with homogeneous electric conductivity. There are 2 major difficulties inherent in this method: (1) The low electric conductivity of the skull causes systematic shifts of the optimal dipole positions from the true positions of concentrated sources; (2) the optimal dipoles cannot specify diffuse source positions. The first difficulty is overcome by using the numerical correction obtained by comparing the known dipole positions generated within a human head with their optimal ones. The second difficulty is removed to a certain extent by comparing the optimal dipole positions obtained with the 1-dipole and 2-dipole models together with their dipolarity. We have obtained criteria for the validity of the dipole approximation and source concentration.
Musha
T
T
Department of Applied Electronics, Tokyo Institute of Technology, Yokohama, Japan.
Homma
S
S
D001921:Brain / Q000502:physiology*; D003198:Computer Simulation; D004569:Electroencephalography; D006801:Humans; D008959:Models, Neurological