MEG/EEG hybrid method for source localization of a dipole with radial component

MEG is a useful method to know brain activities, and has a remarkable feature that the source location is estimated with high accuracy. However MEG is insensitive to a radial current dipole, because a radial dipole in a sphere makes no magnetic field outside the sphere and the shape of human head resembles a sphere. To realize more accurate localization, there is a requirement to estimate a radial current dipole or a dipole with radial component in human brain. On the other hand, EEG detects the potential at surface of scalp regardless of a dipole direction. The accuracy of source estimation by using EEG is not good comparing with that using MEG, because the human head consists of many different electric conductivity tissues such as brain, skull and scalp and it is difficult to know the electrical conductivity exactly. We developed a MEG/EEG hybrid method for source localization of a dipole with radial component [1]. There is a hypothesis that the primary somatosensory area 3b and the area 1 respond to the median nerve stimulation [2]. The activity in the area 1 is considered as a radial current dipole. Then, we carried out source location using somatosensory evoked field (SEF) and somatosensory evoked potential (SEP) by the developed hybrid method to verify the hypothesis. The result shows that the tangential dipole appears in area 3b at the latency of 18-20 ms. The radial component appears and increases gradually during the latency of 2023 ms.