Dipole analysis of epileptic source generator with manual zero potential shifting method

The dipole tracing method is a technique whereby an electric source generator is estimated as an equivalent current dipole (ECD) based on potential distribution on the scalp. To estimate the electric source generator of low amplitude spikes, a manual zero potential shifting (MZPS) method was introduced in which a zero potential is set manually in dipole analysis. The subjects were three patients with localization-related epilepsy with temporal spikes. When low-amplitude spikes (< 50 microV) were analyzed by the conventional mean zero potential method, the dipolarity, an indicator of ECD reliability, had a low value and its locations were scattered. In contrast, when these low-amplitude spikes were analyzed by the MZPS method, ECD showed a high dipolarity value comparable to that obtained when high-amplitude spikes (> or = 50 microV) were analyzed by the mean zero potential method. Furthermore, the locations of the former ECD tended to converge and were almost identical to those of the latter ECD. These findings suggest the usefulness of the MZPS method in dipole analysis in terms of the dipolarity and ECD locations of low-amplitude spikes.