A grand average dipole model over each group of subjects was calculated for the target P300 of the grand average data, and was performed according to the following steps:Referring to the study by Hegerl and Frodl-Bauch (1997), two dipole pairs, one initially placed at bilateral posterior brain regions and the other initially placed at bilateral anterior brain regions, were used to calculate the centers of activity in both hemispheres. The interhemispheric symmetry constraint for coordinates of the dipole pair was used for dipole estimation; locations of dipoles were fitted for the P300 time interval.Because the first two pairs of dipoles already explained the majority of the scalp data (> 93%), the third pair of bilateral dipoles was discarded.
