Tests of EEG localization accuracy using implanted sources in the human brain. The accuracy with which electrical sources in the brain can be localized by using electroencephalograms measured on the scalp is not well known. In this study, tests of localization accuracy were performed by using implanted dipolar sources in the human brain. These dipoles are created by passing a weak (subthreshold) current through intracerebral electrodes implanted in the brains of epileptic patients for seizure monitoring. The locations of these dipoles are accurately known from roentgenographs. First, 16 electroencephalograms produced by these dipoles were recorded, then inverse solutions were calculated for the apparent sources of these electroencephalograms. Finally, the locations of the apparent sources were compared with the known locations of the implanted dipoles to determine localization error. The average localization error for a total of 28 dipoles in 3 subjects was 1.1 cm. These results indicate that good localization accuracy for focal sources in the brain can be provided by scalp electroencephalograms. 