Experimental tests of EEG source localization accuracy in spherical head models.

Objectives: The locations of electrical sources in the brain can be calculated using EEG data. However, the accuracy of these calculations is not well known because it is usually not possible to compare calculated source locations with actual locations since little accurate location information is available about most sources in the brain.

Methods: In this study, sources at known locations are created by injecting current into electrodes implanted in the brains of human subjects. The locations of the implanted and scalp EEG electrodes are determined from CTs. The EEG signals produced by these dipolar sources are used to calculate source locations in spherical head models containing brain, skull, and scalp layers. The brain and scalp layers have the same electrical conductivity while 3 different skull conductivity ratios of 1/80th, 1/40th, and 1/20th of brain and scalp conductivity are used. Localization errors have been determined for 177 sources in 13 subjects.

Results: An average localization error of 10.6 (SD=5.5) mm for all 177 source was obtained for a skull conductivity ratio of 1/40. The average errors for the other ratios are only a few millimeters larger. The average localization error for 108 sources at superior locations in the brain is 9.2 (4.4) mm. The average error for 69 inferior location sources is 12.8 (6.2) mm. There are no significant differences in localization accuracy for deep and superficial sources.

Conclusions: These results indicate that the best average localization that can be achieved using a spherical head model is approximately 10 mm. More realistic head models will be required for greater localization accuracy.