Locations of occipital EEG electrodes verified by computed tomography

Aluminum markers placed according to the 10-20 system (at Cb, O, P, Fp and M sites) and the Queen Square system (at MF, LO, MO and RO) were imaged using the computerized tomography technique in 32 individuals with no skull deformities or brain pathology. There was a small, but reliable, placement error of midline markers with regard to the longitudinal fissure which was maximal for Cbz and Oz. This asymmetry was attributed to variance in the anatomy of the occipital bone and cranio-cerebral topography that cannot be predicted from location of the inion. In over 30% of the sample, difficulties in defining the inion further contributed to location errors. In 11 of 12 subjects there was a reliable asymmetry of the intraparietal sulcus with the left sulcus located about 0.5 cm more medially. Therefore, no matter how perfectly an electrode is aligned with the midline, the lateral electrodes cannot be placed over truly symmetrical sites due to sulcal variance.