A hole in the skull distorts substantially the distribution of extracranial electrical fields in an in vitro model.

The purpose of this study was to quantify the distortion of electrical fields by skull foramina using an in vitro model. Extracranial voltage generated by current dipoles located inside a human calva immersed in saline were measured when a 4-mm hole was open and when it was blocked with paraffin wax. Dipoles were located either along the internal surface of the bone (superficial dipoles) or at increasing distances from the bone (deep dipoles). With the hole open, extracranial signals had a substantially greater amplitude than with the hole blocked. The locations of the largest voltage values recorded outside the skull depended on the distance of the recording electrode from the hole rather than on the location of the internal dipole. For superficial dipoles, voltage values with the hole open were as much as 116 times greater than when the hole was blocked. Furthermore, when the hole was open, the largest extracranial signals were seen at the hole even when the dipole was 5 to 6 cm away from the hole. The effects of skull holes were less prominent for deep dipoles than for superficial dipoles. Skull discontinuities can be major determinants for the distribution of extracranial EEG signals. These results have implications for EEG interpretation and for source localization.