In vivo measurement of skull and brain resistivities with EIT based method and analysis of SEF/SEP data

In this paper we present results of the equivalent brain and skull resistivities (ρ_brain and ρ_skull) for 6 different subjects using 2 different and independent procedures: an EIT based method and the combined analysis of SEF/SEP data. With the EIT based method known currents are injected into the head and the resulting potential distributions are recorded from scalp electrodes. The conductivities are estimated by fitting the conductivity parameters of a 3-sphere head model onto the measured potentials. With the combined SEP/SEF method, a current source is activated inside the brain using a nervous medianus stimulation. The MEG data is used to determine dipole position and tangential orientation, whereas the simultaneously recorded EEG data is used to find the dipole radial component and the electrical conductivities of the brain and the skull. The results show a large variability in the ratio of skull and brain conductivities ρ_skull/ρ_brain over subjects. However, a strong agreement was found between the results of EIT and SEF/SEP methods even though they are quite different, both in theoretical and technical terms. These results indicate that generic conductivity values will result in large systematic errors of EEG inverse modelling. However, the good agreement between the EIT and the SEP/SEF method indicates that the individual´s ρ_skull/ρ_brain ratio can reliably be determined using the EIT method.