In vivo estimation of head conductivities frequency response with IES and SEEG-EEG

Introduction The objective of this research is to see whether the estimated conductivities using intracerebral electrical stimulations (IES) in a FEM head model change with different stimulation frequencies. Method In this study, the estimation of in vivo conductivities, as shown in Fig. 1 , was performed by minimizing the relative difference measurement (RDM) equation that compares the distribution of the SEEG (and the EEG) potentials resulted from IES with the distribution of the simulated potentials. The simulated potentials are represented by the generated potentials from a homogenous and isotropic five-compartment FEM head model using a simulated IES source. These five compartments are: scalp, skull, cerebrospinal fluid (CSF), gray matter (GM) and white matter (WM), and their initial conductivities before optimization were set to: 0.33, 0.008, 1.79, 0.33 and 0.14 S/m, respectively. The resulted signals from twelve different bipolar square current stimulations of frequency 55 Hz were considered from about 120 SEEG (and EEG) electrodes. For each stimulation, the main frequency component (55 Hz) and its 1st and 2nd harmonics (110 Hz and 165 Hz) were considered. Results It is clear from Fig. 2 that the average estimated conductivities and their variances change with the frequency. Moreover, the different values of variances, shown in Fig. 2 , can be explained by the difference in the stimulation positions and the signal-to-noise ratio. The introduction of EEG measurements increases the variance certainly because EEG measurements are subjected artefacts of exogenous sources. Even though the variance is large, the trends of the averages are preserved with or without EEG measurements. Conclusion The results show differences in the estimation of conductivity according to the frequencies and the matters. However, it is necessary to remain careful on interpretation of these preliminary results because a capacitive effect can be generated in the interface between the stimulation electrode and the cerebral mediums even if the frequencies are low.