We investigated the effect of electrode type and stimulation condition (voltage stimulation and current stimulation) in bi-polar subdural cortical stimulation (SCS). For this study, we developed a 3D realistic head model using MRI data with 1 mm3 spatial resolution and simulated the model using the finite element method (FEM). For each study, we used three types of electrodes - disc, ring, and covered-disc - and three efficiency measures - effective depth of penetration, effective volume, and amount of CSF leakage current - to compare the effectiveness of the stimulation between two stimulation conditions. With voltage stimulation, there was no difference in effectiveness between the disc and ring electrodes. However, the amount of CSF leakage current for the covered-disc type was lower than that for the others. The effective depth of penetration and volume for the ring and disc type electrodes were higher than those for the covered-disc type. The current stimulation using the covered-disc electrode penetrated deeper than the other types of electrodes, and the CSF leakage current was still low. The result for voltage and current stimulation was quite different, as the substrate design manipulated the impedance and output current. In the current simulation, if the electrode was covered with the substrate, more current flowed to the cortex. On the other hand, with voltage stimulation, this substrate design makes the impedance between electrodes high, and the total current is reduced.
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