Forward model of rat electroencephalogram: influence of inhomogeneous and anisotropic skull

The forward modeling of electroencephalogram (EEG) plays a crucial role in solving source localization and, consequently, in the detection of EEG connectivity networks in the rat brain. We investigated the impact of skull tissue properties-inhomogeneities, anisotropy, and thickness-on measured skull potentials. Five cuboid head phantoms with identical geometry, differing in skull material parameters, and a realistic head phantom with varying skull anisotropy ratios were tested. Simulations were verified by four electromagnetic solvers and an EEG measurement on a physical phantom. The cuboid phantom with sutures yields similar electric potentials as the isotropic one, but noticeable differences appear at electrodes near sutures, particularly in real brain sensing of somatosensory, motor, and visual cortices, as well as the retrosplenial dysgranular cortex, for a dipole located close to the surface. Skull thickness reduces potentials at the top electrodes but increases them at side electrodes, in real brain sensing somatosensory, auditory, association, and entorhinal cortices. Skull anisotropy alters electric potentials with differences up to 60% for dipoles near electrodes at the limiting anisotropy ratio of 10:1, and its effect decreases with increasing distance. Varying anisotropy ratios (10:1 to 1.3:1) show that nearby dipoles are the most affected, while the impact is reduced for lower, more realistic ratios (1.3:1 to 1.8:1) and for more distant sources. This trend was consistently observed in realistic head phantoms as well. Tangential dipoles are most strongly affected in the vicinity of the electrodes and gradually become radial at more distant positions near the inner skull surface.