Figure 3
Electrical conditions in the cochlea in response to cochlear-implant stimulation for three hypothetical cases of modiolus porosity and conductivity. (a) Distribution of electric-field intensities in the modiolus. Case1: Non-porous modiolus, electrically isotropic of very low conductivity (0.0334 S m−1), Case2: Inhomogeneous porous modiolus (50% overall porosity), mapped (non-isotropic) conductivity (Fig. 2b), Case3: Homogeneous porous modiolus (50% overall porosity), effective (isotropic) conductivity 0.6334 S m−1. The stimulus voltage is 1 V DC. The electric field intensity magnitude is expressed in natural logarithms relative to 1 V m−1. Several subdomains are kept hidden for better visualization. Inset: Distributions of the transmembrane potentials over the surfaces of the spiral ganglion neurons (SGNs) for Case1. The position of the SGNs is delineated with a rectangular box in Rosenthal’s canal (RC) for Case1; they are located directly opposite the second electrode contact. Notice the fine structure in the electric field distribution for the case of inhomogeneous porosity (Case2). (b) Comparison of the electric field distribution pattern on the modiolus for the three cases as a function of a 4-mm arc length of the modiolus beginning from the basal-most region. c, Maximum transmembrane potential, Vmax, for the five SGNs (C1‒C5) highlighted with the inset. The red dotted line joining the Vmax of C3 is drawn to aid visual comparison; it does not represent a functional relationship among the three cases. For the model, a generalized CI design with high-resistance silicon with 20 platinum electrode pads was implemented. The CI had a diameter of 0.6 mm and the electrode contacts had a length of 0.3 mm; their centres were separated by 0.9 mm and were located at a distance of 0.3 mm from the modiolus.
