## 2 Methods
### Tissue Dielectric Properties  
The response of the biological tissue to an applied electrical signal is defined by its dielectric properties, such as relative permittivity (\(\varepsilon_{r}\)) and conductivity (\(\sigma\)). These parameters were described for a wide frequency spectrum (from 10 Hz to 20 GHz) for biological tissues [21,32]. However, many computational studies analyzed the electrode array insertion based on the standard dielectric properties of the cochlea layers as shown in  
Figure 3: Electrode array (EA) modeling. (**A**) shows a border-placed EA model. (**B**) shows a center-placed EA model. The center-placed EA model is highlighted. The simulation waveform and current levels are highlighted. The simulation waveform was applied to each electrode contact. _Pulse (\(\omega\)t)_ shows a current pulse. Electrode contacts are numbered from bottom (16) to top (1).  
Table 2. The tissue dielectric characteristics were defined based on current flows from a stimulating electrode to a return electrode for a specified frequency level [15]. Most of these values were obtained from guinea pig or gerbil animal models and some of these values were assumed to be representative of the human case. However, the dielectric properties of the bone varied between species. Thus, the value was updated to be like the human case [15].  
\begin{tabular}{l c} \hline \hline
**Anatomical Layers** & **Conductivity (S/m)** \\ \hline Scalas & 1.43 \\ \hline Basilar membrane & 0.0125 \\ \hline Spiral ligament & 1.67 \\ \hline Cochlea wall & 0.3 \\ \hline Bony layer & 0.0156 \\ \hline \hline \end{tabular}  
There are no available data that show the dielectric parameters of the cochlea layers based on the various frequency levels. Since it is always desired to place the EA at the center of the ST, extraction of the results based on the electrical parameters of this layer would greatly help in understanding current spread and electrode insertion guidance during cochlear electric stimulation. It has been shown that dielectric properties of cerebrospinal fluid (CSF) can be used for the ST as both layers are filled with the fluid (perilymph) [20,25]. The SV and ST were considered as one structure with the same dielectric properties of the CSF layer based on different frequency ranges as shown in Table 3. The remaining layers of dielectric properties in Table 3 were obtained from [21].  
\begin{tabular}{l c c} \hline \hline
**Anatomical Layers** & **Conductivity (\(\sigma\))** & **Relative Permittivity (\(\epsilon_{p}\))** \\ \hline Scalas & 1.789–2.46 & 16.6–68.4 \\ \hline Basilar membrane & 2.02 \(\times\) 10\({}^{-1}\)–9.78 \(\times\) 10\({}^{-1}\) & 2.57 \(\times\) 10\({}^{7}\)–5.48 \(\times\) 10\({}^{1}\) \\ \hline Spiral ligament & 2.51 \(\times\) 10\({}^{-1}\)–7.6 \(\times\) 10\({}^{-1}\) & 1.99 \(\times\) 10\({}^{7}\)–4.56 \(\times\) 10\({}^{1}\) \\ \hline Cochlea wall & 4.08 \(\times\) 10\({}^{-2}\)–3.12 \(\times\) 10\({}^{-1}\) & 3 \(\times\) 10\({}^{4}\)–6.2 \\ \hline Bony layer & 2.04 \(\times\) 10\({}^{-2}\)–1.56 \(\times\) 10\({}^{-1}\) & 5.22 \(\times\) 10\({}^{2}\)–1.24 \(\times\) 10\({}^{1}\) \\ \hline \hline \end{tabular}  
Also, the electrical parameters of the other layers of the cochlea were kept constant based on the defined frequency, and the conductivity of the ST was parametrized to examine the impact of the electrical parameters of the ST on the outcome as detailed in the result section. It was noted that frequency-dependent permittivity was found by averaging over frequency. It is noted that all tissue layers are assumed to be homogeny and isotropic as there are no data in the literature on the anisotropy of the layer conductivities of the cochlea.