## 5 Discussion
### Solution for Local Stimulation for the Central Part of the Eyeball Surface  
In this subsection, a method for achieving better placement of electrode 3 placements, which had difficulty performing local stimulation, is presented. As stated in Section 5.1, local stimulation potentially depends on the adjustment of the positional relationship between the electrodes and the eyeball. If a wide area of eyeball surface is stimulated, the electrode located near the eyeball should be moved to the side of the expected stimulated area, and the electrode located on the cheek should be moved close to the jaw. Therefore, in this subsection, a method for locally stimulating the lower central eyeball surface by adjusting the positional relationships between the electrodes and the eyeball is discussed. As mentioned in Section 3, there is a correspondence between the position of the electrode and the stimulated eyeball surface. When stimulating the temporal right eyeball, it is necessary to position an electrode close to the temporal right eyeball. Similarly, positioning an electrode close to the nasal side of the eyeball is necessary in order to stimulate the nasal side. Figure 7c shows that, although electrode 3 is able to stimulate the lower central area of the right eyeball, it also stimulates the temporal area. Therefore, it can be hypothesized that moving the anode of electrode 3, located under the eyeball, to the nasal side may make it possible to stimulate only the lower central area of the right eyeball. On the other hand, the electrode cheek electrode of electrode 3 is already positioned near the jaw. Since the cathode of electrode 3 does not have a large effect on the electric field value on the eyeball surface due to its large distance from the eye, it is not necessary to move the cathode of electrode 3. In addition, it was found in a previous study [15] that the stimulation value on the eyeball surface can be adjusted by reducing the current value. Therefore, it is conceivable that a more limited stimulation may be enabled by performing stimulation with a smaller current than the 1 mA used in the simulation. Therefore, the coordinates of the anode used in electrode 3 were moved to the nasal side for verification. Moreover, the stimulated position on the eyeball surface when using 1 mA was compared with that when using 0.8 mA. In evaluating the simulation results obtained using the revised coordinates of electrode 3, the simulation was performed on the basis of the simulation conditions described in Section 4.1, except that the electric current value in the simulation was different. Table 5 shows the revised coordinates of electrode 3, shown in Figure 4, locally stimulating the lower central area of the right eyeball. The revised electrode placement is shown in Figure 9. The pink-colored electrode is the anode, and the blue-colored electrode is the cathode. In order to evaluate the simulation results, they were converted using the method described in Section 4.2. However, step 2, indicated in Figure 6, was skipped in order to compare the stimulus intensity between simulations performed at 1 mA and at 0.8 mA. The simulation results obtained for the stimulated area on the eyeball surface using the revised electrode 3 are presented in Figure 10. Figure 10a shows the simulation results obtained at 1 mA, and Figure 10b shows the simulation result obtained at at 0.8 mA. It can be seen that by moving the anode below the right eyeball to the nasal side, the stimulated area on the eyeball surface can be limited to the lower central area. These simulation results indicate that the stimulated area on the eyeball surface can be localized by moving the electrode close to the eyeball in the direction of the target stimulation position. Therefore, it can be stated that the hypothesis mentioned earlier in this subsection