## 5 Discussion
### Comparison of the Stimulated Areas between Previously Described Electrodes and Proposed Electrodes
Figure 8: Simulation results obtained with electrodes for the presentation of phosphenes in three positions in the visual field: (**a**) simulation results for electrode A; (**b**) simulation results for electrode B; (**c**) simulation results for electrode C; (**d**) simulation results for electrode D; (**e**) simulation results for electrode E; (**f**) simulation results for electrode F.  
area on the eyeball surface. However, electrodes A and F still face challenges with respect to local stimulation. On the basis of the simulation results shown in Figure 8, we determine why, even though all anodes were placed on the forehead and all cathodes were placed on the cheek, the electrode placements determine whether the upper or lower side of the eyeball will be stimulated. Moreover, we discuss why it was possible to stimulate the target area on the lower eyeball surface by all of the proposed electrode placements. In electrodes A and F, which stimulate the temporal side of the eyeball, the electric current flows from the electrode on the forehead to the electrode on the cheek, stimulating the temporal eyeball surface along the shape of the facial surface. The anode of electrodes A and F, as used in the previous study [11], is located on the forehead in a region as close to the ear as possible, and the cathode is located just below the eyeball. Therefore, electrodes A and F broadly stimulate the region on the temporal side of the eyeball surface, and then stimulate the central region of the eyeball surface near the cathode. On the other hand, as shown in Figure 4, electrodes 1 and 2, proposed in this study, which stimulate the temporal side of the eyeball surface, have the anode located significantly lower than the anode of electrodes A and F described in the previous study [11]. It can be seen that it is possible to avoid stimulating the upper side of the eyeball surface by locating the electrode at coordinates below the eyeball. In addition, by moving the cathode of electrodes A and F, used in the previous study, from the position under the eyeball to the jaw, it is possible to avoid stimulating the central eyeball surface, which is an unintended stimulation area, thus enabling local stimulation. When placing the electrode to stimulate the nasal area of the eyeball surface, such as in electrodes C and D, the anode is located at the point between the eyebrows, which is close to the upper nasal area of both eyeballs, and the cathode is located on the lower cheek in consideration of the shape of the facial surface. Therefore, the upper side of the eyeball surface will be more strongly stimulated, since the anode is located closer to the eyeball than the cathode. On the other hand, as shown in Figure 4, electrode 5, which stimulates the nasal area of both eyeball surfaces, has the anode located just on the nasal bridge, which is close to the lower nasal area of both eyeballs. It is considered that, using the proposed electrode placement 5, it is possible to intensively stimulate the nasal part of both eyeballs due to the location of the anode. Moreover, the cathode is located near the jaw, so that it is possible to stimulate only the lower side of the eyeball surface. When placing the electrode to stimulate the central eyeball surface, such as in electrodes B and E, the anode is located just above the center of the surface of the eyeball, and the cathode is located on the lower cheek in consideration of the shape of the facial surface. Therefore, the upper eyeball surface will be more strongly stimulated, since the anode is located closer to the eyeball than the cathode. On the other hand, as shown in Figure 4, electrodes 3 and 4, which stimulate the center of the eyeball surface, have the anode located just below the center of the surface of the eyeball. It is considered that, using the proposed electrode placements 3 and 4, it is possible to intensively stimulate the lower central part of the eyeball due to the location of the anode.