## 3 Method for Designing Electrode Placements to Induce Phosphenes in the Lower Visual Field  
In Section 2.2, methods for placing electrodes in order to induce phosphenes in three positions in the human visual field were described. In reality, it is necessary to determine electrode placements for the presentation of phosphenes in three positions in the lower visual field so that users will be able to avoid tripping hazards. The previous study [11] on the presentation of phosphenes in three positions never discussed whether the phosphenes were presented in the upper or lower visual field. This section presents hypotheses related to the design of electrode placements with the aim of presenting phosphenes in three positions in the lower visual field. These hypotheses were developed in the following two steps. * Hypothesize which target areas of the eyeball need to be stimulated in order to present phosphenes in the lower visual field. * Hypothesize electrode placements that will stimulate the hypothesized target areas. The part of the retina that controls the lower portion of the visual field is located in the upper half of the eyeball. Its contralateral side corresponds to the lower part of the eyeball surface. This is the part that needs to be stimulated in order to induce a phosphene in the lower portion of the human visual field, according to a previous study [15]. Moreover, as described in Section 2.1, when a phosphene is to be presented on the right side of the visual field, it is necessary to stimulate the temporal right eyeball surface. When a phosphene is be presented on the left side of the visual field, it is necessary to stimulate the temporal left eyeball. Therefore, in order to present a phosphene on the right side of the lower visual field, the lower temporal right eyeball surface should be stimulated, as shown in Figure 3(1). In order to present a phosphene on the left side of the lower visual field, the lower temporal left eyeball surface should be stimulated, as shown in Figure 3(2). Since stimulating the central cornea of both eyeballs results in the presentation of a phosphene in the center of the visual field [6,11], by stimulating the lower central eyeball surface, as shown in Figure 3(3),(4), it is possible to induce phosphene presentation in the lower central visual field. On the other hand, when the nasal side of both eyeball surfaces is stimulated, the temporal retinas of both eyes are stimulated. As shown in Figure 1, the retina on the temporal side of each eye controls a wide area, including the central visual field. Therefore, along with the method described above, namely, the stimulation of the lower central region of the eyeball surface, shown in Figure 3(3),(4), it is possible to induce phosphene presentation in the lower central visual field by stimulating the lower nasal side of both eyeball surfaces, as shown in Figure 3(5). Now, we will briefly describe our hypotheses regarding the placements of electrodes that with allow the desired target areas be stimulated. In the case of Figure 3(1), in which a phosphene is presented in the lower visual field on the right side, the electrode placement can be determined with reference to that which presents a phosphene on the right side of the visual field, as used in the previous study [11] and shown in Figure 2A. In Figure 2A, the area around the temporal right eyeball might be subjected to a broad area of stimulation. It is presumed that a more local stimulus would be possible by locating the electrode on the forehead closer to the lower side of the right eyeball, in accordance with method 1. Similarly, in the case of Figure 3(2), in which a phosphene is presented on the left side of the lower visual field, the electrode placement can be determined with reference to that which presents a phosphene on the left side of the visual field, as used in the previous study and shown in Figure 2F. In the case of Figure 3(3)-(5), the phosphenes are presented in the lower central visual field.