## 2 Related Work and Background Knowledge
### Presentation Position of Phosphene  
The location at which phosphenes are perceived when stimulating the eyeball with electrodes placed around the eyes and applying a square wave alternating current of 0.3 mA and 10 Hz was investigated by [PERSON] et al. [10]. In their investigation, the electrodes were placed on the top, bottom, left, and right of both eyes. As mentioned in Section 1, it was reported that phosphenes were perceived at locations near the electrodes. In other words, when the electrodes were placed sandwiching the right eyeball at the top and bottom, is the phosphenes were also perceived in the upper and lower parts of the right visual field. Therefore, there is a correspondence between the position at which the phosphene is presented and the electrode placement. The stimulation method used in their study was similar to that used in tACS, and the current was assumed to flow along the eye's, stimulating the optic nerve. However, there is a discrepancy between the inferred current pathway and the experimental results. In the work by [PERSON] et al. [10], the phosphene was perceived on the right side of the visual field when the electrode stimulated the temporal right eyeball, even though the temporal retina of the right eyeball controls the left side of the visual field, as shown in Figure 1[14]. The current path around the eyeball was verified with respect to the discrepancy between the current pathway and the phosphene presentation position in our previous work. [PERSON] et al. focused on the fact that the visual retina was not exposed to the anterior side when the eyeball was gazing forward and investigated the path of the current around the eyeball [15]. The human eye achieves vision by applying a light stimulus to the visual retina, which has photoreceptors. However, this visual retina is exposed to the anterior side only when eye movement is performed. When performing eye movement towards the right side, the right eyeball's nasal retina and the left eyeball's temporal retina are exposed to the anterior side. In the eye movement to the left side, the right eyeball's temporal retina and the left eyeball's nasal retina are exposed to the anterior side. Therefore, a hypothesis was posited in this study [15] that the visual retina exposed to the anterior side can only be stimulated during eye movement to the left or right. In this study, the position of the phosphene was investigated with three electrodes placed on the right side of the forehead and three electrodes placed on the left side of the forehead. Using this setup, phosphenes were perceived on the side in the visual field area that was processed by the exposed retina, regardless of the electrode placement. Additionally, area in which the phosphene was perceived when gazing forward was different from that when gazing towards the left or the right, even though the same electrode placement was used. This result indicates that the current path around the eyeball directly stimulates only the area  
Figure 1: Visual field and the processing retinae in the human eyeball. exposed to the anterior side, and that the current does not flow along the sclera of the eyeball. Since the visual retina is not exposed to the anterior side when gazing forward, only the cornea is directly stimulated by the electric current. In this stimulus condition, a phosphene is perceived near the electrode, as in the previous study [10], even though the visual retina is not stimulated directly. Methods in which phosphenes are generated without directly stimulating the visual retina, such as when gazing forward, can be referred to as indirect stimulation. A previous study in which TES was used to stimulate the cornea reported that phosphenes were generally perceived in the central visual field [6]. As shown in Figure 1, the central visual field is processed in the retina located on the contralateral side of the stimulated cornea.