## 1 Introduction  
Visually impaired people mostly use white sticks as a support tool while walking. Since the conventional white stick has limitations with respect to its area of detection, various wearable support systems incorporating audio and tactile sense navigation have been introduced [1]. For example, electronic travel aids obtain outside information using sonar and convey this information through another sense, such as through audio or electronic tactile stimulation. Although these existing wearable support systems are able to provide information on the locations of obstacles, the user's senses are disturbed because the locations of obstacles are presented using those senses. Moreover, some support systems use sensors attached to the hands. As a result, the user needs to keep their hands in place [1]. Therefore, hands-free walking support tools that do not interfere with other senses are needed. As a methodology to overcome the above issues, methods for communicating information on obstacle locations using phosphenes, which are visual information that can be perceived by visually impaired people, have been investigated. This visual information enables visually impaired people to achieve intuitive obstacle recognition and to live their other senses being free from interference. In addition, presenting the locations of obstacles with visual information makes it possible to realize hands-free walking support tools. Hence, phosphenes are an effective way of presenting obstacle locations for the visually impaired. Phosphenes are a phenomenon in which light can be recognized by magnetic and electric stimulation of the visual pathway. Generally, non-invasive stimulation methods include TMS (transcranial magnetic stimulation) [2, 3, 4], TES (transcorneal electrical stimulation) [5, 6], and tACS (transcranial alternating current stimulation) [3, 7]. TMS uses magnetic stimulation, and it is possible to generate an induced current in a specific brain region using a coil through which an electric current is passed [2, 3]. However, the TMSdevice is costly, and it has been reported that TMS can cause severe side effects such as convulsions during stimulation [4]. TES stimulates the cornea on the eyeball surface by means of a contact lens with embedded coils being worn [5]. Since stimulation by TES directly stimulates the cornea, it is necessary to protect the cornea with a drug. In addition, since the electrodes are placed on the cornea, it is difficult to change the position of the electrodes. Additionally, the phosphene induced by TES is generally perceived in the center of the visual field, because TES generally stimulates the retina located on the contralateral cornea [6]. tACS is a method using two electrodes, and the area beneath the electrodes is stimulated with alternating current [3,7]. The side effects of tACS have been discussed in several studies, and include dizziness, sensations on the skin, perception of pressure, itching at the stimulus location, aching, headache, sensation of heat, and discomfort [8,9]. However, no severe side effects, such as those reported when using TMS, have been reported. In other words, it is considered to be possible to present a phosphene with a greater degree of safety than when using TMS or TES. Therefore, in this study, we discuss the presentation of phosphenes using tACS. The position at which the phosphene is presented when using tACS has been reported previously. [PERSON] et al. placed electrodes around the eyeball and reported the position of the presented phosphene [10]. Their report states that when the electrodes were located around the eyeball, the phosphene was perceived to be close to the electrodes. In other words, when the electrodes were located below the eyeball, the phosphene was perceived to be in the lower visual field. Our previous study also described a method by which a phosphene could be presented in three different locations, namely, to the right, center, and left of the visual field, by means of electric field simulation [11].