Figure 1. The general diagram of the experimental methods. (A) Illustrations of the given tACS and otDCS waveforms. Waveforms in the upper row have the same carrier frequency, and waveforms in the lower row have the same AM frequency. (B) The electrode montages that were used in the experiment. Electrodes over the occipital scalp were selected and assigned into two groups marked in different colors. In the anodal otDCS, electrodes in the red group served as the anode, while the blue group was the cathode. The assignment reversed during the cathodal otDCS. In the tACS session, the two groups of electrodes switched polarities according to the given waveform. (C) The current distribution simulation was conducted based on the electrode configuration described in Figure (1B), applying an intensity level of 2000 μA. This simulation identified the occipital stimulation montage as producing the highest current density, particularly around the primary visual cortex located in the bilateral occipital lobes. The intensity levels of the simulated current are represented through color coding on the diagrams. The modeling of the head’s current flow was carried out with the ROAST software (Y. Huang et al., 2019) (D) The trial structure of the phosphene task. Participants fixated their eyes on a fixation (0.7 visual degrees) on the center of the screen for 2 to 3 seconds, followed by a 5-second current stimulation. Participants were instructed to maintain focus on the fixation point and to signal phosphene perception by pressing the space key. Subsequently, they utilized a mouse to depict the observed phosphene pattern using scattered dots, with the dot positions being captured for subsequent analysis of the pattern’s size. Following this, participants advanced to a new screen to evaluate the flash’s brightness by adjusting a horizontal slider located at the bottom of the display from 0 (none) to 10 (strongest). In the flash rate scoring, a square was presented at the center of the screen, which flashed at frequencies varying from 1 Hz (left pole of the sliding bar) to 40 Hz (right pole of the sliding bar). Participants were required to move the sliding bar to adjust the flash rate, so the flashing square reproduced the flash rate of their phosphene. Finally, the participant moved the sliding bar to rate the confidence level of their response from 0 (not confident at all) to 10 (very confident).