2.2 Experimental design and stimulation parameters
The current study employed a within-subject design. Each participant visited the lab three times, with a one-week gap in between each visit. During each visit, participants received stimulation of one polarity (anodal otDCS or cathodal otDCS or tACS) in four stimulation blocks: threshold-level sinusoidal wave (18 Hz), threshold-level AM wave (2 Hz amplitude-modulated 18 Hz, 2 AM 18 Hz), supra-threshold level sinusoidal, and supra-threshold AM wave (the waveforms are illustrated in Figure 1A). The carrier and AM frequencies were selected based on our previous finding in which participants reported the most phosphene responses with these frequencies (Hsu et al., 2023). The order of stimulation polarities and blocks was counterbalanced within participants.