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.