2.5 Data processing and statistical methods
For the dataset of each participant, trials with RT exceedingly twice the standard deviation were excluded from further analysis. Responses to the same stimulation condition were then averaged into mean values. In cases where no trials survived in one condition, the missing values were replaced by the cross-subject mean value.
For phosphene size estimation, we adopted the “im2bw” and “bwarea” functions built in MATLAB 2021b (Pratt, 1991). These functions converted the phosphene images into grayscale and computed the brightness value for the pixels from 0 (black) to 1 (white). Pixels with brightness values greater than 0.5 were counted as 1, while others were counted as 0. The phosphene size was then calculated as the sum of the entire screen.
For each measure, we conducted a within-subject 3 × 2 × 2 ANOVAs which employed factors of stimulation polarity (anodal otDCS, cathodal otDCS, and tACS), AM condition (0 and 2 Hz, where 0 Hz AM indicates a sinusoidal waveform), and intensity (threshold: 100% and suprathreshold: 120%). Further paired contrasts were conducted when the main effects or interactions were significant.