2.4 Experimental procedures
Each stimulation block included 10 trials. The threshold intensities
were defined by a pre-experimental test that used the modified binary
search (MOBS) method (Anderson & Johnson, 2006; Tyrrell & Owens, 1988)
(also see Appendix S1: Supplementary 4 of (Hsu et al., 2023) for
details) with the upper and lower bound at peak-to-peak 1000 and 2000 μA
respectively. The threshold intensity was verified with another 10
trials to ensure its response rate exceeded 50%; otherwise, the
threshold was re-defined again with MOBS. The suprathreshold intensity
was defined as 120% threshold intensity.
In every trial, participants were stimulated for 5 seconds. They were
asked to press the space key when seeing phosphene for an RT recording.
They then drew the phosphene pattern on the screen with a mouse. After
the stimulation, participants entered a new frame to report (1)
brightness, (2) flash rate, and (3) confidence level (as illustrated in
Figure 1D).