not-yet-known not-yet-known not-yet-known unknown 1 Discussion The current study investigates the neural mechanisms underlying phosphene perception induced by otDCS and tACS. This is achieved by manipulating stimulation polarity and intensity and observing how these factors interact with the AM frequency effect (Hsu et al., 2023). We examined the necessity of current oscillation in phosphene generation with otDCS and tACS. The hypothesis posits that if current oscillation plays an essential role in phosphene perception, the polarity in otDCS will not interact with the AM condition or stimulation intensity, reliably eliciting phosphene perception. We also investigated the role of neural alignment in phosphene flash rate with stimulation intensity modulation. A significant interaction between intensity and AM condition would support the hypothesis that the percept of phosphene flash rate is constituted by alignment to the carrier frequency only; otherwise, the AM frequency is hypothesized to be encoded independently from carrier frequency and contributes to the perceived flash rate. Finally, the study explores whether the polarity of otDCS affects phosphene perception in different perceptual measurements. This exploration may imply that the underlying mechanisms supporting the percept are influenced by cortical excitability. Our results, as summarized in Table 2, indicate that the polarity of oscillations did not affect the phosphene threshold but did affect the response time and brightness ratings. Furthermore, it interacted with the polarity of stimulation in flash rate scoring and with polarity and AM in confidence ratings. AM affected threshold, response time, and flash rate but not brightness perception or participants’ confidence level in their flash rate scoring. Intensity modulates response time, brightness, and confidence levels but does not affect flash rate scoring. In the following sections, we discuss how these results address the abovementioned questions. Table 2. Summary of all effects in each index. The polarity effect indicates whether the current polarity is positive (anodal otDCS), negative (cathodal otDCS), or switching (tACS). AM indicates whether the current is a sinusoidal (18 Hz) or an amplitude-modulated (2 AM 18 Hz) waveform. Intensity indicates the stimulation is at its threshold (100%) or suprathreshold (120%) amplitude. Mark “O” indicates significant main effects or interactions were found, although the paired contrast may not be significant in the polarity factor. Mark “X” indicates there were null results in the statistical tests. 1 Discussion