The oscillatory transcranial electric stimulation and the amplitude-modulated frequency dictate the quantitative features of phosphenes

The study explores the crucial role of current oscillation and neural alignment to amplitude-modulation frequency in phosphene induction through various forms of transcranial electric stimulation. Previous research has shown that transcranial alternating current stimulation (tACS) can modulate phosphene perception. The application of tACS introduces rhythmic electric field changes and alternating polarity, making the oscillatory mechanism behind phosphene perception still unclear. To dissociate the effects of changing electric field vs. alternating polarity, the present study employs oscillatory transcranial direct current stimulation (otDCS) to eliminate the influence of polarity switching. We administered scalp electric stimulations using tACS and otDCS in anodal or cathodal polarities over the occipital lobe. All stimulations were conducted with sinusoidal (18 Hz) or amplitude-modulated (2 Hz AM embedded in an 18 Hz carrier) waveforms at threshold or suprathreshold intensities. The results revealed no difference between stimulation polarities, suggesting the importance of current oscillation rather than polarity alteration in phosphene induction. Furthermore, amplitude-modulated stimulation consistently produced slower phosphene flash rates, unaffected by intensity, indicating the dominance of amplitude-modulation frequency in phosphene perception. Our findings suggest (1) current oscillation, rather than polarity switching, is crucial for phosphene generation; (2) amplitude-modulation frequency effects on perception threshold, response time, and perceived flash rate are robust irrespective of the oscillatory stimulation protocols; (3) amplitude-modulation information is encoded in phosphene perception generation independently of the carrier frequency. This study provides direct evidence of the link between phosphene occurrence and oscillatory current activity, underscoring the robustness and independence of amplitude-modulation coding in visual perception.