Oscillatory Transcranial Electrical Stimulation and the Amplitude-Modulated Frequency Dictate the Quantitative Features of Phosphenes

Previous research demonstrated that transcranial alternating current stimulation (tACS) can induce phosphene perception. However, tACS involves rhythmic changes in the electric field and alternating polarity (excitatory vs. inhibitory phases), leaving the precise mechanism behind phosphene perception unclear. To disentangle the effects of rhythmic changes from those of alternating polarity, this study employs oscillatory transcranial direct current stimulation (otDCS), in which the current oscillation remains confined to either a positive or negative polarity, thereby eliminating the influence of polarity switching. We applied scalp electrical stimulations using both polarity-switching (tACS) and non-polarity-switching (otDCS) methods, with anodal or cathodal polarities, targeting the occipital lobe. All stimulations were performed using sinusoidal or amplitude modulation (AM) waveforms at threshold or suprathreshold intensities. Our results show that tACS results in faster response times compared to cathodal otDCS, but not anodal otDCS, while anodal otDCS elicits greater brightness perception than both cathodal otDCS and tACS. Additionally, AM frequency induced a higher threshold than the sinusoidal frequency, and response times were slower in the AM condition across all positive, negative, and polarity-switching stimulations. However, stimulation intensity in the anodal AM condition could influence speed ratings, unlike in cathodal or tACS conditions. Our findings reveal that both tACS and otDCS induce phosphenes, with significant differences between polarities and current oscillation types, indicating that both mechanisms are critical in phosphene induction. This study provides evidence linking phosphene occurrence to oscillatory current activity and highlights the robustness and impact of AM coding in visual perception.