Human achromatic flickers and phosphenes thresholds under extremely low frequency electric stimulations.

Human exposure to extremely low-frequency (< 300 Hz) electric/magnetic fields elicits a stroboscopic visual perception called electro/magneto phosphenes. The induction of phosphenes is the most exhaustively documented effect of in-situ electric fields. Thus, they are used by international guidelines as the basis for limiting human exposure to extremely low-frequency electric and magnetic fields. This study aimed to estimate the phosphene perception locus and threshold during an electric current stimulation for four different frequencies (20, 50, 60, and 100 Hz) and to estimate the associated in-situ electric field. Phosphene perception probabilities were calculated in 20 volunteers using binary logistic regressions applied to perceptual responses resulting from non-invasive transcranial alternating current stimulation between 0 and 2 mA delivered at 20, 50, 60, and 100 Hz. A dosimetry analysis was done to study the in-situ electric field induced in the retina during the electric stimulation. The data indicate that the stimulation current plays a significant role in the model's predictions across all frequencies. Phosphene perception thresholds were lowest at 20 Hz, while no perceptible phosphene were observed at 100 Hz. The findings of this study are crucial for understanding the mechanisms of phosphene induction and further support the retinal origin of phosphenes. The observed thresholds and trends will inform updates to international guidelines and standards.