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

Overview

Human exposure to extremely low-frequency (< 300 Hz) electric and magnetic fields can cause a distinct stroboscopic visual sensation known as electro/magneto phosphenes. The induction of these visual effects represents one of the most comprehensively documented outcomes of in-situ electric field interaction with human tissues. Notably, international safety guidelines use phosphenes as a key biological marker to set exposure limits for extremely low-frequency (ELF) electric and magnetic fields.

Methods

• Objective: Estimate the location (locus) and threshold for perceiving phosphenes during electric current stimulation at four frequencies: 20, 50, 60, and 100 Hz.
• Participants: 20 volunteers underwent non-invasive transcranial alternating current stimulation, ranging from 0 to 2 mA at the specified frequencies.
• Analysis: Logistic regressions were used on perceptual data to calculate probabilities of phosphene perception, and dosimetry analyses quantified in-situ electric field levels in the retina.

Findings

• The intensity of the stimulation current had a significant impact on phosphene perception across all frequencies.
• The lowest threshold for phosphene perception occurred at 20 Hz.
• No perceivable phosphenes were reported at 100 Hz.
• The data reinforces the retinal origin of phosphenes and highlights the sensitivity of the human visual system to ELF electric stimulation.

Conclusion

These results are vital for decoding the underlying mechanisms of phosphene induction due to ELF field exposure. Furthermore, the identified perception thresholds and trends offer essential insights to inform revisions of international safety standards for human exposure to electromagnetic fields. Importantly, this study underscores a clear connection between ELF field exposure and specific neurological responses, signifying the necessity for cautious exposure limits to protect public health.