Year

2023

Degree Name

Doctor of Philosophy

Department

School of Psychology

Abstract

Exposure to electromagnetic fields (EMF) can produce illusory perceptions of light referred to as phosphenes. Various exposure guidelines around the world use phosphene perception as an indicator that environmental EMF may be affecting the central nervous system, however many of them are based on low quality legacy literature. While exposure guidelines should consider all commonly encountered ambient lighting conditions, there are no studies examining electrophosphenes in commonly encountered mesopic (i.e., dim) lighting conditions. As a result, conclusions drawn from these guidelines may not be reliable or encompass all plausible EMF exposure conditions. Additionally, the roles of the retina and visual cortex in electrophosphene generation have not yet been adequately separated. Given the importance of understanding the effects of EMF exposure on human health, it is crucial to investigate the factors that affect sensitivity to phosphenes in a rigorous and systematic manner. This thesis examined the effects of transcranial electrical stimulation (tES) using different electrode placements, stimulation parameters, and ambient lighting conditions on phosphene detection thresholds using a large sample size, as well as robust experimental and analytical techniques. Detection thresholds across the three experiments (presented in Chapters 2 – 4) showed that up to 74% less current had to be applied to induce phosphenes in mesopic conditions compared to well-lit and dark conditions, indicating that existing guidelines have used relatively insensitive scenarios to determine safe levels of EMF exposure. Lower phosphene detection thresholds in frontal montages suggested that the retina was the most likely source of tES-induced phosphenes. However, the double dissociation analysis in Chapter 3 showed that additional stimulation over the visual cortex lowered the current strength required to induce phosphenes by stimulation near the retina (from 130.7 μA to 87.5 μA). It appears then that electrical stimulation over the cortex can facilitate phosphene detection. Chapter 4 showed that phosphenes were more readily perceived when stimulation was set to specific frequencies in each of the dark (10 Hz), mesopic (16 Hz) and well-lit (20 Hz) conditions. Frequency dependence in these well-lit and dark conditions was in-line with: 1) previously reported dominant electroencephalograph (EEG) frequency bands in the cortex; and 2) sensitivity to stimulation found in rod and cone photoreceptors in the retina. All three experiments found that stimulation at 16 Hz produced the strongest electrophosphenes in mesopic conditions. While this does not align with any known EEG frequency response in the visual cortex, it closely aligns with the rod-cone phase delay mechanism found in the retina at 15 Hz, suggesting that the frequency component of tES-induced phosphenes may be driven by the frequency dynamics of retinal photoreceptors. Overall, the findings of this thesis indicate that exposure guidelines for EMF need to consider mesopic lighting if they intend to encompass all plausible exposure scenarios. Additionally, tES over the visual cortex can influence phosphene perception. Finally, ambient lighting conditions strongly affect the frequency dynamics and current strength required for tES to produce phosphenes.

FoR codes (2020)

510501 Biological physics, 510303 Electrostatics and electrodynamics

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.