Computational estimation of threshold currents for electrophosphenes

Electrophosphenes are subjective sensations of light that are generated through electric stimulation at frequencies lower than 100 Hz. During electrical stimulation, an electric current flows through the human body between stimulation electrodes. Part of the current passes through the eye, where the current can activate or alter the function of retina, which results in the perception of electrophosphenes. Human exposure limits in international guidelines for the electric field in the central nervous system are set with the objective of preventing the generation of phosphenes. However, while it is possible to measure thresholds for phosphenes in terms of the applied electrode current, there is no straightforward way for determining the local threshold current density on the retina from measured data. In this study, current distributions induced by electrode setups of a previous experimental study are analyzed computationally in four anatomical human models. Computed results together with experimental data are used for determining local electrophosphene thresholds. At the frequency of highest sensitivity, 20 Hz, the threshold in terms of the maximum radial current density on the retina appears not to be less than 10-30 mA m^-1, where the variation is mainly caused by uncertainty due to electrode positioning. The threshold in terms of the total current that flows through the eyeball is 2-4 μA. These thresholds are comparable with those previously determined for magnetically induced phosphenes.