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Effects of transcranial alternating current stimulation (tACS) on the central nervous system are well-established. However, tACS can elicit retinal phosphenes due to current spread to the eyes complicating brain-centred working mechanisms of neuromodulation. Electric field simulations on realistic head models confirm current leakage to the eyes and potentially the optic nerve. In addition, results from vision science point toward retina-induced effects on the brain outside perceptual awareness. Retinal phosphenes and changes in retinal sensitivity as corollaries of tACS need to be considered to understand the mechanisms by which exogenous oscillatory field potentials establishes their effects in the brain. • Transcranial alternating current stimulation can elicit retinal phosphenes due to current spread to the eyes. • E-field simulations in realistic head models indicate that the optic nerve may be an additional source for peripheral phosphene induction. • Changes in retinal excitability by tACS may interact with the central effects of tACS in the brain. • Electric currents which are too low to surpass the retinal phosphene threshold can influence visual brain processing.
Schutter et al. (Fri,) studied this question.