More than meets the eye: Retinal phosphenes as confounds in transcranial alternating current stimulation

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.