Retinal origin of phosphenes to transcranial alternating current stimulation

ObjectiveTo examine possible retinal contributions to cortically induced phosphenes by transcranial alternating current stimulation (tACS) involving the visual cortex.MethodsSelf-reported phosphene ratings and voltage-related potentials from the canthus, supra-orbital and sub-orbital regions of the right eye were measured to 2, 10 and 20 Hz tACS at 250 and 1000 μA intensities in healthy volunteers.ResultsQualitatively similar, but more intense phosphenes were reported during frontalis–vertex tACS as compared to occiput–vertex tACS. In addition, voltage-related potentials were recorded at the canthus and orbit regions of the eye during frontalis–vertex, occiput–vertex and occiput-right shoulder tACS.ConclusionsThe experience of phosphenes during tACS involving the visual cortex is influenced by volume conductions effects of the scalp.SignificanceRetinal effects should be taken into account when studying the cortical modulatory effects of tACS.