Retinal origin of phosphenes to transcranial alternating current stimulation

Objective: To examine possible retinal contributions to cortically induced phosphenes by transcranial alternating current stimulation (tACS) involving the visual cortex. Methods: Self-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 mu A intensities in healthy volunteers. Results: Qualitatively 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. Conclusions: The experience of phosphenes during tACS involving the visual cortex is influenced by volume conductions effects of the scalp. Significance: Retinal effects should be taken into account when studying the cortical modulatory effects of tACS. (C) 2010 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.