Retinal Origin of Electrically Evoked Potentials in Response to Transcorneal Alternating Current Stimulation in the Rat

PURPOSE. Little is known about the physiological mechanisms underlying the reported therapeutic effects of transorbital alternating current stimulation (ACS) in vision restoration, or the origin of the recorded electrically evoked potentials (EEPs) during such stimulation. We examined the issue of EEP origin and electrode configuration for transorbital ACS and characterized the physiological responses to CS in different structures of the visual system. METHODS. We recorded visually evoked potentials (VEPs) and EEPs from the rat retina, visual thalamus, tectum, and visual cortex. The VEPs were evoked by light flashes and EEPs were evoked by electric stimuli delivered by two electrodes placed either together on the same eye or on the eyeball and in the neck. Electrically evoked potentials and VEPs were recorded before and after bilateral intraorbital injections of tetrodotoxin that blocked retinal ganglion cell activity. RESULTS. Tetrodotoxin abolished VEPs at all levels in the visual pathway, confirming successful blockage of ganglion cell activity. Tetrodotoxin also abolished EEPs and this effect was independent of the stimulating electrode configurations. CONCLUSIONS. Transorbital electrically evoked responses in the visual pathway, irrespective of reference electrode placement, are initiated by activation of the retina and not by passive conductance and direct activation of neurons in other visual structures. Thus, placement of stimulating electrodes exclusively around the eyeball may be sufficient to achieve therapeutic effects.