Comparison of Physiological Brain Responses Evoked by Visual and Electrical Stimulation.

Purpose: Noninvasive current stimulation (nCS or electrical stimulation) is a rapidly developing technique to support recovery from eye and brain dysfunctions. One of the most commonly used forms of nCS for treating the visual system is transcranial and transcorneal alternating current stimulation. This technique can exert neuromodulatory effects on the brain through eye stimulation. The mechanism of such stimulation is still poorly understood. Materials and Methods: To understand the pattern of activation evoked by nCS, a series of electrical impulses were delivered directly to the rat eye, alternating with the visual stimulus (VS), and subsequent responses were tested in the Superior colliculus and the primary visual cortex. Additionally, we tested two stimulation electrode placements, eyeball-eyeball, and eyeball-neck. Results: The results indicate that nCS and VS evoke different activation patterns in the recorded structures. In particular, the electrically evoked potentials are characterized by shorter latency and a different shape than the corresponding visually evoked potentials. The transcorneal alternating current stimulation (tACS) evoked shorter sinks and sources in all recorded structures than the visual stimulation. This suggests emerging of a different pattern of extracellular current flow in response to different stimulations. Conclusions: We demonstrate that the eye-eye paradigm of electrical stimulation elicited responses more similar to those evoked by VS. Individual transcorneal electrical impulses evoke a consistent pattern of neuronal activation across the visual system. This consistency is particularly promising for the development of neurotherapy aimed at restoring or improving vision, nCS can effectively activate visual circuits despite variations in stimulus delivery and shape.