quality:correction, linguistic:voice_change
Visual prosthesis

Theoretical and experimental clinical evidences suggest that direct electrical stimulation of the retina might be able to provide some vision to subjects who have lost the photoreceptive elements of their retina. Therefore, visual prostheses are developed to restore vision for the blind by using the stimulation. Depending upon which visual pathway location is targeted for neural stimulation, different approaches have been considered. Visual pathway consists mainly of the eye, optic nerve, lateral geniculate nucleus (LGN), and visual cortex. Therefore, retinal, optic nerve and visual cortex stimulation are the three different methods used in visual prostheses. Retinal degenerative diseases, such as retinitis pigmentosa (RP) and age-related macular degeneration (AMD), are two likely candidate diseases in which retinal stimulation may be helpful. Three approaches called intraocular epiretinal, subretinal and extraocular transretinal stimulation are pursued in retinal devices that stimulate remaining retinal neural cells to bypass lost photoreceptors and allow the visual signal to reach the brain via the normal visual pathway. In epiretinal approach, electrodes are placed on the top side of the retina near ganglion cells, whereas the electrodes are placed under the retina in subretinal approaches. Finally, the posterior scleral surface of the eye is the place in which extraocular approach electrodes are positioned. Second Sight and the Humayun group at USC are the most active groups in the design of intraocular retinal prostheses. The ArgusTM 16 retinal implant is an intraocular retinal prosthesis utilizing video processing technologies.Regarding to the visual cortex stimulation, Brindley, and Dobelle were the first ones who did the experiments and demonstrated that by stimulating the top side of the visual cortex most of the electrodes can produce visual percept. More recently Sawan built a complete implant for intracortical stimulation and validated the operation in rats.