Ill Since antiquity, people have dreamed of restoring vision to the blind. Only recently, however, with the development of optical prostheses, has this prospect become a foreseeable reality. This observation spurred investigation into the use of electrical stimulation to overcome visual loss. Current approaches to optical prosthesis involve stimulation of nerves at either the retina or the visual cortex. While both approaches are theoretically feasible, retinal prostheses have the advantage of being far less invasive and are the focus of this essay.
Visual impairment can result from lesions anywhere along the visual pathway.
In the normal eye, light passes through the cornea, anterior chamber, lens, and vitreous and then stimulates the photoreceptors. The photoreceptors, which comprise the outer layer of the retina, transduce light energy into an electrical signal and propagate this signal through the layers of the retina to the retinal ganglion cells. From there, the electrical signal travels along the optic nerve, through the visual pathways, and eventually reaches the visual cortex, where sight perceptions are formed. Retinal approaches to prosthetic vision are most readily applicable to those causes of blindness that involve injury to the outer retinal layer, where the photoreceptors are located. In age-related macular degeneration and retinitis pigmentosa, the photoreceptors of the outer retinal layer are destroyed and the inner retinal layer is preserved.