Past, present, future: a review on visual prostheses.

This review presents the historic background, the present status and the future prospects of several visual implant types such as retinal implants, as well as optic nerve and thalamus stimulation, and stimulation of the visual cortex. Already achieved milestones, such as improved object recognition and letter reading, give hope that a breakthrough can be achieved in the coming years. Furthermore, clinical results from recent studies are presented in order to describe the obstacles that have to be overcome in the future. Currently, inner eye implants placed within the retina are the preferred option for electrical stimulation of the optic pathway. However, the limited number of stimulating electrodes and the non-focal activation of target neurons are still hindering the generation of percepts of higher quality. In the future, optogenetic approaches may be used to activate retinal neurons with a high temporal and spatial accuracy. The main challenge for all types of visual implants will be to utilize as many parts of the remaining optic pathway as possible and to focally activate functionally different pathways.