Retinal implants to restore vision in blind people

It has been shown that electrical stimulation of retinal ganglion cells yields visual sensations. Therefore, a retina implant for blind humans suffering from Retinitis Pigmentosa based on this concept seems to be feasible. Research groups have been working on the realization of retinal implants world wide. There are two main approaches for stimulating retinal ganglion cells. In the epiretinal approach an extraocular part of these systems records visual images. The images are transformed by a neural net into corresponding signals appropriate for electrical stimulation of the retinal ganglion cells. These signals are transmitted to an implanted receiver unit by magnetic coupling. Integrated circuitry of this unit decodes the signals and transfers the data to a stimulation circuitry that selects stimulation electrodes placed in the eye onto the retina and generates current pulses to the electrodes. By this, action potentials in retinal ganglion cells are evoked, causing a visual sensation. In the subretinal approach the space between sclera and the retinal nerve cells is used by replacing the degenerated photoreceptors by a stimulation electrode array. Meanwhile successful experiments have been performed on patients with both approaches. This paper wants to give an overview about technical aspects of these different activities.