Ours is the only approach where the photodiode–amplifier–electrode set is contained within a single pixel of the MPDA such that each electrode provides an electrical stimulus to the remaining neurons nearby, thereby reflecting the visual signal that would normally be received via the corresponding, degenerated photoreceptor. While the first approach typically requires external image and data processing due to bypassing retinal image analysis, the second seeks to replace the function of degenerated photoreceptors directly by translating the light of the image falling onto the retina point by point into small currents that are proportional to the light stimulus. The latter type of microchip senses light and generates stimulation signals simultaneously at many pixel locations, using microphotodiode arrays (MPDAs ). Two fundamentally different approaches have been taken in this area: (i) implantation of electrode arrays which interface epiretinally with retinal ganglion cells that form the retinal output pathway, and (ii) implantation of microchips under the transparent retina to substitute the degenerated photoreceptors. While gene therapy and application of neuro-protective factors may help maintain vision in the early stages of degeneration, survival of the inner retina encouraged us and others to attempt a partial restoration of visual function using electric stimulation of the remaining retinal network. Although blindness owing to photoreceptor degeneration presently remains incurable, inner retinal nerve cells may continue to function for many years despite neuronal remodelling. Retinitis pigmentosa (RP) and age-related macular degeneration are diseases that predominantly affect photoreceptors of the retina and cause progressive vision loss-leading eventually to blindness in over 15 million people worldwide. These results demonstrate for the first time that subretinal micro-electrode arrays with 1500 photodiodes can create detailed meaningful visual perception in previously blind individuals. Without a training period, the regained visual functions enabled him to localize and approach persons in a room freely and to read large letters as complete words after several years of blindness. One of these patients was able to correctly describe and name objects like a fork or knife on a table, geometric patterns, different kinds of fruit and discern shades of grey with only 15 per cent contrast. Subsequently, three previously blind persons could locate bright objects on a dark table, two of whom could discern grating patterns. The chip generates a corresponding pattern of 38 × 40 pixels, each releasing light-intensity-dependent electric stimulation pulses. Visual scenes are projected naturally through the eye's lens onto the chip under the transparent retina. At the implant's tip, another array of 16 wire-connected electrodes allows light-independent direct stimulation and testing of the neuron–electrode interface. The implant contains an array of 1500 active microphotodiodes (‘chip’), each with its own amplifier and local stimulation electrode. A light-sensitive, externally powered microchip was surgically implanted subretinally near the macular region of volunteers blind from hereditary retinal dystrophy.
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