such implants are asked to not move their eyes, but rather scan the visual field with their head. Additionally, encoding visual information at the ganglion cell layer requires very sophisticated image processing techniques in order to account for various types of the retinal ganglion cells encoding different features of the image. Clinical study The first epiretinal implant, the ARGUS device, included a silicon platinum array with 16 electrodes. The Phase I clinical trial of ARGUS began in 2002 by implanting six participants with the device. All patients reported gaining a perception of light and discrete phosphenes, with the visual function of some patients improving significantly over time. Future versions of the ARGUS device are being developed with increasingly dense electrode arrays, allowing for improved spatial resolution. The most recent ARGUS II device contains 60 electrodes, and a 200 electrode device is under development by ophthalmologists and engineers at the USC Eye Institute. The ARGUS II device received marketing approval in February 2011 (CE Mark demonstrating safety and performance), and it is available in Germany, France, Italy, and UK. Interim results on 30 patients long term trials were published in Ophthalmology in 2012. Argus II received approval from the US FDA on April 14, 2013 FDA Approval. Another epiretinal device, the Learning Retinal Implant, has been developed by IIP technologies GmbH, and has begun to be evaluated in clinical trials. A third epiretinal device, EPI-RET, has been developed and progressed to clinical testing in six patients. The EPI-RET device contains 25 electrodes and requires the crystalline lens to be replaced with a receiver chip. All subjects have demonstrated the ability to discriminate between different spatial and temporal patterns of stimulation. Subretinal implants Design principles Subretinal implants sit on the outer surface of the retina, between the photoreceptor layer and the retinal pigment epithelium, directly stimulating retinal cells and relying on the normal processing of the inner and middle retinal layers. Adhering a subretinal implant in place is relatively simple, as the implant is mechanically constrained by the minimal distance between the outer retina and the retinal pigment epithelium. A subretinal implant consists of a silicon wafer containing light sensitive microphotodiodes, which generate signals directly from the incoming light. Incident light passing through the retina generates currents within the microphotodiodes, which directly inject the resultant current into the underlying retinal cells via arrays of microelectrodes. The pattern of microphotodiodes activated by incident