light therefore stimulates a pattern of bipolar, horizontal, amacrine, and ganglion cells, leading to a visual perception representative of the original incident image. In principle, subretinal implants do not require any external hardware beyond the implanted microphotodiodes array. However, some subretinal implants require power from external circuitry to enhance the image signal. Advantages A subretinal implant is advantageous over an epiretinal implant in part because of its simpler design. The light acquisition, processing, and stimulation are all carried out by microphotodiodes mounted onto a single chip, as opposed to the external camera, processing chip, and implanted electrode array associated with an epiretinal implant. The subretinal placement is also more straightforward, as it places the stimulating array directly adjacent to the damaged photoreceptors. By relying on the function of the remaining retinal layers, subretinal implants allow for normal inner retinal processing, including amplification, thus resulting in an overall lower threshold for a visual response. Additionally, subretinal implants enable subjects to use normal eye movements to shift their gaze. The retinotopic stimulation from subretinal implants is inherently more accurate, as the pattern of incident light on the microphotodiodes is a direct reflection of the desired image. Subretinal implants require minimal fixation, as the subretinal space is mechanically constrained and the retinal pigment epithelium creates negative pressure within the subretinal space. Disadvantages The main disadvantage of subretinal implants is the lack of sufficient incident light to enable the microphotodiodes to generate adequate current. Thus, subretinal implants often incorporate an external power source to amplify the effect of incident light. The compact nature of the subretinal space imposes significant size constraints on the implant. The close proximity between the implant and the retina also increases the possibility of thermal damage to the retina from heat generated by the implant. Subretinal implants require intact inner and middle retinal layers, and therefore are not beneficial for retinal diseases extending beyond the outer photoreceptor layer. Additionally, photoreceptor loss can result in the formation of a membrane at the boundary of the damaged photoreceptors, which can impede stimulation and increase the stimulation threshold. Clinical studies Optobionics was the first company to develop a subretinal implant and evaluate the design in a clinical trial. Initial reports indicated that the implantation procedure was safe, and all subjects reported some perception of light and mild improvement in visual function. The current version of this device has been implanted in 10 patients,