The visual system is an extremely complex system.  Visual prosthetics are underway to aid people who are afflicted with either impaired vision or no vision at all.  The visual prosthetic has not progressed as steadily as the cochlear implant due to the complexity of the visual system.  Our eye takes the information we see and converts it to electrical signals and the brain processes all this information.  One needs to take a moment and look around us, the eye somehow manages to take the colors, depth, shape, shadows, etc... and sends all this information to our brain and somehow our brain takes this information and we are capable of sight.  It is unclear as to what pathways are used, what cells are used and how everything is encoded but this is the challenge that faces the world of visual prosthetic.  This site was used to illustrate the tip of the iceberg.  Retinal prosthetics are but one form of visual prosthetics.  Within retinal prosthetics we have seen that several different forms exist, epiretinal and subretinal.  We are also faced with different issues, such as biocompatibility and limited electrode technology.  The effects of long term retinal electrode implantation is also not known.  Will it damage the surrounding cells, will the visual pathway change due to the aid of an electrode?  These and more questions will be solved in the near future as technology improves and will allow for better electrode arrays to aid in the quest for a visual prosthetic.