Prosthetic retinas are leaving the experimental stage and now are in live animal testing to shake the bugs out. A number of housecats with a condition similiar to retinitis pigmentosa, which causes blindness by killing the rod and cone cells that make up the retinas have been implanted with silicon chips 2mm on a side that replicate some of the functions of organic retinas. The chips are covered with microscopic photodiodes that register light levels, produce miniscule electrical impulses, and feed directly into the optic nerve. Similiar implants have been used in a small number of humans with this disorder, with varying degrees of success. The eyesight of some patients has improved markedly, while others have not seen much improvement in their vision. One of the goals of the animal experimetation series is to get the implantation procedure down; another is to refine the designs so that the resolution of the image transmitted to the visual cortex is closer to that of normal, unassisted vision.