A visual prosthesis is a device that actively provides some type of sight to a person who is blind. These devices are usually implanted into the eye in a particular spot, often the retina or visual cortex. The most successful and widespread visual prosthesis is known as a Dobelle Eye and uses cameras worn by the patient to transmit images that are then passed to the visual cortex, allowing an image to be seen. Patients who benefit the most from such a prosthesis are those who have, at some point, had sight. The use of the devices is less successful on those who were born blind.
When functioning normally, the eye receives light from the surrounding area and focuses it through a pair of transparent structures called the cornea and the lens. A dark hole in the center called the pupil lets the light enter the internal structure of the eye. The focused light travels to the rear of the eye, where it hits a lining known as the retina. From there, the retina translates the light into electrical impulses that travel along the optical nerves to the brain. A visual prosthesis seeks to simulate one or more parts of this process to compensate for degenerative conditions or to restore sight, in general.
One form of visual prosthesis that has received ample research involves the concept of inserting a photosensitive chip over the retina. These chips are able to detect light coming in from the eye naturally and transmit that information to the brain. Although there are several working versions of this concept, such as the Argus™ retinal prosthesis, the technology behind it remains very expensive.
Another type of visual prosthesis implants a type of computer controller on the visual cortex itself. This method relies on some type of external sensor or camera to transmit information to the controller. The controller then stimulates the nerves of the eye and causes a field of lights to appear before the person. This field of lights appears as a very rough, pixel-like representation of the image the camera is transmitting.
There are some problems with using a visual prosthesis. One is that, no matter how small the light-detecting diodes are, the resolution of the image the person sees is still incredibly grainy. Another problem involves how the brain interprets the image. Some patients who have a visual prosthesis have great difficulty determining depth and distance. Finally, some prostheses can cause the image being transmitted to flicker or merge into large bars of light, creating temporary blind spots.