Famed singer Stevie Wonder can't see his fans dancing at his concerts. He can't see the hands of his audience as they applaud wildly at the end of his Superstition. Blind from birth, Wonder has waited his whole life for a chance to see. Recently, Wonder visited Mark Hamayan, a vision specialist. He thought that a new device currently being studied by Humayan might offer him that chance. The device, a retinal prosthesis, is a tiny computer chip implanted inside a patient's eye. The chip sends images to the brain and allows some sightless people to see shapes and colors. Wonder hoped the retinal prosthesis might work for him. 'I've always said that if ever there's possibility of my seeing,' said Wonder, 'then I would take the challenge.' Unfortunately for Wonder, that challenge will have to wait. Humayan explained that the device isn't ready for people who have been blind since birth. Their brains may not be able to handle signals from a retinal prosthesis because their brains have never handled signals from a healthy eye. The retinal prosthesis and other devices, however, show great promise in helping many other sightless people who once had vision see again. Perhaps one day soon, some formerly sightless people may be in Wonder's audience looking up—and seeing him—for the very first time. Wonder's willingness to take part in retinal prosthesis studies and the results of those studies are giving new hope to people who thought they would be blind for the rest of their lives. More than one million people in the United States are considered legally blind, meaning that their eyesight is severely impaired. Another one million are totally blind. Two types of specialized cells in the retina—rods and cones—are critical for proper vision. Light enters the eye and falls on the rods and cones in the retina. Those cells convert the light to electrical signals which travel through the optic nerve to the brain. The brain interprets those signals as visual images. Rods detect light at low levels of illumination. For instance, rods allow you to see faint shadows in dim moonlight. Cones, on the other hand, are most sensitive to color. Some diseases can damage cells in the retina. For instance, macular degeneration causes blindness and other vision problems in 700,000 people in the United States each year. The condition is caused by a lack of adequate blood supply to the central part of the retina. Without blood, the rods, cones, and other cells in the retina die. Devices such as the retinal prostheses won't prevent or cure our eye diseases, but they may help patients who have eye disorders regain some of their vision. Different forms of retinal prostheses are currently being developed. On one type, a tiny computer chip is embedded in the eye. The chip has a grid of about 2,500 light-sensing elements called pixels. Light entering the eye strikes the pixels, which convert the light into electrical signals. The pixels then send the electrical signals to nerve cells behind the retina. Those cells send signals via the optic nerve to the brain for interpretation. Many people who have had a retinal prosthesis implanted say they can see shapes, colors, and movements that they couldn't see before. 'It was great,' said Harold Churchey, who received his retinal prosthesis 15 years after he became totally blind. 'To see light after so long—it was just wonderful. It was just like switching a light on.' (572 words. Current Science. April 7, 2000) Why did stevie Wonder visit Mark Humayan?