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silicon light sensors directly into the eyes of test subjects.The sensors convert light into electrical impulses that activate nerve cells.

A more radical method brings visual information directly into the brain, which means the technique could cure blindness due to a damaged eye or optic nerve, as well as blindness arising from retinal problems.William Dobelle, an independent scientist who operates his own laboratories in the United States and Portugal, has developed an electrode array that is implanted on the surface of the brain, where it stimulates the visual cortex. The array is connected to an electrical socket mounted on the outer surface of the skull, into which is plugged a video camera.

None of the methods described above is a complete bionic cure for blindness. Many questions remain, such as how well the body accepts the implants. However, these initial efforts are providing glimmerings of vision to the blindin one case, apparently sufficient to allow the implantee to drive a car under controlled conditionsalthough not yet anything close to full restoration of sight. One prob- lem is low resolution, because the number of electrodes or sensors in each implant is minuscule compared to the millions of rods and cones in the natural retina. Advances in nanoelectronics will undoubtedly improve the resolution, but a more fundamental difficulty remains. The retina contains a complex multilayered system of neurons that respond to the impulses from the rods and cones and thereby analyze visual information even before it reaches the brain.This retinal pro- cessing tracks movement and the edges of objects, both significant elements in any visual scene. None of the implant schemes tested so far performs this essential first step in visual thinking, but this impor- tant point is being addressed by researchers working onbiomorphicor neuromorphicchips that copy biological functioning.

Kwabena Boahen, at the University of Pennsylvania, has gone beyond merely simulating the retina to actually copying it. Using tran- sistors etched in a silicon chip, which are interconnected and made to operate in a way that mimics the layered retinal neurons, he has repro- duced the edge- and motion-detection carried out by a natural retina. There is still a long path ahead, however, before this chip is ready to

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