John Wyatt, a professor of electrical engineering at the Massachusetts Institute of Technology, says it is precisely because “the standards we need to be useful are quite low” that artificial vision is feasible at all. Wyatt’s MIT lab is home to another artificial retina project, which is taking a slightly different approach than the Hopkins/NCSU team.
The effort got under way in 1988, when Joseph Rizzo, a Harvard Medical School neuro-ophthalmologist, approached Wyatt to find out if the engineer could help him build a retinal implant. Wyatt, who had some experience in retinal neurophysiology from his doctoral studies at Berkeley, was initially skeptical. The retina looked like a pretty flimsy circuit board. But his fascination with the eye’s circuitry made the project too tempting to pass up, and he has since become more optimistic. Wyatt gives most of the credit to advances in microelectric fabrication technology, which, he says, “open up the ability to make little delicate things that you might be able to put in the eye [and are] the only reason there’s any hope of doing this. Without that, it would just be a complete dream.”
Although Rizzo and Wyatt have conducted only two human tests so far (both with inconclusive results), the pair think they know what a retinal implant will ultimately look like. The system, says Wyatt, will start by taking digital pictures with a small camera that can mount on a pair of glasses. Off-the-shelf technology that could do the trick already exists in the form of charged couple devices (CCD) found in conventional camcorders, as well as the newer, smaller and more energy-efficient active pixel sensor (APS) technology that debuted with digital cameras.
A small computer would probably be needed to process the image, which would then have to be sent to the implant inside the eye. The wireless system on Wyatt’s drawing board uses a diode laser, also mounted in a pair of glasses, to flash the images captured by the camera onto an array of photovoltaic cells built into the front of the implant. The laser beam would also provide power.
The implant itself, according to Wyatt, will be a silicon chip, loaded with transistors, sitting on the surface of the retina. In this “epiretinal” configuration, the side covered with photovoltaic cells faces outwards, while the other face, studded with 100 or more electrodes, would ride right on the retinal surface close to the layer of ganglion cells. The Johns Hopkins/NCSU implant has a similar overall design, except that it uses radio frequencies instead of a laser to transmit data and power.
The researchers plan to treat each electrode as one picture element, or pixel, with which to build an image. To squeeze the most out of each pixel/electrode, Wyatt hopes that changing the electrical current to each electrode will control the intensity of each spot a patient sees. “The idea is to convey various shades of gray, rather than just light or dark,” he says. With just a ten-by-ten grid of electrodes, each providing four to six levels of gray, Wyatt says it should be possible to “start making sense of an image, especially if it moves.”