The Why Files The Why Files --

The Substitution Solution: Restoring vision, hearing and movement


And the blind shall see
Retinitis pigmentosa (RP) is a hereditary disease that gradually robs the vision. Starting at the edges, RP inexorably shuts down the entire light-sensitive layer of the retina. But because the rest of the visual system is largely intact, Electrodes carry a picture from a camera to the retina over tiny wires.RP also makes a good test case for an electronic vision system that replaces the dead light-detectors with signals from a tiny television camera.

The retinal implant signals the ganglion cells, which normally get their signals from the photoreceptors -- the rods and cones. The signal moves into the eye, and the electrode activates ganglionic cells on the retina, producing a simple image. View 2.7 MB movie Courtesy Mark Humayun, University of Southern California

Experiments by Mark Humayun, a professor of ophthalmology at the University of Southern California, show that stimulating the ganglion cells (which are normally signaled by the light-sensitive rods and cones) can restore some vision in people who are blinded by RP. Many nerve cells survive in RP patients, he says. "Because they have the wiring relatively intact, are completely blind, and had sight before, that makes them really good candidates for the first implants."

Already, six patients have received implants. The 16 channels of information they carry is paltry by the standard of normal vision, but still useful, Humayun says. "Typically, they see dots of light like pixels, so when you string them together, they create patterns, and that begins to give an outline of form, which can help you determine motion in the environment."

Graphic follows impulse from alphabet poster, through electrode wires to retina to produce vision.
A vision-replacement system tested at the University of Southern California takes information from a camera, processes it, and transfers data to an implant tacked to the retina. Courtesy Mark Humayun, University of Southern California

Time is on my side
Restoring even that limited vision takes time, he adds. "If they haven't seen in decades, it takes a little while even to see the visual perception, to get used to seeing again. We have seen in a month considerable progress, but it continues to get better over one to two years" (see #3 in the bibliography).

Patients who saw just 16 pixels of light could detect motion, find the exit, locate a table or chairThe next step, already approved by the Food and Drug Administration, is to implant 60-channel electrodes, Humayun says. After a few years, if everything goes smoothly, he hopes to move toward 256 or 1,000 channels. At that point, a prosthesis may make sense for a wider group of patients, such as those with macular degeneration, a common disease that can prevent reading and recognizing faces. These devices will be made by Second Sight, Inc., of Sylmar, Cal.

To the totally blind, 16 pixels can produce a surprising benefit, Humayun says. "It's unbelievable. A number have told me that seeing a single light tells you where the exit is. You can tell motion, how tall your family member is, where the table or chair is. It's very hard for us to appreciate, we have so much resolution, so much color. Sixteen pixels are not enough, and the level of function these patients have achieved is not enough, but to them, it makes a big difference."

Can an electronic hearing device make a bigger difference?


Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive

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