Two new devices -- versions of an artificial retina -- hold the promise of restoring vision to the blind.
For blind people, recovering even the slightest bit of vision would be a revolutionary step. Just being able to see the outline of a doorway would propel blind people into a different world.
We’re getting closer to this goal. A number of research teams around the world are working on various versions of the artificial retina, formally known as the intraocular retinal prosthesis. While the devices are still undergoing rigorous testing and development, two types have been implanted in a handful of blind people -- the Argus and the Artificial Silicon Retina microchip. The results to date have raised hopes as well as cautions.
Some promising news: In recent research conducted in Southern California with the Argus, blind people who have had the device implanted report that they can see some light and detect objects and motion. This is vision in its most rudimentary form. The six blind people tested were able to distinguish large, simple shapes (such as the capital letter L) 60–80% of the time. They did better with objects and were able to indicate whether a cup, a plate, or a knife had been placed before them.
The Argus, which is considered the furthest along in development, has several components. A thin chip with 16 electrodes on it is implanted in the eye, and a second chip is implanted behind the ear. The final component is a pair of sunglasses equipped with a miniature video camera and a microprocessor. When the blind person wears the glasses, the video camera captures images and converts them to electrical impulses that are transmitted to the chip behind the ear, which then relays them to the retinal chip.
The retinal chip functions in lieu of damaged rods and cones and sends impulses directly to the retinal ganglion cells, which pass them along to the visual cortex of the brain. For each electrode that is stimulated on the chip, the blind person sees a dot of light. To say that one of the blind people in the Argus study “sees” a plate or cup, then, means something different than what we think of when we think of vision: They see one or more points of light, perhaps a string of lights, not a formed image. Still, for someone who is blind, this is a thrilling advance.
Other artificial retinas in development around the world are similar in theory to the Argus. But the team developing the Artificial Silicon Retina microchip (ASR) has taken a completely different tack. The ASR -- which is implanted in the sub-retinal space, between the pigment epithelial layer of the retina and the outer layer -- is powered solely by natural, incident light. As a result, it needs no external source of power.
The surface of the ASR is coated with 5,000 microscopic solar cells. The ASR’s developers say that these solar cells convert the light that passes through the eye into electrical currents that stimulate visual signals in remaining functional retinal cells. These signals are then sent by the optic nerve to the brain. Critics of the ASR question whether light entering the eye is strong enough to power the miniature solar cells and produce any vision at all. However, results with the first six blind people who received the implant showed that all reported improvements in their vision. To date, the ASR has been implanted in more than 20 blind people.
Both the Argus and the ASR have been implanted in patients with retinitis pigmentosa (RP, a rare, inherited disease that leads to tunnel vision and then blindness) and researchers hope to eventually test them in people who are blind from age-related macular degeneration. While RP and AMD damage the structures of the eye that normally convert light to electrical impulses, they spare the neural paths to the brain that transport the electrical signals. Blind people with diseases that damage the optic nerve wouldn’t be eligible for the artificial retina.
What’s Next? This research has raised many questions, including the following:
- How much current can the retina tolerate?
- What’s the best site for the chip to be implanted in terms of yielding the best perception and requiring the lowest, safest electrical charge needed to stimulate neurons?
- Will the retinal neurons tolerate long-term electrical stimulation without being altered in any way?
- Another question researchers have been grappling with is how many light-producing electrodes can fit on a chip without damaging the retina? That is, how do you provide enough power, but not so much that it generates tissue--destroying heat?
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Posted in Vision on April 27, 2007
Reviewed May 2007
