WASHINGTON -- Attempts to restore sight to people with damaged retinas are turning toward signaling the brain the way nature does it, using chemicals to deliver signals to nerve endings.
Experiments already under way with retinal implants seek to use electrical signals to make the nerves send information to the brain.
But doctors from Michigan and California described a different method last week, using retinal implants that respond to images by releasing nerve-stimulating chemicals.
Dr. Raymond Iezzi of Wayne State University and Dr. Harvey A. Fishman of Stanford University discussed their separate research projects at a science writers seminar on ophthalmology.
Neither is close to testing the idea in people. Both indicated animal experiments may be a year away.
Nerves carry messages to or from the brain by electrical impulses, but nerves are stimulated to send those signals by chemicals generated by organs or relayed from other nerve endings.
About half of all blindness is a result of damage to the retina, the inner part of the eye containing cells that react to light by releasing chemicals that cause the optic nerve to send signals to the brain.
Using electronic chip
Experiments using an electronic chip implanted in the retina, sending an electric current to stimulate the optic nerve, are in early stages.
But, instead of direct electrical signals, Fishman and Iezzi turned their attention, in slightly different ways, to the chemicals that the body uses to get those nerves to send signals.
Iezzi's research is focusing on a retinal implant that can deliver what he calls an array of "chemical pixels" through tiny holes, somewhat like a very small, gentle, inkjet printer or shower head, stimulating nerves to relay an image to the brain.
There are several neurotransmitter chemicals and Iezzi is using glutamate in his tests. He said the final product may use a cocktail of these chemicals.
As he envisions it, the chip would receive a supply of neurotransmitters from a reservoir under the skin behind the ear. It would react to signals from a small digital camera, perhaps worn like an eyeglass.
"Once we prove the basic concept, we can go on and refine the design," he said.
Fishman is developing what he calls an artificial synapse chip, an implant that also would deliver minute amounts of chemical transmitters.
But in his case, the chip is designed to direct the growth of nerve cells into tiny openings in the chip where they can be stimulated by the release of chemicals in response to light. The chemicals are held within the chip.
Fishman said his team is working on ways to get the nerve cells to grow into the chip. Experiments with a carpet of carbon tubes have been promising, he said: "This gives us a lot of hope that in animals we can direct the process."
The two spoke at a conference sponsored by the New York-based organization, Research to Prevent Blindness.