“We were able to create a small dot of light exactly under the stimulating electrode,” says Humayun. Churchey told his interrogators the spot looked to be the size of a pea seen at arm’s length. Worried that it might be some kind of artifact (possibly of Churchey’s imagination) the doctors changed the frequency of the pulse, asking him to count out loud if he saw the light. He did, and also reported that the spot moved when the electrode did, proving there was some degree of spatial resolution.
Buoyed by the results of their first human test, de Juan and Humayun set out to determine whether a blind patient could be induced to see multiple spots of light, something that would be crucial if they were ultimately to create a useful image. In their second human test, another volunteer was able to see three spots of light produced by three probes with an edge-to-edge separation of about 300 microns, or the width of a few human hairs.
The next challenge was to answer what Humayun calls “the million-dollar question.” Namely, how many electrodes would they need to produce usable images?
When cochlear implants-the predecessor to visual implants, which have given hearing to many deaf people-were being developed, some experts believed that at least 1,000 electrodes would be needed to create coherent sound. Yet six electrodes proved enough to help many patients. “This points to the fact that there is incredible plasticity in the ability of the human brain to take a somewhat crude sensory input generated by a man-made machine and make good use out of it,” says Humayun.
Evidence of the brain’s forgiving nature had already come out. The volunteers had reported that the electrodes were producing flickering dots of light. To create a steady image, the doctors simply turned up the frequency of the pulse; just as a movie appears continuous, even though it is made up of a series of still pictures, the brain was compensating by keeping an image in mind until the next pulse came along.
In a 1996 experiment, Churchey’s third, the two physicians, who had by then moved to Johns Hopkins, placed a 25-electrode array (a 5-by-5 square, with a slightly convex surface allowing it to match the contour of the retina) in Churchey’s eye and attempted to create an image of the letter “U” by stimulating the electrodes in a dot-matrix-like format. They had picked the wrong letter. They couldn’t round the edges of the “U,” and Churchey reported seeing an “H.” Since then, de Juan and Humayun have conducted one more experiment, stimulating the outermost electrodes of a square array-the patient reported seeing a matchbox shape.
Although they’ve been able to create only the crudest kind of image, Humayun says the initial successes have “really lit a fire” to move from hand-held electrodes to an actual implant. The know-how gleaned so far about how to stimulate retinal nerves is now being turned into a prototype device by a collaborating team led by Professor of Electrical Engineering Wentai Liu at North Carolina State University (NCSU).
All the researchers interviewed for this story emphasized that the results at Johns Hopkins, while exciting, do not mean blind people will be able to read newspapers, or even recognize a face anytime soon. But, says Ronald Carr, a New York University professor of ophthalmology and retina expert, a retinal implant that could allow some blind people to see light and dark now appears “feasible.” Ultimately, they may even be able to perceive enough of the shapes around them to walk without a dog or cane. “Obviously, this is never going to approach what one sees with the human eye,” says Carr. “But there’s a huge difference between seeing nothing, and being able to see outlines. Anything that could be done is a marked improvement.”