Innovation Spotlight - Stimulating Vision

Implant for stimulating vision on the horizon

Mark Humayun, MD

By Lynda Charters
Reviewed by Mark Humayun, MD

Baltimore-Implantation of a device onto the retina to stimulate vision has long been a dream of vision researchers that may be realized in the near future.
Mark Humayun, MD, recounted the history of the development of the device and the experimental stages that have brought him and his colleagues to the brink of success.
"After 10 years of the Retinal Prosthesis Project, we believe that we are now within 2 to 5 years of developing a prototype suitable for use in humans," said Dr. Humayun, associate professor of ophthalmology, Wilmer Eye Institute, Johns Hopkins University, Baltimore.
Dr. Humayun and his colleagues began by addressing the issue of whether electrodes or extracellular electrical stimulation can be used to excite the remaining retinal neurons and stimulate vision in a retina with photoreceptor degeneration.

The basis for the stimulated vision is a device that is composed of a camera, video chip, receiver chip, and probe. The camera is placed at the level of the spectacle frame to capture visual information and encode it on a video chip. That video chip then transmits information and power to a receiver chip that has been implanted inside the eye. It decodes information and provides electrical retinal stimulation.
"When considering using this device, one of the first questions was whether there were remaining retinal neurons that could be electrically stimulated, given the fact that there are very few photoreceptors in severe retinal degeneration," according to Dr. Humayun.
"To answer this, we studied retinas from patients with no light perception and/or bare light perception vision, after they died," he said. "Histologic study indicated that despite that fact that there was less than 2% of the outer nuclear layer present, about 80% of the inner nuclear layer and about 30% of the ganglion cells were present."
Another practical issue that the investigators tackled was the type of visual information that the electrical current would produce inside the eye.
Dr. Humayun explained that because of fluid inside the eye and dissipation of the electrical current, the potential existed for only streaks of visual information to be transmitted rather than the desired dots of visual information that allow an increase in the number of pixels and subsequently increased vision.
In patients who were blind as a result of retinitis pigmentosa, Dr. Humayun and his colleagues put electrodes of different types onto the patients' retinas and used an external computer to stimulate the probe. The frequency of the stimulation was adjusted up and down and when the patients were able to see the stimulus from the computer, they confirmed this computer-generated vision by counting out loud, which correlated with the computer pulse.
"These completely blind patients were able to see a small yellow-green dot of light when electrical current was passed through the electrodes," Dr. Humayun said. "When the electrode was moved on the retina, the patients were able to report the direction of the movement. That was very encouraging."
The investigators then used a compound electrode and the patients reported that they were able to see three individual lights. The investigators later stimulated the perimeter of an electrode and the patients were able to discern a match box and letters-a step closer to the goal of seeing forms.
He and his colleagues have worked with 15 patients, most of whom had retinitis pigmentosa and the remainder severe macular degeneration with large retinal hemorrhages. All the experiments have taken place in an operating room setting in 45-minute sessions.
Great advances have been made in the technology since the first attempts to stimulate vision. According to Dr. Humayun, the fourth generation of the computer chip incorporates 100 electrodes, the telemetry is far more powerful than the first, and the 1-inch camera fits into a pair of spectacles.
Dr. Humayun reported the development of a flexible silicone electrode array that can be attached to the retina and does not damage the delicate tissue. This will overcome the hurdles of directly implanting a computer chip on the retina that may potentially tear the retinal tissue.
In animals, use of the flexible silicone electrode array has solved the problem of pressure necrosis that could have developed as a result of the attachment of a heavier device (computer chip) to the retina in animals.
"Three companies are in the process of making this device," he said. "We are planning its use in an animal model of retinal degeneration."

Ophthalmology Times / APRIL 15, 2000