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Cortical Implant Research in
from the Foundation Fighting Blindness
Vision researchers are developing high-tech prosthetic devices that can be surgically implanted in the brain or the retina to partially restore lost vision to people who are blind. Although highly experimental, these devices might one-day restore ambulatory vision, thereby giving people the freedom to walk without the assistance of a cane or guide dog.
In the current issue of The Journal of the American Society of Artificial Internal Organs, Dr. William Dobelle, of the Dobelle Institute in New York, reports preliminary findings from one blind patient using an experimental visual prosthesis called the "Dobelle Eye". This device consists of a video camera and distance sensor wired via a computer to platinum electrodes implanted on the surface of the visual cortex of the brain. The visual cortex is the part of the brain that processes visual images. Because it interfaces directly with the brain, the Dobelle Eye is an example of what researchers call a cortical implant.
In a set of preliminary experiments, Dr. Dobelle reports that a patient, who in 1978 was surgically implanted with a set of 68 electrodes, reported seeing flashes of light or "phosphenes" when the electrodes were stimulated. The patient also reported perceiving two-inch large letters through a very small island of vision from a distance of five feet, possibly approximating a visual acuity of 20/400.
The Foundation is hopeful that one patient's experience can be objectively reproduced in many patients. However, it is important to note that these findings have yet to be published in a peer-reviewed medical journal and so the claims have yet to be subjected to the rigorous scientific scrutiny of other researchers. Before a cortical implant device can be made available to patients, it must be further tested and refined in the laboratory and then evaluated in human clinical trials to insure its safety and effectiveness.
One of the largest scientific hurdles facing cortical implants is the massive amount of electrical power needed to excite the neurons that process visual information deep within the visual cortex. In early experiments, researchers found that surface stimulation of the visual cortex with many electrodes might cause seizures and other complications.
Researchers are now developing smaller and more complex cortical implants that can be placed closer to the neurons within the visual cortex. This new generation of cortical implants requires less power to produce meaningful images and thereby transmits safer electrical signals. For example, Dr. Richard Normann of the University of Utah has developed a state-of-the-art cortical implant made of silicon rather than platinum. Silicon allows for the design of wafer-thin stimulating electrodes that are smaller than the thickness of a human hair. Hopefully, such a small device can be safely implanted within the visual cortex. With support from The Foundation Fighting Blindness, Dr. Normann is testing this device in animal models. Researchers at the National Institutes of Health are also working to develop a cortical implant.
Nonetheless, Dr. Dobelle's research is encouraging. First, long-term implantation of electrodes in the brain appears not to evoke immune responses or cause discomfort or pain. Second, although further confirmation is needed, the electrodes appear to still function after 20 years. These findings bode well for long-term use of a cortical device. Lastly, because cortical implants bypass the retina and relay images directly to the brain, it may be possible to restore vision to patients with vision loss from eye trauma and a wide variety of eye diseases, including retinal degenerative diseases.
Date last modified January 23, 2000