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Biology 202
2001 Second Web Report
On Serendip

Visual Prostheses and the Retinal Implant Project

Sadie White

Visual percepts are the final product of a rich interplay of stimulus processing that occurs without the intervention of one's consciousness. While this is a fascinating issue to consider, especially as it pertains to the philosophical and practical definitions of ideas like the "self," the converse is equally interesting to me. In this modern era of exploding technological ingenuity, the sum of which is a product of the conscious brain, increasingly more opportunities exist for the brain to design the input it receives. One method by which this occurs is observable in the treatment of visual pathologies. A development of particular interest to me is the use of visual prosthetic devices in the treatment of some forms of progressive blindness. Research in this area raises numerous conflicts within the realm of bioengineering, but promises, at least, to challenge the boundaries of current microtechnology and instigate further integration of the rapidly expanding fields of electronics and medicine.

In 1988, a multidisciplinary research team called the "Retinal Implant Project," spanning the knowledge bases of Harvard Medical School, the Massachusetts Eye and Ear Infirmary, and the Massachusetts Institute of Technology's Department of Electrical Engineering and Computer Science, was formed with the explicit goal of creating an intraocular retinal prosthetic device to combat the effects of certain types of progressive blindness (4). The prostheses are intended to stimulate retinal ganglion cells whose associated photoreceptor cells have fallen victim to degradation by macular degeneration or retinitis pigmentosa, two currently incurable but widespread conditions (2). Their most recent work has been to orchestrate short-term clinical trials in which blind volunteers receive a temporary intraocular prosthetic implant and undergo a series of tests to determine the quality of visual percepts experienced over a two- to three-hour period (2). The leaders of the Retinal Implant Project, while enthusiastic about their progress, do not anticipate the realization of a workable prosthetic within the next five years (2).

The goal of retinal prosthetic proposed by the collaborators is to bypass degenerate photoreceptors by providing electrical stimulation directly to the underlying ganglion cells (2,3,4,5). The ganglion cell axons compose the optic nerve, which travels from the eye and terminates in various regions of the brain, where the combined input is processed along multiple routes and ultimately results in the experience of sight (4). Ganglion cell excitation will be accomplished by attaching a two-silicon-microchip system onto the surface of the retina, which will be powered by a specially designed laser mounted on a pair of glasses worn by the patient (4,5). This laser will also be receiving visual data input from a small, charge-coupled camera, whose output will dictate the pattern intensity of the laser beam (4,5). The laser's emitted radiation will be collected by the first microchip within the eye on an array of photodiodes and transferred to the second chip, which will be responsible for electrically stimulating a set of retinal ganglion cells via fine microelectrodes (4,5). Because the ganglion cells in a healthy retina are stimulated by photoreceptors, this activation process is designed to mimic the electrical activity within a retinal ganglion cell corresponding to a visual stimulus, with the hope that some measure of sight can be restored to individuals with faulty photoreceptors (3,4,5).

The team selected the retina as the site of artificial stimulation after careful consideration of the effects of the target diseases and the successes and limitations of electrical excitation at various regions along the visual pathway (4). Dr. T. Hambrecht of the National Institutes of Health and Dr. R. Normann of the University of Utah are two neurobiologists examining the effects of microelectrode stimulation of various regions of the visual cortex, a portion of the brain believed to be involved in visual perception (4). Upon administration of electrical stimuli to subsurface regions of the visual cortex of a blind patient, the patient identified spots of light, called phosphenes, which varied in color and depth, depending on the location of the stimulus (4). While this is exciting in its implications for elucidating the physical arrangement of the neuronal cells involved in the visual pathway, it fails to replicate the experience of sight because the stimuli are independent of external factors. Also, the visual percept is the product of neuronal activity in more than one brain region, a fact that renders the proposition that artificial stimulation in any single cortical area (or small collection of cortical areas) could recreate the elaborate perception of vision rather dubious.

The researchers involved with the Retinal Implant Project hypothesize that higher quality visual perceptions will be experienced with retinal than with intracortical stimulation (4). Joseph Wyatt and John Rizzo, III, the co-heads of the Retinal Implant Project write, ". . . in principle, the earlier the electronic input is fed into the nerves along the visual pathway, the better, inasmuch as neural signals farther down the pathway are processed and modified in ways not entirely well understood" (5). This hypothesis is validated by the observation that photoreceptors are the sole neurons decimated by macular degeneration and retinitis pigmentosa, leaving the remaining cells involved in the visual process unharmed (4). Therefore, with the proper artificial input, it is reasonable to expect that those with prosthetic photoreceptive apparatuses will experience some returned vision.

While this proposal is exciting in its scope and purpose, it is not without drawbacks and complications. While the prosthetic's design offsets many potential biological problems by having most of its functional parts external to the body, this fails to solve every obstacle attendant upon the insertion of an inorganic and electrically active device into a living eye. Rizzo and Wyatt explain, "Biocompatibility, which encompasses biological, material, mechanical, and electrochemical issues, is the most significant obstacle to the development of a visual prosthesis" (4).

Specifically, the electrical components of the prosthesis must be sequestered from all intraocular fluids, which could corrode the thin metal of the diodes and ruin the chips' ability to transmit electrical impulses from the laser to the retinal ganglion cells (4,5). Likewise, the by-products of electrical impulse transmission through metallic electrodes are toxic to living cells, and must be diminished in order to insure minimal chemical devastation of the retina (4,5). The electrodes themselves must also be anchored to the retina with sufficient strength to accommodate physical agitation due to daily activity (3,4,5). This promises to be a trying procedure. The retina is a slim 0.25 millimeters thick, a dauntingly thin fabric onto which to stitch a complex, albeit tiny, piece of machinery (3,5). As in all retinal surgical procedures, the implantation of a prosthetic poses a risk of retinal detachment and infection of the associated membranes, both of which would exacerbate, rather than prevent, vision loss (1,4). These concerns have not been seriously addressed in this stage of the research, because no long-term clinical trials of the prosthesis have been undertaken (4).

A final barrier to the project, and perhaps the most complex to troubleshoot, is determining whether the engineered apparatus will be effective in restoring sight with chronic implantation. Although short-term tests of the photodiode array have been undertaken, their success was only measured in the ability of the diodes to generate output once inserted into the eye (4). While this was a necessary experimental step to prove the short-term mechanical soundness of the diode apparatus to fluids of the inner eye, the diodes have never been attached to the retinal tissue, and therefore, their viability as conduits of visual information has not been examined (4). The data the researchers cite in their preliminary investigations and those of their colleagues report that the single visual percept accomplished by artificial stimulation to date is phosphene recognition (4). This, however, is not equivalent to true sight, and certainly falls short of the lofty goal claimed by its spearheads: to "improve quality of life by providing gross perception with some geometric detail that would increase independence by making it easier for a blind person to walk down the street, for instance" (5).

Clearly the project's goal of developing an "innocuous and seamless apposition of [their] device to the retina," and its still-hypothetical benefits have yet to be realized (4). However, the promise of sight to those with macular degeneration or retinitis pigmentosa seems to merit this intensity of inquiry, despite technical and biomechanical difficulties the researchers are encountering. If nothing else, the investigation is redefining the limits of medical mechanics and blurring the borders between the responsibilities of the conscious and the subconscious - a task that could enhance both.

WWW Sources

1) Encyclopedia of Life Sciences: Macular Degeneration. Encyclopedia of Life Sciences Online, A great overview of macular degeneration: statistics, treatments, and effects.

2)The Retinal Implant Project Website The Retinal Implant Project Website, An interesting collection of publications and links regarding retinal implantation, blindness, and neurotechnology.

3)AccessScience: Neurotechnology AccessScience: Neurotechnology, General information about the field of neurotechnology, as well as specific references to visual prostheses.

4) A link to an online edition of an article by the heads of the Retinal Implant Project featured in "The Neuroscientist." "Prospects for a Visual Prosthesis" by Joseph Rizzo, III and John Wyatt , An outline of the development of the retinal prosthesis, as well as discussion of its goals and associated complications.

5)A link to an article by Rizzo and Wyatt featured in an online edition of "Spectrum," a publication of the Institute of Electrical and Electronics Engineers, Inc. "Ocular Implants for the Blind," by Joseph Rizzo, III, and John Wyatt, A discussion of the Retinal Implant Project from a more mechanical perspective.




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