Most blind people wear sunglasses, but what if their glasses could actually restore their vision? Such a feat seems miraculous, but the development of new bionic prostheses may make such miracles a reality. These devices work in two ways: by replacing non-functional parts of the visual pathway or by creating alternative neural avenues to provide vision.
When attempting to repair or restore lost vision, it is important to understand how we normally receive and process visual information. Light enters the eye and is refracted by the cornea to the lens, which focuses the light onto the retina. The cells of the retina, namely photoreceptors, convert the light into electrical impulses, which are transmitted to the primary visual cortex by the optic nerve. In short, this process serves to translate light energy into electrical energy that our brain can interpret. For patients suffering from impaired or lost vision, one of the steps in this process is either malfunctioning or not functioning at all.1,2
Many patients with non-functional vision can be treated with current surgical techniques. For example, many elderly individuals develop cataracts, in which the lens of the eye becomes increasingly opaque, resulting in blurred vision. This condition can be rectified fairly simply with a surgical replacement of the lens. However, loss of vision resulting from a problem with the retina or optic nerve can very rarely be corrected surgically due to the sensitive nature of these tissues. Such pathologies include retinitis pigmentosa, an inherited degenerative disease affecting retinal photoreceptors, and head trauma, which can damage the optic nerve. In these cases, a visual prosthesis may be the solution. These devices, often called “bionic eyes,” are designed to repair or replace damaged ocular functions. Such prostheses restore vision by targeting damaged components in the retina, optic nerve, or the brain itself.
One set of visual prostheses works by correcting impaired retinal function via electrode arrays implanted between the retinal layers. The electrodes serve as substitutes for lost or damaged photoreceptors, translating light energy to electrical impulses. The Boston Retinal Implant Project has developed a device involving an eyeglass-mounted camera and an antenna implanted in the skin near the eye.3 The camera transmits visual data to the antenna in a manner reminiscent of a radio broadcast. Then, the antenna decodes the signal and then sends it through a wire to an implanted subretinal electrode array, which relays it to the brain. The problem with this system is that the camera is fully external and unrelated to the eye’s position, meaning the patient must move his or her entire head to survey a scene. Germany’s Retinal Implant AG team seeks to rectify this problem with the Alpha IMS implant system. In this system, the camera itself is subretinal, and “converts light in each pixel into electrical currents.”2
The Alpha IMS system is still undergoing experimental clinical trials in Europe, but it is facing some complications. Firstly, the visual clarity of tested patients is around 20/1000, which is well below the standard for legal blindness. Secondly, the system’s power supply is implanted in a very high-risk surgical procedure, which can endanger patients. In an attempt to overcome the problems faced by both The Boston Retinal Implant Project and Retinal Implant AG, Dr. Daniel Palanker at Stanford and his colleagues are currently developing a subretinal prosthesis involving a goggle-mounted video camera and an implanted photodiode array. The camera receives incoming light and projects the image onto the photodiode array, which then converts the light into pulsed electrical currents. These currents stimulate nearby neurons to relay the signal to the brain. As Dr. Palanker says, “This method for delivering information is completely wireless, and it preserves the natural link between ocular movement and image perception.”2 Human clinical trials are slated to begin in 2016, but Palanker and his team are confident that the device will be able to produce 20/250 visual acuity or better in affected patients.
A potentially safer set of visual prostheses includes suprachoroidal implants. Very similar to the aforementioned subretinal implants, these devices also replace damaged components of the retina. The only difference is that suprachoroidal implants are placed between the choroid layer and the sclera, rather than between the retinal layers. This difference in location allows these devices to be surgically implanted with less risk, as they do not breach the retina itself. Furthermore, these devices are larger compared to subretinal implants, “allowing them to cover a wider visual field, ideal for navigation purposes.” Development of suprachoroidal devices began in the 1990s at both Osaka University in Japan and Seoul National University in South Korea. Dr. Lauren Ayton and Dr. David Nayagam of the Bionic Vision Australia (BVA) research partnership are heading more current research. BVA has tested a prototype of a suprachoroidal device in patients with retinitis pigmentosa, and results have been promising. Patients were able to “better localize light, recognize basic shapes, orient in a room, and walk through mobility mazes with reduced collisions.” More testing is planned for the future, along with improvements to the device’s design.2
Both subretinal and suprachoroidal implants work by replacing damaged photoreceptors, but they rely on a functional neural network between the retina and the optic nerve. Replacing damaged photoreceptors will not help a patient if he or she lacks the neural network that can transmit the signal to the brain. This neural network is composed of ganglion cells at the back of the retina that connect to the optic nerve; these ganglion cells can be viewed as the “output neurons of the eye.” A third type of visual prosthesis targets these ganglion cells. So-called epiretinal implants are placed in the final cell layer of the retina, with electrodes directly stimulating the optic nerve. Because these devices are implanted in the last retinal layer, they work “regardless of the state of the upstream neurons”.2 So the main advantage of an epiretinal implant is that, in cases of widespread retinal damage due to severe retinitis pigmentosa, the device provides a shortcut directly to the optic nerve.
The most promising example of an epiretinal device is the Argus II Visual Prosthesis System, developed by Second Sight. The device, composed of a glasses-mounted camera that wirelessly transmits visual data to an implanted microelectrode array, received FDA marketing approval in 2012. Clinical trials have shown a substantial increase in visual perception and acuity in patients with severe retinitis pigmentosa, and the system has been implanted in more than 50 patients to date.
The common limitation of all these visual prostheses (subretinal, suprachoroidal, and epiretinal) is that they rely on an intact and functional optic nerve. But some blind patients have damaged optic nerves due to head trauma. The optic nerve connects the eye to the brain, so for patients with damage in this region, bionics researchers must find a way to target the brain itself. Experiments in the early 20th century showed that, by stimulating certain parts of the brain, blind patients could perceive light flashes known as phosphenes. Building from these experiments, modern scientists are working to develop cortical prostheses implanted in either the visual cortex of the cerebrum or the lateral geniculate nucleus (LGN), both of which are key in the brain’s ability to interpret visual information. Such a device would not truly restore natural vision, but produce artificial vision through the elicitation of phosphene patterns.
One group working to develop a cortical implant is the Monash Vision Group (MVG) in Melbourne, Australia, coordinated by Dr. Collette Mann and co. MVG’s Gennaris bionic-vision system consists of a glasses-mounted camera, a small computerized vision processor, and a series of multi-electrode tiles implanted in the visual cortex. The camera transmits images to the vision processor, which converts the picture into a waveform pattern and wirelessly transmits it to the multi-electrode tiles. Each electrode on each tile can generate a phosphene; all the electrodes working in unison can generate phosphene patterns. As Dr. Mann says, “The patterns of phosphenes will create 2-D outlines of relevant shapes in the central visual field.”2 The Illinois Institute of Technology is developing a similar device called an intracortical visual prosthesis, termed the IIT ICVP. The device’s developers seek to address the substantial number of blind patients in underdeveloped countries by making the device more affordable. The institute says that “one potential advantage of the IIT ICVP system is its modularity,” and that by using fewer parts, they “could make the ICVP economically viable, worldwide.”4
These visual prostheses represent the culmination of decades of work by hundreds of researchers across the globe. They portray a remarkable level of collaboration between scientists, engineers, clinicians, and more, all for the purpose of restoring vision to those who live without it. And with an estimated 40 million individuals worldwide suffering from some form of blindness, these devices are making miracles reality.
References
- The Scientist Staff. The Eye. The Scientist, 2014, 28.
- Various Researchers. The Bionic Eye. The Scientist, 2014, 28.
- Boston Retinal Implant Project. http://biomed.brown.edu/Courses/BI108/2006-108websites/group03retinalimplants/mit.htm (accessed Oct. 9, 2015)
- Intracortical Visual Prosthesis. http://neural.iit.edu/research/icvp/ (accessed Oct. 10, 2015)
