Honours Neuroscience, Class of 2021, McMaster University
Dr. Macknik and Dr. Martinez-Conde have been working on a new approach to implant devices into the human brain to restore vision in patients that experience blindness. Their new prosthetic system, called OBServ, should perform better and restore vision to levels unreached by traditional visual prosthetics.1
To understand how this prosthetic works, we first need to understand vision. Light from the environment enters our eyes and hits the retina where photoreceptors convert the light into electrochemical signals. This process is called phototransduction. These signals propagate through nerves and eventually reach neurons in the visual cortex where they are processed and the perception of vision is formed.
People may experience poor vision or blindness if any part of this pathway stops functioning properly. A common form of vision loss that develops as we get older is known as AMD, or age-related macular degeneration. AMD is a vision disease characterized by the degradation of photoreceptors in the macula, the central part of the retina, due to aging.2,3
Individuals with AMD are primarily treated with traditional visual prosthetics such as retinal implants. Retinal implants are small devices that are surgically implanted to replace retinal photoreceptors with sensors and electrodes that convert light into electrical signals. Although clinical trials have shown that these implants are effective in restoring vision, the efficacy is limited as the degree of restored vision has not been sufficient to be truly useful.4 Vision is still blurry as the prosthesis only allows the user to see edges and areas in the visual field with varying contrasts. Dr. Weiland, a professor of ophthalmology and biomedical engineering, explained that as of now, the electrodes do not have the resolution and energy to mimic retinal neurons.5 While this implant would restore the visual field and allow patients to differentiate between simple objects, the patients would not be able to confidently navigate themselves in a new environment.
A new prosthetic system in development, called OBServ, uses an alternate approach that may overcome the limitations of retinal implants. This system uses a cortical implant: a device that is surgically implanted into the brain.
The perception of vision is ultimately produced by neurons in the visual cortex. Therefore, in individuals that have damaged photoreceptors, as long as specific neurons in the visual cortex are activated, the correct visual perception should be formed and vision should be restored. While the traditional retinal implants functioned by targeting and replacing neurons in the retina, the OBServ system uses a cortical implant to stimulate neurons in the visual cortex.6
The OBServ prosthetic system works in three steps.
First, the user wears a pair of glasses with cameras that track eye movement and record visual information that the eye is centrally focused on.
Second, gene therapy is used to convert normal neurons in the visual cortex into photoreceptors that respond to light, similar to the ones in the retina.
Third, two prosthetic devices are surgically implanted in the back of the head where they are facing the visual cortex. The recorded visual information from the cameras are wirelessly transmitted to a prosthetic device that processes this information and projects light onto the newly converted photoreceptors, which convert the light into electrical signals like normal.
The neuronal projections from the retina are laid out in the visual cortex in a specific pattern that matches the visual field that the person is looking at. In other words, specific areas of the visual cortex process certain parts of the visual field and adjacent areas process adjacent parts of the visual field.1 This means that the cortical implants can project and stimulate only the neurons that would be activated by looking at a specific part of the visual field. Instead of using electrodes to mimic photoreceptors, the OBServ system simply creates new photoreceptors that perform phototransduction normally. This allows vision to be processed with the same acuity as normal.
It’s important to note that because the cameras only record regions of the visual field the eyes are fixated on, the OBServ system only restores vision to a portion of the visual field where the eyes are directly focused on. In fact, the designers expect that a patient would have a window of vision around the size of a thumbnail at arm’s length.6 While the OBServ system can’t restore the complete visual field, the image that is restored is expected to have normal visual acuity levels. The primary objectives of these treatments are to provide patients with the ability to independently manage day-to-day tasks, such as reading and navigating through rooms. Even with a full visual field, if vision is blurry, the user may still have difficulties with these tasks. Instead, by providing clear vision to a limited portion of the visual field, patients should be able to accomplish these tasks, even if it takes longer.
This being said, it is still in question whether this new technique can really be successful.
One area of concern is if the implant can be safely inserted into the head and remain without being disrupted by brain tissue. A common neurosurgical procedure, similar to the technique used in deep brain stimulation (DBS), can be used for insertion. The DBS procedure has been widely used for treatments in many disorders such as Parkinson’s, OCD, and epilepsy. Improvements to surgical methods, such as faster insertion times, ensure that minimal tissue damage occurs during the procedure.7 It’s also true that most implants are inserted for shorter time frames and not many studies have looked at the effects of long-term cortical prosthetics. New techniques are in development to ensure that these implants will have long-term durability; a few studies have looked at coating the device with a bioactive layer or with specific protein sequences to reduce interaction with tissue.8,9
Another area of concern is the power source. In some cases, batteries or surgically inserted wires are used for power; however, batteries need routine surgery for replacement and there have been cases where wired power sources lead to infection. This brings up the question: Is it possible to safely provide energy to the device on a long-term scale? The proposed solution is for the OBServ system to use a wireless system that utilizes remote powering. This is achieved by using inductive coupling, a process where power is transferred between two coils.10 This would allow the researchers to power the device wirelessly without having to perform surgery again.
Visual prosthetics and cortical implants have been in development for several years and have been shown to restore vision. Although current technologies are limited, restoring enough vision for patients to function by themselves is within arms-reach. Although we might not see this OBServ prosthesis in use for some time as further development and clinical studies are required, by approaching the problem in a unique fashion, the OBServ prosthesis system may finally achieve the limitations of previous visual prosthetics.
- Macknik S. A New Type of Visual Prosthesis [Internet]. Scientific American Blog Network. 2021 [cited 13 March 2021]. Available from: https://blogs.scientificamerican.com/illusion-chasers/a-new-type-of-visual-prosthesis/
- WHO . Vision impairment and blindness [Internet]. Who.int. 2021 [cited 13 March 2021]. Available from: https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment
- NIH . Age-Related Macular Degeneration | National Eye Institute [Internet]. Nei.nih.gov. 2021 [cited 13 March 2021]. Available from: https://www.nei.nih.gov/learn-about-eye-health/eye-conditions-and-diseases/age-related macular-degeneration
- Chow A. The Artificial Silicon Retina Microchip for the Treatment of Vision Loss From Retinitis Pigmentosa. Archives of Ophthalmology. 2004;122(4):460.
- Karmel M. Retinal Prostheses: Progress and Problems [Internet]. American Academy of Ophthalmology. 2021 [cited 13 March 2021]. Available from: https://www.aao.org/eyenet/article/retinal-prostheses-progress-problems
- Collins F. The Amazing Brain: Making Up for Lost Vision [Internet]. NIH Director’s Blog. 2021 [cited 13 March 2021]. Available from: https://directorsblog.nih.gov/2019/08/27/the-amazing-brain-making-up-for-lost-vision/
- Kringelbach M, Jenkinson N, Owen S, Aziz T. Translational principles of deep brain stimulation. Nature Reviews Neuroscience. 2007;8(8):623-635.
- Zhong Y, Bellamkonda R. Controlled release of anti-inflammatory agent α-MSH from neural implants. Journal of Controlled Release. 2005;106(3):309-318.
- Olbrich K, Andersen T, Blumenstock F, Bizios R. Surfaces modified with covalently-immobilized adhesive peptides affect fibroblast population motility. Biomaterials. 1996;17(8):759-764.
- Amar A, Kouki A, Cao H. Power Approaches for Implantable Medical Devices. Sensors. 2015;15(11):28889-28914.