Visual Prosthesis

By Eva Borras

In 2019 it was reported that at least 2.2 billion people worldwide have blindness or visual impairment. In 1 billion of these cases, moderate or severe vision impairment could have been prevented. The likelihood of having a vision impairment was 4 times higher in low and middle-income regions than high-income regions. (Vision impairment and blindness, 2019)

The visual system consists of various parts, one being the retina. There are five different classes of neurones in the retina, including rods and cones photoreceptors. Rods are for night vision and cones for day vision. Another set of retinal cells are ganglion cells, which extend to the central nervous system. In a fully functional eye, the visual information is captured by the photoreceptors and passed to the optical fibre via ganglion cells. (Schiller and Tehovnik, 2008) Degeneration of these cells, either because of disease or aging, results in vision loss.

The idea of stimulating the nervous system to create artificial vision was introduced in 1929 by Foerster, a German neurosurgeon. He directly stimulated the visual cortex to produce phosphene, an impression of light without light entering the eye directly that is felt when you apply pressure on the eyeball with the lid closed. (Definition of PHOSPHENE, 2020) The goal of the experiment was to excite phosphenes to help facilitate visual perception, Foersters’ results  concluded that appearance of phosphene perceptions were located where the cortex was stimulated. However, after experiments where electrodes were implanted into the cortex by chronic stimulation, it was concluded that resolution of results was very low and there was a high risk of seizures being induced in patients. For this reason, cortical stimulation didn’t make it past clinical use. (Sabel, Henrich-Noack, Fedorov and Gall, 2011)

Retinal prostheses were another proposed solution to vision impairment. A retinal prosthesis is a complex device that works by capturing visual images and communicating them to electronic components that interface with the retina. Electrical pulses are then delivered to the retina to create vision. Although nonelectrical means of stimulation such as neurotransmitters could be less toxic to the user, studies have shown electrical stimulation has larger potential to restore vision. (Memon and Rizzo, 2010) However, studies have also shown that individuals that have had visual loss later in life are the best candidates to get back vision by means of implanted visual prosthetics, giving fewer hopes for the early-blind. (Collignon, Champoux, Voss and Lepore, 2011)

Bionic eye implants are compared to cochlear implants used to restore hearing loss, nonetheless, the development for visual prosthesis is still at its beginnings. More recently, retinal prosthesis targeted at patients suffering from retinitis pigmentosa have been developed. Retinitis pigmentosa is an inherited retinal degeneration that affects 1 in 4000 people worldwide, or 1.5 million people. It is the predominant cause of inherited blindness where a subset of patients completely lose sight, typically starting in childhood with a gradual decline in vision. Retinitis pigmentosa targets photoreceptors in the retina causing retinal ganglion cells to lose photoreceptors and degenerate. Nonetheless, usually enough cells survive so that electrodes can be implanted to electrically stimulate them and therefore restore rudimentary forms of vision. (Shepherd, Fallon and McDermott, 2014) 

The retinal prosthesis, or bionic eye, is the ‘Argus II Retinal Prosthesis System’ developed in the USA that has been implanted to over 190 users with a retinitis pigmentosa. Results have shown that patients with recessively inherited retinitis pigmentosa have restored some visual perception such as distinguishing shapes and light using the Argus system. This system consists of a pair of eyeglasses with an integrated camera that take the visual information from the surroundings. In the glasses, there is a processor that converts visual information to electrical signals. The most crucial part of the system are 60 electrodes that are implanted in the back of the eye in the retina which receive these electrical signals wirelessly. The optimal size of electrodes was determined to be 200–300 μm. (Hornig and Velikay-Parel, 2013) Due to the fact that retinitis pigmentosa leaves the optic nerve almost intact, the signals are passed to it and processed by the brain. One drawback to this technology is the lack of resolution, resulting in colours not being distinguished. This is because an implant of 1 million electrodes -as opposed to 60- would be needed to mimic natural sight. (Guide to Bionic Eyes: Implants, Lenses & the Status in 2020 | NVISION Eye Centers, 2020)

In conclusion, what lies ahead for retinal prosthesis is finding out how to optimise reliability and quality of images. Nonetheless, it is clear that when visual prostheses undergo major improvements, results will be promising. 

References:

Collignon, O., Champoux, F., Voss, P. and Lepore, F., 2011. Sensory rehabilitation in the plastic brain. Enhancing Performance for Action and Perception – Multisensory Integration, Neuroplasticity and Neuroprosthetics, Part I, pp.211-231.

Hornig, R. and Velikay-Parel, M., 2013. Retina implants. Implantable Sensor Systems for Medical Applications, pp.469-496.

Memon, M. and Rizzo, J., 2010. Visual prostheses and other assistive devices. Ocular Disease, pp.590-598.

Merriam-webster.com. 2020. Definition Of PHOSPHENE. [online] Available at: <https://www.merriam-webster.com/dictionary/phosphene&gt;.

NVISION Eye Centers. 2020. Guide To Bionic Eyes: Implants, Lenses & The Status In 2020 | NVISION Eye Centers. [online] Available at: <https://www.nvisioncenters.com/education/bionic-eyes/&gt;.

Sabel, B., Henrich-Noack, P., Fedorov, A. and Gall, C., 2011. Vision restoration after brain and retina damage: The “residual vision activation theory”. Progress in Brain Research, pp.199-262.

Schiller, P. and Tehovnik, E., 2008. Visual Prosthesis. Perception, 37(10), pp.1529-1559.

Shepherd, R., Fallon, J. and McDermott, H., 2014. Medical Bionics. Comprehensive Biomedical Physics, pp.327-341.

Who.int. 2019. Vision Impairment And Blindness. [online] Available at: <https://www.who.int/news-room/fact-sheets/detail/blindness-and-visual-impairment&gt;. 

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