Written by Mayuri Ramnarain MBChB IV and Kaylee Stella Harris MBChB III
Figure 1 (1)
There are currently 49.1 million blind people worldwide.(2) In Africa, it is estimated that 26.3 million people have some form of visual defect with 6 million of those people being completely blind.(3) The following figure shows a representation of the number of people that are affected by vision loss. The largest concentrations can be seen in countries such as India and China, although Africa has only 2 million cases less than China.
Figure 2(4)
Figure 3(5)
Restoring vision by using implantable neural prosthetic devices which bypass the damaged visual pathways could be ground-breaking for the field of ophthalmology.(6) While some visual pathologies may be successfully treated, treatments do not exist for all causes of blindness.(6)
New approaches to artificial vision are centred around electrical stimulation of the retina, optic nerve, lateral geniculate nucleus or visual cortex. Currently, retinal prostheses are the most successful and several devices have been approved for use. However, most blind people have damage to the neurons which connect the retina to the occipital lobe. Therefore, retinal prostheses will not restore their vision.(6)
Other approaches to restore vision which bypass the retina need to be explored. Since the neurons in the higher visual regions of the brain are generally spared from damage, visual prostheses which directly stimulates the primary visual cortex are being developed.(6)
Figure 4(4): The visual pathway. Visual prostheses may directly stimulate the primary visual cortex.
History of Intracortical Electrodes
In 1929, neurosurgeon Otfried Foerster electrically stimulated the occipital area in an awake patient under local anaesthesia. This led to the patient seeing small spots of light. However, such stimulation may lead to side effects such as epileptic seizures. These issues were caused by the large surface area of the electrodes, which needed high electrical currents to induce these small spots of light: an image called phosphenes. These large electrodes also produced a low resolution of the phosphenes, making shape identification, which is the crux of sight, impossible.(6)
Current advances in Intracortical Electrodes
Cortical artificial vision was unattainable until a technique to localise the stimulation of neurons in the cortex was established, thus avoiding the aforementioned side effects. This led to the development of smaller intracortical electrodes which were comparable in size to neuron cell bodies. An example is the Utah Electrode Array (UEA).(6) The UEA contains 100 tiny electrode spikes, each 1mm tall, which penetrate the surface of the brain when inserted. One electrode is calibrated at a time by increasing the current until the patient says when and where they see a phosphene.(8) This would theoretically effectively treat glaucoma, optic atrophy and brain injuries or strokes that result in damage of the central visual pathways.(9)
Figure 5(2, 9): The Utah Electrode Array
Challenges
1. Disturbances to brain tissue
The cortical neurons cannot be reached without disrupting the adjacent tissue and the brain has a complex network of blood vessels that might be injured during surgical implantation. The insertion of foreign material also provokes an inflammatory response. In addition, friction between throbbing neural tissue (due to the pulse and breathing) and the stationary implant produces microhaemorrhages. As a consequence, long-term biotolerability is a key issue and most implanted cortical microelectrodes have a maximum lifespan of several months to a few years.(6)
2. Information delivery
Another problem is the way the brain decodes synthetically programmed information. This area of research is still in early its stages of development and the result is that prostheses provide very poor vision and a low spatial resolution. In the future, intelligent signal and image processing strategies and artificial intelligence could drastically influence the interpretation of these signals.(6)
Patient experience
A visual cortical prosthesis was implanted into a 57-year old woman named Bernardeta Gómez in 2018. When she was 42, toxic optic neuropathy left her totally blind and incapable of perceiving light.
The 100 electrode UEA was implanted in her right visual cortex near the occipital pole for 6 months. Gómez wore a customised pair of glasses attached with a camera. The glasses recorded a live video feed of her surroundings and this video was sent to a computer that converted the feed into electronic signals. The computer was linked to a port implanted in the back of her skull which conveyed the electronic signals generated to the UEA implanted in her visual cortex.(8)
Figure 6 (9): Bernardeta Gómez wearing the modified glasses.
The result was that Gómez could perceive a very low-resolution impression of the world, denoted by bright yellow spots and shapes. This crude view enabled her to identify lights, letters, basic shapes and people. Gómez’s breakthrough is accredited to decades of research by Prof. Eduardo Fernandez, director of neuroengineering at the University of Miguel Hernandez in Spain. Gómez is the first human recipient, but over the next few years implants will be installed in five more blind people.(9)
Conclusion and Future Perspectives
While the complete restoration of vision is improbable in the near future, an intracortical microelectrodes device could generate significant visual insights – a lifechanging phenomenon for blind people. Ongoing research on the local cellular environment, materials science and artificial intelligence will undoubtedly advance this technology and bring science one step closer to curing blindness.
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Bibliography
- The Royal Society of Medicine. Neuro-ophthalmology for the neurologist and ophthalmologist. The Royal Society of Medicine; 2020.
- Bourne RRA, Adelson J, Flaxman S, Briant P, Bottone M, Vos T, et al. Global Prevalence of Blindness and Distance and Near Vision Impairment in 2020: progress towards the Vision 2020 targets and what the future holds. Investigative Ophthalmology & Visual Science. 2020;61(7):2317-.
- Eye health [Internet]. WHO | Regional Office for Africa. [cited 2021 Jun 27]. Available from: https://www.afro.who.int/health-topics/eye-health#:~:text=Approximately%2026.3%20million%20people%20in
- A Way Out of the Dark [Internet]. The Ophthalmologist. [cited 2021 Jun 27]. Available from: https://theophthalmologist.com/subspecialties/a-way-out-of-the-dark
- International Centre for Eye Health, London School of Hygiene & Tropical Medicine. Global Blindness: Epidemiology and visual impairment [Internet]. International Centre for Eye Health. 2016 [cited 2021 Jun 27]. Available from: https://www.slideshare.net/InternationalCentreforEyeHealth/epidemiology-andvisualimpairment
- Fernández E, Alfaro A, González-López P. Toward Long-Term Communication With the Brain in the Blind by Intracortical Stimulation: Challenges and Future Prospects. Frontiers in neuroscience. 2020;14:681.
- Nieto MP. A simplified schema of the human visual pathway.: Wikimedia Commons; 2015.
- Juskalian R. A new implant for blind people jacks directly into the brain 2020 [cited 2021 24 June]. Available from: https://www.technologyreview.com/2020/02/06/844908/a-new-implant-for-blind-people-jacks-directly-into-the-brain/.
- Fernandez E, Normann R. Introduction to Visual Prostheses [Internet]. Nih.gov. University of Utah Health Sciences Center; 2016 [cited 2019 Dec 3]. Available from: https://www.ncbi.nlm.nih.gov/books/NBK391004/
- Utah Array [Available from: https://www.brainlatam.com/manufacturers/microelectrodes/utah-array-335.
