Blindness affects about 50,000 Australians, the majority of whom are elderly. The most common causes of blindness in this group are degenerative disease such as glaucoma, age-related macular degeneration, cataract and diabetic retinopathy.
Dr Anthony Hall
Among people of working age, blindness is the result of very different, and often genetic, causes. While some cases of blindness are treatable, there’s a group of patients for whom the development of a bionic eye is the only possible treatment.
The process of perceiving vision by the human eye and brain is very complex. It involves the raw perception of light by light-receiving cells in the retina, known as rods and cones. These cells lie in an orderly grid at the back of the retina and convert light into an electrical impulse. This impulse is then processed by other cells in the retina.
The impulse is sent down the optic nerve to the initial reception area in the brain, the thalamus. Here, the signals from the two eyes join and there’s further processing of motion, depth and colour. Nerve fibres run from the thalamus to the primary visual cortex at the back of the brain. This is the part of the brain ultimately responsible for vision.
Unlike the small representation of images on the retina, in the primary visual cortex the picture is spread over a large area over the surface of the brain. This is important for developing a bionic eye because the large surface allows space for an implant.
Data is sent from the primary visual cortex to a variety of areas for the processing of shape, edge and motion, among other things. In theory, a device to help a blind person see could be placed at any point on this pathway to replicate the natural electrical image seen by the brain at that point.
In reality, the complexities of the distribution of the electrical signals and of access mean research has been concentrated on devices that sit under the retina (subretinal or supra choroidal), on top of the retina (pre-retinal or epiretinal), or on the visual cortex at the back of the brain.
Approaches to bionic eyes have many features in common. Most of them, for instance, rely on external capture of the visual image by a digital camera and a small computer to arrange the message in a way the brain will recognise.
Each also has a method of transferring new electrical messages to the retina or brain (this can be wireless or via a physical wire). Finally, there’s the actual implant that’s inserted into the body (either the retina or brain). This delivers the new visual message to the patient, creating artificial vision.
The image from the camera is modified by a computer to mimic the human electrical signal at the point of the visual pathway (described above) where the device is inserted. This signal is transmitted to an electrode grid inserted at that point in the pathway.
Placing the device in the sub-retinal space (under the retina adjacent to the rods and cones) allows for a relatively secure mechanical fixing. It also allows the device to replace the function of the rods and cones for diseases that cause them not to work or to degenerate, such as retinitis pigmentosa (a common and important inherited cause of blindness).
But this technique relies on other cells in the retina and optic nerve to still be working well. It also requires the implanted device to be miniscule and to have densely placed electrodes, as at this point the visual image is presented over a tiny area (around 25 mm²).
This is one type of bionic eye being designed and manufactured by Bionic Vision Australia.
Placing the device on top of the retina is technically easier, but fixing it securely and close enough to the retina to be effective is much more challenging.
This technique allows direct stimulation of the optic nerve cells and bypasses other diseased cells in the retina but it needs the optic nerve to be intact. And we know that a number of diseases that cause blindness also destroy the optic nerve, such as glaucoma – the second commonest cause of blindness around the world.
Again, there’s the need for a very fine and dense electrode array.
Research into this type of bionic eye is the most advanced internationally – it’s the approach used by Second Sight in America and by IMI Intelligent Medical Implants and Epiret in Germany.
The last type of bionic eye has the device placed on the brain. The advantage of having the device on the visual cortex in the brain is that there’s a much broader spatial representation of the visual image (over an area of around 3000 mm²) so, in theory, it should be possible to use a larger implant to give a more detailed picture.
This device also bypasses the retina completely and is appropriate for patients blinded by severe retinal disease or optic nerve disease, such as glaucoma.
In normal vision, the image is highly processed at this point in the visual pathway and, so, much more sophisticated computer processing of the image is needed prior to transferring the message. But the visual cortex in the brain is folded and its position makes it hard to access, so a device that’s inserted here will need to be very malleable, and the surgery to insert it will be complex.
This is the style of prosthesis being developed by the Monash Vision Group.
There’s been a massive growth in research around the world on bionic eyes so it’s likely some of the crude early implants currently in use will soon be superseded.
The American second sight Argus II prosthesis is the most advanced commercially and is now licensed for commercial use in Europe. It’s currently the only bionic eye licensed for use anywhere in the world.
It’s predicted the German preretinal implants will also be commercially licensed in Europe next year. Meanwhile Bionic Vision Australia is planning to start human trials in 2013 and Monash Vision is planning human trials in 2013.
To date, progress has been rapid and effective but we are still in the early stages of the marathon. It seems likely that both the eye- and brain-placed implants will be used and needed in future. For the thousands of blind Australians there is finally hope – but unfortunately the wait continues.
Dr Anthony Hall is the Head of the Department of Ophthalmology at the Alfred Hospital and Associate Professor in the Faculty of Medicine, Nursing and Health Sciences at Monash University.
This article has previously appeared in The Conversation.