Introduction to the Eye
There are other large problems too. Too much oxygen given to premature babies can cause similar issues to AMD, resulting in life-long visual impairment and blindness. Insufficient daily sunlight can produce sight-threatening Myopia and increase the risk of Multiple Sclerosis. The Vision ACTion team and their Partners are working on diagnostics and treatments for these and other diseases.
Once the optic nerves of the two eyes reach the brain they spread out in bundles, called the optic radiations, to connect to multiple brain areas. These brain areas specialise in things like colour, texture, motion and shape. Each area is a sheet of brain tissue that has a projection of the retina on it. That is, every patch of retinal sheet has a corresponding patch on the sheets of each of the brain areas. That means that diseases affecting a patch of these brain areas, can result in reduced visual function in the corresponding patch of the visual field.
Brain conditions like stroke, optic neuritis, idiopathic intracranial hypertension (IIH), and multiple sclerosis (MS) can affect vision. MS and IIH mainly effect women beginning at about 30 years of age. MS patients now have normal lifespans, leaving about half with 40 to 50 years of disability, at great cost to them and the community. The Vision ACTion team and their Partners are working on diagnostics or treatments for these brain disorders and others including migraine, epilepsy, Parkinson’s and Alzheimer’s disease.
Our vision is produced by a thin neural tissue, the retina, which wallpapers the inside surface of our eyeball, like the film in a camera (Figure 1). It detects the light focused onto it by the clear parts of our eye and performs a set of calculations to “get the exposure right”. We have all taken disappointing photos where the exposures are wrong: shadows you hadn’t noticed obscure faces; the colours seem all wrong. We don’t notice these things before we take the photo because our retinas perform calculations that fix these problems, better than any camera. The eye is basically a camera and the pupil is a hole that allows light in. It usually appears black because the retina absorbs almost all the light that enters the eye through the pupil.
In the developing eye of a foetus new blood vessels grow into and out of the eye through a small hole in the back of the eyeball. Little patches of retina from every part of the retina each sprout a tiny nerve-fibre and 1 million of these make their way to the same hole, exiting the eye in a sheaf that is called the optic nerve. Thus, each tiny patch of retina is attached to the brain, giving us our sight. Looking into the eye from the front we can see a pale looking round filled hole against the redder retina. That pale object is called the optic disc (Figure 1). The central 9 mm of the retina produces about half the nerve-fibres, densely sampling the image formed by the clear optical parts of the eye, the cornea and lens.
Figure 1. The eye. The parts we can see are the cornea, pupil (grey), and iris (teal). The clear cornea and lens focus light onto the retina (red) a neural sheet that wallpapers inside of the eye. The 1 mm diameter fovea provides our high visual acuity. The optic disc is where the optic nerve (tan), made up of 1 million nerve-fibres, leaves the eye and goes to the brain, which creates our visual perception. This image by Casey Henley is available under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 License.
Figure 2. A high resolution Optical Coherence Tomograph angiography (OCTa) image of the whole macula. The white lines are where blood is moving inside tiny blood vessels in a living eye. Disease can disturb those flows. The dark central spot is the Foveal Avascular Zone (FAZ), site of the fovea which has no vessels that might obscure our most acute vision. The axes show that this central 6 mm of the retina projects to the central 20 degrees of our visual field.
The central 6 mm of those 9 mm is called the macula and is more important. It is covered in a yellow sun-screening pigment. The central 1 mm of that, the fovea, creates our acuate vision for things like reading and face-recognition. The eyes scan the world with this little patch of clear vision and the brain stiches the information together to give us the impression that the whole world is clear. To give us the clearest vision possible the centre of the fovea does not have surface blood vessels (Figure 2). This makes our most important patch of retina susceptible to insufficient oxygen.
Optical technologies for detailed inspection of the living retina and its layers have improved greatly in recent decades (e.g. Figure 2). Methods for objectively determining which parts of the retina are functioning normally, or abnormally, have lagged behind. Damage to the broader macula can spread into the central fovea, destroying acute vision. We therefore need to monitor visual function in multiple parts of the macula and broader retina to watch for expanding damage that may spread to the fovea.
Genetically regulated biomarkers can also inform treatment or underpin treatments. Glaucoma is the loss of nerve-fibres from the optic nerves, progressively detaching the eyes from the brain. Diabetes and Age-related Macular Degeneration (AMD) either kill patches of retina or grow malformed and leaky blood vessels in response to the retina thinking it doesn’t have enough oxygen. Diabetes, Glaucoma, and AMD are the big-3 causes of visual impairment and blindness in the Western World, costing Australia 10s of billions each year and lowering quality of life for millions. About half of all cases are undiagnosed and so receive treatment too late.