The eyeball is a magnificent contraption of brain tissue that protrudes during development to form said ball. It works to capture and process photons in light, and transmit the compressed information via the optic nerve to the visual cortex in the brain. This journey starts in the retina, which is the innermost layer at the back of the eye containing the light receptors and cells involved in this process.
Having two eyes provides binocular vision which enhances depth perception, gives a wider visual field and better distance perception.
The eye has many different components that work in unison to enable vision. On its surface and lining the eyelids, the conjunctiva secretes mucus and tears, lubricating the eye and providing immune surveillance via the many microvessels. The sclera is the white of the eye and contains collagen and elastic fibre that protects the eye.
The ciliary body allows the eye to focus, while the suspensory ligament ensures the eye doesn’t fall downwards. The ability to focus is given by the added curvature given to the lens. The cornea covers the central part of the eye and refracts light, giving the eye’s optical power. Alongside the cornea, the lens, of course, refracts light.
The pupil allows the light to strike to retina, with the amount controlled by the contracting iris.
The aqueous humour is replaced frequently and consists of a plasma-like solution without proteins. On the other hand, the vitreous humour is a gel and does not get refreshed. As such, debris can accumulate in it, causing floaters. With age, it can also liquefy and collapse.
The choroid supplies the retina with oxygen and nutrients via the blood supply. Additionally, it contains pigmented cells (with melanin) to prevent internal reflection. The retina contains photoreceptor cells like cone cells which perceive visual information. This is transmitted to the brain via the optic nerve. The optic nerve creates a region without photoreceptor cells, called the blind spot because one cannot perceive images through it. The fovea is an area dense in cone cells, and serves as the key processor of central vision. This vision provides high focus on detail e.g. in reading.
The retina lines the inside of the eyeball. The macula is the high-density photoreceptor area of the retina found towards the back of the eye. The structure of the retina, width-wise, contains several types of cell. These are interconnected one to the next towards the optic nerve side, as well as horizontally connected amongst them.
Notice the “vertical” connections from the rods and cones to bipolar cells (lime) to ganglion cells (purple/red), as well as the “horizontal” connections from amacrine cells to multiple ganglion cells, and from horizontal cells to multiple rods and cones. The rainbow beam represents the entry and direction of light.
The signals transmitted from the rods and cones travels down this pathway of multiple cells, as it is being processed before reaching the brain’s visual cortex via the optic nerve. Some of these cells, such as the ganglion and amacrine cells, are able to generate action potentials, while the other only transmit them.
They are the two main specialised eye photon receptors and differ in their distribution and sensitivity.
Rods are extremely sensitive in low light and can perceive as few as 6 photons, while cones sense more abundant light and therefore can identify different colours.
The signal starts in the rods and cones. They contain photoreceptors such as rhodopsin which chemically shift under different light environments. Rods are responsible for low-light vision and hence contain rhodopsin, which is very sensitive to light (cones contain photopsin). In daylight, it is completely photobleached and it can take half an hour for a human eye to chemically process it back to original functionality under low light (adaptation).
The photons in light initiate a response from rhodopsin that cascades down an enzyme pathway (G-coupled protein) to result in membrane depolarisation and the generation of an electrical signal.
Have a look above: action potentials are generated by polarised membranes as a result of light entering the eye and causing a chemical reaction in the respective cells.
Note the cone cell is connected to a single neurone while a few rod cells share the same neurone. Why could this be? When very little light enters the eye, it isn’t sufficient to generate an action potential per rod cell. So the cumulative light from a few rods creates a response great enough to trigger an action potential in one neurone.
The downside of this arrangement for rod cells is that visual acuity, that is the ability to tell two points apart, decreases.
Routine eye tests
Monitoring visual function is needed to manage widespread conditions such as short-sightedness, as well as to preventively assess many eye conditions that would not present symptoms before potentially causing vision loss (e.g. glaucoma and macular degeneration). Eye tests can also point to less related health conditions such as high blood pressure, diabetes, dementia and tumours.
Tests can be highly personalised and assess eye health in people to best address possible eye disease, as well as optimise vision itself through prescription glasses, contact lenses, etc.
The patient’s own input regarding their vision is used alongside tests to produce a diagnosis and best course of treatment. Visual acuity is one of the most basic traits tests, and refers to the ability to distinguish separate points. In photography, this would be equivalent to resolution.
Visual acuity can be tested via traditional methods such as the Snellen chart or modern equipment that determines the eye’s resolving ability digitally. The Snellen chart assigns visual acuity based on the ability to discern an object at a given distance, that is considered “normal” based on what someone without impaired visual acuity would see. 20/20 vision refers to the ability to see at 20 feet what one would be expected to see at 20 feet with visual acuity that was not impaired. If one can only see at 20 feet what one would be expected to be able to see at 80 feet, the vision is termed 20/80.
Modern equipment can test visual acuity in addition to other functions. One of these is colour vision. Colour vision refers to the ability to distinguish all colours from each other. It is given by the varying cone cells which process different light wavelengths associated with the different colours.
The most common forms of colour “blindness” are deuteranopia, protanopia and tritanopia. Deuteranopia results in a lack of sensitivity to green, with increased sensitivity to yellow, orange and red. Protanopia is the reverse, where there is a lack of sensitivity to red, with increased sensitivity to green, yellow and orange.
Tritanopia is a lack of differentiating between shades of blue and yellow. The genes responsible for the development of photopigments in the eye that determine colour vision are carried on the X chromosome, explaining why impaired colour vision occurs more commonly (1 in 12 as opposed to 1 in 200) in people with a single X chromosome as opposed to two.
Optical coherence tomography (OCT) is an imaging technique used in ophthalmology as well as other fields (cardiology, oncology, etc.) which uses a low-brightness (low coherence) near-infrared wavelength of light to achieve superimposition (interferometry) that provides information on biological structures such as the eye. This creates 3D images of the tissue examined and can be carried out quickly and non-invasively.
Commonly, the retina is imaged in order to see whether a patient is developing macular degeneration, diabetic macular edema or glaucoma.
Drops may be administered in order to dilate the pupil. The head is placed on a support to prevent movement, and the scan takes place without touching the eyes. It takes up to 10 minutes.
Scans can be saved from each patient so that a long-term evolution of eye health can be tracked, enabling prompt intervention should signs show up that may indicate eye problems.
Since OCT relies on light to produce the data, conditions that block light such as cataracts may render this technique unusable.
OCT can help provide insight into optic nerve fibre damage such as in the case of glaucoma, as well as detect multiple eye conditions e.g. diabetic retinopathy, macular hole, macular edema, age-related macular degeneration (AMD) and others.