Broadly speaking, vision is the ability to see.
Light enters the EYE by passing through the transparent cornea, then through the aqueous humour filling the anterior chamber. It then passes through the pupil, through the lens and the vitreous humour to reach the retina. In the retina, the rod and cone photoreceptors detect light and relay messages in the form of electrochemical impulses through the various layers of the retina to the nerve fibres. The nerve fibres carry messages via the optic nerve, optic chiasma, optic tract, lateral geniculate body and finally the optic radiations to the visual cortex. Here in the visual cortex these messages are interpreted. It is therefore the visual cortex of the BRAIN that ‘sees’.
Two points will not be seen as two unless they are separated by a minimum distance. This distance is such that the objects are so far apart that the lines joining them to the eye enclose between them (subtend) an angle of at least one minute of a degree. This amount of separation allows the images of the two points to fall on two separate cones (if the light from two points falls on one cone, the two points would be seen as a single point). There are many tests of visual acuity. One of the more common is the Snellen test type. This is made up of many letters of different size. By conventions the chart is placed 6 metres away from the patient. Someone able to see the lowest line at this distance has a visual acuity of 6/4. If they are only able to see the top letter they have 6/60 vision. ‘Normal’ vision is 6/6 (or 20/20 in those countries not using the metric system).
‘White light’ is made up of component colours. These can be separated by a prism, thereby producing a spectrum. The three cardinal colours are red, green, and blue; all other colours can be produced by a varying mixture of these three. Colour vision is a complex subject. The trichromatic theory of colour vision suggests that there are three types of cones, each type sensitive to one of the cardinal colours. Colour perception is based on differential stimulation of these cone types. The opponent colour theory suggests that each cone type can generate signals of the opposite kind. Output from some cones can collaborate with the output from others or can inhibit their action. Colour perception results from these various complex interactions.
Defective colour vision may be hereditary or acquired, and can occur in the presence of normal visual acuity.
is more common in men (7 per cent of males) than women (0.5 per cent of females). Men are affected, but women convey the abnormal gene (see GENES) to their children. It occurs because one or more of the photopigments of the retina are abnormal, or the cones are damaged. Red-green colour defect is the most common.
is the result of disease of the cones or their connections in the retina, optic nerve or brain – for example, macular disease, optic neuritis. Colour vision can be impaired, but not lost, as a result of corneal opacification or cataract formation (see under EYE, DISORDERS OF).
These use specially designed plates displaying numbers made of coloured dots surrounded by dots of confusing colour.