Pathway of light from the eye to the brain Light enters the EYE by passing through the transparent cornea, then through the aqueous humour ?lling the anterior chamber. It then passes through the pupil, through the lens and the vitreous 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 ?bres. The nerve ?bres carry messages via the optic nerve, optic chiasma, optic tract, lateral geniculate body and ?nally 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’.
Visual acuity 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 di?erent 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.
Colour vision ‘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 trichromat 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 di?erential 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 the action of other cones. 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. HEREDITARY DEFECTIVE COLOUR VISION 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. ACQUIRED DEFECTIVE COLOUR VISION is the result of disease of the cones or their connections in the retina, optic nerve or brain – for example, macular disease, optic neuritis. Col-our vision can be impaired but not lost as a result of corneal opaci?cation or cataract formation (see under EYE, DISORDERS OF).
TESTS OF COLOUR VISION These use specially designed numbers made of coloured dots surrounded by dots of confusing colour (e.g. plates).... vision
Defects in the visual ?eld (scotomas) can be produced by a variety of disorders. Certain of these produce speci?c ?eld defects. For example, GLAUCOMA, some types of brain damage and some TOXINS can produce speci?c defects in the visual ?eld. This type of ?eld defect may be very useful in diagnosing a particular disorder. The blind spot is that part of the visual ?eld corresponding to the optic disc. There are no rods nor cones on the optic disc and therefore no light perception from this area. The blind spot can be found temporal (i.e. on the outer side) of the ?xation point. (See also EYE.)... vision, field of
between about 400 and 700 nanometres. Different wavelengths produce sensations of violet, indigo, blue, green, yellow, orange, and red when they fall on the retina and stimulate nerve signals, which are processed in the brain.
As light falls on the retina, it strikes light-sensitive cells called rods and cones. The rods can detect all visible light, but only the cones can distinguish colour. There are 3 types of cones: red-sensitive, blue-sensitive, and green-sensitive. Each responds more strongly to a particular part of the light spectrum. Because the cones are most concentrated in a central area of the retina called the fovea, colour vision is most accurate for objects viewed directly and is poor at the edges of vision. When light hits a cone, it causes the cone to emit an electrical signal, which passes to the brain via the optic nerve. Colour perception requires a minimum level of light, below which everything is seen as shades of grey. (See also colour vision deficiency; eye; perception; vision.)... colour vision
Progressive loss of visual clarity is common with advancing age and may be due to a number of disorders (see vision, disorders of).
Sudden loss of vision may be caused by disorders such as hyphaema, severe uveitis, vitreous haemorrhage, or retinal haemorrhage.
Optic neuritis can reduce vision in 1 eye.
Damage to the nerve connections between the eyes and brain, or to the visual area of the brain, can cause loss of peripheral vision and may be a result of embolism, ischaemia, tumour, inflammation, or injury.... vision, loss of
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