Colour Vision: From 1 Different Sources
The ability to see different parts of the colour spectrum. Light perceived by the human eye consists of electromagnetic radiation (energy waves) with a spectrum of different wavelengths
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.)
See VISION – Defective colour vision.... colour blindness
See SQUINT.... double vision
Broadly speaking, vision is the ability to see.
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
Most vision tests examine a person’s sharpness of VISION (visual acuity) and often of the ?eld of vision (see VISION, FIELD OF). Refraction tests assess whether a person has an error that can be corrected with glasses such as ASTIGMATISM, HYPERMETROPIA or MYOPIA. Visual acuity is tested using a Snellen chart when the patient tries to read letters of di?ering standard sizes from 6 metres away. The optician will prescribe lenses to correct any defects detected by vision tests.... vision tests
The list of disorders resulting in poor or dim vision is huge. Disturbance of vision can result from an uncorrected refractive error, disease or injury of the cornea, iris, lens, vitreous, retina, choroid or sclera of the EYE. It may also result from disease or injury to the structures comprising the visual pathway from the retina to the occipital cortex (see VISION – Pathway of light from the eye to the brain) and from lesions of the structures around the eye – for example, swollen lids, drooping eyelids. (See EYE, DISORDERS OF.)... vision, disorders of
Refer: ALCOHOLISM, CATARACT, CONJUNCTIVITIS, DIABETES, ECLAMPSIA, GLAUCOMA, IRITIS, MIGRAINE, MULTIPLE SCLEROSIS, RETINITIS, SHOCK. ... blurred vision
Loss of the peripheral visual field to the extent that only objects straight ahead can be seen clearly. Tunnel vision is most commonly caused by chronic glaucoma. Retinitis pigmentosa is another possible cause.... tunnel vision
the ability to focus both eyes on an object at the same time, so that a person sees one image of the object he is looking at. It is not inborn, but acquired during the first few months of life. Binocular vision enables judgment of distance and perception of depth. See also stereoscopic vision.... binocular vision
see Doppler ultrasound.... colour flow ultrasound imaging
see visual field.... field of vision
an unusual phenomenon in newborn babies characterized by transient red colour changes to half of the body, well demarcated at the midline. It is seen usually 2–5 days after birth and can last from 30 seconds to 20 minutes before fading away. It may recur when the infant is placed on his or her side as the intensity of the erythema appears to be gravity-dependent.... harlequin colour change
(stereopsis) perception of the shape, depth, and distance of an object as a result of having *binocular vision. The brain receives two distinct images from the eyes, which it interprets as a single three-dimensional image.... stereoscopic vision
Any abnormality in colour vision that causes difficulty distinguishing between certain colours. Total absence of colour vision (monochromatism) is rare. The most common types of colour vision deficiency are reduced discrimination of red and green. Most cases of red and green colour vision deficiency are caused by defects in the light-sensitive cells in the retina. These defects are usually inherited, although occasionally defects are caused by retinal or optic nerve diseases or injury. The inherited defects tend to be sex-linked (see genetic disorders), which means that the majority of sufferers are male. A person with a severe green deficiency has difficulty distinguishing oranges, greens, browns, and pale reds. In severe red deficiency, all reds appear dull. A much rarer deficiency in which blue cannot be distinguished may be inherited or may be due to degeneration of the retina or optic nerve.... colour vision deficiency
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