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Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. Vision has a specific sensory system.
There is disagreement as to whether or not this constitutes one, two or even three distinct senses. Some people make a distinction between "black and white" vision and the perception of
color, and others point out that vision using rod cells uses different physical detectors on the retina from cone cells. Some argue that the perception of depth also constitutes a sense, but others argue that this is really cognition (that is, post-sensory) function derived from having stereoscopic vision (two eyes) and is not a sensory perception as such.
The eye is the light-sensitive organ that is the first component of the sensory system. The eye's retina performs the first stages of visual perception processing, with the remaining stages of visual perception occurring in the optic nerve and the visual cortex of the brain.
Focusing
Light from a single point of a distant object and light from a single point of a near object being brought to a focus.In order for light rays to be brought to a focus they must be refracted. The amount of refraction required depends on the distance of the object which is being viewed. A distant object will require less bending of light than a nearer one. Most of the refraction occurs at the cornea which has a fixed curvature. The remainder of the required refraction occurs at the lens. The lens can be pulled flatter or rounder by muscles, which adjust the power of the lens. As we age we lose this ability to adjust the focus. Such a condition is known as presbyopia. There are other refraction errors arising from the shape of the cornea and lens, and from the length of the eyeball. These include myopia, hyperopia, and astigmatism.
Presbyopia is a condition that occurs with growing age and results in the inability of the human eye to focus on objects up close. Presbyopia is most likely the result of a decrease in the flexibility of the lens of the eye, changes in its curvature from continual growth, and loss of power of the ciliary muscles (the muscles that bend and straighten the lens).
Presbyopia is not a disease as such, but a condition that affects everyone at a certain age. The first symptoms are usually noticed at the age of 40-50. It can start with difficulty reading fine print, particularly if the lighting is poor, or with eyestrain when reading for long periods. More advanced presbyopes may complain that their arms have become "too short" to hold reading material at a comfortable distance.
Myopia, also known as nearsightedness or short-sightedness, is a refractive defect of the eye, where the person affected usually can see nearby objects clearly but distant objects appear blurred. The opposite of myopia is farsightedness or hyperopia.
Myopia is the most common eyesight problem in the world. About one quarter of the adult population in the United States has myopia. In places like Japan, Singapore and Taiwan, as many as one in three or one in two of the adult population is myopic.
Myopia is measured in diopters; specifically, the strength of the corrective lens that must be used to enable the eye to focus distant images correctly on the retina. Myopia of 6.00 diopters or greater is considered high, or severe, myopia. People with high myopia are at greater risk of more acute eye problems such as retinal detachment or glaucoma. They are also more likely to experience floaters.
The most widely held theory of the cause of myopia is that it is mainly hereditary. Measures of the heritability of myopia have yielded figures as high as 89%, and recent research has identified genes that may be responsible: defective versions of the PAX6 gene seem to be associated with myopia in twin studies. Under this theory, the eye is slightly elongated front to back as a result of faults during development, causing images to be focused in front of the retina rather than directly on it. It is usually discovered during the pre-teen years between eight and twelve years of age. It most often worsens gradually as the eye grows during adolescence and then levels off as a person reaches adulthood.
Mainstream ophthalmologists most commonly correct myopia through the use of corrective lenses, such as spectacles or contact lenses. It may also be corrected by refractive surgery, such as LASIK. The corrective lenses have a negative dioptric value (i.e. are concave) which compensates for the excessive positive diopters of the myopic eye.
Another theory is that myopia is caused by a weakening of the ciliary muscle which controls the eye's lens. The weak muscle is unable to adjust the lens enough to see far distances, causing far-off things to be blurred. This theory states that the muscle's weakness is usually caused by doing lots of "nearwork", like reading books or using a computer screen. Since the eye rarely has to focus on far distances, the muscle is rarely used and, as a result, becomes weak. Since corrective lenses do the ciliary muscle's work for it, proponents of this theory suggest that they make it even weaker, increasing the problem. Instead, they recommend a variety of eye exercises to strengthen the muscle. A problem with this theory is that mainstream ophthalmology and medicine hold that the ciliary muscle is used when focussing at close distances, and is relaxed when accommodating for distant vision. Other theories suggest that the eyes become strained by the constant extra work involved in "nearwork" and get stuck in the near position, and eye exercises can help loosen the muscles up thereby freeing it for far vision.
Regardless of the accuracy of the ciliary muscle theory, it should be understood that even a high heritability of myopia (as for any other condition) does not mean that environmental factors and lifestyle have no effect on the development of the condition. High heritability simply means that most of the variation in a particular population at a particular time is due to genetic differences. If the environment changes - as, for example, it has by the introduction of televisions and computers - the incidence of myopia can change as a result, even though heritability remains high.
One 2002 study (http://news.bbc.co.uk/1/hi/health/1909075.stm) suggested that myopia may be caused by over-consumption of bread in childhood. So far, this has not been supported by further research results.
Hyperopia (or more rarely, hypermetropia), also known as farsightedness or longsightedness, is a defect of vision caused by an imperfection in the eye (often when the eyeball is too short), causing inability to focus on near objects, and in extreme cases causing a sufferer to be unable to focus on objects at any distance. As an object moves towards the eye, the eye must increase its power to keep the image on the retina. If the power of the cornea and lens is insufficient, as in hypermetropia, the image will appear blurred. It is corrected by using eyeglasses with lenses of positive curvature (magnifying glasses).
Astigmatism is a refraction error of the eye characterized by an aspherical cornea in which one axis of corneal steepness is greater than the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can be often corrected by glasses with a cylindrical lens or by toric contact lenses.
Astigmatism occurs when either the cornea or the lens of the eye is not perfectly spherical. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retina in the horizontal plane, but in front of the retina in the vertical plane. Once diagnosed, astigmatism is corrected by wearing glasses with lenses having different radii of curvature in different planes. They are commonly called cylindrical lenses.
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This article is licensed under the GNU Free Documentation License.
It uses material from the Wikipedia article "Eye"
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