dichromacy


Also found in: Dictionary, Thesaurus, Legal, Financial, Encyclopedia, Wikipedia.
Related to dichromacy: deuteranopia, color blindness, protanopia, Anomalous trichromacy, color deficiency

vision

 [vizh´un]
the faculty of seeing; called also sight. adj., adj vis´ual. The basic components of vision are the eye itself, the visual center in the brain, and the optic nerve, which connects the two. (See also Plate 17.)
How the Eye Works. The eye works like a camera. Light rays enter it through the adjustable iris and are focused by the lens onto the retina, a thin light-sensitive layer which corresponds to the film of the camera. The retina converts the light rays into nerve impulses, which are relayed to the visual center. There the brain interprets them as images.



Like a camera lens, the lens of the eye reverses images as it focuses them. The images on the retina are upside down and they are “flipped over” in the visual center. In a psychology experiment, a number of volunteers wore glasses that inverted everything. After 8 days, their visual centers adjusted to this new situation, and when they took off the glasses, the world looked upside down until their brain centers readjusted.

The retina is made up of millions of tiny nerve cells that contain specialized chemicals that are sensitive to light. There are two varieties of these nerve cells, rods and cones. Between them they cover the full range of the eye's adaptation to light. The cones are sensitive in bright light, and the rods in dim light. At twilight, as the light fades, the cones stop operating and the rods go into action. The momentary blindness experienced on going from bright to dim light, or from dim to bright, is the pause needed for the other set of nerve cells to take over.

The rods are spread toward the edges of the retina, so that vision in dim light is general but not very sharp or clear. The cones are clustered thickly in the center of the retina, in the fovea centralis. When the eyes are turned and focused on the object to be seen the image is brought to the central area of the retina. In very dim light, on the other hand, an object is seen more clearly if it is not looked at directly, because then its image falls on an area where the rods are thicker.
Color Vision. Color vision is a function of the cones. The most widely accepted theory is that there are three types of cones, each containing chemicals that respond to one of the three primary colors (red, green, and violet). White light stimulates all three sets of cones; any other color stimulates only one or two sets. The brain can then interpret the impulses from these cones as various colors. Man's color vision is amazingly delicate; a trained expert can distinguish among as many as 300,000 different hues.



Color vision deficiency (popularly called “color blindness”) is the result of a disorder of one or more sets of cones. The great majority of people with some degree of deficiency lack either red or green cones, and cannot distinguish between those two colors. Complete color vision deficiency (monochromatic vision), in which none of the sets of color cones works, is very rare. Most deficiencies of color vision are inherited, usually by male children through their mothers from a grandfather with the condition.
Stereoscopic Vision. Stereoscopic vision, or vision in depth, is caused by the way the eyes are placed. Each eye has a slightly different field of vision. The two images are superimposed on one another, but because of the distance between the eyes, the image from each eye goes slightly around its side of the object. From the differences between the images and from other indicators such as the position of the eye muscles when the eyes are focused on the object, the brain can determine the distance of the object.



Stereoscopic vision works best on nearby objects. As the distance increases, the difference between the left-eyed and the right-eyed views becomes less, and the brain must depend on other factors to determine distance. Among these are the relative size of the object, its color and clearness, and the receding lines of perspective. These factors may fool the eye; for example, in clear mountain air distant objects may seem to be very close. This is because their sharpness and color are not dulled by the atmosphere as much as they would be in more familiar settings.
Impaired Vision. This may consist of loss of visual acuity, visual field, ability to distinguish colors, motion of the eye, or any other function related to sight. (See also blindness.) Farsightedness, or hyperopia, results when the eyeball is shorter than normal and the image focuses behind the retina. Nearsightedness, or myopia, results when the eyeball is longer than usual from front to back, so that the image focuses in front of the retina. astigmatism is impaired vision caused by irregularities in the curvature of the cornea or lens.
Patient Care. Visually handicapped persons who are visiting a clinic for the first time or being admitted to a hospital room require orientation to their environment. Ambulatory patients can be walked around to familiarize them with the location of the bathroom and any other facility they may need to use.



Patients who are in bed following surgery or for therapeutic rest should have articles on their bedside table arranged in the same way all of the time so that they can be found easily. If only one eye is affected, articles should be placed within reach on the unaffected side and persons communicating with the patient also should stand on that side. If peripheral vision is limited, objects and persons should be positioned in the patient's line of vision.

Some patients, especially the elderly, may experience increased sensitivity to glare. Wearing sunglasses outdoors, adjusting the window blinds to deflect the sun, and using indirect lighting can help avoid discomfort. This does not mean that the patient should be in a darkened room. For most, increased illumination makes it easier to see. It is the glare that impairs their vision.

Whenever it is necessary to do something for the visually impaired person, explain beforehand what will be done. This helps reduce confusion and establishes trust in the caregiver. (For patient care, see also blindness.)

Patients with impaired vision may also benefit from such low-vision aids as convex or magnifying lenses that are hand held or mounted on a stand or clipped to the eyeglasses. Adjustable lamps, large-print reading matter, reading stands, writing guides and lined paper, and felt-tipped pens can facilitate reading and writing and improve the quality of life of a person with limited vision.

Categories of nursing diagnoses associated with impaired vision include Anxiety, Ineffective Coping Patterns, Fear of Total Blindness, Impaired Home Maintenance Management, Potential for Physical Injury, Impaired Physical Mobility, Self-Care Deficit, and Self-Imposed Social Isolation.
Top, Anatomy of the eye. Vision is the reception of images by the eye as a result of the passage of light into the eye. Light is focused by the lens on the retina, where it is converted into nerve impulses that are transmitted to the centers in the brain where images are interpreted.
achromatic vision monochromatic vision.
anomalous trichromatic vision color vision deficiency in which a person has all three cone pigments but one is deficient or anomalous; it may be either inherited as an X-linked recessive trait or acquired as a result of a retinal, cerebral, systemic, or toxic disorder.
binocular vision the use of both eyes together, without diplopia.
central vision that produced by stimulation of receptors in the fovea centralis.
color vision see vision.
day vision visual perception in the daylight or under conditions of bright illumination.
dichromatic vision color vision deficiency in which one of the three cone pigments is missing altogether. The most common forms are protanopia and deuteranopia, which are transmitted by X-linked inheritance. A third form, tritanopia, is very rare. A fourth form is also thought to exist, called tetartanopia. Called also dichromatism.
double vision diplopia.
indirect vision peripheral vision.
low vision impairment of vision such that there is significant visual handicap but also significant usable residual vision; such impairment may involve visual acuity, visual fields, or ocular motility.
monochromatic vision color vision deficiency in which the person cannot distinguish hues, so that all the colors of the spectrum appear as shades of gray. Popularly known as complete or total color blindness.
monocular vision vision with one eye.
multiple vision polyopia.
night vision visual perception in the darkness of night or under conditions of reduced illumination.
oscillating vision oscillopsia.
peripheral vision that produced by stimulation of receptors in the retina outside the macula lutea; called also indirect vision.
vision therapy technician an allied health professional who evaluates clients and plans and implements vision therapy programs under the supervision of an optometrist
trichromatic vision
1. any ability to see all three primary colors of light (red, green, and blue).
2. normal color vision; called also trichromacy and trichromatism.
tunnel vision
1. that in which the visual field is severely constricted. When it is due to organic causes, such as retinitis pigmentosa or glaucoma, the visual field expands as it is tested at increasing distance from a constant object but when it is due to psychogenic disorders, such as conversion disorder or malingering, the field remains constant or contracts as the distance increases.
2. in psychiatry, restriction of psychological or emotional perception to a limited range.

dichromacy

/di·chro·ma·cy/ (di-kro´mah-se) dichromatic vision
Mentioned in ?