vision

vision,

physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived.

Vision in Humans

The human eyeeye,
organ of vision and light perception. In humans the eye is of the camera type, with an iris diaphragm and variable focusing, or accommodation. Other types of eye are the simple eye, found in many invertebrates, and the compound eye, found in insects and many other
..... Click the link for more information. functions somewhat like a camera; that is, it receives and focuses light upon a photosensitive receiver, the retina. The light rays are bent and brought to focus as they pass through the cornea and the lens. The shape of the lens can be changed by the action of the ciliary muscles so that clear images of objects at different distances and of moving objects are formed on the retina. This ability to focus objects at varying distances is known as accommodation.

The Role of the Retina

The retina—the embryonic outgrowth of the brain—is a very complex tissue. Its most important elements are its many light-sensitive nerve cells, the rods and cones. The cones secrete the pigment iodopsin and are most effective in bright light; they alone provide color vision. The rods, which secrete a substance called visual purple, or rhodopsin, provide vision in dim light or semidarkness; since rods do not provide color vision, objects in such light appear in shades of gray.

Light rays brought to focus on the rods and cones produce a chemical reaction in those cells, in which the two pigments are broken down to form a protein and a vitamin A compound. This chemical process stimulates an electrical impulse that is sent to the brain. The structural change of pigment is normally balanced by the formation of new pigment through the recombination of the protein and vitamin A compound; thus vision is uninterrupted.

The division of function between rods and cones is a result of the different sensitivity of their pigments to light. The iodopsin of cone cells is less sensitive than rhodopsin, and therefore is not activated by weak light, while in bright light the highly sensitive rhodopsin of rod cells breaks down so rapidly that it soon becomes inactive. There is a depression near the center of the retina called the fovea that contains only cone cells. It provides the keenest possible vision when an object is viewed directly in bright light. In dim light objects must be viewed somewhat to one side so the light rays fall on the area of the retina that contains rod cells.

The Role of the Optic Nerve and Brain

The nerve impulses from the rods and cones are transmitted by nerve fibers across the retina to an area where the fibers converge and form the optic nerve. The area where the optic nerve passes through the retina is devoid of rods and cones and is known as the blind spot. The optic nerve from the left eye and that from the right eye meet at a point called the optic chiasma. There each nerve separates into two branches. The inner branch from each eye crosses over and joins the outer branch from the other eye. Two optic tracts exit thereby from the chiasma, transferring the impulses from the left side of each eye to the left visual center in the cerebral cortex (see brainbrain,
the supervisory center of the nervous system in all vertebrates. It also serves as the site of emotions, memory, self-awareness, and thought. Anatomy and Function
..... Click the link for more information. ) and the impulses from the right half of each eye to the right cerebral cortex. The brain then fuses the two separate images to form a single image. The image formed on the retina is an inverted one, because the light rays entering the eye are refracted and cross each other. However, the mental image as interpreted by the brain is right side up. How the brain corrects the inverted image to produce normal vision is unknown, but the ability is thought to be acquired early in life, with the aid of the other senses.

Color and Stereoscopic Vision

Color vision is based on the ability to discriminate between the various wavelengths that constitute the spectrum. The Young-Helmholtz theory, developed in 1802 by Thomas Young and H. L. F. Helmholtz, is based on the assumption that there are three fundamental color sensations—red, green, and blue—and that there are three different groups of cones in the retina, each group particularly sensitive to one of these three colors. Light from a red object, for example, stimulates the cones that are more sensitive to red than the other cones. Other colors (besides red, green, and blue) are seen when the cone cells are stimulated in different combinations. Only in recent years has conclusive evidence shown that the Young-Helmholtz theory is, indeed, accurate. The sensation of white is produced by the combination of the three primary colors, and black results from the absence of stimulation.

Humans normally have binocular vision, i.e., separate images of the visual field are formed by each eye; the two images fuse to form a single impression. Because each eye forms its own image from a slightly different angle, a stereoscopic effect is obtained, and depth, distance, and solidity of an object are appreciated. Stereoscopic color vision is found primarily among the higher primates, and it developed fairly late on the evolutionary scale.

Defects of Vision

Defects of vision include astigmatismastigmatism
, type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina.
..... Click the link for more information. , color blindnesscolor blindness,
visual defect resulting in the inability to distinguish colors. About 8% of men and 0.5% of women experience some difficulty in color perception. Color blindness is usually an inherited sex-linked characteristic, transmitted through, but recessive in, females.
..... Click the link for more information. , farsightednessfarsightedness
or hyperopia,
condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is
..... Click the link for more information. , and nearsightednessnearsightedness
or myopia,
defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it.
..... Click the link for more information. . The absence of rods causes a condition known as night blindness; an absence of cones constitutes legal blindness.

Bibliography

See A. Hughes, The Visual System in the Evolution of Vertebrates (1977); G. S. Wasserman, Color Vision: An Historical Introduction (1978); M. Fineman, The Inquisitive Eye (1981); D. H. Hubel, Eye, Brain, and Vision (1988).

Vision

The sense of sight, which perceives the form, color, size, movement, and distance of objects. Of all the senses, vision provides the most detailed and extensive information about the environment. In the higher animals, especially the birds and primates, the eyes and the visual areas of the central nervous system have developed a size and complexity far beyond the other sensory systems.

Visual stimuli are typically rays of light entering the eyes and forming images on the retina at the back of the eyeball (Fig. 1). Human vision is most sensitive for light comprising the visible spectrum in the range 380–720 nanometers in wavelength. In general, light stimuli can be measured by physical means with respect to their energy, dominant wavelength, and spectral purity. These three physical aspects of the light are closely related to the perceived brightness, hue, and saturation, respectively.

Diagram showing the eyes and visual projection system

Anatomical basis for vision

The anatomical structures involved in vision include the eyes, optic nerves and tracts, optic thalamus, primary visual cortex, and higher visual areas of the brain. The eyes are motor organs as well as sensory; that is, each eye can turn directly toward an object to inspect it. The two eyes are coordinated in their inspection of objects, and they are able to converge for near objects and diverge for far ones. Each eye can also regulate the shape of its crystalline lens to focus the rays from the object and to form a sharp image on the retina. Furthermore, the eyes can regulate the amount of light reaching the sensitive cells on the retina by contracting and expanding the pupil of the iris. These motor responses of the eyes are examples of involuntary action that is controlled by various reflex pathways within the brain. See Eye (vertebrate)

The process of seeing begins when light passes through the eye and is absorbed by the photoreceptors of the retina. These cells are activated by the light in such a way that electrical potentials are generated. These potentials serve to generate nerve responses in various successive neural cells in the vicinity of excitation. Impulses emerge from the eye in the form of repetitive discharges in the fibers of the optic nerve, which do not mirror exactly the excitation of the photoreceptors by light. Complex interactions within the retina serve to enhance certain responses and to suppress others. Furthermore, each eye contains more than a hundred times as many photoreceptors as optic nerve fibers. Thus it would appear that much of the integrative action of the visual system has already occurred within the retina before the brain has had a chance to act.

The optic nerves from the two eyes traverse the optic chiasma. Figure 1 shows that the fibers from the inner (nasal) half of each retina cross over to the opposite side, while those from the outer (temporal) half do not cross over but remain on the same side. The effect of this arrangement is that the right visual field, which stimulates the left half of each retina, activates the left half of the thalamus and visual cortex. Conversely the left visual field affects the right half of the brain. This situation is therefore similar to that of other sensory and motor projection systems in which the left side of the body is represented by the right side of the brain and vice versa.

The visual cortex includes a projection area in the occipital lobe of each hemisphere. Here there appears to be a point-for-point correspondence between the retina of each eye and the cortex. Thus the cortex contains a “map” or projection area, each point of which represents a point in visual space as seen by each eye. Other important features of an object such as its color, motion, orientation, and shape are simultaneously perceived. The two retinal maps are merged to form the cortical projection area. This allows the separate images from the two eyes to interact with each other in stereoscopic vision, binocular color mixture, and other phenomena. In addition to the projection areas on the right and left halves of the cortex, there are visual association areas and other brain regions that are involved in vision. Complex visual acts, such as form recognition, movement perception, and reading, are believed to depend on widespread cortical activity beyond that of the projection areas. See Brain

Scotopic and photopic vision

Night animals have eyes that are specialized for seeing with a minimum of light. This type of vision is called scotopic. Day animals have predominantly photopic vision. They require much more light for seeing, but their daytime vision is specialized for quick and accurate perception of fine details of color, form, and texture, and location of objects. Color vision, when it is present, is also a property of the photopic system. Human vision is duplex; humans are in the fortunate position of having both photopic and scotopic vision. Some of the chief characteristics of human scotopic and photopic vision are enumerated in the table.

Scotopic vision occurs when the rod receptors of the eye are stimulated by light. The outer limbs of the rods contain a photosensitive substance known as visual purple or rhodopsin. This substance is bleached away by the action of strong light so that the scotopic system is virtually blind in the daytime. In darkness, however, the rhodopsin

Characteristics of human vision
Characteristic	Scotopic vision	Photopic vision
Photochemical substance	Rhodopsin	Cone pigments
Receptor cells	Rods	Cones
Speed of adaptation	Slow (30 min or more)	Rapid (8 min or less)
Color discrimination	No	Yes
Region of retina	Periphery	Center
Spatial summation	Much	Little
Visual acuity	Low	High
Number of receptors per eye	120,000,000	7,000,000
Cortical representation	Small	Large
Spectral sensitivity peak	505 nm	555 nm

is regenerated by restorative reactions based on the transport of vitamin A to the retina by the blood. One experiences a temporary blindness upon walking indoors on a bright day, especially into a dark room. As the eyes become accustomed to the dim light the scotopic system gradually begins to function. This process is known as dark adaptation. Complete dark adaptation is a slow process during which the rhodopsin is restored in the rods. A 10,000-fold increase in sensitivity of is often found to occur during a half-hour period of dark adaptation. By this time some of the rod receptors are so sensitive that only one photon is necessary to trigger each rod into action. Faulty dark adaptation or night blindness is found in persons who lack rod receptors or have a dietary deficiency in vitamin A. This scotopic vision is colorless or achromatic.

Normal photopic vision has the characteristics enumerated in the table. Emphasis is placed on the fovea centralis, a small region at the very center of the retina of each eye.

Foveal vision is achieved by looking directly at objects in the daytime. The image of an object falls within a region almost exclusively populated by cone receptors, closely packed together in the central fovea, each of which is provided with a series of specialized nerve cells that process the incoming pattern of stimulation and convey it to the cortical projection area. In this way the cortex is supplied with superbly detailed information about any pattern of light that falls within the fovea centralis.

Peripheral vision takes place outside the fovea centralis. Vision extends out to more than 90^° from center, so that one can detect moving objects approaching from either side. This extreme peripheral vision is comparable to night vision in that it is devoid of sharpness and color.

There is a simple anatomical explanation for the clarity of foveal vision as compared with peripheral vision. The cones become less and less numerous in the retinal zones that are more and more remote from the fovea. In the extreme periphery there are scarcely any, and even the rods are more sparsely distributed. Furthermore, the plentiful neural connections from the foveal cones are replaced in the periphery by network connections in which hundreds of receptors may activate a single optic nerve fiber. This mass action is favorable for the detection of large or dim stimuli in the periphery or at night, but it is unfavorable for visual acuity (the ability to see fine details of an object) or color vision, both of which require the brain to differentiate between signals arriving from closely adjacent cone receptors.

Vernier and stereoscopic discriminations of space

Space and time perception

Vernier and stereoscopic discrimination are elementary forms of space perception. Here, the eye is required to judge the relative position of one object in relation to another (Fig. 2). The left eye, for example, sees the lower line as displaced slightly to the right of the upper. This is known as vernier discrimination. The eye is able to distinguish fantastically small displacements of this kind, a few seconds of arc under favorable conditions. If the right eye is presented with similar lines that are oppositely displaced, then the images for the two eyes appear fused into one and the subject sees the lower line as nearer than the upper. This is the principle of the stereoscope. Again it is true that displacements of a few seconds of arc are clearly seen, this time as changes in distance. The distance judgment is made not at the level of the retina but at the cortex where the spatial patterns from the separate eyes are fused together. The fineness of vernier and stereoscopic discrimination transcends that of the retinal mosaic and suggests that some averaging mechanism must be operating in space or time or both.

The spatial aspects of the visual field are also of interest. Good acuity is restricted to a narrowly defined region at the center of the visual field. Farther out, in the peripheral regions, area and intensity are reciprocally related for all small sizes of stimulus field. A stimulus patch of unit area, for example, looks the same as a patch of twice the same area and half the luminance. This high degree of areal summation is achieved by the convergence of hundreds of rod receptors upon each optic nerve fiber. It is the basis for the ability of the dark-adapted eye to detect large objects even on a dark night.

In daytime vision, spatial inhibition, rather than summation, is most noticeable. The phenomenon of simultaneous contrast is present at a border between fields of different color or luminance. This has the effect of heightening contours and making forms more noticeable against their background.

The temporal characteristics of vision are revealed by studying the responses of the eye to various temporal patterns of stimulation. When a light is first turned on, there is a vigorous burst of nerve impulses that travel from the eye to the brain. Continued illumination results in fewer and fewer impulses as the eye adapts itself to the given level of illumination. Turning the light off elicits another strong neural response. The strength of a visual stimulus depends upon its duration as well as its intensity. Below a certain critical duration, the product of duration and intensity is found to be constant for threshold stimulation. A flash of light lasting only a few milliseconds may stimulate the eye quite strongly, providing its luminance is sufficiently high. A light of twice of the original duration will be as detectable as the first if it is given half the original luminance.

Voluntary eye movements enable the eyes to roam over the surface of an object of inspection. In reading, for example, the eyes typically make four to seven fixational pauses along each line of print, with short jerky motions between pauses. An individual's vision typically takes place during the pauses, so that one's awareness of the whole object is the result of integrating these separate impressions over time.

A flickering light is one that is going on and off (or undergoing lesser changes in intensity) as a function of time. At a sufficiently high flash rate (called the critical frequency of fusion, cff) the eye fails to detect the flicker, and the light pulses seem to fuse to form a steady light that cannot be distinguished from a continuous light that has the same total energy per unit of time. As the flash rate is reduced below the cff, flicker becomes noticeable, and at very low rates the light may appear more conspicuous than flashes occurring at higher frequency. The cff is often used clinically to indicate a person's visual function as influenced by drugs, fatigue, or disease. See Color vision, Perception

Vision

(pop culture)

According to Abraham Van Helsing, the voice of authority on vampires in Dracula (1897), the vampire can see in the dark. Although this is not mentioned in the folk literature, it was a logical conclusion because vampires were nocturnal creatures who moved about freely in the darkness of the evening hours. Some of the vampire’s attributes were derived from its association with the bat. Bats, for instance, have a radar system that makes them extremely well-adapted night creatures. Dracula was pictured as regularly leaving his castle each evening to feed and return with food for his vampire brides. He also used his acute sight in his attacks on Lucy Westenra and Mina Murray. Modern vampire writers have cited night vision as one of the positive characteristics of the vampiric existence, frequently mentioned as allowing vampires to feel natural and at home in the nocturnal world. Night vision counterbalanced the blinding effect of direct sunlight.

Vision

perception by an organism of the external world— that is, the reception of information about it by special organs of vision. These organs intercept light that is reflected from or radiated by objects.

The visual apparatus includes a peripheral part located in the eye (the retina, which has photoreceptors and nerve cells) and the central parts connected to the peripheral part (some areas of the mesencephalon and dieocephalon and also the visual area of the cerebral cortex). Vision makes possible the organization of appropriate behavior based on analysis of external conditions. Through vision the organism receives information on the direction of certain light beams, their intensity, and the like. Light is absorbed by the eye’s photoreceptors, which have a visual pigment that converts the energy of light quanta into nerve signals; the range of light perceived depends on the absorption spectrum of the pigments. Man perceives electromagnetic rays in the wavelength range of 400–700 nanometers (nm), some insects discern ultraviolet rays (up to 300 nm), and some lizards perceive infrared light. In the process of animal evolution vision has undergone a complex development: from the ability to distinguish only the degree of illumination (earthworm) or the direction of the light source (snail) to diverse forms of image analysis. Uniquely arranged are the compound eyes of crustaceans and insects, which produce a “mosaic” image and are adapted for discerning the shapes of close objects. The eyes of a number of invertebrates are capable of distinguishing the plane of polarization of light.

The vertebrate eye has an optical system that refracts light: a cornea, crystalline lens, vitreous body, and iris with pupil. By means of a special muscle the curvature of the crystalline lens, and consequently its refractive power, changes (eye accommodation); this ensures the sharpness of the image on the fundus oculi. The interior surface of the eyeball is occupied by the light-sensitive part of the eye, the retina. Behind the photoreceptors—the rod and cone cells—is a system of several levels of nerve cells that analyze the signals received from the photoreceptors. The nerve cells of the retina generate bioelectric potentials, which can be recorded in the form of electroretinograms. Analysis of the electrical activity of the retina and its separate elements is one of the most important ways of studying its function and condition. The area of the retina with the highest discrimination in man, the yellow spot (macula lutea) and especially its central depression (the fovea), the density of whose receptors (cones) reaches 1.8 x 105 per sq mm, ensures a high degree of spatial resolution of the eye, or visual acuity. (In man under optimal illumination it averages one angular minute.) On the periphery of the retina rods predominate, large groups of which are each connected to one nerve cell; visual acuity is significantly lower here. Consequently, the periphery of the field of vision serves for general orientation, while the center is for detailed examination. In addition to man and apes, birds also have a fovea (some have two in each eye).

Cones with three different curves of spectral sensitivity have been discovered in man, apes, and fish; the maximums of the curves of spectral sensitivity in man are in the violet, green, and yellow bands of the spectrum. According to the Young-Helmholtz theory, the three-dimensionality of color vision is explained by the fact that light of different spectral composition produces reactions of different intensities in the three types of cone; it is this that leads to the perception of one or another color. When there is intense stimulation of all the photoreceptors, one may perceive the color white. Three-dimensional or two-dimensional color vision is characteristic of many vertebrates and some insects. An important property of vision is%physiological adaptation, adaptation to functioning in sharply changing conditions of illumination. This ensures the maintenance of a high degree of contrast sensitivity of the eye—that is, the eye’s ability to perceive small differences in brightness (in man, 1 percent) within a broad range of illumination. A number of adaptive mechanisms are known: change in pupillary diameter (dilation and constriction), retinomotor effect (screening of receptors with grains of opaque pigment), breaking down and resynthesizing of visual pigment in the rods, and reorganization in the nerve structures of the retina. In twilight only the more sensitive rod system functions (hence, color vision is absent and visual acuity is decreased); in daylight the cone and rod systems both function. In the retinas of nocturnal animals rods dominate; in diurnal animals the retina is either mixed or cones dominate. Visual organs of various animals differ in their postexcitation refractory period and hence the resolving power in time. Thus, the frog perceives flashes of frequencies up to 15–20 hertz (Hz), man up to 50–60 Hz (in bright illumination), and some insects (for example, flies) up to 250–300 Hz.

One may distinguish monocular and binocular vision. In monocular vision, one eye functions; in binocular vision, the visual fields of the two eyes partially overlap. Owing to the difference in the angles at which the same object is observed by both eyes, binocularity leads to stereoscopic perception, which is one way of judging size and distance. Eye movements, which are accomplished by the eye muscles controlled by the mesencephalon, play an important role in vision, especially in higher vertebrates. Movements may be voluntary or involuntary. The latter are divided into three types: slow drift, high-frequency tremor (80 Hz), and saccadic movement. Objects whose image is immobile in relation to the retina are not perceived by man; hence vision is practically impossible without eye movements.

Signals from the eye travel through the optic nerve along two principal pathways: into the mesencephalon, which in fish and amphibians serves as the higher level since their prosencephalon is poorly developed, and into the prosencephalon (through the lateral geniculate body into the occipital area of the cerebral cortex), which in mammals is very highly developed. The processing and analysis of visual signals is accomplished on all levels of the visual system, including the retina. In various animals fibers of the optic nerve (“detectors”) have been discovered that transmit to the brain such signals as movement, direction of movement, and the presence in the visual field of a dark spot or horizontal strip. The signals of the retinal detectors are probably used in the mesencephalon to organize simple automated reactions characteristic of lower and, to some extent, higher vertebrates (for example, movements of the eyes and head when in danger or while watching a moving object). Analysis performed in the cerebral cortex is considerably more diverse and subtle. A property of vision that is essential for analysis is its constancy, owing to which the characteristics of objects (their color, dimensions, shape) are perceived as constant, regardless of fluctuations in the intensity and spectral composition of the illumination, the distance to the object, the angle of vision, and the like.

REFERENCES

Kravkov, S. V. Glaz i ego rabota. Moscow-Leningrad, 1950.
Glezer, V. D., and I. I. Tsukerman. Informatsiia i zrenie. Moscow-Leningrad, 1961.
larbus, A. L. Rol’ dvizhenii glaz v protsesse zreniia. Moscow, 1965.
Byzov, A. L. Elektrofiziologicheskie issledovaniia setchatki. Moscow, 1966.
Mazokhin-Porshniakov, G. A. Zrenie nasekomykh. Moscow, 1965.
Gregory, R. L. Glaz i mozg: Psikhologiia zritel’nogo vospriiatiia. >Moscow, 1970. (Translated from English.)
Cornsweet, T. N. Visual Perception. New York-London [1970].

A. L. BYZOV

What does it mean when you dream about vision?

Vision is a common metaphor for insightfulness, perceptiveness, and point of view. Any of these meanings could be indicated in a dream emphasizing vision. To experience an obstruction to one’s vision could indicate that the dreamer is having difficulty perceiving such things as errors in judgment, or how significant and important their role is in the world. The term “vision” can also be used in the sense of an apparition, which in a dream could indicate that a spiritual message is being given to the dreamer.

vision

[′vizh·ən] (physiology) The sense which perceives the form, color, size, movement, and distance of objects. Also known as sight.

vision

[vizh´un] the faculty of seeing; called also sight. adj., adj vis´ual. The basic components of vision are the eye" >eye itself, the visual center in the brain, and the optic nerve" >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" >rods and cones" >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 (vision" >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 blindness" >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 optometristtrichromatic vision 1. any ability to see all three primary colors of light (red, green, and blue).2. normal vision" >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.

vis·ion

(vizh'ŭn), The act of seeing.
See also: sight. [L. visio, fr. video, pp. visus, to see]

vision

(vĭzh′ən)n.1. The faculty of sight; eyesight.2. The manner in which one sees or conceives of something.

vision

Vox populi 1. The act of seeing.2. Visual acuity. See Binocular vision, Computer vision, Tunnel vision.

vis·ion

(vizh'ŭn) The act of seeing.
See also: sight[L. visio, fr. video, pp. visus, to see]

vision (V)

1. The appreciation of differences in the external world, such as form, colour, position, etc. resulting from the stimulation of the retina by light. 2. See unaided visual acuity.
achromatic vision See achromatopsia.
alternating vision See contact lens.
ambient vision Vision mediated primarily by the peripheral retina and involved in spatial orientation and recognition of motion. See focal vision.
anomalous trichromatic vision See anomalous trichromatism.
binocular vision (BV) Condition in which both eyes contribute towards producing a percept which may or may not be fused into a single impression. See sensory fusion; monoblepsia; critical period; bar reading test; FRIEND test; hole in the hand test; Worth's four dot test; Worth's classification of binocular vision; binocular vision single zone of clear.
binocular single vision See single binocular vision.
blue vision See chromatopsia.
blurred vision Vision characterized by poor visual acuity or in which the edges of objects are indistinct. It may be due to uncorrected or poorly corrected ametropia or presbyopia, anomalies of the ocular media (e.g. cataract, corneal opacity, haemorrhage in the vitreous), amblyopia, excess lacrimation, spasm of accommodation, optic neuritis, angle-closure glaucoma, diabetes, multiple sclerosis, migraine, etc.
central vision Vision of objects formed on the foveola or the macula. See sensory fusion.
chromatic vision See colour vision.
colour vision (CV) Vision in which the colour sense is experienced. Syn. chromatic vision. See Hering's of colour vision theory; Young-Helmholtz theory.
daylight vision See photopic vision.
defective colour vision See defective colour vision.
deuteranomalous vision See deuteranomaly.
dichromatic vision See dichromatism.
distance vision (DV) Vision of objects situated either at infinity or more usually at some 5 or 6 m. See Snellen chart; near vision.
diurnal vision See photopic vision.
double v . See diplopia.
eccentric v . See eccentric fixation; peripheral vision.
entoptic v . See entoptic image.
extrafoveal vision See peripheral vision.
field of vision See visual field.
focal vision Vision mediated by, primarily, the macular area of the retina and involved in the examination and identification of objects. See ambient vision.
green v . See chromatopsia.
gun barrel v . See tunnel vision.
haploscopic vision Vision as obtained by looking in a haploscope.
indirect vision See peripheral vision.
industrial vision The branch of optometry concerned with vision and perception by the individual at work, the evaluation of visual performance in a given occupation, the prescribing of protective ocular devices and the determination of the optimum environment (e.g. illumination) to accomplish a visual task efficiently.
intermediate vision Vision of objects situated beyond 40 cm from the eye but closer than, say, 1.5 m. See distance vision; near vision.
island of vision A description of the visual field as a three-dimensional hill surrounded by a sea of darkness. Stimuli that fall within the island can be seen, whereas stimuli that fall outside the island cannot be seen. The height of the island represents the sensitivity of the eye, with the highest acuity at the top of the hill corresponding to foveal vision and declining progressively towards the periphery (when the eye is light-adapted). See visual field.
low vision Vision impairment even after correction by conventional lenses, resulting from either congenital anomalies or ocular diseases such as cataract, glaucoma, age-related macular degeneration, pathological myopia, trachoma, onchocerciasis, etc. The correction and rehabilitation of patients with low vision is achieved by special aids called low vision aids (LVA) such as a telescopic lens, and appropriate counselling (e.g. about illumination and reading distance). The criteria that the health authorities normally use to classify a person as having partial sight take into consideration not only the corrected visual acuity but also the extent of visual field loss (generally less than 20º). Syn. partial sight; subnormal vision.The World Health Organization (WHO) defines low vision as visual acuity less than 6/8 (20/60) and equal to or better than 3/60 (10/200) in the better eye with best correction. See low vision aids; blindness; bracketing; Bailey-Lovie chart; contrast sensitivity chart; clipover; deaf-blind; halogen lamp; cross-cylinder lens; telescopic lens; apparent magnification; relative distance magnification; relative size magnification; magnifier; Kestenbaum's rule; magnifying spectacles; pinhole spectacles; galilean telescope; Pepper test; typoscope; eccentric viewing.
mesopic vision Vision at intermediate levels between photopic and scotopic vision, and corresponding to luminances ranging from about 10⁻³ to 10 cd/m². Syn. twilight vision.
monochromatic vision Synonym of monochro-matism. See monochromat.
monocular vision Vision of one eye only.
multiple vision See polyopia.
near vision (NV) Vision of objects situated 25-50 cm from either the eye, or more commonly the spectacle plane. See Jaeger test types; distance vision.
night vision; nocturnal vision See scotopic vision.
panoramic vision Vision of some animals whose eyes are located laterally so that the two visual fields overlap only slightly or are adjacent, thus providing vision over a much larger region of the environment than if the two lines of sight were aimed in the same direction.
peripheral vision Vision resulting from stimulation of the retina outside the fovea or macula. Syn. eccentric vision; extrafoveal vision; indirect vision. See sensory fusion; central vision.
photopic vision Vision at high levels of luminance (above 10 cd/m²) and resulting from the functioning of the cones. Syn. daylight vision; diurnal vision. See duplicity theory; differential threshold.
protanomalous vision See protanomaly.
red vision See chromatopsia.
vision science The scientific study of how the visual system contributes to an understanding of the environment by processing and interpreting the light stimulation to the eye. Various disciplines contribute to vision science including anatomy, biology, optics, physiology and psychology.
scotopic vision Vision at low levels of luminance, below about 10⁻³ cd/m² and resulting from the functioning of the rods. Syn. night vision; nocturnal vision; scotopia. See duplicity theory.
vision screener An instrument used to measure various visual functions rapidly and inexpensively. There are various models, but most are modified stereoscopes with an internally illuminated set of targets and an optical system or variable target positioning to simulate either a near or far testing distance. Most of these instruments measure visual acuity, heterophoria, fusion, stereopsis, colour vision and visual field. See photorefraction.
simultaneous vision See contact lens.
single binocular vision (SBV) Condition in which both eyes contribute towards producing a single fused percept. See sensory fusion.
spatial vision See depth perception.
stereoscopic vision See stereopsis.
subnormal vision See low vision.
telescopic vision See tunnel vision.
vision therapy; vision training See visual training.
tritanomalous vision See tritanomaly.
tunnel vision Vision limited to the central part of the visual field as though one were looking through a hollow tube. It may be a symptom of hysteria, malingering, the final stage of either open-angle glaucoma or retinitis pigmentosa, etc. Syn. gun barrel vision; telescopic vision. See hysterical amblyopia; visual expander field.
twilight vision See mesopic vision.
Worth's classification of binocular vision For the purpose of visual rehabilitation, binocular vision is often classified into three grades: (1) simultaneous binocular vision (first-degree fusion or superimposition); (2) fusion (sensory fusion or second-degree fusion or flat fusion); (3) stereopsis (third-degree fusion). See sensory fusion; superimposition.
yellow vision See xanthopsia.

vis·ion

(vizh'ŭn) The act of seeing. [L. visio, fr. video, pp. visus, to see]

Patient discussion about vision

Q. What age does eye sight stabilizes? I was just wondering at what age does your eye sight usually level off and stop getting worse?Any ideas much appreciated!A. It will stabilize in a few years-age of 26-27.That is the average age people perform lasik surgery.
It is also the age the eye is fully grown.
Don't worry it will not get much worse maybe about -0,75.
Take care

Q. What can you do to make your eye sight better? My eye sight isn’t that great. Is there anyway to improve it, like eating certain foods or drinking certain drinks?A. There is something called the Bates Method. It is a combination of relaxation and exercises with the eyes. Depending on the problems you have with your eyes it may help. Do an internet search on Bates Method and you will get some info on it and a couple years ago I actually found a place I downloaded a description of some of the exercises.

Q. My vision is blurry and I see zigzag lines, what is it from? Every now and again, usually after sitting a few hours in front of the computer or not sleeping enough at night, I start getting blurry vision. I see zigzag lines in front of my eyes and it can take sometimes an hour to go away. What is this from?A. It sounds like you have a migraine. A migraine that comes with blurry vision and zigzags (an aura) is called a classic migraine. The aura of migraine typically lasts from 20 minutes to an hour. Some patients have prolonged aura symptoms that can last hours to days. The aura also typically ends before the headache itself begins. When migraine aura symptoms are prolonged or last into or through the headache phase these types of migraine are commonly referred to as complicated migraines.

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noun image

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noun sight

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noun foresight

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noun picture

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noun the faculty of seeing

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verb to form mental images of

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noun a vivid mental image

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noun the ability to see

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noun the perceptual experience of seeing

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