Vision (redirected from direct vision)

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

Vision in Humans

The human eye 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 brain

Anatomy and Function

..... ) 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 astigmatism, color blindness, farsightedness, and nearsightedness. 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).

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Vision

See Vision Server.

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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