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vision, physiological sense of sight by which the form, color, size, movements, and distance of objects are perceived.
Vision in Humans
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
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
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).
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.
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
|Characteristic||Scotopic vision||Photopic vision|
|Photochemical substance||Rhodopsin||Cone pigments|
|Speed of adaptation||Slow (30 min or more)||Rapid (8 min or less)|
|Region of retina||Periphery||Center|
|Number of receptors per eye||120,000,000||7,000,000|
|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.
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
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.
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.
REFERENCESKravkov, 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 .
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.