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'How do you find Weedin?' 'Totally,' Dougal said, 'lacking in vision. It is his fatal flaw. Otherwise quite sane.'—Muriel Spark, 1960.

This meaning, 'statesmanlike foresight', grew out of a much older set of meanings to do with intellectual perception as a metaphor based on physical seeing. Fowler dubbed it a 'vogue word', a status that to some extent it still has.

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(1923-24), a periodical edited by Jack Lindsay, Kenneth Slessor and Frank C. Johnson, was published in Sydney. It lasted only four issues but was of major importance. Jack Lindsay was the main force behind the magazine's production, but its inspiration derived chiefly from Norman Lindsay, who also provided the title and contributed illustrations and essays. The editors, who looked forward to a 'Renaissance' of 'creative passion' beginning in Australia, opposed both European modernism and the nationalist strain in Australian writing. Looking back to the Greeks and such European writers as Shakespeare, Byron, Burns, Villon, Browning, Rabelais and Nietzsche, they espoused a romantic, vitalist credo. The same ideas were continued in two other periodicals, the London Aphrodite (1928-29) and the Australian Outline (1933-34). Contributors to Vision included Kenneth Slessor, Jack Lindsay, Hugh McCrae, Adrian Lawlor, Dorothea Mackellar, R.D. FitzGerald, Dulcie Deamer, Louis Lavater, Philip Lindsay and Blamire Young. An important dispute between Jack Lindsay, Kenneth Slessor and Norman Lindsay on the circumstances of the magazine's publication and its influence later took place in Southerly (1952 and 1953).

Previous:	Vinegar Hill, Victorian: A Weekly Journal of Politics, Literature, Irish and Catholic Intelligence, Colonial, Foreign and General Information, Victorian Review
Next:	Vision: A Magazine of the Arts, Science and Australiana, Vista, Voice: The Australian Independent Monthly

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. See also Color; Light.

Diagram showing the eyes and visual projection system. The visual angle θ is measured in degrees.

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 also 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 also 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.

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

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 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. See also Vitamin A.

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.

Vernier and stereoscopic discriminations of space.

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 also Color vision; Perception.

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The process of vision is mediated by a pigment derived from vitamin A bound to a protein (opsin). The pigments are variously known as visual purple (because of the colour), rhodopsin (in the rod cells of the retina), and iodopsin (in the cone cells). The action of light on rhodopsin causes a chemical change, with loss of the purple colour (known as bleaching), which results in the initiation of a nerve impulse from the retina to the brain. See also dark adaptation; night blindness.

Vision is the task of understanding the world through our eyes. It is probably the most difficult thing that we do with our brains, yet we do it every waking moment, and it is virtually effortless. Just open your eyes and the universe is there, in all the richness of its shapes and colours, its brightness, distance and movement. But the analysis that underlies seeing involves about one third of the entire human cerebral cortex — more than a billion nerve cells. That is one indication of the magnitude of the task of vision.

Using their eyes, most people can thread a needle, recognize thousands of faces, read a newspaper, drive a car, see an orange as orange whatever the colour of the illuminating light. Some people can fly a jet plane at three times the speed of sound, return a tennis ball served at 200 km an hour, distinguish a thrush from a female blackbird at 100 m, or an early Cubist still life by Picasso from one by Braque. Each of these is an accomplishment of staggering complexity. Even the most sophisticated of computer vision systems, which interpret signals from cameras mounted on robots, seem like idiots compared with the genius of normal human vision. This is another indication of the scale of the task of vision.

Vision involves the detection of light — electromagnetic, non-ionizing radiation, ranging in wavelength from about 400 to about 750 nanometres. The main natural source of light is stars, especially our own sun. Full sunlight appears white, but light consisting of a limited range of wavelengths appears coloured. Short wavelengths look blue, long wavelengths red. Most of the light that enters our eyes does not come directly from the sun but is reflected from the surfaces of objects. Most surfaces (except mirrors and pure white objects) absorb part of the spectrum of light, changing the wavelength composition of the reflected light, thus making the surfaces appear coloured.

Vision has humble origins. In its very simplest form, it probably appeared near the start of life on Earth, with single-celled organisms that produced photopigments — molecules that change shape when they absorb light, and trigger chemical reactions in the cell. The mere detection of light can be useful to organisms, enabling them to regulate their activity according to the time of day or the seasons of the year, and even allowing them to orientate themselves towards or away from the source of light. Eyes — organs for collecting light — exploit the fact that light travels in straight lines. They use a lens, a mirror, or even just a pinhole, to cast an image on to receptor cells containing photopigment (photoreceptors). The crucial feature of an image is that it contains information about individual objects in the scene and their relative positions, thus affording the animal an opportunity to recognize and respond to those objects, as long as it has the apparatus in its head to analyze the information. The other huge value of vision is that it works at a distance, and hence serves to predict the future:

For I dipt into the future, far as human eye could see,
Saw the Vision of the world, and all the wonder that would be.

Alfred, Lord Tennyson, Locksley Hall

All vertebrate eyes are built to a common plan. Rather like cameras, they have a lens system that forms an inverted image on a layer of photoreceptor cells in the back of the retina, which lines the eyeball. In front of the receptors are alternating layers of nerve fibres and cells, forming a complex network through which signals from the receptors are passed. Each photoreceptor absorbs light over a particular band of wavelengths, thus providing, between them, a pattern of activity that can be used to retrieve the brightness and colour of light. Essentially, the photoreceptors pixellate the information in the image, reducing it to a point-by-point description of intensity and wavelength, rather like that on a computer screen. The grain of photographic emulsion in camera film does much the same. But cameras do not see. Vision depends on the interpretation of the patterns of activity from the photoreceptors, across space and time.

Part of the process of interpretation occurs within the retina itself. The essential function of all vertebrate retinas is to reduce the overwhelming flood of information that pours into the eye. In the human eye there are about 120 million rod photoreceptors, which work only in dim light, and 6 million cones, which respond under brighter conditions and are of three types, sensitive to light in the blue, green or red part of the spectrum. Each photoreceptor produces a signal, dependent on the intensity and wavelength composition of the light that it catches. In computer terminology, this translates into many megabytes of information every second. Evolution, ever the master of tricks and short-cuts to efficiency, has discovered ways in which unneeded information is removed, during processing in the retina, so that only the essential skeleton of the message is transmitted to the brain.

First, the overall number of ‘pixels’ is dramatically reduced. The signals are passed from the photoreceptors through several connections, to the last retinal cells in the chain, the ganglion cells, which cover the inner surface of the retina and whose axons stream out through a hole in the eyeball to form the optic nerve. Each ganglion cell in the fovea (the central part of the retina, which we point towards objects when we look directly at them), receives its main input from just one cone photoreceptor, perfectly conserving the fine-grain detail of that part of the image. But, compared with the roughly 125 million photoreceptors, there are a mere 1.5 million or so ganglion cells. Those in the peripheral parts of the retina pool signals from very large numbers of receptors. In effect the output of the retina is like very coarse-grain film for the peripheral parts, and very fine-grain just in the middle. The constant jerky movements of the eye, which occur about 3 times every second, deliver one part of the image after another to the high-resolution fovea.

The second function of the retina is to ‘filter’ the image in space and in time, through procedures somewhat similar to those used to ‘compress’ the information of an entire movie on to a DVD. Everyone is familiar with a phenomenon called dark adaptation: if you go from a bright environment into a dark room it is initially very hard to see anything, but vision gradually improves, over the course of fully half an hour. In other words, the eye changes its sensitivity over time to suit the average brightness of the scene — rather like having camera film that can constantly change its speed to match the light conditions. On a shorter time-scale, the eye transmits signals only when the image has just changed, for example, after an eye movement. Indeed, if the image is held absolutely stationary on a person's retina (by means of optical or electronic techniques), perception fades out completely within a few seconds.

Our detailed knowledge of the visual system has come largely from the study of animals, and especially from the use of tiny microelectrodes to record impulses from individual nerve cells or fibres. Retinal ganglion cells have been much studied in this way in totally anaesthetized animals (in which the retina, indeed much of the visual pathway, continues, surprisingly, to respond to visual stimulation). Each ganglion cell responds to changes of light intensity over a limited area of the retina, called the cell's receptive field, corresponding to the group of photoreceptors that influence the cell, via the network of connections in the retina. Roughly half the retinal ganglion cells respond with a burst of impulses when the centre of the receptive field is illuminated (ON cells). The other half respond to a decrease in illumination (OFF cells). Thus the output of the retina signals the relative brightness and darkness of each point or patch in the visual field.

Horace Barlow discovered (in the frog) and Steven Kuffler (in the cat) that ganglion cells also ‘filter’ the image in space (as well as in time), to achieve further information-compression. Essentially, the signals from each group of photoreceptors that feed the central part of the receptive field are inhibited by signals from surrounding photoreceptors, a process called lateral inhibition. This means that each ganglion cell signals the difference of illumination, or contrast, between the central and the surrounding part of its receptive field. Any cell whose receptive field happens to view a part of the image with uniform brightness (e.g. the sky on a cloudless day) will be fairly inactive, while those whose receptive fields lie at the boundary of a change of intensity in the image will send strong signals to the brain.

It is almost as if the retina reduces the image to a line drawing of the visual scene. Perhaps this accounts for the fact that simple outlines are so powerful in their ability to evoke rich perception: just think of the how much can be seen in a line drawing or etching by Rembrandt or Matisse.


Fig. 1 The power of outline to evoke visual perception. A bison drawn between 10 000 and 15 000 years ago on a cave wall in France


In the retina of old-world monkeys (e.g. rhesus monkeys), assumed to be very similar to the human retina, the ON and OFF classes of ganglion cell can be further sub-divided into two main groups, called P cells and M cells (read on to discover the origin of these terms). P ganglion cells receive the central part of their receptive fields from one, or sometimes two (but not all three) types of colour-selective cone photoreceptors, and thus are colour-selective in their responses. M cells, which generally have larger receptive fields, receive input from all cone classes: they are not colour selective but are exquisitely sensitive to contrast and hence to movement of images on the retina. To some extent, this division of function between P and M cells is maintained through the visual pathway, and into the domain of visual perception.

The real business of vision is in the brain. Each optic nerve (the second cranial nerve) passes through a hole at the back of the bony orbit (the cavity in the skull that contains the eyeball), and the two nerves meet to form a distinctive cross-shaped structure, the optic chiasma, directly underneath the hypothalamus. (Actually, a small number of fibres branch off at this point to provide information about ambient light level to nerve cells of the suprachiasmatic nucleus, the heart of the body clock mechanism in the brain.)

In the optic chiasma, roughly half the nerve fibres cross over to the opposite side, and the rest continue on to the same side. It was Isaac Newton who first described this anatomical curiosity, and recognized its functional importance:

Are not the Species of Objects seen with both Eyes united where the optick Nerves meet before they come into the Brain, the Fibres on the right side of both Nerves uniting there, and after union going thence into the Brain in the Nerve which is on the right side of the Head, and the Fibres on the left side going into the Brain in the Nerve which is on the left side of the Head. (Opticks, Book 3, Part 1, 14th edition, 1730)

Thus, the arms of the optic chiasma that point towards the brain, called the optic tracts, contain a mixture of fibres from geometrically corresponding halves of the two retinas, which, because of optical inversion of the image, view the opposite half of the visual world. Essentially this arrangement splits the representation of the visual field neatly into two. The right side of the field is viewed by the left cerebral hemisphere, the left side by the right. This fits with a general rule, that the left hemisphere is concerned with everything to the right of the body — the skin of right side, control of the muscles of the right side, even sounds coming from the right — while the right hemisphere is devoted to the left side of the body.

This means that damage to the visual pathway on one side of the brain causes blindness or partial blindness in both eyes, on the opposite side of the visual field. Interruption of one optic tract causes total blindness in the opposite half of the visual field — hemianopia. Nothing at all is visible to one side of a precise vertical line through the middle of whatever the patient is looking at. Remarkably, patients with this condition are sometimes unaware that they are half-blind: they complain of not being able to read normally, or not being able to drive as well as they used to! This points up a sensible but surprising property of vision — that it is concerned with what we can see, and not with what we cannot see. Think of how indifferent we are to the fact that we cannot see behind our heads. Equally, we are normally unaware that most of the visual field (except that part falling on the fovea) is represented in the brain with very poor detail and colour.


Fig. 2 The base of the human brain (drawn by Christopher Wren) from Cerebri anatome (1664) by Thomas Willis showing the optic nerves (E) meeting in the optic chiasm and the optic tracts continuing into the hemispheres of the brain. Wellcome Institute Library, London


In Dickens' Pickwick Papers, Sam Weller says:

Yes I have a pair of eyes … and that's just it. If they was a pair o' patent double million magnifyin' gas microscopes of hextra power, p'raps I might be able to see through a flight o' stairs and a deal door; but bein' only eyes, you see, my wision's limited.

Indeed, our ‘wision’ is limited — by the resolution of the optics of our eyes and the structure of the retina, by the range of wavelengths to which our photoreceptors are sensitive, and by the capacity of our brains to fathom, from the mere shadows that flit across the retina, what is there in the outside world. But mercifully we are normally blissfully unaware of those limitations of sight.

This leads to a more general conclusion. Visual experiences are externalized, i.e. they happen outside the body, not inside the head. The visual properties of objects appear to belong to them, not to be the products of the brain. We are hardly even aware of our eye movements, which cause the image to jerk and slew continuously across the retina. The task of vision is to inform about the outside world, not about the nature of vision.

The nerve fibres in the optic tract (the axons of retinal ganglion cells) terminate in two main areas of the brain. A minority project to a structure called the superior colliculus (the upper little hill), which can be seen as a bump, one on each side, on the roof of the midbrain, as well as to nearby tiny clusters of nerve cells (in the pretectum). This general region, the mammalian vestige of the principal visual centre in amphibia, reptiles, birds and fish, is concerned mainly with visual reflexes. It contains regions that regulate the size of the pupil of the eye in bright and dim conditions, and that make the eye involuntarily follow large moving objects. The main function of the two superior colliculi is to control the automatic tendency of the eyes, the head and the body, to turn towards objects of interest — so-called orienting responses. They are, in fact, centres for sensory integration, since they receive input from the ears and the skin as well as the eyes, all helping to guide such reactions.

The bulk of the fibres of the optic nerve reach the lateral geniculate (meaning knee-shaped) nucleus (the LGN) in the thalamus (an egg-shaped mass of grey matter through which virtually all information passes on its way to the cerebral cortex). In monkeys, the LGN has six layers. The information from the two eyes remains separate, each eye sending its fibres to three of the layers. The lower two layers are called magnocellular, because the nerve cells in them are relatively large. The neurons of the magnocellular layers receive input from the fibres of the M class of ganglion cells (that is why they are called M cells), and hence they are also sensitive to contrast and motion, but not colour. The upper four, parvocellular layers (two for each eye) contain relatively small nerve cells, and receive input (one-to-one connections in some cases) from the axons of P ganglion cells. Hence the parvocellular layers transmit information about colour and fine detail.

The fibres of the roughly 1.5 million cells in the LGN fan backwards and upwards in a bundle of white matter called the optic radiation, which passes to the back of the hemisphere to reach the region of cerebral cortex, called the primary visual cortex (or striate cortex, or area 17, or V1). During the First World War, the British neurologist Gordon Holmes examined the visual deficits of soldiers who had suffered shrapnel injuries to this region. If a tiny fragment had entered the back of the skull on one side, there was a corresponding blind patch, a scotoma, in the opposite side of the visual field. This implies that there is a kind of ‘map’ of the retinal image across the surface of the primary visual cortex. Indeed, individual nerve cells in the grey matter receives input, directly or via the network of connections in the cortex, from a limited group of cells in the LGN. Thus each cortical cell also has its own receptive field — a patch of retina, and hence visual field, through which it responds to appropriate visual stimuli.

Nerve cells in the middle layers of the cortex, where the incoming fibres mainly terminate, respond to brightening or darkening of a particular spot in the visual field, very much like neurons in the LGN. Indeed there are separate sub-layers receiving input from P-type and M-type cells. Input from the two eyes is still kept separate at this point, with axons from the right- and left-eye layers of the LGN terminating in a remarkable alternating pattern. Each eye's input occupies regions that form curving, branching ocular dominance stripes, each about 0.3 mm wide, running across the middle layers of the cortex. Alternate stripes are dominated by right eye, then left, forming a pattern similar to a fingerprint impressed on the visual cortex. Neighbouring stripes have input from roughly the same point in the visual field, seen through the two eyes.


Fig. 3 The visual pathway, depicted by the great Spanish neuroanatomist Ramón y Cajal (1899). Optic nerve fibres from the nasal half of each retina (the half closer to the nose) cross over in the optic chiasma. So, the optic tract contains fibres from corresponding half-retinas of the opposite eye (c) and the eye on the same side (d). The fibres contain nerve cells in the lateral geniculate nucleus (g) whose fibres run up to the primary visual cortex at the back of the hemispheres. The right visual cortex (Rv) views the left side the visual field, the left views the right side


Extraordinary things happen as the information is passed up and down within the grey matter, to the many other neurons in the cortex. David Hubel and Torsten Wiesel won the Nobel Prize in 1981 for their pioneering work on the physiology of the visual cortex. They discovered, first in cats and later in monkeys, that these neurons respond not just to light or dark spots, like the neurons that drive them, but selectively to lines or edges, falling on, or moving over, the receptive field. Each cell prefers a line stimulus, at a particular orientation, and the preferred orientation varies from cell to cell. Somehow, the property of orientation selectivity is created by the combination of all the nerve fibres that converge on each cell. These orientation-selective cells are arranged into a beautiful system of columns, presumably created by the fact that most connections within the cerebral cortex run up and down radially within the grey matter. The selective neurons within each column, perhaps 0.1 mm across, running from the surface down to the white matter, all prefer the same orientation. And the preferred orientation shifts progressively from column to column, across the cortex.


Fig. 4 Vision depends on inference. The bright, white triangle in this illusory figure of Kanizsa is 'invented' by the brain on the basis of the evidence from the other featurer in the image


Orientation-selective neurons remain perhaps the best example of feature detection — the notion that sensory neurons are ‘programmed’ (partly through innate control of the ‘wiring’ of the pathway, partly through the effects of sensory experience early in life) to respond to particular information-rich features of the sensory world. The primary visual cortex starts the process of ‘dissecting’ the retinal image, so as to encode its essential structure. In normal conditions, these cells respond to the boundaries of objects in space, or to elements of the texture of surfaces, presumably describing these features to the rest of the brain. This is the beginning of a process that has been called inverse optics — inferring from the flat retinal image the true shapes and distribution of the objects that generated the image, ‘reversing’ the optical process that made the image.

Hubel and Wiesel also discovered that the vast majority of these orientation-selective neurons are also ‘binocularly driven’: they have receptive fields in roughly corresponding positions on both retinas, and are remarkably similar in their preferences for visual stimuli, whichever eye is open. Thus, in normal viewing conditions, these cells will be stimulated simultaneously through both eyes, by the two images of individual objects in space. This presumably accounts for the fact that we see only one, fused visual world, despite the fact that two eyes are viewing it.

Because our two eyes are horizontally separated in the head, when we view a three-dimensional scene, their retinal images are not absolutely identical. Binocular parallax, as it is called, creates tiny differences in the relative positions on the two retinas of the images of individual objects that lie at different distances from the eyes. Sir Charles Wheatstone first described, in 1838, the fact that we can interpret these minute differences between the two retinal images to perceive the solidity of objects and their relative distances in space. This skill, called stereopsis or stereoscopic vision, is a wonderful example of inverse optics. The brain has evolved mechanisms for analysing not just the individual retinal images, but also the differences between them, so as to understand the world.

Now, it turns out that, although the two receptive fields of individual visual cortical cells, on average, lie on geometrically corresponding points in the two retinas, there is a little variation in their relative positions. This, combined with the fact that the responses of neurons are often strongly enhanced when both receptive fields are stimulated simultaneously, means that individual such cells respond best to the boundaries of objects at particular distances, behind or in front of whatever the eyes are fixating. Thus, the processing that underlies stereopsis appears to start with the binocular neurons of the primary visual cortex.


Fig. 5 The brain infers a three-dimensional world from the flat retinal image. But in the case of the Necker cube, it is unable to decide between two equally likely interpretations. The cube spontaneously alternates in depth, depending on which face appears closer


The existence of a visual area in the back of the cerebral hemispheres was known in the nineteenth century. But at that time, the vast continent of uncharted cortex in between the major sensory and motor regions was thought simply to combine information, in some ill-defined way. It was called association cortex. Work on monkeys, starting in the 1960s, has shown that the entire association cortex of the rear part of the hemispheres is in fact devoted exclusively to the analysis of vision. It is divided into a huge patchwork of individual areas, each containing a representation of all or part of the visual field. These are known as extrastriate visual areas, to distinguish them from the striate cortex — the primary visual cortex. Virtually all the fibres from the LGN, carrying information from the eyes, reach only the striate cortex, and these other visual areas receive their input mainly from cortico-cortical connections, forming a complex network, with fibres running back and forth linking the striate cortex to the other areas.

While damage to the primary visual cortex leads to blindness in the corresponding area of the visual field, injury in extrastriate areas generally leads to more subtle deficits in perception. It must, however, be said that people rendered clinically blind by damage to the striate cortex can nevertheless sometimes respond unconsciously to a visual stimuli, by moving their eyes towards it, particularly if it moves rapidly or is of very high contrast. Indeed, some can ‘guess’ reliably the direction of movement of the stimulus and whether a flashed line is vertical or horizontal, even though they deny actually seeing it. This curious residual visual capacity, called ‘blindsight’, may be mediated by surviving connections from the eyes to other parts of the brain, perhaps via the superior colliculus.

Broadly speaking, the extrastriate areas of the cortex form two broad processing ‘streams’, both originating in the striate cortex. The ‘ventral stream’, which runs downwards into the lower parts of the temporal lobe, is dominated by the P-cell system, and thus contains information about colour and fine detail, while the ‘dorsal stream’, monopolized by M-cell input, runs up into the parietal lobe, and is concerned with the analysis of movement, and the detection of the position of objects in space. The ventral and dorsal streams have been dubbed ‘what’ and ‘where’ systems, although this is an over-simplification.


Fig. 6 In the brain of a person imagining the movement of an elephant certain (light grey) areas are active. When imagining the colour of an elephant, separate (dark grey) areas are active. The areas at the back of the brain are those that become active when one actually sees real movement or colour. The activity in the front of the brain is associated with the acts of imagining


The ventral stream does seem to be mainly concerned with the recognition of objects, and it feeds signals to parts of the brain, especially the hippocampus, thought to be responsible for conscious visual memory. Neurons in some areas within the ventral stream have remarkable properties. In an area called V4, for instance, some cells respond selectively to surfaces of a particular colour, regardless of the spectral composition of the illuminating light. This correlates with the fact that we see the colours of objects as more or less constant, whatever the illumination — a phenomenon called colour constancy. To achieve this property, these neurons must somehow take account of the wavelength composition of light reflected from surrounding surfaces, a ‘computation’ that cannot be done in the primary visual cortex. Further south, in parts of the temporal lobe, are populations of nerve cells that respond selectively to the appearance of monkey or human faces, somehow detecting the combination of features that define a face. Even deeper into the ventral stream cells can ‘learn’ to respond specifically to one stimulus out of a series of objects or abstract shapes that the animal is shown as part of a memory task. This all suggests that the ventral stream is concerned with identifying and remembering objects.

This work on monkeys has underpinned the recent study of visual areas in the human brain, making use of the new imaging techniques of Positron Emission Tomography (PET) and functional Magnetic Resonance Imaging (fMRI), which essentially detect the small local changes of blood flow associated with activity in neurons. There may be as many as 50 different extrastriate visual areas in humans, and those occupying the lower part of the occipital and temporal lobes also seem to be concerned with the analysis of colour, faces and the identification of objects. Damage in these regions, caused, for instance, by STROKE, causes various, selective deficits in visual understanding, such as central achromatopsia, a form of colour blindness, or prosopagnosia, the inability to recognize individual faces. In extreme cases, damage of the ventral stream leads to the frightening condition of visual agnosia, in which patients simply cannot recognize familiar objects, despite all their basic visual functions being normal.

The dorsal stream in monkeys also has areas with distinctive physiological properties. One, called the middle temporal area (MT) or V5, seems deeply involved in the analysis of motion. Neurons here almost all respond to movement in a particular direction, and they probably also play a part in stereoscopic vision. Neighbouring areas are concerned with analysing the flow of patterns across the retina produced by movements of the head or the whole body through space. Even higher up, in the parietal lobe, cells respond to the positions and movements of objects in ways that imply that they are concerned with guiding hand and eye movements. Again, similar functional areas have been found in the upper parts of the human occipital lobe and the parietal lobe. Damage in these regions can produce such conditions as akinetopsia (deficiency in the perception of motion) and visual neglect (failure to attend to objects on the opposite side of visual space).

It has been argued that the dorsal stream is more concerned with unconscious visually-guided reactions, such as manipulating objects with the hands, while the ventral stream underlies the conscious perception of objects. Evidence for this view comes from the fact that some individuals with ventral stream damage, while unaware of the differences between particular objects, can nevertheless shape their hands correctly when asked to pick them up. Equally, some patients with dorsal stream damage make clumsy hand movements when they try to pick up objects that they can recognize perfectly well.

The huge adaptive value of vision has driven its explosive evolution. Its machinery dominates our brains; its impressions dominate our subjective lives. Indeed, for the sighted, it is hard to imagine life without it. Language is full of visual metaphors that bear testimony to the fact that vision is the main route to the mind. ‘I see what you mean’; ‘A person of vision’; ‘My point of view’, ‘A picture is worth a thousand words’. Moreover, vision not only underpins our understanding of the world around us but also sets the scale of beauty and ugliness. The view from a mountaintop, the skyline of New York, sunset in the south of France, Botticelli's Birth of Venus (see Venus). It is seeing that makes those things breathtaking. Vision rules our aesthetic lives.

Vision has been a favourite topic of some of the most eminent individuals in the history of science, including such physicists as Isaac Newton, James Clerk Maxwell, Thomas Young, Hermann von Helmholtz and Ernst Mach. Arguably, we know more about vision than any other high-level function of the brain. Yet much remains mysterious. How does the brain arrive at reliable interpretations of objects? How is the identity of every object we can distinguish represented in the brain? How is the subjective experience of seeing related to, and generated from, the activity of neurons? Indeed, what, if anything, does conscious experience add to the purely computational process of vision?

— Colin Blakemore

Bibliography

• Gregory, R. L. (2001) Eye and brain: the psychology of seeing, 5th edition. Oxford University Press, Oxford.
• Hubel, D. H. (1988) Eye, brain and vision. Scientific American Library/W. H. Freeman, San Francisco.
• Zeki, S. (1999) Inner vision. Oxford University Press, Oxford

See also blindness; blindness, recovery from; colour blindness; consciousness; eye movements; eyes; illusions sensory receptors.

noun

1. The faculty of seeing: eye, eyesight, seeing, sight. Archaic light¹. See see/not see.
2. Unusual or creative discernment or perception: farsightedness, foresight, prescience. See foresight.
3. An illusory mental image: daydream, dream, fancy, fantasy, fiction, figment, illusion, phantasm, phantasma, reverie. See real/imaginary.
4. Something that is foretold by or as if by supernatural means: divination, oracle, prophecy, soothsaying, vaticination. See foresight.

verb

To form mental images of: conceive, envisage, envision, fancy, fantasize, image, imagine, picture, see, think, visualize. Informal feature. See thoughts.

n

Definition: ability to perceive with eyes
Antonyms: blindness, sightlessness

n

Definition: apparition
Antonyms: actuality, fact, reality

1. Sight; the ability to perceive with the eye. The eye is the most important sense organ for supplying information about the external world and is classified as an exteroceptor. It has also been shown to be an important proprioceptor (see visual proprioception).

2. The ability of a coach to set realistic goals and to understand the steps that lead to achieving these goals.

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Ocular vision is the perception of material objects in accordance with optical laws from a definite point in space. Difficult to classify are those rare cases when the sense of sight is transposed and the subject "sees" with his elbows, forehead, fingertips or stomach, since it is not clear what mechanism of vision is involved.

Inner vision is independent of space, objective existence, and, seemingly, optical laws. The simplest type of inner vision is presented by memory images, waking dreams, and images of imagination. The latter type may attain such an intensity as to emerge spontaneously and reach the pitch of hallucination.

Hallucination is the widest extent of inner vision. Dreams represent the primary type. They are hallucinations of low intensity. Generally, hallucinations appear to conform to all factors of ocular vision—space, optical laws, objectivity. The images appear externalized in space.

Indeed, objectivity in some cases of hallucinations may be more than an appearance, as some believe that a camera may register an apparition when outwardly nothing is visible and the vision must have taken place internally (see psychic photography). A still stronger proof of objectivity is furnished by cases of veridical visions in which the perception is afterward found to be a true visual representation of incidents taking place at a distance.

On the other hand, no objectivity is discoverable in degenerative hallucinations, the dogs and snakes of the drunkard, the scarlet fire of the epileptic, or the visions of the psychotic.

Inner vision may be developed empirically in crystal gazing and afford fruitful study for the determination of what elements are externalized from the subconscious mind of the scryer or of discarnate intelligences. Visions may also be distinguished as either spontaneous or induced.

IN BRIEF: Sight. Also: The power of imagination.

Where there is no vision, there is no hope. — George Washington Carver (1864-1943).

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Quotes:

"Far away there in the sunshine are my highest aspirations. I may not reach them, but I can look up and see their beauty, believe in them, and try to follow where they lead." - Louisa May Alcott

"Why should you be content with so little? Why shouldn't you reach out for something big?" - Charles L. Allen

"No vision and you perish; No Ideal, and you're lost; Your heart must ever cherish Some faith at any cost. Some hope, some dream to cling to, Some rainbow in the sky, Some melody to sing to, Some service that is high." - Harriet Du Autermont

"The ultimate function of prophecy is not to tell the future, but to make it. Your successful past will block your visions of the future." - Joel A. Barker

"Your successful past will block your visions of the future." - Joel A. Barker

"Vision without action is merely a dream. Action without vision just passes the time. Vision with action can change the world." - Joel A. Barker

See more famous quotes 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.

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The faculty of seeing; sight.
The basic components of vision are the eye itself, the visual center in the brain, and the optic nerve, which connects the two. Abnormalities of vision in animals can only be inferred by an assessment of the animal's response to a variety of visual stimuli. The commonly used tests of vision are the menace reflex test, the watching of a moving object and the obstacle test. These can all be performed in subdued light as a test for night blindness.

• achromatic v. — vision characterized by lack of color vision.
• aphakic v. — vision after lens removal.
• binocular v. — the use of both eyes together, without diplopia.
• central v. — that produced by stimulation of receptors in the fovea centralis.
• day v. — visual perception in the daylight or under conditions of bright illumination.
• double v. — diplopia.
• half v. — hemianopia.
• monocular v. — vision with one eye.
• night v. — visual perception in the darkness of night or under conditions of reduced illumination.
• panoramic v. — 360° vision conferred on grazing herbivora by the lateral placement of their eyes.
• peripheral v. — that produced by stimulation of receptors in the retina outside the macula lutea.
• photopic v. — vision in bright illumination.
• scotopic v. — vision in low illumination.
• v. test — see visual acuity test.

(vizh′ən)
n

Sight; the faculty of seeing.

For a list of words related to vision, see:

• Intuition and Imagination - vision: perception with one’s imagination; extraordinary or vivid discernment or insight; supernatural appearance that provides insight
• Occult States and Experiences - vision: supernatural, often revelatory appearance in dream, trance, or ecstasy
• Mystical States and Experiences - vision: supernatural, revelatory appearance in dream, trance, or meditative state

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See words rhyming with "visional."

  See crossword solutions for the clue Vision.

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)

Dansk (Danish)
n. - vision, syn, synsevne, drømmebillede, åbenbaring, klarsyn, fremsyn
v. tr. - forudse, åbenbare

idioms:

• vision mixer    billedmixer

Nederlands (Dutch)
gezicht, visie, vooruitziende blik, visioen, inzicht, gezichtsvermogen, zich voorstellen

Français (French)
n. - vision, sagacité, idée, vue, image, (TV) image
v. tr. - (US) image

idioms:

• vision mixer    réalisateur de direct, mélangeur d'images

Deutsch (German)
n. - Sehvermögen, Phantasien, Bild, Vision
v. - erschauen

idioms:

• vision mixer    Bildmixer

Ελληνική (Greek)
n. - όραση, όραμα, ευγενής φιλοδοξία, ενόραση, οπτασία, διορατικότητα, οξυδέρκεια, οπτικό πεδίο

idioms:

• double vision    (παθολ.) διπλωπία
• vision mixer    μίκτης εικόνας (στην τηλεόραση)

Italiano (Italian)
vista, visione

idioms:

• vision mixer    vision mixer

Português (Portuguese)
n. - visão (f), fantasma (m), faculdade de ver (f)

idioms:

• vision mixer    equipamento misturador de imagens

Русский (Russian)
зрение, проницательность, дальновидность, мечта, предвидеть, воображать

idioms:

• vision mixer    человек, переключающий изображение с одной камеры на другую

Español (Spanish)
n. - vista, visión, clarividencia, perspicacia, sueño
v. tr. - ver o imaginarse

idioms:

• vision mixer    mezclador de imágenes

Svenska (Swedish)
n. - syn, vision, bildfrekvens

中文（简体）(Chinese (Simplified))
视觉, 眼光, 先见之明, 梦见, 显示, 想象

idioms:

• vision mixer    电视或电影的混合图像镜头

中文（繁體）(Chinese (Traditional))
n. - 視覺, 眼光, 先見之明
v. tr. - 夢見, 顯示, 想象

idioms:

• vision mixer    電視或電影的混合圖像鏡頭

한국어 (Korean)
n. - 시력, 선견지명, 몽상
v. tr. - 환상으로 보다, 꿈에 보다, 뇌리에 그리다

日本語 (Japanese)
n. - 視覚, 見ること, 想像力, 夢想, 幻, 幻影, 未来像, 見方, 見えるもの
v. - 夢に見る, 幻想する

idioms:

• twenty-twenty vision    正常な視力
• vision mixer    フィルム編集者

العربيه (Arabic)
‏(الاسم) رؤيه, بصر, مشهد, منظر, طيف, رؤيا, الكشف‏

עברית (Hebrew)
n. - ‮ראייה, ראות, חזון, מעוף, מראה, מחזה מרהיב, דמיון, חלום, הזיה‬
v. tr. - ‮ראה או הציג בחזון או כחזון‬

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