perception

n.

1. The process, act, or faculty of perceiving.
2. The effect or product of perceiving.
3. Psychology.
   1. Recognition and interpretation of sensory stimuli based chiefly on memory.
   2. The neurological processes by which such recognition and interpretation are effected.
4. 1. Insight, intuition, or knowledge gained by perceiving.
   2. The capacity for such insight.

[Middle English percepcioun, from Old French percepcion, from Latin perceptiō, perceptiōn-, from perceptus, past participle of percipere, to perceive. See perceive.]

perceptional per·cep'tion·al adj.

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Those subjective experiences of objects or events that ordinarily result from stimulation of the receptor organs of the body. This stimulation is transformed or encoded into neural activity (by specialized receptor mechanisms) and is relayed to more central regions of the nervous system where further neural processing occurs. Most likely, it is the final neural processing in the brain that underlies or causes perceptual experience, and so perceptionlike experiences can sometimes occur without external stimulation of the receptor organs, as in dreams.

In contemporary psychology, interest generally focuses on perception or the apprehension of objects or events, rather than simply on sensation or sensory process. While no sharp line of demarcation between these topics exists, it is fair to say that sensory qualities are generally explicable on the basis of mechanisms within the receptor organ, whereas object and event perception entails higher-level activity of the brain. See also Hearing (human); Sensation; Vision.

Since objects or events are not experienced only through vision, the term perception obviously applies to other sense modalities as well. Certainly things and their movement may be experienced through the sense of touch. Such experiences derive from receptors in the skin (tactile perception), but more importantly, from the positioning of the fingers with respect to one another when an object is grasped, the latter information arising from receptors in the muscles and joints (haptic or tactual perception). The position of the parts of the body are also perceived with respect to one another whether they are stationary (proprioception) or in motion (kinesthesis), and the position of the body is experienced with respect to the environment through receptors sensitive to gravity such as those in the vestibular apparatus in the inner ear. Auditory perception yields recognition of the location of sound sources and of structures such as melodies and speech. Other sense modalities such as taste (gustation), smell (olfaction), pain, and temperature provide sensory qualities but not perceptual structures as do vision, audition, and touch, and thus are usually dealt with as sensory processes. See also Olfaction; Pain; Proprioception.

Constancy

By and large, these perceptual properties of objects remain remarkably constant despite variations in distance, slant, and retinal locus caused by movements of the observer. This fact, referred to as perceptual constancy, is perhaps the hallmark of perception and more than any other, serves to characterize the field of perception.

Examples of perceptual constancy are: size (except at very great distances, an object appears the same size whether seen nearby or far away, although the size of its image on the retina can be very different); shape (a circle seen from the side is perceived as a circle, although it appears as an ellipse on the retina); orientation (objects appear to keep the same orientation in space, independently of the orientation of the observer's head); and position (a fixed object remains perceived as stationary even when its image on the retina moves because of eye or head movements).

Motion perception

Perceived movement cannot simply be explained by the motion of an object's retinal image since image motion caused by observer or eye movement does not lead to perceived object movement. Moreover, an object tracked by smooth-pursuit eye movements will appear to move, although in that case there is essentially no motion of the object's image over the retina. Similarly, an afterimage will appear to move during eye movement even in a completely darkened room. Where ordinarily the movement of the retinal image caused by the moving eye is computed to signify “no object motion,” thus yielding position constancy (since the image motion and eye motion are equal in magnitude), the same computational rule must signify “object motion” in the case of the afterimage.

Form perception

Form perception means the experience of a shaped region in the field. Recognition means the experience that the shape is familiar. Identification means that the function or meaning or category of the shape is known. For those who have never seen the shape before, it will be perceived but not recognized or identified. For those who have, it will be perceived as a certain familiar shape and also identified. Recognition and identification obviously must be based on past experience, which means that through certain unknown processes, memory contributes to the immediate experience that one has, giving the qualities of familiarity and meaning.

The figure of a 4 in Fig. 1a is seen as one unit, separate from other units in the field, even if these units overlap. This means that the parts of the figure are grouped together by the perceptual system into a whole, and these parts are not grouped with the parts of other objects. This effect is called perceptional organization. There are other problems about form perception that remain to be unraveled. For example, the size of a figure can vary, as can its locus on the retina or even its color or type of contour, without affecting its perceived shape (Fig. 2).

Perceptual organization, (a) The figure of a four is immediately and spontaneously perceived despite the presence of other overlapping and adjacent lines, (b) The four, although physically present, is not spontaneously perceived and is even difficult to see when one knows it is there.

Transposition of form; the two shapes clearly look the same despite the difference in size.

A further fact about form perception is that it is dependent upon orientation. It is a commonplace observation that printed or written words are difficult to read when inverted, and faces look very odd or become unrecognizable when upside down. Simple figures also look different when their orientation is changed: a square looks like a diamond when tilted by 45°.

Geometrical illusions

Related to the topic of form perception is the misperception of the size or direction of parts of figures that constitutes many of the geometric illusions. In an illusion figure, one particular part is perceived to be either longer or shorter than another part, although they are objectively equal (Fig. 3a); or the direction of a contour is perceived to be different from that of another contour although they are the same (Fig. 3b). For reasons still not understood, the background or context of the rest of the figure affects these parts.

unequal, (b) The Poggendorff illusion in which the two oblique line segments are aligned with one another (that is, are collinear) but appear to be misaligned.">
Geometrical illusions, (a) The Ponzo illusion in which the two horizontal lines of equal length appear unequal, (b) The Poggendorff illusion in which the two oblique line segments are aligned with one another (that is, are collinear) but appear to be misaligned.

Innate or learned?

A central problem is whether the perception of properties such as form and depth or the achievement of veridical perception as in the constancies is innately determined or is based on past experience. By “innate” it is meant that the perception is the result of evolutionary adaptation and thus is present at birth or when the necessary neural maturation has occurred. By “past experience” it is meant that the perception in question is the end result of prior exposure to certain relevant patterns or conditions, a kind of learning process. Despite centuries of discussion of this problem, and considerable experimental work, there is still no final answer to the question. It now seems clear that certain kinds of perception are innate, but equally clear that past experience also is a determining factor. See also Intelligence.

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Our senses probe the external world, and they also tell us about ourselves as they monitor the positions of our limbs and the balance of our bodies. Through pain they signal injury and illness.

How we experience and know about external objects is a question that was discussed by the Greek philosophers and has been ever since. Planned experiments on perception, in the spirit of the physical sciences, were hardly attempted before the mid nineteenth century. They have revealed a surprising complexity of physiological and cognitive (knowledge-based) processes, of which we are normally unaware, though many can be demonstrated simply and dramatically, especially through the phenomena of illusions.

There is a long-standing tradition in philosophy that perception, especially touch and vision, gives undeniably true knowledge. For philosophers have generally sought certainty and often claimed it, whereas scientists (who are used to their theories being upset by new data) are more ready to settle for today's best bet. Many scientific instruments have been developed because of the unreliability or inadequacy of perception.

Perceptions are separate, and in several ways different, from conceptual understanding, for perception must work very fast (whereas we may take minutes or hours to ‘make up our minds’, and years to form new concepts). Also, it would be impossible for perception to draw upon all of our knowledge; and perceptions are of individual objects and events in present time, while concepts are abstract and generally timeless.

The evolution of mechanisms for the perception of objects and events at a distance (most completely through vision and hearing) freed organisms from the tyranny of reflex responses to immediate situations, and no doubt was a necessary precursor of all intelligence. It is a fairly new notion that perception itself is an intelligent activity, requiring still only partly understood problem-solving to infer the objective world from sensory signals. Earlier accounts, especially British Empiricism, portrayed sensory perception very differently, as a passive, undistorting window, through which the mind accepts sensations directly from objects. This is not consistent with physiological knowledge of the senses and the brain, nor with many phenomena, such as illusions of vision and hearing and touch. The notion of ‘direct perception’ is, however, still maintained by some followers of the American psychologist J. J. Gibson, perhaps by taking this aspect of his important writings too literally. Perception is not traditionally thought of as an intelligent activity; though the power, especially of vision, to probe distance gains the time needed for intelligent behaviour and for the intelligence of perception itself.

Are perceptions simply picked up by the senses passively, or are they created actively by the brain, or mind? This issue between passive or active perception is a long-standing debate, with significant implications, such as: what is ‘objective’ and what ‘subjective’? The philosopher John Locke (1690) suggested that there are primary characteristics, such as hardness and mass and extension of objects, in space and time — in the world before life, existing apart from mind — and secondary characteristics, created in minds or brains. Thus colours are not in the world, but are created within us, though related in complex ways to light and to the surfaces of objects. Sir Isaac Newton (Opticks, 1704) expressed clearly that red light is not itself red, but is: ‘red-making’:

…there is nothing else than a certain power and disposition to stir up the sensation of this or that colour. For as sound in a bell or musical string … is nothing but a trembling motion.

Then (in Query 23 of Opticks) Newton speculates on something like a neural mechanism of vision:

Is not vision perform'd chiefly by the Vibrations of this (Eatherial) Medium, excited in the bottom of the eye of Rays of Light, and propagated through the solid, pellucid and uniform Capillamenta of the optic Nerves in the place (the ‘Sensorium’) of Sensation?

The Empiricist school (of which, in their different ways, Locke and Newton were founders) also rejected the notion that minds can receive knowledge by direct intuition quite apart from sensory experience. Mind was now regarded as essentially isolated from the physical world — linked only by tenuous threads of nerve and by fallible inferences of what might be ‘out there’. Some people find this too unsettling to be true. But it is now generally accepted that perception depends on active, physiologically based, intelligent processes. This is not intuitively obvious, since perception seems so simple and easy and we know nothing of the processes in our brains by introspection. Seeing happens so fast and so effortlessly that it is hard to conceive the complexity of the processes that we now know are needed to interpret the nature of the visual world from sensory signals — processes that remain largely beyond the capabilities of the most advanced computers.

Paradoxically, this takes us to concepts familiar to engineers and useful for physiology. We may describe the organs of the senses as ‘transducers’, which accept patterns of energy from the external world, signalling them as coded messages to be read by the brain, which uses these patterns to infer the state-of-play of the surrounding world, and something of the body's own states. Another useful engineering concept is that of ‘channels’. The various senses feed specialized ‘brain modules’ through neural channels, discovered by physiological and ‘psychophysical’ (perceptual) experiments. Thus, as Thomas Young suggested in 1801, colour vision is created from information about the wavelength of light transmitted through three channels, red, green, and blue, responding to light of long, medium, and short wavelength, respectively. All the hundreds of colours we can see are interpretations by the brain of the relative activity of these three colour channels. The three colour channels correspond, initially, to three kinds of light-catching photopigment in the photoreceptors, called cones, in the retina.

There are similar neural channels representing the orientation of lines and edges, and for movement, as first shown by direct physiological recording from nerve cells in the visual cortex of cats by the physiologists D. H. Hubel and T. N. Wiesel in 1962. There are channels for many other visual characteristics: stereoscopic (3-D) depth, texture, spatial size, etc. The ear has many different frequency channels, and there are scores of channels for the sense of ‘touch’, including those for various kinds of pain, for tickle, and for monitoring the positions of the limbs and the stretch of muscles in order to control movement. We are unaware of activity in these sensory channels themselves. Somehow outputs from the many channels are combined to give consistent perceptions. Small discrepancies — such as the delay in sound between seeing a ball hit a bat and hearing the impact — are rejected or pulled into place to maintain a consistent world. Equally, whole objects are somehow assembled from the many signals in different sensory channels that define them. But how this (‘the binding problem’) is done is not understood.

The theory that perception is ‘cognitive’, depending on inferences from essentially inadequate sensory signals, was first clearly proposed by the German polymath physicist, physiologist, and psychologist, Hermann von Helmholtz (1821-94). He called perceptions ‘unconscious inferences’. We might say that they (our most intimate experiences and knowledge) are simply hypotheses, essentially like the predictive hypotheses of science — though not always agreeing in particular accounts.

More recently, attempts to program computers to see (an important component of artificial intelligence) has shown how hard it is to infer objects from sensed data. The most influential attempt, by physiologist David Marr, suggested that object shapes are derived from the retinal images via three essential stages:

(i) the ‘primal sketch(es) ’, describing intensity changes and locations or critical features and local geometric relations;
(ii) the ‘2 ¹/₂-D sketch’, giving a preliminary analysis of depth, surface discontinuities, and so on, in a frame that is centred on the viewer;
(iii) the ‘3-D model representation’, in an object-centred co-ordinate system, so that we see objects much as they really are in 3-D space, though they are presented from just one viewpoint. Marr supposed that this last stage is aided by restraints on the range of likely solutions to the problem of what is ‘out there’. These information-processing constraints are set by assuming typical object shapes; for example, that the shapes of many objects, such as other human beings, are modified cylinders. Interestingly, the painter Paul Cézanne came close to this notion in 1904:

Treat nature by the cylinder, the sphere, the cone, everything in proper perspective so that each side of an object or a plane is directed towards a central point … nature for us men is more depth than surface.

David Marr stressed the importance of immediate, passive processing of sensory signals, over active, cognitive ‘top down’ application of knowledge gained from the past. This is a central controversy, currently moving towards greater cognitive ‘top down’ contributions, especially for vision. When computers (or the form of computing known as ‘neural nets’) can access vast amounts of knowledge appropriately, in real time, they might share our miracle of perception.

Artists and scientists can teach each other secrets of perception (as by Gombrich, 1960), though such cross-cultural communication is not easy for most of us.

— Richard L. Gregory

Bibliography

• Gibson, J. J. (1950). Perception of the visual world. Houghton Mifflin, Boston MA.
• Gibson, J. J. (1966). The senses considered as perceptual systems. Houghton Mifflin, Boston MA.
• Gombrich, E. (1960). Art and illusion. Phaidon, London.
• Gregory, R. L. (1966, fifth edn 1998). Eye and brain. Oxford University Press.
• Hubel, D. and Wiesel, T. N. (1962). Receptive fields, binocular interaction and functional architecture in the cat's visual cortex. Journal of Physiology, 160, 106-54.
• Marr, D. (1982). Vision. W. H. Freeman, San Francisco.
• Zeki, S. (1993). A vision of the brain. Blackwell, Oxford

See also illusions; sensation; senses, extensions of; sensory integration; vision.

noun

1. The condition of being aware: awareness, cognizance, consciousness, sense. See knowledge/ignorance.
2. That which exists in the mind as the product of careful mental activity: concept, conception, idea, image, notion, thought. See thoughts.

A fundamental philosophical topic both for its central place in any theory of knowledge, and its central place in any theory of consciousness. Philosophy in this area is constrained by a number of properties that we believe to hold of perception. (i) It gives us knowledge of the world around us. (ii) We are conscious of that world by being aware of ‘sensible qualities’: colours, sounds, tastes, smells, felt warmth, and the shapes and positions of objects in the environment. (iii) Such consciousness is effected through highly complex information channels, such as the output of the three different types of colour-sensitive cells in the eye, or the channels in the ear for interpreting pulses of air pressure as frequencies of sound. (iv) There ensues even more complex neuro-physiological coding of that information, and eventually higher-order brain functions bring it about that we interpret the information so received. (Much of this complexity has been revealed by the difficulties of writing programs enabling computers to recognize quite simple aspects of the visual scene.) The problem is to avoid thinking of there being a central, ghostly, conscious self, fed information in the same way that a screen is fed information by a remote television camera. Once such a model is in place, experience will seem like a veil getting between us and the world, and the direct objects of perception will seem to be private items in an inner theatre or sensorium. The difficulty of avoiding this model is especially acute when we consider the secondary qualities (see primary/secondary qualities) of colour, sound, tactile feelings, and taste, which can easily seem to have a purely private existence inside the perceiver, like sensations of pain. Calling such supposed items names like sense data or percepts exacerbates the tendency. But once the model is in place, the first property, that perception gives us knowledge of the world around us, is quickly threatened, for there will now seem little connection between these items in immediate experience and any independent reality. Reactions to this problem include scepticism and idealism.

A more hopeful approach is to claim that the complexities of (iii) and (iv) explain how we can have direct acquaintance of the world, rather than suggesting that the acquaintance we do have is at best indirect. It is pointed out that perceptions are not like sensations, precisely because they have a content, or outer-directed nature. To have a perception is to be aware of the world as being such and such a way, rather than to enjoy a mere modification of sensation. But such direct realism has to be sustained in the face of the evident personal (neurophysiological and other) factors determining how we perceive. One approach is to ask why it is useful to be conscious of what we perceive, when other aspects of our functioning work with information determining responses without any conscious awareness or intervention. A solution to this problem would offer the hope of making consciousness part of the natural world, rather than a strange optional extra. See also observation, myth of the given.

The mental process by which the brain interprets and gives meaning to information it receives from sense organs. Perception depends on both the psychological and physiological characteristics of the perceiver, in addition to the nature of the stimuli.

Our senses probe the external world. They also tell us about ourselves, as they monitor positions of the limbs and the balance of our bodies, and through pain they signal injury and illness. More subtly, there are innumerable internal signals monitoring everyday physiological activities, and conveying and maintaining our well-being; though little of this enters our consciousness. In perception of objects and pictures, as Sir Ernst Gombrich (1950) realized to such good effect, art and science meet.

Just how we know things through sensory experience is a question that was discussed by the Greek philosophers and has continued to be discussed ever since. But, perhaps curiously, planned experiments in the spirit of the physical sciences were hardly attempted much earlier than the mid-19th century. Since then, the experimental study of perception has yielded fundamental knowledge for physiology and psychology, especially from the outstanding work of Hermann von Helmholtz (1867). It has revealed many surprises in the form of processes of which we are unaware, though they can often be demonstrated simply and dramatically as by the phenomena of illusions. The study of perception, especially of vision and hearing and touch, has allowed psychology to grow from its philosophical roots into an experimental science; yet deeply puzzling philosophical questions remain, especially over the role of consciousness. It is puzzling, both that we are aware of so little of perception, and that we have any awareness or consciousness.

There is a long-standing tradition in philosophy that perception gives undeniably true knowledge. Philosophers have traditionally sought certainty, and often claimed it, whereas scientists — who are used to their theories being modified and upset by new data — generally settle for today's best bet. Philosophers have a heavy investment in the reliability of perception, for they stake their all on the certainty of knowledge from the senses to provide secure premisses for their arguments based on experience. Scientists, on the other hand, who are used to errors in measurement and observations, have found it necessary to check, and compare, and repeat observations as they do not expect reliability from the senses. Indeed, many scientific instruments have been developed precisely because of the limitations or the unreliability of perception: for it is easy to produce and demonstrate all manner of dramatic illusions, which could hardly occur if perception were direct reliable knowledge. Yet although illusions of object perception have been discussed by philosophers from Aristotle to Berkeley and more recently, philosophy generally has paid more attention to errors of logic and ambiguity of language, than to fallibilities of perception.

Philosophers are traditionally impressed by the undeniability of the 'raw experience' of sensations. The sensation of toothache may be undeniable; but are perceptions of objects, and things happening, similarly infallible? One would think so if one believed that perceptions are simply sensations; but we now regard perceptions as giving us knowledge, albeit surprisingly indirectly, of the causes or sources of sensations — such as the states of our bodies, and especially objects in the environment — rather than of sensations themselves. It is now clear that there are vast, still largely mysterious perceptual jumps, intelligent leaps of the mind, which may land on error. One can, indeed, be wrong about the cause of toothache!

It is worth asking why we have both perceptions and conceptions of the world. Why is perception somehow separate, and in several ways different from our conceptual understanding? Very likely it is because perception, in order to be useful, must work very fast; whereas we may take minutes, hours, or years, forming concepts.

Perception is not traditionally thought of as an intelligent activity, though the power, especially of vision, to probe distance gains time needed for intelligent reactions to on-going events. It can be argued (see stimulus) that the development of distance perception freed organisms from the tyranny of reflexes, and allowed perception to be intelligent.

Are perceptions picked up by the senses, or are they created internally by the perceiver? This question about the passivity or activity of perception is long-standing, and still debated. If sensations are created by the brain, a notion that receives strong support from physiology (see visual brain in action; visual system: environmental influences; visual system: organization), we should expect to find, as we do, a vast amount of brain activity for perception. Whereas, if perceptions are simply 'picked up' (Gibson 1950, 1966), the brain would have little to do. But a very great deal goes on, physiologically and cognitively.

This raises the old question: what is 'objective' and what is 'subjective'? The philosopher John Locke (1690), who was well aware of the new science of his time, suggested that there are two kinds of characteristics: primary characteristics, such as hardness, mass, and extension of objects in space and time — being in the world before life, and quite apart from mind — and secondary characteristics, which are created by brain–mind. Thus colours are not in the world, but are created within us, though they are related in complex ways to light and the surfaces of objects.

It is generally accepted that Locke's 'primary' characteristics are present independently of mind and perception, and it is clear that 'secondary' characteristics are affected by states of the sensing organism; because colours change with adaptation, and everything appears tinged with yellow if we have jaundice. Isaac Newton, writing on sensations of colour in Opticks (1704), agreed with his friend Locke, saying that red light is not itself red, but is 'red-making'. Spelling this out, he said of light rays: 'there is nothing else than a certain power and disposition to stir up the sensation of this or that colour. For as sound in a bell or musical string ... is nothing but a trembling motion.' Then (in Query 23), he specifies something of the neural mechanism of vision that leads to the mysterious seat of sensation: 'Is not vision perform'd chiefly by the Vibrations of this (Eatherial) Medium, excited in the bottom of the eye by Rays of Light, and propagated through the solid, pellucid and uniform Capillamenta of the optic Nerves in the place (the "Sensorium") of Sensation?'

The empiricist school, of which Locke and Newton were founders, rejected the notion that had been the basis of much philosophy, that minds can receive knowledge by direct intuition, quite apart from perception. Mind was now regarded as essentially isolated from the physical world, linked by tenuous threads of nerve, sending signals to the brain, which has to make sense of sensations. At the same time, there were attempts to discover 'laws' of mind, corresponding in some ways to the laws of physics, though seldom if ever seen as being in quite the same category. Newton did, however, write (in a letter to Henry Oldenburg, secretary of the Royal Society): 'I suppose the Science of colours will be granted Mathematicall and as certain as any part of Optiques'. Laws of colour mixture were developed later, especially following the work of Thomas Young, who made the important discovery in 1801 that all the spectral colours can be produced by mixture of various intensities of only three spectral lights. This took the sensations of colour somewhat outside the realm of physics, and yet they were seen as bound by certain laws. So evidently there could be a lawful science of sensation and of mind (see colour vision: brain mechanisms; colour vision: eye mechanisms). Newton fully appreciated that colour sensations are not always given by light, as he said (Opticks, Query 16): 'When a Man in the dark presses either corner of his Eye with his finger, he will see a Circle of Colours like those of a Peacock's Tail.' At the same time, much like Pythagoras linking music with the physics of vibrating strings, Newton tried to describe aesthetics according to physical principles (Query 14):

• May not the harmony and discord of Colours arise from the proportions of the Vibrations propagated through the Fibres of the optick Nerves into the Brain, as the harmony and discord of Sounds arise from the proportions of the Vibrations of the Air? For some Colours, if they are view'd together, are agreeablezto one another, as those of Gold and Indigo, and others disagree.

So we find long-standing attempts to explain perceptual experience — from sensations to aesthetics — by physical principles of the natural sciences. But though, for example colour mixture, is linked to the physics of light, it is not derivable from optical principles. As the direct realism of immediate experience of the object world has been (almost universally) abandoned, we are left with having to devise bridging theories of perception to relate mind to the matter of the universe.

It is now generally accepted that perception depends on active physiologically based processes; but this notion is non-intuitive, for we know nothing of such processes or mechanisms by introspection, by consciousness. Moreover, perceiving objects around us seems so simple and easy! It happens so fast, and so effortlessly it is hard to conceive the complexity of the processes that we now know are involved.

This takes us to concepts familiar to engineers. It is not misleading to describe the organs of the senses — the eyes, ears, touch receptors, and so on — as 'transducers' that accept and signal patterns of energy from the external world, as coded messages, read by the brain to infer the state-of-play of the surrounding world. Another useful engineering concept is that of 'channels'. The various senses: touch and vision and hearing, and so on, are each subdivided into channels which generally can only be discovered by experiment. Thus, for example, though this was not at all realized before Young's (1801) colour mixture experiment, colour vision works with just three channels, responding to long-wavelength red, medium-wavelength green, and short-wavelength blue, light, respectively. All the hundreds of colours we see are, neurally, mixtures from these three colour channels (see yellow.) Then there are channels representing the orientation of lines and edges, and channels for movement, as shown by direct physiological recording from the visual cortex, demonstrated dramatically by David Hubel and Torstin Wiesel (1962), who received the Nobel Prize for this outstanding work. By less physiologically direct methods, such as selective adaptation, it has been found that there are more or less independent channels for spatial frequency and many other visual characteristics. The ear has many frequency channels (see hearing), and there is a score of channels for touch, various kinds of pain, tickle, and for monitoring the positions of the limbs and setting muscle tensions for moving them appropriately (see pain; tickling). Small discrepancies, such as delay in sound between seeing a ball hit by a bat and hearing the impact, are rejected or pulled into place, to maintain a consistent world. Here, the constancies (see colour perception: constancy and contrast) are very important; they modify sensations and perceptions to fit what should be there!

For signalling by the senses, as from instruments, it is important to appreciate the range of likely or possible objects that may be present (see information rate of vision; information theory). The eye receives all sorts of irrelevant stimuli, which are mainly disregarded, just as unwanted data and random disturbances are rejected whenever possible by scientific instruments, and in computer signal-processing. Sometimes, though, what is rejected turns out to be just what is needed. The immense difficulties encountered in current attempts to program computers to recognize objects from signals provided by television cameras indicate the incredible complexity and subtlety of animal and human perception (see object perception). A key, surely, is the vast knowledge needed for sophisticated perception; but as yet this is inadequate, and hard to access as needed, in computers (see artificial intelligence).

David Marr (1980) suggested that object shapes are derived from images via three essential stages: (i) the 'primal sketch'(es), describing intensity changes, locations of critical features such as terminal points, and local geometrical relations; (ii) the '2½-D sketch', giving a preliminary analysis of depth, surface discontinuities, and so on, in a frame that is centred on the viewer; (iii) the '3-D model representation', in an object-centred coordinate system, so that we see objects much as they are in three-dimensional space though they are presented from just one viewpoint. Marr supposed that this last stage is aided by restraints on the range of likely solutions to the problem of what is 'out there', the information-processing restraints being set by assuming typical object shapes, for example that many objects such as human beings, are modified cylinders, spheres, and cones. Interestingly, the painter Paul Cezanne came close to this notion in 1904: 'Treat nature by the cylinder, the sphere, the cone, everything in proper perspective so that each side of an object or a plane is directed towards a central point ... nature for us men is more depth than surface'.

The limited variety of typical objects may set restraints that are useful both for brains for perceiving, and for the artist to represent objects, and for the artificial intelligence endeavour to program computers to see. But although it can be difficult to represent or see some atypical objects (or even familiar objects from atypical viewpoints), perhaps it is not clear that these difficulties reflect perceptual restraints based on assuming cylinders, spheres, or cones, etc., for very different shapes can generally be seen without special difficulty.

Looking 'inwards' by introspection, we seem to know that perceptions are made of sensations, although from physiological and psychological experiments, as well as from the engineering approach, it has to be denied that sensations are the data for perceptions. The data are neural signals, from the transducer senses, transmitted by many parallel channels — we may say to generate predictive hypotheses, which are our perceptual reality of the object world (see perceptions as unconscious influences).

It was generally thought that perception occurs passively from inputs from the senses. It is now, however, generally accepted that stored knowledge and assumptions actively affect even the simplest perceptions. The relative importance of what are called passive 'bottom-up' processes to active 'top-down' processes, is a central controversy among those who study perception. There is more and more evidence for top-down knowledge, carried to lower-level perceptual mechanisms, some of this evidence being physiological. Psychological evidence, bearing on this, is discussed in the entry on illusions. The changes of shape of wire cubes which reverse spontaneously in depth (see Necker cubes) is clear evidence of top-down processes affecting what used to be regarded as simple sensory characteristics, such as shape and brightness. This is an example of how illusory phenomena can reveal processes of perception far removed from the world we perceive. Yet the brain is a physical system, the most wonderful machine we know.

(Published 1987)

— Richard L. Gregory

Bibliography
• Cezanne, P. (1904). Letter from Aix-en-Provence. In Rewald, J. (ed.) (1941), Letters, and in Goldwater, R., and Treves, M. (eds.) (1976). Artists on Art.
• Gibson, J. J. (1950). Perception of the Visual World.
• — —  (1966). The Senses Considered as Perceptual Systems.
• Gombrich, E. (1960). Art and Illusion.
• Gregory, R. L. (1966, 5th edn. 1997). Eye and Brain.
• Helmholtz, H. von (1867). Handbuch der Physiologischen Optic. Hamburg. (3rd edn. 1909; trans. 1924 by Southall, J. P. C., with additions, as Helmholtz's Treatise on Physiological Optics, repr. 1962).
• Hubel, D. H., and Wiesel, T. N. (1962). 'Receptive field, binocular interaction and functional architecture in the cat's visual cortex'. Journal of Physiology, 160.
• Locke, J. (1690). Essay Concerning Human Understanding.
• Marr, D. (1982). Vision: A Computational Investigation into the Human Representation and Processing of Visual Information.
• Newton, I. (1704). Opticks. (4th edn. 1730).

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The conscious mental registration of a sensory stimulus. The ability of animals to perceive is apparent from their responses to the application of stimuli but the nature of the perceptivity is only surmised. The difficulty in examining an animal is to decide whether a failure to respond to a stimulus is due to lack of perception, inability to respond or disinclination to do so.

IN BRIEF: Knowledge through the senses of the existence and properties of matter or the external world.

Possession diminishes perception of value, immediately. — John Updike

Quotes:

"Perception is a mirror not a fact. And what I look on is my state of mind, reflected outward." - A Course In Miracles

"To perceive means to immobilize... we seize, in the act of perception, something which outruns perception itself." - Henri L. Bergson

"If the doors of perception were cleansed everything would appear to man as it is, infinite. For man has closed himself up, till he sees all things thru chinks of his cavern." - William Blake

"Nothing exists until or unless it is observed. An artist is making something exist by observing it. And his hope for other people is that they will also make it exist by observing it. I call it creative observation. Creative viewing." - William S. Burroughs

"You are only as wise as others perceive you to be." - M. Shawn Cole

"To see what is right, and not do it, is want of courage, or of principle." - Confucius

See more famous quotes about Perception

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Dansk (Danish)
n. - sanseopfattelse

Nederlands (Dutch)
gewaarwording, besef, perceptie (filosofie), interpretatie/ opvatting

Français (French)
n. - (Philos, Psych) perception, idée, perspicacité, finesse, (Comm, Fisc) perception

Deutsch (German)
n. - Wahrnehmung, Empfindung, Wahrnehmungsvermögen, Vorstellung

Ελληνική (Greek)
n. - αντιληπτικότητα, αντίληψη (δια των αισθήσεων), αίσθηση

Italiano (Italian)
percezione, visione, intuizione

Português (Portuguese)
n. - percepção (f), idéia (f)

Русский (Russian)
осознание, понимание, восприятие

Español (Spanish)
n. - percepción, idea, noción, sensibilidad, comprensión

Svenska (Swedish)
n. - iakttagelseförmåga, varseblivning

中文（简体）(Chinese (Simplified))
知觉, 领悟力, 感觉

中文（繁體）(Chinese (Traditional))
n. - 知覺, 領悟力, 感覺

한국어 (Korean)
n. - 지각, 인지

日本語 (Japanese)
n. - 知覚, 理解, 認識, 知覚されたもの

العربيه (Arabic)
‏(الاسم) إدراك حسي, إدراك للحقيقه‏

עברית (Hebrew)
n. - ‮תפיסה, הבחנה, תחושה, השגה, קיבול‬

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