PERCEPTION

in humans, the process whereby sensory stimulation is translated into organized experience. That experience, or percept, is the joint product of the stimulation and of the process itself. Relations found between various types of stimulation (e.g., light waves and sound waves) and their associated percepts suggest inferences that can be made about the properties of the perceptual process; theories of perceiving then can be developed on the basis of these inferences. Because the perceptual process is not itself public or directly observable (except to the perceiver himself, whose percepts are given directly in experience), the validity of perceptual theories can be checked only indirectly. That is, predictions derived from theory are compared with appropriate empirical data, quite often through experimental research. Historically, systematic thought about perceiving was the province of philosophy. Indeed, perceiving remains of interest to philosophers, and many issues about the process that were originally raised by philosophers are still of current concern. As a scientific enterprise, however, the investigation of perception has especially developed as part of the larger discipline of psychology. Philosophical interest in perception stems largely from questions about the sources and validity of what is called human knowledge (see epistemology). Epistemologists ask whether a real, physical world exists independently of human experience and, if so, how its properties can be learned and how the truth or accuracy of that experience can be determined. They also ask whether there are innate ideas or whether all experience originates through contact with the physical world, mediated by the sense organs. For the most part, psychology bypasses such questions in favour of problems that can be handled by its special methods. The remnants of such philosophical questions, however, do remain; researchers are still concerned, for example, with the relative contributions of innate and learned factors to the perceptual process. Such fundamental philosophical assertions as the existence of a physical world, however, are taken for granted among most scientific students of perceiving. Typically, researchers in perception simply accept the apparent physical world particularly as it is described in those branches of physics concerned with electromagnetic energy, optics, and mechanics. The problems they consider relate to the process whereby percepts are formed from the interaction of physical energy (for example, light) with the perceiving organism. Of further interest is the degree of correspondence between percepts and the physical objects to which they ordinarily relate. How accurately, for example, does the visually perceived size of an object match its physical size as measured (e.g., with a yardstick)? Questions of the latter sort imply that perceptual experiences typically have external referents and that they are meaningfully organized, most often as objects. Meaningful objects, such as trees, faces, books, tables, and dogs, are normally seen rather than separately perceived as the dots, lines, colours, and other elements of which they are composed. In the language of Gestalt psychologists, immediate human experience is of organized wholes (Gestalten), not of collections of elements. A major goal of Gestalt theory in the 20th century was to specify the brain processes that might account for the organization of perception. Gestalt theorists, chief among them the German-U.S. psychologist and philosopher, the founder of Gestalt theory, Max Wertheimer and the German-U.S. psychologists Kurt Koffka and Wolfgang Khler, rejected the earlier assumption that perceptual organization was the product of learned relationships (associations), the constituent elements of which were called simple sensations. Although Gestaltists agreed that simple sensations logically could be understood to comprise organized percepts, they argued that percepts themselves were basic to experience. One does not perceive so many discrete dots (as simple sensations), for example; the percept is that of a dotted line. Without denying that learning can play some role in perception, many theorists took the position that perceptual organization reflects innate properties of the brain itself. Indeed, perception and brain functions were held by Gestaltists to be formally identical (or isomorphic), so much so that to study perception is to study the brain. Much contemporary research in perception is directed toward inferring specific features of brain function from such behaviour as the reports (introspections) people give of their sensory experiences. More and more such inferences are gratifyingly being matched with physiological observations of the brain itself. Many investigators relied heavily on introspective reports, treating them as though they were objective descriptions of public events. Serious doubts were raised in the 1920s about this use of introspection by the U.S. psychologist John B. Watson and others, who argued that it yielded only subjective accounts and that percepts are inevitably private experiences and lack the objectivity commonly required of scientific disciplines. In response to objections about subjectivism, there arose an approach known as behaviourism that restricts its data to objective descriptions or measurements of the overt behaviour of organisms other than the experimenter himself. Verbal reports are not excluded from consideration as long as they are treated strictly as public (objective) behaviour and are not interpreted as literal, reliable descriptions of the speaker's private (subjective, introspective) experience. The behaviouristic approach does not rule out the scientific investigation of perception; instead, it modestly relegates perceptual events to the status of inferences. Percepts of others manifestly cannot be observed, though their properties can be inferred from observable behaviour (verbal and nonverbal). One legacy of behaviourism in contemporary research on perception is a heavy reliance on very simple responses (often nonverbal), such as the pressing of a button or a lever. One advantage of this Spartan approach is that it can be applied to organisms other than man and to human infants (who also cannot give verbal reports). This restriction does not, however, cut off the researcher from the rich supply of hypotheses about perception that derive from his own introspections. Behaviourism does not proscribe sources of hypotheses; it simply specifies that only objective data are to be used in testing those hypotheses. Behaviouristic methods for studying perception are apt to call minimally on the complex, subjective, so-called higher mental processes that seem characteristic of adult human beings; they thus tend to dehumanize perceptual theory and research. Thus, when attention is limited to objective stimuli and responses, parallels can readily be drawn between perceiving (by living organisms) and information processing (by such devices as electronic computers). Indeed, it is from this information-processing approach that some of the more intriguing theoretical contributions (e.g., abstract models of perception) are currently being made. It is expected that such practical applications as the development of artificial "eyes" for the blind may emerge from these man-machine analogies. Computer-based machines that can discriminate among visual patterns already have been constructed, such as those that "read" the code numbers on bank checks. the process whereby sensory stimulation is translated into organized or meaningful experience. The perceptual process is not directly observable, but relations can be found between the various types of stimulation and their associated experiences or percepts. Empirical demonstration of the difference between sensation and perception has been a classical problem, largely because of a lack of agreement about the definition of the two terms. A common distinction is that sensations are simple sensory experiences while percepts are complex constructions of simple elements that have been joined through association. Another distinction is that perceiving is subject to the influence of learning. Percepts also have been characterized as relating to external objects while sensations are more subjective and are internally localized experiences. An anatomical distinction identifies sensation with neural events occurring near the sense organ, while percepts happen at the level of the brain. Experimental evidence suggests that percepts follow a measurable, developmental time course, and may even change with time or yield more than one percept. Such devices as the tachistoscope, which permit the duration of visual stimuli to be precisely controlled, have indicated that in human beings there is a brief period (100 to 200 milliseconds at most) during which a percept is highly vulnerable to disruption. Theorists have been divided as to whether perceptual organization is primarily innate or learned. Research has suggested that some basic visual functions, such as pattern and depth perception, are innate, but that visual experience is also important to perceptual development. Structuralist theory states that percepts are structured or synthesized from sensations. To study this concept, Edward Bradford Titchener devised a means of taking percepts apart to reveal their constitutional elements. Through the use of a device that moves the image source along with the eye, it was discovered that stabilized images seem to disappear and that some movement in retinal image is needed to maintain perception over extended periods of time. The Gestalt theorists Max Wertheimer, Kurt Koffka, and Wolfgang Khler rejected the assumption that perceptual organization was the product of learned relationships. They agreed that simple sensations could comprise organized percepts, but they maintained that percepts were basic to experience and that the human experience is of organized wholes (Gestalten) rather than collections of elements. The mind tends to fill in small gaps in a figure in order to perceive a logical whole. Not only do patterns have properties that are not inherent in the elements themselves, but, according to the principle of Prgnanz, the perceived pattern will be as good as prevailing conditions permit. A good configuration is inferred to have such properties as simplicity, stability, regularity, symmetry, continuity, and unity. Gestalt theory states that the ground (background) of a figure provides a great deal of perceptual information. The apparent brightness of a stimulus, for example, depends on the surrounding stimulation as well as the figure's own luminance. Perceptual processes may become fatigued by prolonged visual exposure to an image. Figural aftereffects refer to changes in the perceived shape or location of a figure following its inspection. Most objects tend to appear stable despite continually changing stimulus features. Perceptual constancy prevails, given the appropriate contextual cues, enabling an observer to match an object as it is perceived with the object as it is understood to exist. Perceptual functioning varies among cultures, among individuals, and even within the same individual. Perceptions may be influenced by expectancies, needs, unconscious ideas, values, and conflicts; people have a tendency to impose order and meaning upon their experiences. See also movement perception; space perception; time perception. Additional reading A clear presentation of Gestalt theory and its case against structuralism and behaviourism is found in Wolfgang Khler, Gestalt Psychology: An Introduction to New Concepts in Modern Psychology (1947, reissued 1992). A comprehensive historical overview is available in Edwin G. Boring, Sensation and Perception in the History of Experimental Psychology (1942, reissued 1977). Illustrations and discussions of Gestalt principles of organization along with material on illusions, context effects, and related phenomena are provided in William N. Dember and Joel S. Warm, Psychology of Perception, 2nd ed. (1979); James J. Gibson, The Perception of the Visual World (1950, reissued 1974); and Julian E. Hochberg, Perception, 2nd ed. (1978). Also of interest are studies by Hermann von Helmholtz, Helmholtz's Treatise on Physiological Optics, ed. by James P. Southall, 3 vol. (1924, reissued 1962; originally published in German, 3rd ed., 1909-11), the classic work on visual perception and its physiological basis; Shimon Ullman, The Interpretation of Visual Motion (1979), an original and accessible account of how we connect successive views of a moving object; and David Marr, Vision: A Computational Investigation into the Human Representation and Processing of Visual Information (1982), the book that triggered the computer revolution in vision science.Implications of research on early experience for perceptual and intellectual development are spelled out in J. McVicker Hunt, Intelligence and Experience (1961). Two excellent collections of technical articles, covering a wide range of topics, are Ralph Norman Haber (compiler), Contemporary Theory and Research in Visual Perception (1968), and Information-Processing Approaches to Visual Perception (1969). A scholarly discussion of depth perception and a lucid description of an elegant series of experiments are contained in Bela Julesz, Foundations of Cyclopean Perception (1971). Louis Jolyon West The Editors of the Encyclopdia Britannica