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The area of psychology associated with the functioning of sensory systems and how information from the external world is interpreted.

Psychologists have identified two general ways in which humans perceive their environment. One involves what is called "top-down" processing. In this mode, what is perceived depends on such factors as expectations and knowledge. That is, sensory events are interpreted based on a combination of what occurs in the external world and on existing thoughts, experience, and expectations. When a perception is based on what is expected, it is called a perceptual set, a predisposition to experience an event in a particular way. One example of such a predisposition involves hearing potentially disturbing words or phrases when rock music is played backwards. Although most people will not detect such words or phrases when they first listen to the backward sounds (when they do not have a perceptual set), these same people will hear them quite clearly if they are then told what to listen for. Psychologists regard this process as involving a perceptual set because perception of the distressing message does not occur until the individual is primed to hear it.

Motivation can also influence the way an event is perceived. At sporting events, the same episode can be interpreted in exactly opposite ways by fans of two different teams. In this instance, people are interpreting the episode with what they regard as an open mind, but their subjectivity colors their perceptions. The alternate approach is "bottom-up" processing that relies less on what is already known or expected and more on the nature of the external stimulus. If there are no preconceived notions of what to expect, cues present in the stimulus are used to a greater extent. One part of this process is called feature analysis, which involves taking the elementary cues in a situation and attempting to put them together to create a meaningful stimulus. When children listen to an initially unfamiliar set of sounds, like the "Pledge of Allegiance," they often hear words and phrases that adults (who use top-down processing) do not hear. Thus, the phrase "one nation indivisible," may be heard by a child as "one naked individual." The child has heard the correct number of syllables, some key sounds, and the rhythm of the phrase, but too many features are unclear, resulting in an inaccurate perception. In general, many psychologists have concluded that perceptual abilities rely both on external stimuli and on expectation and knowledge.

Much of the research in perception has involved vision for two general reasons. First, psychologists recognize that these this sense dominates much of human perception and, second, it is easier to study than audition (hearing) or the minor senses like taste, smell, touch, and balance. Other perceptual research has investigated the way people pay attention to the world around them and learn to ignore information that is irrelevant to their needs at any given moment.

Within the realm of vision, several areas have especially captured the attention of psychologists: depth perception, form perception, perceptual constancy, and perceptual organization. When a visual scene contains information that includes conflicting information about depth, form, and organization, the result is a visual illusion, commonly referred to as an optical illusion. Such illusions can occur when there is too little information available to generate an accurate interpretation of the stimulus; when experience leads to the formulation of a specific interpretation; or when the sensory systems process information in a consistent, but inaccurate, fashion. Illusions are completely normal, unlike delusions that may reflect abnormal psychological processes.

Another aspect of perception that psychologists have studied intensively is attention. Often, people can selectively attend to different aspects of their world and tune others out. In a loud, crowded room, for example, a person can understand a single speaker by turning his or her attention to the location of the speaker and concentrating on the frequency (pitch) of the speaker's voice; the individual can also use the meaning of the conversation to help in concentration and to ignore irrelevant speech. In some cases, however, we seem incapable of ignoring information. One common example is the "cocktail party phenomenon." If something is holding our attention but an individual within earshot speaks our name, our attention is quickly diverted to that individual. When we perceive a stimulus that is important to us (like our name), our attention switches. One famous example that involves an inability to ignore information is the Stroop effect. If words are printed in colored ink, it is normally an easy task to name the color of the ink. If the words are color names, however, (e.g., "RED") that appear in a different ink color (e.g., the word "RED" in green ink), we have difficulty naming the ink color because we tend to read the word instead of paying attention to the ink color. This process seems entirely automatic in proficient readers.

Research on the perceptual capabilities of young children is more difficult because of insufficient communication skills. At birth, infants can see objects clearly only when those objects are about eight inches (20 cm) from the eye, but distance vision improves within the first month. Infants also exhibit depth perception and appear to have some color vision. Similarly, infants can detect speech sounds shortly after birth and can locate the origin of sounds in the environment, as is smell and taste. Within a few days following birth, breast-fed babies can differentiate their own mother's milk from that of another mother, and also prefer odors that adults like and respond more negatively to the types of odors adults do not like.

Further Reading

Chapman, Elwood N. Attitude: Your Most Priceless Possession. 2nd ed. Los Altos, CA: Crisp Publications, 1990.

Eiser, J. Richard. Social Psychology: Attitudes, Cognition, and Social Behaviour. New York: Cambridge University Press, 1986.

Additional topics

Psychology EncyclopediaPsychological Dictionary: Ibn Bajjah (Abu-Bakr Muhammad ibn-Yahya ibn-al-Saʼigh, c.1106–38) Biography to Perception: cultural differences