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Perception is the process by which we receive and interpret information from the world around us. The world around us consists of various kinds and levels of physical energy. Our knowledge of the world comes through our sense organs, which react to these energies. Certain wavelengths of electromagnetic radiation stimulate our eyes. Our ears sense certain kinds of mechanical vibrations in the air. Our noses and tongues are sensitive to certain chemical stimuli. Sense organs in our skin respond to pressure, temperature changes, and various stimuli related to pain. Sense organs in our joints, tendons, and muscles are sensitive to body movement and position. 

The sense organs change the various environmental energies into nervous impulses, which go to the brain. Through the psychological process of perception, the patterns of energies become known as objects, events, people, and other aspects of the world. 

The process of perception does not reveal objects and events of the world. We see light and colour, but there is no light or colour in the electromagnetic waves that stimulate the eyes. In the same way, there is no music or noise in the vibrations that stimulate the ear. The brain organizes and interprets nervous impulses from the eyes as light and colour, and impulses from the ears as sound. Together, the sense organs and the brain transform physical energy from environmental stimuli into information about the events around us. 

When looking at the illustration on this page, you may first see only a complicated pattern of dark and light areas. As you study the pattern, your first perception may change, particularly if you are told that a bearded man is in the picture. After you have seen the man, it will be almost impossible not to see him when you look at the picture again. 

This picture emphasizes two important points about perception. First, stimulation of the sense organs alone does not determine the nature of what is perceived. Second, perception is a dynamic process of "working on" sensory data to produce perceptual objects and events. The "work" involves many physical, physiological, and psychological factors. 

Factors affecting perception 

Various factors influence what and how we perceive. Our perceptions are influenced by the ways our bodies are structured to receive and process stimuli from the environment. Our perceptions also reflect our emotions, needs, expectations, and learning. 

Receptors. Each sensory system, such as vision, hearing, or touch, has its own specialized body parts. These parts are called receptors, and they change energies from the environment into nervous impulses. The human eye, for example, has two major kinds of receptors in the retina (the light-sensitive part of the eye). These receptors are called rods and cones. The rods respond to the intensity of light, but not to different frequencies of light (different colours). The cones do respond to different frequencies of light, and are called colour receptors. The rods allow us to see in dim light, and the cones enable us to see colours and sharp detail in bright light. Thus, the particular ways that receptors are structured and function help determine the perceptual effects related to them. 

The brain. Certain physical and functional features of the brain also determine some aspects of perception. The part of the brain that serves vision has different kinds of cells that respond only under certain conditions of stimulation. Some of these cells respond only when a light goes off. Others respond when a light comes on, but they stop responding if the light stays on. Such cells also are arranged in special ways in the brain, and this fact is related to how we perceive. For example, some cells are arranged in columns or in clusters. Such arrangements are related to how we perceive edges and forms. Experiments suggest that some cells in the brain allow us to perceive movement. Thus, the structure of the brain is an important element in perception. 

Learning, emotion, and motivation. 

Much evidence points to the conclusion that early experience, learning, emotion, and motivation are important in defining what and how we perceive. Part of this accumulating evidence comes from experiments that compare how people in different cultures perceive things. The perception of such things as form, colour, pain, and touch may differ from culture to culture, depending on habits and customs, and training of children. 

A simple example of how learning can affect perception is provided by reading the phrases inside the two triangles in the illustration on the next page. Did you fail to see the duplicate word in each phrase? Most people do, and some continue to do so even with many repeated readings. In learning to perceive words and sentences, we learn not to perceive each letter and word separately. Instead, we become able to scan the overall pattern and "fill in" the remainder. A poor reader is more likely than a good reader to see the duplicate word in each phrase. 

Some illusions are related to learning and past experience. An illusion is not a false perception, as many people believe, but one that is inconsistent with another perception. Since perception does not literally reveal the environment, no sensory system is closer to some absolute truth than any other. We tend to check visual illusions against touch, but touch can involve illusory effects, too. Look at the two triangular patches of grey containing black and white detail in the illustration on this page. If you see the patches as being different shades of grey, you are experiencing an illusion. The patches are the same shade of grey. 

Emotions and motivation can have an important effect on perception. Sometimes a severe emotional disturbance can prevent perception completely, as when emotional shock causes individuals to lose their hearing temporarily. We are more likely to perceive those aspects of our environment that are related to our motives. For example, motivation can affect the perceived characteristics of objects. To hungry people, food may appear larger or more colourful than usual. 

Understanding perception 

Types of perception. 

Perception has three levels of complexity: (1) detection, (2) recognition, and (3) discrimination. Detection refers to whether people can sense that they are being stimulated by some form of energy. For example, a light may be so dim they can barely detect its presence. Recognition means being able to identify as well as detect a particular pattern of stimulation. Discrimination means being able to perceive one pattern of stimulation as different from another. For example, a person may hear slight differences between two similar musical tones. 

The field of study that deals with levels of perception is called psychophysics. Experimental psychologists investigate the relationships between the physical properties of stimulus patterns and the perceived effects of the stimuli. For example, they may study the relationship between sound frequency and the perceived pitch of sound. 

Principles of perception. 

There are a number of general principles that help us understand the process of perception. One of the most important is the principle of closure. It tells us that we have the general tendency to perceive things as complete and unified. We tend to "fill in" parts that are missing, or parts that conform to an overall impression. 

The principle of constancy states that despite changes that occur in stimulation, we have a strong tendency to perceive objects as constant in size, shape, colour, and other qualities. For example, an orange will be perceived with its characteristic colour under different kinds of light. 

The opposite of the principle of constancy is also important. Sometimes an object or pattern of stimulation will remain constant, but the perceived effect will vary. Look at the grey and black cubes in the illustration on this page. At one moment you will see three complete cubes, and at another you may see five. 

Another important principle relates to perceptual context. The perception of an object or event depends in part on the context (surrounding conditions). Look at the two rectangles containing the words World Book in the illustration on this page. The words are printed with the same ink. Do they look the same? Background intensity and colour may affect the colour and intensity of elements upon it. To most people, grey surrounded by black appears brighter and somewhat larger. This effect is called visual induction. Notice, too, that the effect is opposite to that observed with the two grey triangles with black and white detail. In this case, the grey with black detail appears darker rather than brighter.



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