Perception refers to how the brain organizes and interprets sensory information. Until fairly recently, perception was considered by the school of psychology called behaviorism to be largely a passive and inevitable response to stimuli. Today's cognitive scientists, however, explain perception as an activeprocess in which the brain treats external stimuli as raw material to be shaped, aided by our experience. Earlier in this century gestalt psychologists made a major contribution to the theory of perception by studying the ways people organize and select from the multitude of stimuli that are presented to them.

The brain receives information from the environment by way of specialized sensors called receptors. These receptors respond to physical stimuli such as light, sound, touch, taste, and smell. Nature has conveniently distributed these receptors in places on the body where they will be most useful, for example, in the retina, tongue, ears, nose, and skin--what we call our sensory apparatus. Environmental inputs are received by the senses and distributed to different parts of the brain for analysis. By a process that is not understood, the brain assembles the different elements into the perceptual experiences that make up our everyday lives.

Vision is our most important sense. If our brains had to process every bit ofsensory stimulus we receive from the world, we would soon be overwhelmed. Selective attention helps us to focus only on stimuli that are needed or wantedat any instant, and to ignore less important ones. Perceptual constancy explains our tendency to interpret one object in the same way, no matter how nearor far away it is, the angle we are viewing it from, or how bright it is. Inother words, the world should look chaotic, but it doesn't.

Context--the setting in which something happens--is important to perception because we do not perceive objects in isolation. So how near one stimulus element (object in the environment) is to another, how similar the elements are,the human tendency to see complete figures, and our ability to distinguish important figures from the background will all contribute to the pattern that we perceive. Perception is also influenced by the intensity and physical dimensions of the stimulus, our own past experience, how ready we are to respond,and our motivation and emotional state.

Some perceptual abilities appear to be innate. For example, six-month-old infants are able to perceive depth. Similarly, experiments with young animals inthe laboratory show that they are reluctant to step off the edge of what appears to be a steep cliff. But learning is also assumed to play a role in perception, since infants who are deprived of sensory experience show impaired perception.

Normally the brain is able to seamlessly integrate its mental equivalent of the world outside our bodies, based on an interplay between the physiologicalactivity of the brain and external sensory stimuli. When the interplay breaksdown, however, owing to a variety of causes, perceptual disturbances can result.

Sometimes these disturbances are benign, as in certain auditory or visual illusions. An illusion is a false impression of an object or event. For example,the sound of a siren drops as it moves away from the observer, which we callthe Doppler effect. Another familiar illusion is the perception we have thatwe are moving when we are seated on a stationary train and the train on thenext track begins to pull away.

Disordered perception is also associated with a range of diseases and conditions. Hallucinations are perceptions of objects and events in the absence of any external stimulus or situation. Auditory hallucinations are a cardinal feature of schizophrenia; patients suffering from alcoholic delerium tremens mayfeel insects crawling on their skin; and patients with temporal lobe epilepsy experience certain taste sensations even when they have not eaten anything.In phantom limb syndrome, patients who have had arms or legs amputated continue to feel the arm or leg as though it were still there. Neurological diseases also cause perceptual disturbances. For example, a lesion of the right parietal cortex results in a condition called hemispatial neglect: a patient canonly perceive the right side of things. If you ask them to draw a clock, they will only draw the numbers 12 to 6. In somatoparaphrenia, patients deny possession of their own limbs.

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