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Constancies and Illusions

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Constancies and Illusions
What are Perceptual Constancies? * Tendency for the perception of an object to stay constant despite changes in stimuli * Perceptual constancies rescue us from confusion * They allow us to identify objects with different stimuli
Three types of constancies: * Size * Shape * Brightness

Size Constancy * The most studied of all constancies is size constancy, the fact that an object’s size remains relatively constant no matter what its distance. As an object moves farther away from us, we generally do not see it as decreasing in size. Hold a quarter a foot in front of you and then move it out to arm’s length. Does it appear to get smaller? Not noticeably so. Yet the retinal image of the quarter when it is 24 inches away is half the size of the retinal image of the quarter when it is 12 inches away. We certainly do not perceive the quarter as becoming half its size as we move it an arm’s length. Like other constancies, however, size constancy is not perfect; very distant objects appear to be smaller than the same objects close up, as anyone knows who has looked down from a tall building or from an airplane in flight.

Shape Constancy * Tendency for the perceived shape of an object to remain constant despite changes in its retinal image.
Ex. A book will have the same shape regardless of the angle it is viewed from.

Brightness Constancy * Tendency for the perceived brightness of an object to stay the same as long as it is illuminated by the same amount of light. * Object and background must have same lighting
Ex. A book on a lighted desk is the same as the one in a dark room.


a misrepresentation of a “real” sensory stimulus that is, an interpretation that contradicts objective “reality” as defined by general agreement. For example, a child who perceives tree branches at night as if they are goblins may be said to be having an illusion. An illusion is distinguished from a hallucination, an experience that seems to originate without an external source of stimulation. Neither experience is necessarily a sign of psychiatric disturbance, and both are regularly and consistently reported by virtually everyone.

The nature of illusions

Illusions are special perceptual experiences in which information arising from “real” external stimuli leads to an incorrect perception, or false impression, of the object or event from which the stimulation comes.
Some of these false impressions may arise from factors beyond an individual’s control (such as the characteristic behaviour of light waves that makes a pencil in a glass of water seem bent), from inadequate information (as under conditions of poor illumination), or from the functional and structural characteristics of the sensory apparatus (e.g., distortions in the shape of the lens in the eye). Such visual illusions are experienced by every sighted person.

Another group of illusions results from misinterpretations one makes of seemingly adequate sensory cues. In such illusions, sensory impressions seem to contradict the “facts of reality” or fail to report their “true” character. (For more-profound philosophical considerations, see epistemology.) In these instances the perceiver seems to be making an error in processing sensory information. The error appears to arise within the central nervous system (brain and spinal cord); this may result from competing sensory information, psychologically meaningful distorting influences, or previous expectations (mental set). Drivers who see their own headlights reflected in the window of a store, for example, may experience the illusion that another vehicle is coming toward them even though they know there is no road there.

3 Types of illusion

Optical Illusions An optical or visual illusion is a kind of illusion in which the images perceived through the sense of sight tend to be misleading or deceptive, causing errors in perception. An optical illusion is based on the process through which the brain creates a visual world in one's mind using either or both these two sources: previous memory stored in it and the current presentation of the object in the environment. In order for perception to occur, the brain tries to organize sensory information related to the object as gathered by the eye. This leads to the formation of a percept. If there are any gaps once the percept is created, the brain attempts to fill in such gaps. However, the percept may not represent or interpret the real, physical measurement of the stimulus. Thus, an optical illusion emerges.
Auditory Illusions
While optical illusions deceive the eyes through visual images, auditory illusions mislead the ears through sounds. These sounds are usually those that are not really present in the physical stimulus, but is heard by the ears and perceived as a sound related to the stimulus in the environment. There are also auditory illusions that come from "impossible sounds", such as hearing a missing fundamental frequency, provided that there are other portions of the harmonic series, and different psychoacoustic tricks of lossy audio compression.
Tactile Illusion
While optical and auditory illusions are common manifestations of several psychological disorders such as schizophrenia and psychosis, tactile illusion is experienced by patients who have undergone amputation. The phantom limb is a tactile illusion wherein the patient still 'feels' pain on the leg, arm, or digit that has already been removed.

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