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Subjective constancy or perceptual constancy is the perception of an object or quality as constant even though our sensation of the object changes. [1] While the physical characteristics of an object may not change, in an attempt to deal with the external world, the human perceptual system has mechanisms that adjust to the stimulus.
Color constancy: Colour constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and ...
The MIT Encyclopedia of the Cognitive Sciences explains the illusion as an effect of "size and shape constancy [which] subjectively expand[s] the near-far dimension along the line of sight." [4] It classifies Shepard tables as an example of a geometrical illusion, in the category of an "illusion of size." [4]
The moon illusion is a good example. Before invoking concepts of apparent distance and size constancy, it helps to be sure that the retinal image hasn't changed much when the moon looks larger as it descends to the horizon. Once the signals from the retina enter the visual cortex, a host of local interactions are known to take place.
Unfamiliar objects, however, do not always follow the rules of shape constancy and may change when the perspective is changed. The Shepard tables illusion [ 27 ] is an example of an illusion based on distortions in shape constancy.
In the Müller-Lyer illusion, the visual system would in this explanation detect the depth cues, which are usually associated with 3D scenes, and incorrectly decide it is a 3D drawing. Then the size constancy mechanism would make us see an erroneous length of the object which, for a true perspective drawing, would be farther away.
Their model also demonstrates a high degree of shape constancy conserved between 2D images, which allow the 3D image to be recognized. [10] The 3-D model representations obtained from the object are formed by first identifying the concavities of the object, which separate the stimulus into individual parts.
Perhaps the most famous example of an illusory contour is the triangle configuration popularized by Gaetano Kanizsa. [2]Kanizsa figures trigger the percept of an illusory contour by aligning circles with wedge-shaped portions removed in the visual field such that the edges form a shape.