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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]
Originally developed in 1978 by Vandenberg and Kuse [5] based on the research by Shepard and Metzler (1971), [1] a Mental Rotation Test (MRT) consists of a participant comparing two 3D objects (or letters), often rotated in some axis, and states if they are the same image or if they are mirror images (enantiomorphs). [1]
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. [2]
It was taken up by Wilhelm Wundt, widely regarded as the founder of experimental psychology, and is now universally used. [ 1 ] [ 2 ] [ 3 ] That by 1972 the first edition of Robinson's book devotes 100 closely printed pages and over 180 figures to these illusions attests to their popularity.
It contradicts the Ebbinghaus illusion (1898) and the Obonai square illusion (1954). In fact, the central test shape (a cross) surrounded by large squares appears larger instead of smaller. Sarcone's Cross illusion consists of a cross (the test shape) surrounded by sets of squares of distinct size (the inducing shapes).
Movement also helps; the figure may be moving against a static environment. Color is also a cue because the background tends to continue as one color behind potentially multiple foreground figures, whose colors may vary. Edge assignment also helps; if the edge belongs to the figure, it defines the shape while the background exists behind the shape.
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.
The illusion occurs when a person underestimates the weight of a larger object (e.g. a box) when compared to a smaller object of the same mass.The illusion also occurs when the objects are not lifted against gravity, but accelerated horizontally, so it should be called a size-mass illusion. [6]