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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.
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]
Color constancy and brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color difference or luminosity difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed.
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 also at sunset ...
Emmert's law has been used to investigate the moon illusion (the apparent enlargement of the moon or sun near the horizon compared with higher in the sky). [7] [8] A neuroimaging study that examined brain activation when participants viewed afterimages on surfaces placed at different distances found evidence supporting Emmert's Law and thus size constancy played out in primary visual cortex ...
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.
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]
An example of the Ponzo illusion. Both of the horizontal yellow lines are the same length. The Ponzo illusion is a geometrical-optical illusion that takes its name from the Italian psychologist Mario Ponzo (1882–1960). Ponzo never claimed to have discovered it, and it is indeed present in earlier work.