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Shepard tables (also known as the Shepard tabletop illusion) are an 
To quote ''A Dictionary of Psychology'', the Shepard table illusion makes "a pair of identical
Animation of the illusion
Opticalillusion.net.
{{Optical illusions Optical illusions
Shepard tables (also known as the Shepard tabletop illusion) are an optical illusion
Within visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide v ...
first published in 1990 as "Turning the Tables," by Stanford psychologist Roger N. Shepard
Roger Newland Shepard (January 30, 1929 – May 30, 2022) was an American cognitive science, cognitive scientist and author of the "universal law of generalization" (1987). He was considered a father of research on spatial relations. He studied m ...
in his book ''Mind Sights'', a collection of illusions that he had created. It is one of the most powerful optical illusions, typically creating length miscalculations of 20–25%.
To quote ''A Dictionary of Psychology'', the Shepard table illusion makes "a pair of identical parallelogram
In Euclidean geometry, a parallelogram is a simple (non- self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of eq ...
s representing the tops of two tables appear radically different" because our eyes decode them according to rules for three-dimensional objects.
This illusion is based on a drawing of two parallelograms, identical aside from a rotation of 90 degrees. When the parallelograms are presented as tabletops, however, we see them as objects in three-dimensional space. One "table" seems long and narrow, with its longer dimension receding into the distance. The other "table" looks almost square, because we interpret its shorter dimension as foreshortening
Linear or point-projection perspective (from la, perspicere 'to see through') is one of two types of graphical projection perspective in the graphic arts; the other is parallel projection. Linear perspective is an approximate representation, ...
. The ''MIT Encyclopedia of the Cognitive Sciences'' explains the illusion as an effect of "size and shape constancy hich
Ij ( fa, ايج, also Romanized as Īj; also known as Hich and Īch) is a village in Golabar Rural District, in the Central District of Ijrud County, Zanjan Province, Iran
Iran, officially the Islamic Republic of Iran, and also ...
subjectively expand /nowiki> the near-far dimension along the line of sight." It classifies Shepard tables as an example of a geometrical illusion, in the category of an "illusion of size."
According to Shepard, "any knowledge or understanding of the illusion we may gain at the intellectual level remains virtually powerless to diminish the magnitude of the illusion." Children diagnosed with autism spectrum disorder
The autism spectrum, often referred to as just autism or in the context of a professional diagnosis autism spectrum disorder (ASD) or autism spectrum condition (ASC), is a neurodevelopmental condition (or conditions) characterized by difficulti ...
are less susceptible to the Shepard table illusion than typically developing children but are equally susceptible to the Ebbinghaus illusion
The Ebbinghaus illusion or Titchener circles is an optical illusion of relative size perception. Named for its discoverer, the German psychologist Hermann Ebbinghaus (1850–1909), the illusion was popularized in the English-speaking world by Edw ...
.
Shepard had described an earlier, less-powerful version of the illusion in 1981 as the "parallelogram
In Euclidean geometry, a parallelogram is a simple (non- self-intersecting) quadrilateral with two pairs of parallel sides. The opposite or facing sides of a parallelogram are of equal length and the opposite angles of a parallelogram are of eq ...
illusion" (''Perceptual Organization,'' pp. 297–9). The illusion can also be constructed using identical trapezoid
A quadrilateral with at least one pair of parallel sides is called a trapezoid () in American and Canadian English. In British and other forms of English, it is called a trapezium ().
A trapezoid is necessarily a convex quadrilateral in Eucli ...
s rather than identical parallelograms.
A variant of the Shepard tabletop illusion was named "Best Illusion of the Year" for 2009.
Christopher W. Tyler, among others, has done scholarly research on the illusion.
References
External links
Animation of the illusion
Opticalillusion.net.
{{Optical illusions Optical illusions