Editing Müller-Lyer illusion (section)

==Perspective explanation==
[[Image:Mueller lyer.svg|thumb|right|The Müller-Lyer effect in a non-illusion]]
One possible explanation, given by [[Richard Gregory]],<ref>{{cite book | vauthors = Gregory RL |title=Eye and Brain: The Psychology of Seeing |date=2005 |publisher=Princeton University Press |location=Princeton, N.J |isbn=978-0-19-852412-0 |edition=5th}}</ref> is that the Müller-Lyer illusion occurs because the visual system learns that the "angles in" configuration corresponds to a rectilinear object, such as the convex corner of a room, which is closer, and the "angles out" configuration corresponds to an object which is far away, such as the concave corner of a room. However, in a recent report<ref>{{cite journal | vauthors = Howe CQ, Purves D | title = The Müller-Lyer illusion explained by the statistics of image-source relationships | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 102 | issue = 4 | pages = 1234–1239 | date = January 2005 | pmid = 15657142 | pmc = 544622 | doi = 10.1073/pnas.0409314102 | doi-access = free }}</ref> Catherine Howe and Dale Purves contradicted Gregory's explanation:
<blockquote>Although Gregory's intuition about the empirical significance of the Müller-Lyer stimulus points in the right general direction (i.e., an explanation based on past experience with the sources of such stimuli), convex and concave corners contribute little if anything to the Müller-Lyer effect.</blockquote>

Neural nets in the visual system of human beings learn how to make a very efficient interpretation of [[Three-dimensional space|3D]] scenes. That is why when somebody goes away from us, we do not perceive them as getting shorter. And when we stretch one arm and look at the two hands we do not perceive one hand smaller than the other. Visual illusions are sometimes held to show us that what we see is an image created in our brain. Our brain supposedly projects the image of the smaller hand to its correct distance in our internal 3D model. This is what is called the [[size constancy]] mechanism hypothesis.

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 (visual)|perspective]] drawing, would be farther away.

In the perspective drawing in the figure, we see that in usual scenes the heuristic works quite well. The width of the rug should obviously be considered shorter than the length of the wall in the back.