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The Necker cube is an ambiguous line drawing.
The effect is interesting because each part of the picture is ambiguous by itself, yet the human visual system picks an interpretation of each part that makes the whole consistent. The Necker cube is sometimes used to test computer models of the human visual system to see whether they can arrive at consistent interpretations of the image the same way humans do.
Humans do not usually see an inconsistent interpretation of the cube. A cube whose edges cross in an inconsistent way is an example of an impossible object, specifically an impossible cube (compare Penrose triangle).
With the cube on the left, most people see the lower-left face as being in front most of the time. This is possibly because people view objects from above, with the top side visible, far more often than from below, with the bottom visible, so the brain "prefers" the interpretation that the cube is viewed from above. Another reason behind this may be due to the brain's natural preference of viewing things from left to right, therefore seeing the leftmost square as being in front.
There is evidence that by focusing on different parts of the figure one can force a more stable perception of the cube. The intersection of the two faces that are parallel to the observer forms a rectangle, and the lines that converge on the square form a "y-junction" at the two diagonally opposite sides. If an observer focuses on the upper "y-junction" the lower left face will appear to be in front. The upper right face will appear to be in front if the eyes focus on the lower junction.
|It is possible to cause the switch to occur by focusing on different parts of the cube. If one sees the first interpretation on the right it is possible to cause a switch to the second by focusing on the base of the cube until the switch occurs to the second interpretation. Similarly, if one is viewing the second interpretation, focusing on the left side of the cube may cause a switch to the first.|
The Necker cube has shed light on the human visual system. The phenomenon has served as evidence of the human brain being a neural network with two distinct equally possible interchangeable stable states. Sidney Bradford, blind from the age of ten months but regaining his sight following an operation at age 52, did not perceive the ambiguity that normal-sighted observers do.
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The Necker cube is used in epistemology (the study of knowledge) and provides a counter-attack against naïve realism. Naïve realism (also known as direct or common-sense realism) states that the way we perceive the world is the way the world actually is. The Necker cube seems to disprove this claim because we see one or the other of two cubes, but really, there is no cube there at all: only a two-dimensional drawing of twelve lines. We see something which is not really there, thus (allegedly) disproving naïve realism. This criticism of naïve realism supports representative realism.
A rotating Necker cube was used to demonstrate that the human visual system can recruit new visual cues that affect the way things look.
A doctored photograph purporting to be of an impossible cube was published in the June 1966 issue of Scientific American, where it was called a "Freemish Crate".
References in popular culture 
The Necker cube is discussed to such extent in Robert J. Sawyer's 1998 science fiction novel Factoring Humanity that "Necker" becomes a verb, meaning to impel one's brain to switch from one perspective or perception to another.
In Grant Morrison's Doom Patrol comic book, issue 36/September 1990 page 14, a character named Mr. Jones, leader of one iteration of the Men from N.O.W.H.E.R.E., as well as a former member of the Men in Blue of a shadowy faction of the U.S. military, constructs a three-dimensional Necker cube out of paper, meditates upon it in order to create new agents for his organization, and calling it a "delirium box," uses it as a weapon by making others look into it.
See also 
- Necker, L.A. (1832). "Observations on some remarkable optical phaenomena seen in Switzerland; and on an optical phaenomenon which occurs on viewing a figure of a crystal or geometrical solid". London and Edinburgh Philosophical Magazine and Journal of Science 1 (5): 329–337.
- Einhäuser, W.; Martin, K.A.C.; König, P. (2004). "Are switches in perception of the Necker cube related to eye position?". European Journal of Neuroscience 20 (10): 2811–2818. doi:10.1111/j.1460-9568.2004.03722.x. PMID 15548224.
- Marr, D. (1982). Vision: A Computational Investigation into the Human Representation and Processing of Visual Information. W.H. Freeman. Reprint: The MIT Press. ISBN 0-7167-1284-9.
- Gregory, R. (August 2004). "The Blind Leading the Sighted: An Eye-Opening Experience of the Wonders of Perception" (pdf). Nature 430 (7002). doi:10.1038/430836a. PMID 15318199.
- Sawyer, Robert J. (1998). Factoring Humanity. New York: Tor. pp. 233, 256, 299, et al.
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