The Necker cube is an optical illusion that consists of a two dimensional representation of a three dimensional wire frame cube. It is one of several well-known figures that, for the viewer, flip back and forth between equally possible perspectives of the object represented. Such drawings are known as ambiguous figures. Impossible figures, such as the impossible cube, Penrose triangle, and blivet, are a special class of ambiguous figures in which parts of the picture that are not ambiguous are drawn in incompatible perspectives. Other ambiguous figures that involve changes in content, not just perspective, include Joseph Jastrow's duck-rabbit and the Rubin vase.
Ambiguous figures provide valuable opportunities for research into perception, since the sensory input to the eyes remains constant but the information perceived by the viewer changes. The general interpretation of these occurrences is that perception is not a passive process of sensory input that results in direct perception of the "real world" outside, but rather that the perceiver actively creates reasonable interpretations of the input based on past experience as well as biologically programmed processes. The Necker cube, and other ambiguous figures, provide both opportunities for valuable research into human perceptual processes and to bring joy and fascination to many through their inclusion in works of art, showing humanity's endless fascination with the creative and unusual. Such instances can also help us realize that our own perceptions may be limited or different from those of another person viewing the same thing.
The Necker cube was first described in 1832 by Swiss crystallographer Louis Albert Necker, who observed that ambiguous cubic shapes could spontaneously switch perspective. Necker first described his findings in a letter to Sir David Brewster. Although a cube is generally used to illustrate the illusion, Necker first used a rhomboid.
The Necker cube is an ambiguous line drawing of a wire-frame cube. Drawn in isometric perspective (parallel edges of the cube are drawn as parallel lines), there are no cues to determine whether one line crosses in front of or behind another. This creates an ambiguous situation where there are two possible orientations of the three dimensional cube. When a person looks at a drawing of the Necker cube, it often appears to flip back and forth between the two valid interpretations (an effect often called multistable perception).
Because of the ambiguity of the line drawing, the brain chooses an interpretation of the ambiguous parts that makes the whole figure consistent. It is rare that one will see an inconsistent interpretation of the cube; the brain picks one of the two interpretations that would be possible in the three dimensional world. (A version of the Necker cube where the edges cross in inconsistent ways is found in the impossible cube.)
When viewing the Necker cube, people most often see the lower-left face as being in front. This is possibly because people view objects from above more often than from below. When given a choice, the brain chooses the interpretation that most closely matches everyday experience. It is interesting to note that Sidney Bradford, blind from a very early age but regaining his sight following an operation at age 52, did not perceive the ambiguity that normal-sighted observers do. Additionally, Bradford was unable to perceive depth in the illusion, which supports the idea that the brain interprets visual images based on past experiences.
There is evidence that by focusing on different parts of the figure one can force a more stable perception of the cube. At diagonally opposite corners of the rectangle in the center of the figure are two "y-junctions." By focusing on the "y-junction" in the upper right corner of the central rectangle, the lower left face will appear to be in front. By focusing on the lower junction, the upper right face will appear to be in front (Einhauser, et al., 2004).
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.
The Necker cube, like many perceptual and visual illusions, is used to further the study of how the brain and visual system perceive and interpret information.
Additionally, the Necker cube is often used as an example in epistemology (the study of knowledge). The Necker cube helps to provide a counter-attack against naïve realism, also known as direct or common-sense realism, which 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.
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