The Necker Cube is an optical illusion first published in 1832
by Swiss crystallographer Louis Albert Necker.
The Necker Cube is an ambiguous line drawing. It is a wire-frame drawing of a
cube in isometric perspective, which means that parallel edges of the cube are
drawn as parallel lines in the picture. When two lines cross, the picture does
not show which is in front and which is behind. This makes the picture
ambiguous; it can be interpreted two different ways. When a person stares at the
picture, it will often seem to flip back and forth between the two valid
interpretations (so-called multistable perception).
Necker cube on the left, impossible cube on the right.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 if 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 called an impossible object,
specifically an impossible cube.
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.
The Necker cube can shed light on the human visual system. Sidney Bradford,
blind from birth but regaining his sight following an operation at age 52, did
not perceive the ambiguity that normally sighted observers do.