The Necker Cube is an optical illusion.

  |\       |\ 
  | \      | \
  |  \     |  \
  |   \____|___\
  |   |    |   |
  |___|____X   |
  \   |     \  |
   \  |      \ |
    \ |       \|
Is the corner marked with an X the closest corner to you, or the furthest away? Your perception of the cube can spontaneously shift between these two representations, but you can only perceive one at a time.

This is the result of encoding a 3-dimensional shape in 2D space. There is an ambiguity in the way it can be observed. Visual psychologist Richard Gregory observed that such a shift in perception is harder to obtain with a 3D wireframe of a cube. This is because the binocular vision provided by our two eyes give the depth information necessary to perceive the object correctly. Closing one eye allows the ambiguity to return.

Interestingly, when you touch an object, the sensory experience usually agrees with your vision. However, if you hold a 3D Necker Cube, you can get a perceptual shift in your vision but not in your touch, thus seperating the touch and visual spaces.

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