Any number of drawings are ambiguous, where a single drawing has two or more different interpretations. A famous example is the Necker cube shown off to the right. The Necker cube can either seem as if the upper square is the front face, or the lower square is the front face.

The ambiguity arises in part because our visual system evolved to make sense of a 3-dimensional world, but uses 2-D optical projections to get the job done. Many of the mechanisms we employ to extract depth from an image can be triggered by a flat 2-dimensional display that has no depth. In this case, a bunch of lines on a computer screen appears as a 3-D cube.

A brief presentation of an unambiguous stimulus can influence how an ambiguous stimulus is later interpreted. There also seems to be a fatiguing effect, where the perceived images seem to switch spontaneously from one interpretation to another, sometimes over decreasing time intervals.