Necker cube

[1] It is a simple wire-frame, two dimensional drawing of a cube with no visual cues as to its orientation, so it can be interpreted to have either the lower-left or the upper-right square as its front side.

[6] The phenomenon has served as evidence of the human brain being a neural network with two distinct equally possible interchangeable stable states.

[8][2] During the 1970s, undergraduates in the Psychology Department of City University, London, were provided with assignments to measure their Introversion-Extroversion orientations by the time it took for them to switch between the Front and Back perceptions of the Necker Cube.

[11] The Necker cube is used to illustrate how vampires in Peter Watts' science fiction novels Blindsight (2006) and Echopraxia (2014) have superior pattern recognition skills.

One of the pieces of evidence is that vampires can see both interpretations of the Necker Cube simultaneously, which sets them apart from baseline humanity.

Necker cube on the left, impossible cube on the right
This is an example of two identical Necker cubes, the one on the left showing an intermediate object (blue bar) going in "down from the top" while the one on the right shows the object going in "up from the bottom" which shows how the image can change its perspective simply by changing which face (front or back) appears behind the intervening object.