This is the standard reversible Necker cube, with circles added nearby. As you observer the cube, notice that it sometimes reverses in depth. Also notice that two of the circles seem closer to you, and two further away. When the cube reverses in depth, so do the circles, suggesting that human vision assigns depth to the circles based on the depth it assigns to nearby portions of the cube. This demonstration was devised by Marc Albert, motivated by work of Allan Jepson and Whitman Richards.
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