Rubin's vase is an ambiguous image, which can be seen as vase or as two faces looking at one another. Human viewers tend to see such images flip quasi-periodically between the two interpretations: vase, then faces, then vase again ... This gives clues as to how the brain deals with ambiguity and hence lessons for computer disambiguation.
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Links:
Wikipedia: Rubin vase
Rubin's Vase Ataturk.svg: NevitNevit Dilmen, CC BY-SA 3.0, via Wikimedia Commons
