Coffer Ambiguous Figure
From BurnZero
Take a look at the image to the right. Can you see the 16 squares align horizontally? However, after a brief period of time, your brain's interpretation of the Coffer Illusion may "flip," giving you the impression of 16 circles.
The Coffer Illusion takes advantage of the brain's strong preference for primarily identifying horizontal lines opposed to vertical and round shapes. It is thought that this is a canalization of the optical perception.