Steven Pinker -- How the Mind Works - Hampshire High Italian ...

The Mind's Eye 267The shape at the top right flips between looking like a square and lookinglike a diamond, depending on whether you mentally group it with thethree shapes to its left or the eight shapes below. The imaginary linesaligned with the rows of shapes have become Cartesian referenceframes—one frame aligned with the retinal up-down, the other tilteddiagonally—-and a shape looks different when it is mentally describedwithin one or the other.And in case you are still skeptical about all these colorless, odorless,and tasteless reference frames allegedly overlaying the visual field, I giveyou a wonderfully simple demonstration from the psychologist FredAttneave. What is going on in the triangles on the left: 1Look at them long enough, and they snap from one appearance toanother. They don't move around, they don't reverse in depth, but somethingchanges. People describe the change as "which way they point."What is leaping around the page is not the triangles themselves but amental frame of reference overlaying the triangles. The frame comes notfrom the retina, the head, the body, the room, the page, or gravity, butfrom an axis of symmetry of the triangles. The triangles have three suchaxes, and they take turns dominating. Each axis has the equivalent of anorth and a south pole, which grant the feeling that the triangles arepointing. The triangles flip en masse, as if in a chorus line; the brain likesits reference frames to embrace entire neighborhoods of shapes. The trianglesin the right diagram are even more jumpy, hopping among siximpressions. They can be interpreted either as obtuse triangles lying flaton the page or as right-angle triangles standing in depth, each with a referenceframe that can sit three ways.