Separation

January 10, 2010, 5:12 pm • Tags: , ,

icon_05Rubin’s vase is a famous set of cognitive optical illusions developed around 1915 by the Danish psychologist Edgar Rubin. The illusion generally presents the viewer with a mental choice of two interpretations, each of which is valid. Often, the viewer sees only one of them, and only realizes the second valid interpretation after some time or prompting. When the viewer attempts to simultaneously see the interpretations together, they suddenly cannot see the first interpretation anymore, and no matter how they try they simply cannot encompass both interpretations simultaneously; one occludes the other.

The illusions are useful because they are an intuitive demonstration of the figure-ground distinction the brain makes during visual perception. Rubin’s figure-ground distinction influenced the Gestalt psychologists, who discovered many similar illusions themselves. It involves higher-level cognitive pattern matching in which the overall picture determines its mental interpretation, rather than the net effect of the individual pieces.

Normally the brain classifies images by what surrounds what, establishing depth and relationships. If something surrounds another thing, the surrounded object is seen as figure, and the presumably further away (and hence background) object is the ground, and vice versa. This makes sense, since if a piece of fruit is lying on the ground, one would want to pay attention to the “figure” and not the “ground”.

However, when the contours are not so unequal, ambiguity starts to creep into the previously simple inequality and the brain must begin “shaping” what it sees. It can be shown that this shaping overrides and is at a higher level than feature recognition processes that pull together the face and the vase images. One can think of the lower levels putting together distinct regions of the picture (each region of which makes sense in isolation), but when the brain tries to make sense of it as a whole, contradictions ensue, and patterns must be discarded.

70_rubinsvase1