Rubin vase

One can then state as a fundamental principle: When two fields have a common border, and one is seen as figure and the other as ground, the immediate perceptual experience is characterized by a shaping effect which emerges from the common border of the fields and which operates only on one field or operates more strongly on one than on the other.

Rubin's vase (sometimes known as the Rubin face or the Figure-ground vase) is a famous set of cognitive optical illusions developed around 1915 by the Danish psychologist Edgar Rubin. They were first introduced at large in Rubin's two-volume work, the Danish-language Synsoplevede Figurer ("Visual Figures"), which was very well-received; Rubin included a number of examples, like a Maltese cross figure in black and white, but the one that became the most famous was his vase example, perhaps because the Maltese cross one could also be easily interpreted as a black and white beachball.

The illusion
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 they attempt to simultaneously see the second and first interpretations, 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.



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

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 takes to make sense of it as a whole, contradictions ensue, and patterns must be discarded.

Construction
The distinction is exploited by devising an ambiguous picture, whose contours match seamlessly the contours of another picture (sometimes the same picture; a practice M. C. Escher used on occasion) or more often another picture. The picture should be "flat" and have little (if any) texture to it. The stereotypical example has a vase in the center, and a face matching its contour (since it is symmetrical, there is a matching face on the other side).