Rubin's vase (aka Rubin vase or figure--ground vase) is an optical illusion which generally presents the viewer with two shape interpretations, each of which is consistent with the retinal image, but only one of which can be maintained at a given moment.
This is because the bounding contour will be seen as belonging to the figure shape, which appears interposed against a formless background. If the latter region is interpreted instead as the figure, then the same bounding contour will be seen as belonging to it.
These types of stimuli are both interesting and useful because they provide 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 perceptions themselves.
Normally the brain classifies images by which object surrounds which – establishing depth and relationships. If one object surrounds another object, 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.
The illusion is a famous set of ambiguous or bi-stable (i.e., reversing) two-dimensional forms 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 could also be easily interpreted as a black and white beach ball.