Figure-Ground Perception

Figure-ground perceptions describes how we perceive any object, called the figure, as distinct from it's surroundings, called the ground. In most situations we can clearly determine what is the figure and what is the ground, but in some cases the lines become blurred and we see that the same stimulus can trigger more than one perception. We use cues like the size, shape, and positioning of lines and shapes in the pictures to help us determine what is front and center and what is in the background.

http://en.wikipedia.org/wiki/Figure%E2%80%93ground_(perception)

In this image, it is hard to tell what is the figure and what is the ground. It can be perceived as a white vase set against a black background, or as two black faces in a white background.