Figure-Ground Perception

Figure-ground perception is the tendency of our minds to simplify a scene into a main object (the figure) and its background (ground). Most of the time, figure-ground illustrations can be reversible, and therefore show that it triggers more than one perception.
In the picture below, you might see a vase, or you might see the faces of two people looking at each other. If you see a vase, then the vase would be considered the figure while the black space around it would be considered the ground area. If, however, you see the two faces, the faces would now be considered the figures while the white area behind them would be the ground area. This picture shows how our sense of depth and area can be changed easily depending on what one is willing to interpret.

http://psychology.about.com/od/findex/g/figure-ground-perception.htm