Figure-Ground Perception (Plummer)

Figure-Ground Perception is a term that is used to describe the way our minds interpret images in different sections. The main item we are seeing would be called the 'figure', and anything behind it -- in the background -- would be called the 'ground'. The following is one of the most famous examples of Figure-Ground Perception, it is an illusion called 'the Rubin Vase'. Dependent upon whether you're focusing in on the figure or the ground you will either see a vase or two faces, respectively.

 

http://www.mhhe.com/cls/psy/ch04/rubin1.gif