Figure and Ground

Figure/ ground is the term used to indicate the ability of human to diferrentiate elements according to their contrast. When we look at an image or picture, our visual system tends to simplify visual things into figures. Figure and ground cannot be seen at one time but we can see them sequentially.
Usually, figure occupies a smaller area than does ground.
Understanding how we see thing can be very useful especially in design. It is impossible to have figure without ground. The red circle we draw in white sheet of paper is figure. The white paper left alone is ground. However, the negative space also plays an important role because the white space gives our eyes the space to rest.
This is an example :

http://en.wikipedia.org/wiki/Image:Facevase.JPG

What do you see in this image , a vase or 2 face staring at each other? It depends on you. You can see a vase if you see the figure is in white and the ground is in black and vice versa, you can see 2 faces.

So, Figure and ground plays an important role in when we see picture. We can learn from the principle of figure and ground to make out piece much more interesting. The following is some interesting pictures that trick our eyes :