Shape
As the negative spaces begin to 
approach the scale or complexity of the positive shapes, then the figure/ground relationships can become unstable and ambiguous.    This ambiguity repeatedly reverses our perceptions of what is the figure and what is the ground. 
The face/vase illusion is the most famous example of this. We can perceive the image as either a black vase on a white ground, or as a pair of white faces on a black ground. Even though we can flip back and forth very quickly, we cannot see both vase and faces simultaneously.