Moving Circles Illusion

You can see how this optical illusion seems to move and shift? Most people can see this optical illusion move and shift if they look at it for a short while.
Since we live in a 3D world, our mind tries to convert a 2D object into a 3D one. In this optical illusion, we try to make sense of these 2D objects. The shading used is completely incongruous. The eyes lock onto a part of the illustration that looks appropriately shaded but then find it disjointed with shading in other nearby areas. The eyes keep jumping around the drawing trying to map out a 3-D solution that would make the shading illustration make sense, but it never seems to work.