Moving Plate Optical Illusion

Can you see how this optical illusion seems to move and shift?

See Another Random Optical Illusion

Scroll down for the explanation.

Most people will see this optical illusion move and shift when they look at it for a short while. Many people also report seeing the center section shimmer and shake.

Explanation

We live in a three dimensional world. Our mind tries to convert a two dimensional object into a 3D one. In this optical illusion, our mind tries to make sense of these 2D objects. The shading used in this drawing is completely incongruous. We lock onto a bit of the picture that looks appropriately shaded. Then we find it conflicting with shading in other nearby areas. We keep jumping around the drawing trying to map out a 3D solution that would make the shading make sense, but it never seems to work.