The rings on left and right are identical, except for the four dots. But the rings on the right
look like a single rotating welded 8, whereas those on the left look like two upright rings
sliding over each other. Try to track with your eyes the intersection of the rings
It;s easy on the right, but very hard on the left, where eye tracking errors are 5--10x
greater. Conclusion: Smooth tracking eye movements are under the control of top-
down object parsing.