The coupling of motion and direction is based partially on the objects’ synchronicity in time (and speed). Physiologists have proposed that such perceptual groupings may arise when there is synchronization of nerve impulses that are evoked in multiple brain regions by the individual quartets or ballerinas. If so, what happens if ballerinas in different parts of the visual field spin at speeds slightly different from one another? Would they still get synchronized? What if ballerinas were of nonuniform sizes? Could you uncouple large and small ones then? Have fun!
Note: This article was originally printed with the title, "The Power of Symmertry".