Visual motion aftereffect
Instructions: Stare at the black dot and don't move your eyes! When the dot disappears, the next stimulus ("test stimulus") should appear to move opposite the direction of the previous motion.
Now try adapting to the other direction of motion--the same test stimulus will appear to go the other way!
You can see the test stimulus alone--it is an ambiguous stimulus. You can see it going either up or down!
Check out the cross-modal motion aftereffect demo