ambiguous quartet | biased horizontal | biased vertical

Try this:
1) Make sure you see these dots jumping vertically.
2) Stare at the dots and count to 10.
3) Then, without moving your eyes, click on the "ambiguous quartet" link.

(It helps to position your mouse on this link ahead of time so you can keep your eyes still)


The ambiguous quartet should appear to jump in the opposite direction, horizontally.

What's happening:
This phenomenon shows how your visual system adapts to what it's viewing
and this can cause systematic illusions for what you see next, termed "aftereffects".