Fixate the red point you will see in the center while paying
attention to the rotating ring of gray lines Most observers
perceive that the ring of lines rotates slowly, with brief jumps
of much faster rotation in the opposite direction. The very fast
jumps are illusory. The illusion usually gets stronger after one
or two jumps, and seems to be enhanced by paying close attention
to the lines in the ring. The most famous illusion of movement
is the motion aftereffect: look at uniform motion for a while,
and things will seem to move the other way. It was discovered a
long time ago by staring at a waterfall in Scotland, and, though
important, is a bit boring: the illusory motion is slower than
the real movement that induces it. As it turns out, the Scottish
waterfall actually feeds… Loch Ness, and that just under the
placid surface of the motion aftereffect lurks a true monster,
capable of producing illusory motion a hundred times faster than
the inducing movement. In the Illusion of Sex, two faces are
perceived