Last week I was at the annual meeting of the Vision Sciences Society. One of the more exciting events of this conference was the illusion contest. And of the more exciting illusions of this contest was the one that you can see below, created by Mark Wexler. (The winner of the contest, incidentally, was the awesome illusion by Suchow and Alvarez that I described earlier.)
You will see a slowly (or at least not very quickly) clockwise rotating ring. You will also see intermittent counterclockwise rotations, which are brief and much faster.
So what's going on here? Basically, and as you might have guessed, the counterclockwise jumps are illusionary. The only thing that happens is that for a few frames the coloured squares out of which the ring is composed are completely randomized. Obviously, this randomized "noise" is not really rotating in any particular direction. And yet we perceive a fairly clear counterclockwise rotation.
This illusion bears some resemblance to the traditional motion after effect, in which prolonged exposure to a motion in one direction results in a small, but clear after effect of perceived motion in the opposite direction. But usually the illusory motion is much slower than the real motion that induces the after effect. In contrast, in Wexler's illusion the illusory motion is much faster than the motion that induced the effect.
It seems to be that the "noise" amplifies the motion after effect. Even if the line-segments were not randomized, you would still perceive a regular motion after effect when the ring stops moving. The randomization appears to somehow, in a way that is not understood, greatly increase the speed of illusory motion.
One of the cool things of this illusion is that it still works if you push it to the limit. Below you can see a version of the illusion that is customizable. Just for fun, try the following settings: noise frequency = high, noise duration = very long and inducer speed = very slow.
And in case you're wondering why the illusion has been dubbed "the Loch Ness after effect": This refers to the anecdote that the motion after effect was discovered when Robert Addams looked at the Falls of Foyer, which feed into the Scottish Loch Ness. He must have felt that the world was drifting upwards after he had been staring at the falls for some time.