This illusion of a spinning female silhouette is making the rounds these days. It's oscillating in the sense that you may perceive the figure rotating either clockwise or counterclockwise depending on which leg you are perceiving as the right leg (the one touching the ground or the one held up). It's pretty cool. I stared at it a full five minutes, only seeing it rotating CCW, before it seemed to spontaneously flip.

Here's a related illusion that's easier for me to flip.

These examples are more dramatic, but they are in the same class as the old Neckar cube and the Old Lady/Young Lady illusions.

It doesn't seem to come up much, but I'm mostly reminded of the barber poles at shops I used to get my hair cut in. I remember first being fascinated at the sensation of motion, and then at the weird property of either being able to see the stripes moving up or moving down the pole. (I was also fascinated by the seeming infinite regress of the barber shop having two walls of mirrors facing each other, looking like a corridor stretching infinity...so getting my hair cut wasn't all bad.)

Anyway, a lot of people have their pet theories about how these types of illusions work. The spinning silhouette was making the rounds on the ground that it gave some indication about whether your were more "right-brained" or "left-brained", though this appears to be made-up silliness without any actual data to back it up.

I think it has to do entirely with top-down processing, since the bottom-up information hitting your retina is exactly the same, no matter what you're perceiving. I think part of it is related to attention, which features you are attending to in the image. In the case of the Neckar cube, my perspective flips when I attend more to one of the "flippable" corners than the other. Same with the lady illusion. If you attend more to the features that could be the old lady's eye and the young lady's ear, you're more likely to see the old lady...I think because eyes are more important and salient features than ears in identifying faces. Also, I think when you have competing percepts, context effects activated at higher levels in your cortical hierarchy are pushing down and priming how you perceive the bottom-up information.

I've remember seeing explanations for the Neckar cube arguing that it's proof of the existence of a soul...hmmmmm. Silliness. Many illusions can be explained by the neuroscience...we have no reason to think this class of illusions is any different.