I’ve just found this ‘spinning silhouette‘ visual illusion which took ages to take effect but when it did it was so striking I thought at first it was faked.
The idea is that you keep looking and the woman suddenly ‘flips’ and seems to spin in the opposite direction. It’s very impressive when it happens, but it seemed to happen so randomly that I wondered whether it had been programmed to randomly reverse.
However, I’ve found that if you cover image apart from the shadow of the feet and concentrate on seeing them rotate in the opposite direction, when you uncover the image, it too will seem to be in a reverse spin.
I’m guessing it works because our brain is making the best guess of a 3D shape from a 2D image. The silhouette from a real 3D rotating shape would look identical no matter what way it rotated.
Think about a rotating coin. No matter which way it turns, the silhouette would be the same – it would seem as if a disc was being progressively ‘squashed’ into a line and then back to a disc again.
As with all visual perception, our brain ‘fills in the gaps’ with best guesses, in this case to make it seem like a rotating 3D shape.
However, there’s actually no information about which way its rotating, so it can suddenly ‘flip’ when our perception of the direction becomes unstable and another interpretation takes effect.
It’s like a motion-based necker cube effect.
Link to Spinning Silhouette illusion.