On Oct 23, 5:13 pm, cavelamb himself wrote:

snip

http://www.sonnyradio.com/spinninglady.html

This is extremely weird. Thanks for posting it!

I am usually pretty good at the spatial part of IQ tests, but it took
me several minutes before I could see the figure spinning
anticlockwise. If you can't see the figure spinning in one direction,
it's worth trying for a while, because when your mind makes the switch
it's really weird. Look at her arms, and when you think they're
passing in front of her body, try to see them passing behind instead.
Then once you've seen both directions, it gets a bit weird. At one
point I could see the top half turning one way and the bottom half the
other!

I think the reason the illusion works is because the frames are
ambiguous. I had a look at the frames individually using the Gimp.
With many of the frames it's impossible to tell whether the woman is
facing you or looking away from you. Furthermore, there are many
successive pairs of frames from which it's impossible to tell the
direction of rotation. I think it's a very carefully crafted graphic,
and I don't think it would work if there was more detail that the bare
outline.

Best wishes,

Chris