Have a look at this.
The way the dancer turns depends on how your brain works. I can see her turning both ways, but I can’t control it at all. When she was loading frame by frame, she was spinning anti-clockwise. I blink, and she starts turning in the other direction. I don’t think I’m any the wiser. (Via)
Niles's Blog 
Weird. When she started she was definitely going clockwise, then I concentrated on the words to find out about it and she changed direction.
Looking back hard at the dancer sent her clockwise again, trying to read did the same reversal.
Watching her for too long just makes me feel sick.