My theory (a-hem) about the dancer

For me, she starts to turn counter-clockwise after I pay attention to the shadow. Eventually I ignore it again and then she flips. Maybe that commenter on the neuropsychology site is right: that the shadow is only consistent with the counter-clockwise motion. Perhaps the left/right brain diagnosis is that if you pay attention to physics cues like that you are a left brainer?

Pah.

My mum reports that she sees it both ways. Genius! Anselm sees it only counter-clockwise. Blaise did at first, then switched. My explanation is not intended to explain the data, just the supposed mapping of direction onto "right/left".