The video-based spinning woman illusion (http://flavor8.com/index.php/2007/06/26/spinning-woman-optical-illusion/) has been making the rounds on the internet. Olivia Carter, a neuroscientist at the University of Melbourne in Australia, and her team, found that people's pupils dilate when they switch between two alternative ways of viewing an optical illusion (e.g., the woman seems to switch direction but in fact does not). The effect results from the viewer swapping how they view her. Eye pupils dilate in stressful situations as part of the "fight-or-flight" response. The reflex is mediated by the release of the hormone noradrenalin. Noradrenalin helps you to cement decisions toward which you are moving. Pupil dilation is an outward sign of this.