I think that's exactly right, Hugh. If you watch orthographic projections of a rotating object, the sign of depth is ambiguous, and often those will spontaneously switch the direction of rotation. E.g., if the leftward-moving dots are perceived as if on the front surface, you get one direction of rotation; if on the back, the opposite. After watching for a while, you can get reversals.
The same ambiguity is operating here: if you perceive the leg as coming out of the plane toward you, then you get one direction of rotation; if the other, the other. m ------ "There is no power for change greater than a community discovering what it cares about." -- Margaret Wheatley -----Original Message----- From: Hugh Foley [mailto:[EMAIL PROTECTED] Sent: Tuesday, February 05, 2008 4:35 PM To: Teaching in the Psychological Sciences (TIPS) Subject: Re: [tips] Never Seen This One Before I don't think that there is a single/simple explanation. However, my preference is to think of it as an example of the hypothesis-testing nature of perception. That is, many stimuli are open to multiple interpretations (e.g., multistable images, such as Rubin's face/vase illusion or the old lady/young girl figure). When faced with such ambiguity, our visual systems don't seize and hold a single interpretation, but continue to try out alternative explanations--which will often cause a change in interpretation. I think that anyone who looks at that rotating figure long enough will see a shift in direction of rotation. You can find other examples, such as the rotating ferris wheel. http://www.skidmore.edu/%7Ehfoley/PercLabs/Color&Motion.htm <scroll down to the bottom of #12> There was also a neat example provided by Gilbert, S. (1991) A new kinetic depth illusion..., ToP, 18, 55-56. With a simple BASIC program, Gilbert showed how you could get an illusion of depth, but then the helix-like structure would reverse direction of rotation as you watched it. Cool stuff! Hugh On Feb 5, 2008, at 11:45 AM, Larry Daily wrote: Friends, One of my colleagues here forwarded the attached image file to me and asked if I could explain why the dancer is sometimes seen as spinning to the left and sometimes to the right. I don't buy the left brain/right explanation that came with it, but I have no real idea why it works. Can anyone help me out? Thanks, Larry ************************************************************ Larry Z. Daily Associate Professor of Psychology Psychology Department Chair Department of Psychology White Hall, Room 216 Shepherd University Shepherdstown, West Virginia 25443 Psychology phone: (304) 876-5297 email: [EMAIL PROTECTED] WWW: http://webpages.shepherd.edu/LDAILY/index.html Adam's prize was open eyes His sentence was to see - The Dreamer - Tom Rush -------------------------------------------------- Hugh J. Foley Department of Psychology Skidmore College Saratoga Springs, NY 12866 518-580-5308 http://www.skidmore.edu/~hfoley -------------------------------------------------- "And I still don't know if I'm a falcon, a storm, or an unfinished song." Rilke -------------------------------------------------- --- To make changes to your subscription contact: Bill Southerly ([EMAIL PROTECTED]) --- To make changes to your subscription contact: Bill Southerly ([EMAIL PROTECTED])
