Extracted from:http://theness.com/neurologicablog/?p=27


Take a look at the spinning girl below. Do you see it spinning clockwise or counter-clockwise? I see it spinning counter-clockwise, and I had a hard time getting it to switch direction. Give it a try.
[img]http://www.phy.ntnu.edu.tw/ntnujava/index.php?action=dlattach;topic=2055.0;attach=3749;image[/img]

These kinds of optical illusions are always fun. What they reveal is how our brain processes visual information in order to create a visual model of the world. The visual system evolved to make certain assumptions that are almost always right (like, if something is smaller is it likely farther away). But these assumptions can be exploited to created a false visual construction, or an optical illusion.
The spinning girl is a form of the more general spinning silhouette illusion. The image is not objectively “spinning” in one direction or the other. It is a two-dimensional image that is simply shifting back and forth. But our brains did not evolve to interpret two-dimensional representations of the world but the actual three-dimensional world. So our visual processing assumes we are looking at a 3-D image and is uses clues to interpret it as such. Or, without adequate clues it may just arbitrarily decide a best fit – spinning clockwise or counterclockwise. And once this fit is chosen, the illusion is complete – we see a 3-D spinning image.
By looking around the image, focusing on the shadow or some other part, you may force your visual system to reconstruct the image and it may choose the opposite direction, and suddenly the image will spin in the opposite direction.
The following is the same gif image displayed as a EJS created java simulation.
You can click the check box to watch how image change step by step and find out what really happened!

translate strings in simulation to different language format before download
Full screen applet or Problem viewing java?Add http://www.phy.ntnu.edu.tw/ to exception site list
Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
Download EJS jar file(1146.2kB):double click downloaded file to run it. (15 times by 11 users) , Download EJS source View EJS source


Actually, the above image is a combination of the following two images.


Clockwise imageCounterclockwise image
[img]http://www.phy.ntnu.edu.tw/demolab/phpBB/webdata/177_spin-cw.gif[/img][img]http://www.phy.ntnu.edu.tw/demolab/phpBB/webdata/178_spin-ccw.gif[/img]

The following flash animation let you play the image step by step,too!
Can you find out what is really going on!