[Gathnet] Optical illusion of motion

[SKM]

http://www.cs.toronto.edu/~moraes/illusion.html

I do not know how it works, but I will bet that it is related in part to 
the color of the ovals. If you think of this as a collection of segments 
consisting of two ovals (blue and yellow) with a connecting black 
biconcave lens, all the circles appear to rotate "away from yellow" and 
"towards the blue". If you convert the picture to grayscale, the 
illusion is still present but less strong. If you rotate the image 180 
degrees, the blue and yellow circles are on opposite sides, but the 
rules just mentioned apply. If you make it a "negative" (inverting all 
colors) these rules still apply. You can see this things yourself by 
saving the picture from your browser to your desktop and then 
manipulating it with a utility such as Irfan view.

There is an explanation that I do not understand at 
http://psych.upenn.edu/backuslab/vss/vss2004/backus2004.html
Scott