What is needed
What to Do: Head Shrinker
What to Do: Head Expander
The same thing, but backwards!
What is Happening
This is an example of what is known as the "waterfall effect." If you stare at something brightly colored for a long time, you will see an afterimage of the color - in a complementary (opposite) color. (See the page Ghost of the Little Shop for more details.) This animation shows a similar effect: after watching it for a while, you see an afterimage of the motion - and the afterimage is the opposite of the motion you stared at. If the circles were expanding, the afterimage causes things to look like they are shrinking. If the circles were shrinking, the afterimage causes things to look like they are expanding!
Of course, you can do this with a waterfall - which is where the name for the effect came from! If you stare at a waterfall for a long time and then look at something else, it will appear to rise!
Other Things to Try
As the animations play, motion and color illusions start to occur. I see distinct colors appear - and they are different on the top and the bottom of the animations. These are afterimages of the light and dark areas, and produce an illusion of color as different color sensors in your eyes respond at different rates. A similar effect is produced by something called a "Benham's Disk," something we have in the Little Shop, and something we will put on the Web at some point.
I think how well you see this one might be age-related. I have found a bunch of different optical effects and optical illusions that work well for young people or old people but not both, and I suspect this is one. If you notice any such effects, you might let us know.