On a gray surround, the light and dark sqaures are correctly seen as moving
at constant speed side by side. Now click on the "Background" button. This fills the
with stationary black/white stripes. Now the squares seem to speed up and slow
down in alternation, like the two feet of a walking person. The illusion is strongest
in peripheral vision, when you fixate the red cross.
Reason: CONTRAST (=brightness difference) affects apparent speed!
When the dark square crosses a white stripe its contrast is high and it seems to
speed up. When it crosses a black stripe its contrast falls and it seems to slow down.
The opposite is true for the light square.

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