CONTRAST CAN CHANGE PERCEIVED DIRECTION OF MOTION:
2-D version of the Footsteps Illusion.
Fixate the red cross. The light and dark squares move along obliquely.
Now click on the Background button. This fills the background with a
plaid of vertical and horizontal stripes. Now the squares appear to wiggle in
and out toward and away from each other. Reason: As the dark square hits
a white vertical line (& its horizontal motion speeds up), it also hits a black
horizontal line (& its vertical motion slows down). To see this better, click
on the Explanation button and you ll see a light and a dark square, both
moving in step obliquely. Click on the Background button to fill the
background with a plaid. For the dark square on the left, the top and
bottom edges have high contrast, the left & right edges have low contrast
and so its vertical motion is exaggerated. For the light square on the
right the opposite is true.