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The Purple Nurple Optical Illusion by Walter Anthony
Neatorama reader Walt Anthony shares with us his blog, geared to educate kids about optical illusions. Walt did a good job in explaining the illusions in the post:
Anomalous Motion Optical Illusion aka Peripheral Drift Optical Illusion is characterized by anomalous motion that can be observed in peripheral vision. [...]
Keep in mind that this is a static image. It is not animated in any way. but as your vision moves back and forth the center area seems to be moving toward the center (contracting) and the outer edges seem to be moving away (expanding) from the center. Also worth noting is that if you fixate on a point in the center and don't move your eyes this anomalous motion will stop.
I also applaud him for giving credit where credits are due - most optical illusion blogs rip off images without giving any clue as of the artist.