....sounds pejorative, as if exposing a malfunction of the visual system. Rather, I view these phenomena as bringing out particular good adaptations of our visual system to standard viewing situations. These adaptations are »hard-wired« in our brains, and thus under some artificial manipulations can cause inappropriate interpretations of the visual scene. As Purkinje put it: "Illusions of the senses tell us the truth about perception."
--cited by Teuber, 1960.
Watch the '+' in the middle very closely. You will see the purple dots disappear then move your head slightly off center, the dots will reappear.
Again, if you try to concentrate more on the '+', after few seconds, you see only a single green color dot moving around the '+'.
This illusion was designed by Jeremy L Hinton of Bristol, UK, and this illusion - with a lot more explanation and many other illusions as well - can be found at the web site of Michael Bach.
Note: Don’t distress yourself if you don’t see the effect described, even if trying carefully. For many illusions, there is a small percentage of people with perfectly normal vision who just don’t see it, for reasons currently unknown!
Illusions and Paradoxes - Seeing is Believing?