Season’s Greetings! But why do the circles spin?

Our visual system is primarily set up for the detection of movement sequences. Static conditions are relatively seldom in nature and are not terribly interesting for the predators with whom we share the principles of image processing. Furthermore, our heads are constantly in motion and our eyeballs are also turning relentlessly. It is therefore not surprising that sometimes we think we recognize small movements in static images, which can be explained by the movement of our head and eyeballs, and by the data processing which differs in the retina centre and periphery, and thirdly by the mechanism of the motion sensors in the primary visual cortex.
This movement illusion was designed by Professor Akiyoshi Kitaoka (Department of Psychology, Ritsumeikan University, Kyoto, Japan). It is a variation of a work of art by the Japanese artist Hajime Ouchi. The apparent movements of the circles are triggered at the boundaries between the horizontally and vertically tiled structures. The periodically occurring changes between dark and light image elements are the basic stimulus patterns for our movement detector (More illusions: www.ritsumei.ac.jp/~akitaoka/index-e.html).

Example and description are taken from „Visuelle Wahrnehmung /Visual Perception“ von/by Jörg Nänni; Verlag Niggli, 2008. www.niggli.ch