An Optical Illusion
If you focus your vision on the small dot at the center of the rings in the figure below, and move your head alternately toward and away from the picture, the rings of symbols appear to rotate slightly in opposite directions.
The reason for this illusion can be inferred from the figure below, showing a side view of the optical rays for a single ring at two different distances from the focal point.
As the distance between the focal point and the picture increases, the visual image of the ring of symbols contracts, so each symbol moves radially inward. The result is illustrated below for two diametrically opposite symbols.
Since the ends of each symbol are slanted, the purely radial movement inwards gives the slanted edges a component of motion normal to themselves, which is interpreted optically as circumferential motion of the symbol.