OPTICAL ILLUSIONS: THE LILAC CHASER

The lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac (or pink, rose or magenta), blurred discs arranged in a circle (like the numbers on a clock), around a small black, central cross on a grey background.

The unique thing with this illusion is, when one stares long enough at the cross, (say for about 20 seconds or so), one sees three different things:

1. A gap running around the circle of lilac discs;
2. A green disc running around the circle of lilac discs in place of the gap;
3. The green disc running around on the grey background, with the lilac discs having disappeared in sequence.

Stare at the center cross for at least 30 seconds to experience the three phenomena of the illusion

The chaser effect results from the beta movement illusion, combined with an afterimage effect in which an opposite, complementary, colour—green—appears when each lilac spot disappears (if the discs were blue, one would see yellow)...

Source