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. One of the discs disappears briefly (for about 0.1 seconds), then the next (about 0.125 seconds later), and the next, and so on, in a clockwise direction. When one stares at the cross for at least 30 seconds, one sees three illusions The illusion was created by Jeremy Hinton some time before 2005. It then spread widely over the internet. It is a visual illusion that demonstrates color adaptation or human visual perception. The chaser effect results from the phi phenomenon illusion, combined with an afterimage effect in which an opposite color, or complementary color – green – appears when each lilac spot disappears (if the discs were blue, one would see yellow), and Troxler's fading of the lilac discs.

History

The illusion was created by Jeremy Hinton sometime before 2005. He stumbled across the configuration while devising stimuli for visual motion experiments. In one version of a program to move a disc around a central point, he mistakenly neglected to erase the preceding disc, which created the appearance of a moving gap. On noticing the moving green-disc afterimage, he adjusted foreground and background colors, number of discs, and timing to optimize the effect. In 2005 Hinton blurred the discs, allowing them to disappear when a viewer looks steadily at the central cross. Hinton entered the illusion in the European Conference on Visual Perception's Visual Illusion Contest, but was disqualified for not being registered for that year's conference. Hinton approached Michael Bach, who placed an animated GIF of the illusion on his web page of illusions, naming it the "Lilac Chaser", and later presenting a configurable Java version. The illusion became popular on the Internet in 2005.

Explanation

The lilac chaser illusion combines three simple, well-known effects, as described, for example, by Bertamini. These effects combine to yield the sight of a green spot running around in a circle on a grey background when only stationary, flashing lilac spots have been presented.

Psychophysics

Psychophysical research has used lilac chaser's properties. Hinton optimized the conditions for all three aspects of the illusion before releasing it. He also noted that the color of the green disc could be outside the color gamut of the monitor on which it was created (because the monitor never displays the green disc, only lilac ones). Michael Bach's version of the illusion allows viewers to adjust some aspects of the illusion. It is simple to confirm that the illusion occurs with other colors.