Lilac chaser

Lilac chaser is a visual illusion, also known as [very] cool illusion and as the pacman illusion. It consists of 12 lilac (or pink or magenta-like), blurred disks arranged in a circle (like the numbers on a clock), around a small, black, central cross on a grey background. One of the disks disappears briefly (for about 0.1 second), then the next (about 0.125 second later), and the next, and so on, in a clockwise direction. When one stares at the cross for about 20 seconds or so, one first sees a gap running around the circle of lilac disks, then a green disk running around the circle of lilac disks, then a green disk running around on the grey background, the lilac disks having disappeared.

An interactive version of the illusion may be found at http://www.michaelbach.de/ot/col_lilacChaser/index.html. This version allows viewers to adjust the colour, saturation, and timing of the disks.

The illusion spread around the internet in 2005. One reason for its popularity could be that the perception (of a moving green disk on a grey background) is so different from the stimulus (a largely static collection of lilac disks) that it makes viewers think about their connection with the physical world.

History
The illusion was devised by Jeremy Hinton some time before 2005. He stumbled across the configuration while devising stimuli for visual motion experiments. In one version of a program to move a disk around a central point, he mistakenly omitted to erase the preceding disk, giving the moving gap. On noticing the moving green-disk afterimage, he adjusted foreground and background colours, number of disks, and timing to optimize the effect.

In 2005 Hinton blurred the disks, allowing them to disappear when a viewer looks steadily at the central cross. Hinton entered the illusion in the ECVP Visual Illusion Contest, but was disqualified from his not being registered for that year's conference. He then approached Michael Bach, who placed it on his web page of illusions (http://www.michaelbach.de/ot/col_lilacChaser/index.html), and named it. Then the illusion was duplicated on a web page for Mark Levinson's Design Services (http://www.patmedia.net/marklevinson/cool/cool_illusion.html). Someone noticed this version and promoted it via e-mail or via an electronic bulletin board. Others did the same, spreading the illusion across the internet. Someone at http://www.planetperplex.com/en/color_illusions.html dubbed it the pacman illusion because the green disk appears to eat up the lilac disks. A web search for rotating pink dot (which is the illusion's most widespread nickname) in November 2005 yielded more than 800 replications.

Explanation
Lilac chaser combines three simple, and well-known effects:

First, when a visual event occurs briefly at one place in the visual field, and then a similar event occurs at an adjacent place in the same visual field, we see movement from the first place to the second. This is called apparent movement or beta movement (see also motion perception), because no actual movement has occurred. The visual events are the disappearances of the lilac disks. Initially, we see something moving around the circle of lilac disks, as though something dark has passed over them, the visual event being a disappearance. Apparent movement is the basis of moving neon signs, film, and video. We see movement because such displays stimulate receptors (called Reichardt detectors) in our brains that encode movement.

Second, when a lilac stimulus that is presented to a particular region of the visual field for a longish time (say 10 seconds or so) disappears, a green afterimage will appear. The afterimage lasts only a short time, and in this case is effaced by the reappearance of the lilac stimulus. The afterimage is a simple consequence of adaptation of the rods and cones of the retina. Colour is encoded by the ratios of activities in three types of cones. The cones stimulated by lilac get "tired". When the stimulus disappears, the tiredness of some of the cones means that the ratios evoked by the grey background are the same as if a green stimulus had been presented. Adaptation of rods and cones begins immediately they are stimulated, so afterimages also start to grow. We normally do not notice them because we move our eyes about three times a second, so the image of a stimulus constantly falls on new, "fresh", unadapted rods and cones. In this case, we keep our eyes still, so the afterimages grow and are revealed when the stimulus disappears.

Third, when a blurry stimulus is presented to a region of the visual field away from where we are fixating, and we keep our eyes still, that stimulus will disappear even though it is still physically presented. This is called Troxler's fading. It occurs because although our eyes move a little when we are fixating a point, away from that point (in peripheral vision) the movements are not large enough to shift the lilac disks to onto new neurons of the visual system. Their afterimages essentially cancel the original images, so that all one sees of the lilac disks is grey.

These effects combine to yield the remarkable sight of a green spot running around in a circle on a grey background when only stationary, flashing lilac spots have been presented.

Psychophysics
By December 2005, no systematic study of the stimulus properties of the illusion had been published. Hinton optimized the conditions for all three aspects of the illusion before releasing it. He also noted that the colour of the green disk could be outside the colour gamut of the monitor on which it was created. 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 colours, and that Troxler fading is enhanced by reducing the saturation of the disks.

Other illusions involving colour

 * Same color