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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 about 20 seconds or so, one sees three different things:
- A gap running around the circle of lilac discs;
- A green disc running around the circle of lilac discs in place of the gap;
- The green disc running around on the grey background, with the lilac discs having disappeared in sequence.
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), and Troxler's fading of the lilac discs.
The illusion was created 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 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 colours, number of discs, and timing to optimise 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 ECVP 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.
The lilac chaser illusion combines three simple, well-known effects:
- 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 perceive movement from the first place to the second. This is called apparent movement or beta movement, because no actual movement has occurred. Beta 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. The visual events in the lilac chaser initially are the disappearances of the lilac discs. The visual events then become the appearances of green afterimages (see next).
- When a lilac stimulus that is presented to a particular region of the visual field for a long 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 and brightness are encoded by the ratios of activities in three types of cones (and also the rods under mesopic conditions). 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 to these cones when they are fresh. Adaptation of rods and cones begins immediately when 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 the lilac chaser, we keep our eyes still, so the afterimages grow and are revealed when the stimulus disappears.
- 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 discs to new neurons of the visual system. Their afterimages essentially cancel the original images, so that all one sees of the lilac discs is grey, except for the gap where the green afterimage appears.
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. Occasionally it seems as though the green afterimage has eaten up the lilac discs, this resembles Pac-Man accounting for the illusion's alternative name.
As of May 2011[update], no systematic study of the stimulus properties of the illusion had been published. Hinton optimised the conditions for all three aspects of the illusion before releasing it. He also noted that the colour of the green disc could be outside the colour 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 colours, and that Troxler fading is enhanced by reducing the saturation of the discs.
Other effects 
It is not necessary to fixate the black cross for the effects to occur. As long as the eyes are held steadily on any point of the figure, even the centre of one of the discs, the illusion will occur.
If instead of fixating the black cross, one follows the moving gap with one's eyes, one will see only a moving gap and 12 lilac discs rather than a single green disc. This is because the green disc arises as an afterimage, requiring the eyes to be held steadily to occur, and the disappearance of the lilac discs is from Troxler's fading, also requiring the eyes to be held steadily to occur. Moving the eyes prevents the formation of afterimages and the occurrence of Troxler's fading.
If after looking at the effect for 30 seconds or so, one moves one's eyes elsewhere (e.g., to another point on the figure or to a blank sheet of white paper), one will see a stationary ring of 12 green discs that will fade after a short time. These green discs are the afterimages of the 12 lilac discs.
If one watches the illusion for long enough to see only the green disc and then moves away from the computer screen while keeping the eyes on the cross, one sees larger green spots outside a ring of lilac spots with a smaller green disc running around them. The smaller green disc may merge briefly with the outer green spots, making the spots appear to be radial blobs. The outer green spots soon fade. These outer green spots are afterimages that appear larger because of Emmert's law: the size of an afterimage becomes larger as its viewing distance is increased. They are outside because moving away from the computer screen has decreased the visual angle of the lilac spots. They fade because the lilac discs that constantly refresh the green afterimages are now projected onto a different part of the retina. If one moves towards the screen, the effects are opposite.
If one closes the right eye and moves close to the stimulus so that the nine-o'clock disc falls in the blind spot, one sees that the movement is no longer smooth. There is a noticeable pause when the disappearance of the disc occurs on the region of the retina having no rods or cones. This suggests there are limits to the filling-in that normally prevents us from noticing a black hole in our visual fields at the location of the blind spot.
See also 
- Michael Bach. "Lilac Chaser". Michaelbach.de. Retrieved 2010-11-18.