Lilac chaser

Demonstration of the illusion. Look at the central cross. Three effects will appear in order: First, the gap may appear to move clockwise, or a single disk adjacent to the gap may appear to move anticlockwise. This quickly stabilizes to become a gap moving clockwise. Second, the moving gap appears as a green disk. Third, the moving green disk will appear to wipe out the lilac disks, until only the green disk is visible. This picture however is not genuine: The pink discs don't just disappear - If you look at an individual frame of the animation and zoom in, you can clearly see that there is a slightly blue disc replacing what should be just a gap, possibly to reinforce the illusion.

Lilac chaser is a visual illusion, also known 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 or to have been erased by the green disk.

An interactive version of the illusion may be found here. This version allows viewers to adjust the color, saturation, and timing of the disks.

The illusion spread around the Internet in 2005.

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 colors, 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, and named it. Then the illusion was duplicated on a web page for Mark Levinson's Design Services [1]. 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. A web search for rotating pink dot (which is the illusion's most widespread nickname) in November 2005 yielded more than 800 replications.

Explanation

The lilac chaser illusion 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 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 with the rods). 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 color of the green disk could be outside the color 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 colors, and that Troxler fading is enhanced by reducing the saturation of the disks.

Other effects from lilac chaser

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 (e.g., the centre of the three o'clock disk), they 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 disks rather than a single green disk. This is because the green disk arises as an afterimage, requiring the eyes to be held steadily to occur, and the disappearance of the lilac disks 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 disks that will fade in about 20% of the time spent looking at the effect. These green disks are the afterimages of the 12 lilac disks.

Other illusions involving color

• Same color

Bibliography

• "Michael Bach's explanation
• "Electroneurobiology article". The ontological nature of the color afterimages have been analyzed in this article, "A visual yet non-optical subjective intonation", by Mariela Szirko.

See also

• Subjectivism

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