Lilac chaser

Lilac chaser is a visual illusion, also known as the Pac-Man illusion. It consists of 12 lilac (or pink, rose or magenta), 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 having been erased by the green disk. This is due to an interesting effect in which the colors of the lilac disks are inverted in the optical illusion to a green color. (If the discs were blue, one would see yellow where the gaps were.)

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

The illusion spread around the Internet in 2005 (see External Links).

History

The illusion was invented 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 neglected to erase the preceding disk, which created the appearance of a 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 for not being registered for that year's conference. He then approached [http://www.uniklinik-freiburg.de/augenklinik/live/homede/mit/bach_en.html Michael Bach] , who placed it on his [http://www.michaelbach.de/ot/col_lilacChaser/index.html web page of illusions] 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.

Explanation

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. 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.The visual events in lilac chaser initially are the disappearances of the lilac disks. 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. Color 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 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 disks to new neurons of the visual system. Their afterimages essentially cancel the original images, so that all one sees of the lilac disks 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 disks, this resemblance to Pac-Man accounting for the illusion's alternative name.

Psychophysics

As of 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

The demonstration in this article shows a blurred, very slightly blue-grey disk in each gap. These can be seen by examining one frame of the image under high magnification with any adjoining lilac disks masked off. But these blurred disks are not required for the illusion. The demonstration at [http://www.michaelbach.de/ot/col_lilacChaser/index.html web page of illusions] contains no such disks, but yields the illusion.

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 center 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 after a short time. These green disks are the afterimages of the 12 lilac disks.

If one watches the illusion for long enough to see only the green disk 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 disk running around them. The smaller green disk 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 disks 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.

ee also

General:
*Subjectivism

Other illusions involving color:
*Same color

External links

These example links show the wide spread of the illusion over the internet in 2005.

*http://www.planetperplex.com/en/jeremy_hinton.html
*http://www.fusionlangaming.com/forum/showthread.php?t=7379
*http://www.bsangha.net/articles/lilac_chaser.html Jeremy Hinton's explanation
*http://www.seanbuckley.ca/blog/2005/09/03/purple-dots-mind-trick/
*http://xona.com/opticalillusions/
*http://weblog.frittle.org/article/29/optical-illusions-and-visual-phenomena
*http://www.stumbleupon.com/url/www.michaelbach.de/ot/col_lilacChaser/index.html

Further reading

* [http://www.michaelbach.de/ot/col_lilacChaser/index.html "Michael Bach's explanation]
* [http://electroneubio.secyt.gov.ar/non-optic_visual_intonation.htm "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.