Lilac chaser is a visual illusion, also known as the Pac-Man

Pac-Man

is an arcade game developed by Namco and licensed for distribution in the United States by Midway, first released in Japan on May 22, 1980. Immensely popular from its original release to the present day, Pac-Man is considered one of the classics of the medium, virtually synonymous with video games,...

 illusion. It consists of 12 lilac

Lilac (color)

Lilac is a color that is a pale tone of violet that is a representation of the average color of most lilac flowers. It might also be described as light purple. The colors of some lilac flowers may be equivalent to the colors shown below as pale lilac, rich lilac, or deep lilac...

 (or pink

Pink

Pink is a mixture of red and white. Commonly used for Valentine's Day and Easter, pink is sometimes referred to as "the color of love." The use of the word for the color known today as pink was first recorded in the late 17th century....

, rose

Rose (color)

Rose is the color halfway between red and magenta on the HSV color wheel, also known as the RGB color wheel, on which it is at hue angle of 330 degrees.Rose is one of the tertiary colors on the HSV color wheel...

 or magenta

Magenta

Magenta is a color evoked by light stronger in blue and red wavelengths than in yellowish-green wavelengths . In light experiments, magenta can be produced by removing the lime-green wavelengths from white light...

), blurred discs arranged in a circle (like the numbers on a clock

Clock

A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...

), around a small black, central cross on a grey

Grey

Grey or gray is an achromatic or neutral color.Complementary colors are defined to mix to grey, either additively or subtractively, and many color models place complements opposite each other in a color wheel. To produce grey in RGB displays, the R, G, and B primary light sources are combined in...

 background. One of the discs 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 sees three different things:

1. A gap running around the circle of lilac discs;
2. A green
   Green
   Green is a color, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 520–570 nanometres. In the subtractive color system, it is not a primary color, but is created out of a mixture of yellow and blue, or yellow and cyan; it is considered...
   
    disc running around the circle of lilac discs in place of the gap;
3. The green disc running around on the grey background, with the lilac discs having disappeared in sequence.

The illusion is due to three, well-known effects:

1. An illusion of movement called beta movement
   Beta movement
   The Beta movement is an optical illusion, first described by Max Wertheimer in 1912. Its illusion is that fixed images seem to move, even though of course the image does not change. It might be considered similar to the effects of animation...
   
   ;
2. An afterimage
   Afterimage
   An afterimage or ghost image or image burn-in is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased...
   
    in which an opposite, complementary, colour—green—appears when each lilac spot disappears (if the discs were blue, one would see yellow);
3. Troxler's fading
   Troxler's fading
   Troxler's fading or Troxler's effect is a phenomenon of visual perception. When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point, in peripheral vision, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low...
   
    of the lilac discs.

History

The illusion was invented by Jeremy Hinton

Jeremy Hinton

Jeremy L. Hinton is a vision expert working in the UK as of 2009. In 2005, he invented the popular illusion Lilac chaser. Apart from his interest in visual perception, he is also a keen balloonist.-Bibliography:...

 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

Internet

The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...

 in 2005.

Explanation

The lilac chaser illusion combines three simple, well-known effects:

1. 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
   Motion perception
   Motion perception is the process of inferring the speed and direction of elements in a scene based on visual, vestibular and proprioceptive inputs...
   
    from the first place to the second. This is called apparent movement or beta movement
   Beta movement
   The Beta movement is an optical illusion, first described by Max Wertheimer in 1912. Its illusion is that fixed images seem to move, even though of course the image does not change. It might be considered similar to the effects of animation...
   
   , because no actual movement has occurred. Beta movement is the basis of moving neon sign
   Neon sign
   Neon signs are made using electrified, luminous tube lights that contain rarefied neon or other gases. They are the most common use for neon lighting, which was first demonstrated in a modern form in December, 1910 by Georges Claude at the Paris Motor Show. While they are used worldwide, neon signs...
   
   s, film
   Film
   A film, also called a movie or motion picture, is a series of still or moving images. It is produced by recording photographic images with cameras, or by creating images using animation techniques or visual effects...
   
   , and video
   Video
   Video is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.- History :...
   
   . We see movement because such displays stimulate receptors (called Reichardt detectors) in our brain
   Visual cortex
   The visual cortex of the brain is the part of the cerebral cortex responsible for processing visual information. It is located in the occipital lobe, in the back of the brain....
   
   s that encode movement. The visual events in lilac chaser initially are the disappearances of the lilac discs. The visual events then become the appearances of green afterimages (see next).
2. 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
   Afterimage
   An afterimage or ghost image or image burn-in is an optical illusion that refers to an image continuing to appear in one's vision after the exposure to the original image has ceased...
   
    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
   Adaptation
   An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
   
    of the rods
   Rod cell
   Rod cells, or rods, are photoreceptor cells in the retina of the eye that can function in less intense light than can the other type of visual photoreceptor, cone cells. Named for their cylindrical shape, rods are concentrated at the outer edges of the retina and are used in peripheral vision. On...
   
    and cones
   Cone cell
   Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
   
    of the retina
   Retina
   The vertebrate retina is a light-sensitive tissue lining the inner surface of the eye. The optics of the eye create an image of the visual world on the retina, which serves much the same function as the film in a camera. Light striking the retina initiates a cascade of chemical and electrical...
   
   . 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 lilac chaser, we keep our eyes still, so the afterimages grow and are revealed when the stimulus disappears.
3. 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
   Troxler's fading
   Troxler's fading or Troxler's effect is a phenomenon of visual perception. When one fixates a particular point, after about 20 seconds or so, a stimulus away from the fixation point, in peripheral vision, will fade away and disappear. The effect is enhanced if the stimulus is small, is of low...
   
   . It occurs because although our eyes move a little when we are fixating a point, away from that point (in peripheral vision
   Peripheral vision
   Peripheral vision is a part of vision that occurs outside the very center of gaze. There is a broad set of non-central points in the field of view that is included in the notion of 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 resemblance's to Pac-Man accounting for the illusion's alternative name.

Psychophysics

, 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

The demonstration in this article shows a blurred, very slightly blue-grey disc in each gap. These can be seen by examining one frame of the image under high magnification with any adjoining lilac discs masked off. But these blurred discs are not required for the illusion. Michael Bach's demonstration contains no such discs, 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, 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

Emmert's law

Emmert's Law bears the name of Emil Emmert , since it was first described by him in 1881. Emmert noted that an afterimage appeared to increase in size when projected to a greater distance. It is unclear whether he intended this to mean physical distance or perceived distance, but most authors...

: 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

Visual angle

The visual angle is the angle a viewed object subtends at the eye, usually stated in degrees of arc.It also is called the object's angular size....

 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

Blind spot

Blind spot may refer to:In ophthalmology:*Scotoma, an obscuration of the visual field*Optic disc, also known as the anatomical blind spot, the specific region of the retina where the optic nerve and blood vessels pass through to connect to the back of the eye*Blind spot , also known as the...

, 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.

External links

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