## Lilac Chaser Illusion

posted on 19 Jun 2013 by guy
last changed 25 May 2016

 2 Average: 2 (2 votes) Select ratinghated itdidn't like itliked itreally liked itloved itCancel rating Your vote (click to rate)ages: 10 to 99 yrs budget: $0.00 to$0.00prep time: 0 to 5 min class time: 5 to 20 min The Lilac Chaser illusion is a creation by Jeremy Hinton that relates to the effects of desensitization (and afterimages), motion-induced blindness, and beta movement (perceiving motion from static images). It makes a great follow-up activity after a lesson on negative afterimages.For more examples of optical illusions caused by visual desensitization, check out our summary curriculum on After Images and Optical Illusions.
subjects: Biology, Psychology
keywords: illusion, desensitization, lilac chaser, beta movement

## the illusion

Click on the thumbnail of figure 1 to bring up the full-size image. You'll see a ring of lilac-colored spots. Each of these spots temporarily disappears in a clockwise sequence around the ring.

To enjoy the illusion, stare at the cross in the center of the screen without moving your eyes. After a few seconds, the lilac spots should fade from view and a single green spot should appear, moving clockwise around the ring.

Jeremy Hinton invented this wonderful illusion in 2005, usually called the "Lilac Chaser", and it has since been copied all over the web.

## what's going on?

The Lilac Chaser illusion makes use of three features of human vision to produce its magical effect.

First of all, it uses desensitization of photoreceptor cells in the retina both to make the lilac spots disappear and also to make the green spot appear. Steady light shining on the retina quickly desensitizes your photoreceptors so that the signal they send to your brain gets weaker and weaker. This desensitization (also called "adaptation" or "bleaching") makes the lilac spots appear to fade away. The same desensitization process makes images disappear in our lesson on Cheshire Cat Illusions.

Moreover, desensitization occurs separately for each of three types of color sensitive photoreceptors in the eye. Lilac-colored light will desensitize both blue-sensitive photoreceptors and red-sensitive photoreceptors, but will not affect green-sensitive photoreceptors.  When a lilac spot is temporarily removed from the image, the neutral grey background will try to stimulate each of the three types of photoreceptors equally, but since the blue and red photoreceptors are desensitized, only the green-sensitive photoreceptors will send signals to the brain, giving the appearance of a green afterimage. This same mechanism generates the afterimages described in our lesson on Yang and Yin negative afterimages.

The disappearance of the lilac spots is enhanced by another feature of human vision, which is independent of desensitization, and which has to do with a changing background. Just such a changing background produces disappearances in our lesson on Motion-induced Blindness Illusions. The physiological mechanism behind motion-induced blindness is still a matter of debate, but it does seem to speed up the disappearance of the lilac spots in the Lilac Chaser illusion.

Finally, the sequential appearance of the green afterimages in each of the empty spots, is interpreted by the brain as a continuous movement. The observer seems to see a single green spot moving clockwise around the circle rather than individual green spots appearing and disappearing at particular locations. This ability to interpret discrete images at different locations as continuous motion is called "beta movement", and is the principle behind "chase lighting", which uses carefully timed lights to create the illusion of motion. Examples of chase lighting are often found on movie marquees.

## more illusions

A clever variation of the Lilac Chaser has been put together by Sebastiaan Mathôt (below) with red, purple and green chasers. The afterimage colors are cyan, green and lilac.

Variation of the Lilac Chaser illusion by Sebastiaan Mathôt at http://www.cogsci.nl/illusions/colour-after-effect.

## teaching notes

This illusion makes for a perfect finale to discussions about negative afterimages and motion perception. Consider preceding it with our lessons on Yang and Yin negative afterimages, Cheshire Cat Illusions and Motion-induced Blindness Illusions. Ask students to explain what's going on based on their understanding of negative afterimages.

Use Michael Bach's adjustable demonstration (under further resources below) to allow more active class participation. Project the illusion on a screen for the whole class and ask for suggestions about what changes to make.

## further resources

An excellent display of the Lilac Chaser is available on Michael Bach's site at http://michaelbach.de/ot/col-lilacChaser/index.html. It allows adjusting color, saturation, speed and dot size. Great for class experiments.