The Lilac Chaser Illusion

The Lilac Chaser Illusion. Image by TotoBaggins/Wikimedia Commons, Creative Commons Attribution-Share Alike 3.0

The lilac chaser is a type of visual illusion that was first discovered by vision expert Jeremy Hinton in 2005. In order to view the illusion, start by clicking here to open the image in a new window. Stare at the black center cross for a minimum of 30 seconds and see what happens. Want to learn more? Continue reading to discover how this fascinating illusion works and what it reveals about the brain and perception.

What Do You See?

In the lilac chaser illusion, the viewer sees a series of lilac colored blurry dots arranged in a circle around a focal point. As the viewer stares as the focal point, a few different things are observed.

At first, there will appear to be a space running around the circle of lilac discs. After about 10 to 20 seconds, the viewer will then see a green disc moving around the circle instead of the space. With longer observation, the lilac discs will disappear altogether and the viewer will only see the green disc moving around in a circle.

How Does the Lilac Chaser Illusion Work?

According to its inventor Jeremy Hinton, "the illusion illustrates Troxler fading, complementary colors, negative after-effects, and is capable of showing colors outside the display gamut."

What exactly does this mean? Let's break it down a bit further.

Why do the lilac discs appear to move around the circle?

This is an example of what is known as apparent movement or beta movement.

When we see something in one spot and then again in a slightly different spot, we tend to perceive movement. You can probably think of multiple examples of this in real-life. Motion pictures and neon signs operate based upon this principle. A progressively flashing neon sign is able to create the illusion of movement simply by altering the timing and spacing that the lights are flashed.

Why do we begin to see green discs in place of the gray spaces?

This is an example of a negative afterimage effect. When a color is presented in the visual field for an extended period of time, an afterimage results. An afterimage involves continuing to see colors briefly even after a stimulus is no longer present. In some cases we see the colors exactly as they were in the original image, which is known as a positive afterimage. In other cases, we see the opposing colors of the original image, which is known as a negative afterimage.

In the case of this illusion, we see a green afterimage in place of the lilac discs. We generally don't notice afterimages because we move our eyes frequently enough that they rarely occur in day-to-day experience.

Why do all of the lilac discs eventually disappear?

This is an example of what is known as Troxler fading, which occurs when blurry objects that are located in the periphery of our visual field disappear while we have our eyes fixated on a certain spot.

Why does the green disc appear to fly around in a circle?

After fixating on the center cross for about 30 seconds or so and the lilac discs have disappeared, it seems as if the green disc is now flying around the circle by itself. This can be explained by a Gestalt effect known as the phi phenomenon. The sequential movement of the retinal afterimage (aka, the green disc) causes the illusion of movement.

Sources:

Bach, M. (n.d.). Hinton's lilac chaser. Retrieved from http://www.michaelbach.de/ot/col_lilacChaser/index.html

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