Lilac Chaser

Stare at the black cross for 30 seconds.

The above image is an optical illusion sometimes referred to as the Lilac Chaser. It’s the result of several different known visual phenomena including:

Negative Afterimage – The everyday experience we have when we see something very bright and then continue to see its floating silhouette even after we look away from it. The neutral gray background seen here works to enhance this effect and this interactive site allows you to change the dot’s color to see opposing ghost colors.

Troxler’s Fading – First discovered in 1804 by Ignaz Troxler. Basically, we have a blind spot in the middle of our vision (called Scotoma) but we constantly make up for it by having two eyes, rapid eye movement, and also a brain that fills-in the missing information. Unvarying stimuli, however, is able to escape our focus. Similarly, we may feel our clothes when we first put them on but we soon forget about them even though they are still probably touching millions of neurons on our skin right now. A similar illusion is seen below and you can also test your blind spot at this website.