Spring 2008
Newsletter

VISIONS NEWSLETTER

Optical Illusions

Julie Schnapf, PhD, studies the nature of color vision at the Department of Ophthalmology. The two optical illusions below demonstrate how color signals of our red, green, and blue cone photoreceptors sometimes “trick” us into seeing colors that aren’t there.

The scientific explanation: The nervous system detects color from the relative strength of signals of the three cone types in response to different wavelengths of light. Staring at a colored image for a while produces an afterimage of complementary colors. This is because staring at the image causes signals from the three cone types to differentially weaken in different parts of the image; the resulting alteration in the strength of cone signals alters our perception of color.

Read more about Dr. Schnapf’s research in the Spring 2008 issue of Visions.

1. American flag. Stare at the cross in the center of this image for about 30 seconds. Then, still staring at the cross, move the mouse over the image. An afterimage will appear. Over time the afterimage will fade, but you can revive it by briefly closing your eyes.

2. San Francisco Golden Gate Bridge. Stare at the cross in the image for 30 seconds. Then, still staring at the cross, move the mouse over the image. The black-and-white photo will appear in full color, but only when you are fixating on the cross. If you divert your eyes, the color disappears.

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