Fall 2007
Newsletter

VISIONS NEWSLETTER

Optical Illusions

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

The scientific explanation: The nervous system detects color from the way that the three types of cones respond to varying wavelengths of light. When you stare at the figure for 30 seconds and then move your gaze to a white surface, an afterimage of its complementary color appears. This is because one type of cone becomes desensitized by staring at the figure, lessening its ability to transmit electrical signals to the brain.

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 look at a white wall. An afterimage will appear. Over time the afterimage will fade, but you can revive it by briefly closing your eyes.

2. Strawberries. Stare at the center of the image for about 30 seconds. Then look at a white wall. What color are the berries now? Over time this afterimage will fade, and you can revive it by briefly closing your eyes.

3. San Francisco cityscape. Stare at the cross in the upper image for 30 seconds. Then look at the cross in the lower image. The black-and-white photo will appear in full color, but only when you are fixating the cross. If you divert your eyes, the color disappears.

You are invited to subscribe, free, to this informative newsletter, which reports on advances in ophthalmology research, education and clinical care at UCSF.