The ghostly birds you see here are called afterimages. An afterimage is an image that stays with you even after you have stopped looking at an object. The back of your eye is lined with light sensitive cells, called cones, which are sensitive to certain colors of light. When you stare at the red bird, your red-sensitive cones adapt to the light and lose their sensitivity. When you shift your gaze to the white background of the bird cage, you see white (minus red) where the red-sensitive cells have become adapted. White light minus red light is blue-green light. That's why the afterimage you see is blue-green and in the shape of a parrot. The same thing happens when you stare at the green bird, but this time it's the green-sensitive cones that adapt. White minus green light is magenta light, so you see the afterimage as a magenta cardinal.


Here's another afterimage:
Look at the flag for about 15 seconds, then scroll down to the white space provided below. Notice how the ghostly image of the familar "stars and stripes" appears. Like Bird in a Cage, this afterimage occurs because red, white and blue are the complementary colors of cyan, black and yellow.

Originally rendered in 1994 by Cija Briegleb and Zach Waller, students at San Francisco State University department of Information Arts and Conceptual Design. Contact zwaller@sfsuvax1.sfsu.edu for comments.