The ghostly birds are called afterimages . An afterimage is an image that stays with you even after you have stopped looking at something. As you stare at the red bird, light-sensitive cells at the back of your eyes become less responsive to red light. When you shift your gaze to the birdcage, your visual system “subtracts” red light from the white light that’s being reflected from the white background. White light minus red light is blue-green light. That’s why the afterimage of the parrot is blue-green. A similar thing happens when you stare at the green bird, but this time your eyes become less responsive to green light. White light minus green light is magenta light, so you see the afterimage as a magenta cardinal.

Here’s another afterimage:
Scroll down so you see a white space below the flag. Look at the flag for about 15 seconds, then look at the white space. Notice how the ghostly image of the familiar “stars and stripes” appears. 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.