Introduction
This activity demonstrates the phenomenon of negative afterimages. When the user clicks on one of the small images at the lower left, a larger version of the image appears in the square above it. If the user stares intently and steadily at the center of this adapting stimulus for 12 seconds, the adapting stimulus is replaced by a plain white square. Continued staring at the same spot and causes the perception of an afterimage, which has the opposite colors of the inducing image.
What’s Going On Here, Part I
Negative color afterimages result from the operation of opponent process cells in the visual system. As described in the textbook, the output from the three types of cones combine to form red–green, blue–yellow, and black–white opponent color pairs. Each of the mechanisms works basically the same way with respect to afterimages, so let us consider the stimulus with the word “GREEN” (written in green) on a red background for illustration purposes.
The visual system contains some cells that are excited by red light and inhibited by green light, and other cells that are inhibited by red and excited by green. We will call these R+G– and G+R– cells, respectively. Remember that white light includes all wavelengths (including red and green) in roughly equal proportions. Thus when viewing a white square, the R+G– and G+R– cells normally fire at about the same rate.
What’s Going On Here, Part II
If the user stares at the adapting stimulus for an extended period of time, the R+G–cells in the background and the G+R cells in the text will fire at a very high rate and will eventually become fatigued. At the same time, the G+R–cells responding to the background and the R+G–cells responding to the text will be inhibited. If the user then looks at the white square again, the exhausted R+G– cells will fire at a slower rate than they did before staring at the red circle, while the now liberated G+R– cells will fire slightly faster than they did previously (and vice-versa for the text). The result is an afterimage: The portions of the visual system that receive input from the opponent process mechanisms “see” the G+R– cells firing faster than the R+G– cells and conclude that you are seeing not a plain white square, but a green square with the word “GREEN” written in red in the middle.