Color Afterimages

1. Stimuli

1. Open a blank white slide in a slide editor (software such as PowerPoint or Keynote will suffice).
2. Use the shape tool to make two equally sized stars with no color fill, only a thin black outline. Center them vertically on the slide, and place one on each side (left and right).
3. Place a small black disc in the center of the slide between the stars. This is the fixation point.
4. Now, make a copy of this black and white slide. The second copy of it will be the second slide in your afterimage stimulus. Turn now to making the first slide by selecting the first of the two identical slides.
5. Select the star on the left and fill it uniformly with a bright blue. Select the star on the right and fill it uniformly with a bright yellow.
6. The stimulus is now ready. The first slide should be in color and the second should be entirely black and white. Figure 3 shows what the first slide should look like, and Figure 4 shows the second.

Figure 3. Slide #1 of a two-slide color afterimage. The first slide in an afterimage pair is in color. The dark disc in the center is the fixation point.

Figure 4. Slide #2 of a two-slide color afterimage. The second slide in an afterimage pair is in black and white. But observers will perceive illusory color inside the white fills of the objects in the frame (the stars in this case).

7. Make additional stimuli for investigation following the same procedure. For example, make one with a red star on one side, and a green star on the other. Then, make one with a green star on one side, and a blue star on the other. Any other colors can be used as well.
8. Each stimulus needs to include a color slide followed by an identical slide in black and white. Figure 5 shows two examples of additional slide pairs for inducing color afterimages.

Figure 5. Two additional examples of slides for inducing color afterimages. The first slide in each pair (top row) is always in color. The second slide in each pair (bottom row) is always in black and white.

2. Generating the illusion

1. To cause someone to experience the color afterimage illusion, seat them in front of the computer monitor.
2. Load one of the color slides, e.g., the one with the blue and yellow stars.
3. Ask the observer to fixate on the black disc in the center of the screen, and to avoid moving their eyes.
4. Count out 10 s, then advance the slides forward so that the black and white slide replaces the color one.
5. Instruct the participant to continue to maintain their fixation. They should see the white stars as filled in with the opponent colors of those that filled them in previously. So, the formerly blue star will look yellow, and the formerly yellow one will look blue. Figure 6 schematizes the relationship between the colors in the first slide and the illusory colors that will be perceived.

Figure 6. Illusory colors perceived in relation to a particular slide-pair. In Slide #1, the left star is blue and the yellow star is yellow. Slide #2 is actually black and white. But when toggled to after an observer has fixated on Slide #1, Slide #2 will be perceived with illusory colors. Specifically, the stars will appear filled in by the opponent colors from the ones filling them before, so the left star will be perceived as yellow, and the right one as blue.

6. Once the observer moves their eyes, the illusion fades quickly.
7. Note, it is easy to do this on one's self, and it works.

3. Collecting data

1. As with many visual illusions, the effects are phenomenologically salient, and experienced by nearly all people. So, quantitative data serves mainly to affirm what one experiences.
2. A simple technique for collecting data is to include an X someplace in the white region in the second slide in a pair, and to supply a set of color options for observers to pick from.
3. The task is for the observer to select the color option they perceive where the X is placed. Figure 7 shows the sequence of events in one trial of the experiment.

Figure 7. Sequence of events in a single trial of a color afterimage experiment. The observer fixates on the center disc in Slide #1. After 10 s, the experimenter advances forward to Slide #2. The participant's task is to select the color among the displayed options that best matches the color they perceive in the position of the 'X.'

4. To run a complete version of the experiment, use several different slide pairs. During each trial of the experiment, place an X in a different part of one of the objects in the second slide.
5. Run 5-10 participants in an experiment with 10-20 trials. Tally up the color chosen as a function of the color that was present in an X's position in a trial's slide #1. In other words, relate the color perceived at the position of an X on a white background to the color previously in that position. The color previously in that position is called the 'inducer color,' and the color perceived is called the 'after-color.'