MACULA contains a screen which is alternately illuminated by a white and purple light. The yellowish pigment covering the macula absorbs the purple light, casting a shadow which looks like a ring with a dot in the center. This shadow appears no matter where you look on the screen because it is really in your eye. You normally don't see the shadow because your eye-brain erases it from view. The alternation in color is necessary so that the image does not disappear with fatigue.