The illusions in this section are driven by the way we see things such as high contrast and color. We have many specialized light receptors, but only a few million of them are active at any one time. Nearby, just out of focus, other receptors are at rest, or may be recovering from just being used. Contrast illusions often prey upon sleepy light receptors.
The fuzzy gray balls in the intersections of Herman’s Grid are not really there. The intersections tend to look a little brighter because they are more completely surrounded by black. As your eye moves around the design, an afterimage forms in those brighter areas. The fuzz balls fade when you look right at them because your light receptors are most active when you look right at one area. Light receptors work best when they are most active, so this illusion is like wind-sprints for your eyeballs. It never hurts to do a few laps. Afterimages always appear in the opposite color, so the fuzzes are gray.