Optical Illusions Blind Spot and the "Filling-in" phenomenon Did you know that your eyes have blind spots (and not just at the back of your head)? Each of your eyes contains an area that has no photoreceptors because it is occupied by the optic nerve. You may not have noticed these areas because they are on opposite sides of your visual field. However, this exercise is designed to isolate your blind spot and you will be amazed at the results. Instructions: Close your left eye and fixate your right eye on the cross in the diagram. If your eye is about 12 inches (30 cm) away from the monitor, you should notice that the round dot disappears. This distance may vary according to the screen resolution you have set. You may be surprised to see that the dot is replaced, not by a black region, but rather blank white space. The brain simply "fills in" the most probable stimulus (in this case, a uniform white area) where there is none. south24