Why won't my brain let me see the 12 black dots simultaneously? by Erasmuth Wolf
Answer by Erasmuth Wolf:
These dots are positioned in such a way that it takes advantage of your blind spot (where the optic nerve enters the eye). If you look right at a dot, it's neighbors may be invisible depending on how your eyes developed. Your brain masks this for you by overlapping the input of both eyes, covering each eyes blind spot, unless (as is the case here) the two blind spots happen to overlap.