Ninio's Extinction Illusion
How many black dots do you see?
It may surprise you to learn that there are 12 dots on this grid - because your eyes can only perceive 4 or 5 of them at a time. The reason? Your eye's receptors are stimulated and influenced by the activity of neighboring receptors. In a complex, repetitive grid like this, one receptor can have trouble perceiving the dots accurately because of stimulation occurring in a nearby receptor.
This illusion is a variation on the Hermann Grid, which has sort of the opposite effect of the Ninio's Extinction Illusion. In the Hermann Grid, all of the dots are white, but some appear black based on where your eyes travel.