The image above is called Ninio’s extinction illusion. On first glance, how many black dots do you see?
If you answered anything less than 12, you’re wrong.
There are twelve black dots distributed on the grey pattern, but your brain (probably) won’t allow you to see them all at once. Why?
The illusion demonstrates a weird thing that sometimes happens in our eyes. Basically, the illusion shows that the light receptors in your eye can sometimes mess with the ones next to them, making you see things that aren’t there, or not see things that are there.
Here’s how the illusion’s creator explains it:
When the white disks in a scintillating grid are reduced in size, and outlined in black, they tend to disappear. One sees only a few of them at a time, in clusters which move erratically on the page. Where they are not seen, the grey alleys seem to be continuous, generating grey crossings that are not actually present. Some black sparkling can be seen at those crossings where no disk is seen. The illusion also works in reverse contrast.