To be clear, there really are 12 black dots in the image. But (most) people can’t see all 12 dots at the same time, which is driving people nuts.

"They think, 'It’s an existential crisis,'" says Derek Arnold, a vision scientist at the University of Queensland in Australia. "'How can I ever know what the truth is?'" But, he adds, scientists who study the visual system know that perception doesn’t always equal reality.

In this optical illusion, the black dot in the center of your vision should always appear. But the black dots around it seem to appear and disappear. That’s because humans have pretty bad peripheral vision. If you focus on a word in the center of this line you’ll probably see it clearly. But if you try to read the words at either end without moving your eyes, they most likely look blurry. As a result, the brain has to make its best guess about what’s most likely to be going on in the fuzzy periphery — and fill in the mental image accordingly.

""It’s an existential crisis.""

That means that when you’re staring at that black dot in the center of your field of view, your visual system is filling in what’s going on around it. And with this regular pattern of gray lines on a white background, the brain guesses that there’ll just be more of the same, missing the intermittent black dots.