This optical illusion relies on our eye's blind spot to make a man's head vanish. Check out the video below to see how it works.
Here's a bit about our blind spot, courtesy of io9:
One of the most remarkable things about the blind spot is just how good the brain is at hiding its existence. Our blind spot is absolutely gigantic, covering a portion of our field of vision big enough to hold 17 full moons, and yet nobody in recorded history suspected its existence until the 1660s. The fact that the blind spot only affects part of our peripheral vision is obviously also a big part of why it remained undetected - I'm guessing someone would have noticed if it was right in the middle of our field of vision. The presence of a second eye is also crucial, as the brain can use visual information from one eye to fill in the holes in the others' field of vision.
But when the other eye is covered up, as is the case in the video up top, the brain just makes its best guess based on what surrounds the blind spot. That's why the man's head disappears, while the black bar remains solid - the bar is a simple continuous line that the brain knows how to complete, but it can't infer the existence of a person's head simply based on its body. We might imagine a hypothetical evolutionary path in which the brain adapted to deal with the blind spot by filling in the gaps from our memory, which would mean the head would remain visible. Admittedly, that sounds like a recipe for seeing a lot of things that aren't there, which is probably why vertebrate brains didn't evolve that way.
The blind spot was discovered by a French physicist and priest named Edme Mariotte, who discovered this break in the retina while dissecting an eye in the 1660s. He realized that this gap necessarily meant a substantial part of our field of vision was blocked off, completely unbeknownst to all of humanity. Through a series of experiments that basically involved moving his eye around until a fixed point disappeared - a primitive version of the video up top, essentially, he was able to locate just where the blind spot was in our field of vision.
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