Monday, July 27, 2009

Vanishing head

Tonight's optical illusion:


Sailorcurt said...

When I was in the Navy I spent one tour teaching at the Atlantic Fleet Helicopter Operations schools. One of our courses was on the use of Night Vision Systems. As a part of that we got fairly deep into the physiology of the eye so that students had an understanding of the limitations of NV systems and of their own eyesight...especially at night.

We used a trick similar to that to illustrate a point. We used an "X" and a series of numbers on the dry erase board. By moving the vision across the line of numbers, you could make the X completely disappear.

Everyone has a blind spot in each eye. That spot is where the optic nerve enters the back of the retina. There are no light receptors there. You normally don't notice it because your other eye covers that area. Also, your brain will fill in the area with what it thinks should be there.

That's why the guy's head disappears: His head in the video is small enough to be completely obscured by the blind spot; when you move your head just until the blind spot caused by your optic nerve covers the guy's head, your brain fills the blank spot with the color of the wall that surrounds the spot.

You can see the black bar moving up and down because it extends beyond the edges of the blind spot, the brain fills in the blind spot with what it thinks should be there, so it seems that you can "see" the bar, but can't see the guy's head.

You'll notice that the "X" that you have to look at is on the same plane as the center of the guy's head. That's because the optic nerve enters the eye in a spot on the same plane as the focal point of your lens. If the "X" was just a little bit higher or lower, the illusion wouldn't work because the guy's head wouldn't be completely obscured by the blind spot.

It is a neat trick, but not so mysterious when you understand how your eyes are put together.

David said...

Curt, thanks for the excellent explanation.