"Seeing is believing" is one of our deepest articles of faith. It is, though, faith, and it's not supported by science.

Neuroscientist David Eagleman describes how most of us think of perception:

Intuition suggests that you open your eyes and voilà: there’s the world, with all its beautiful reds and golds, dogs and taxicabs, bustling cities and floriferous landscapes. Vision appears effortless and, with minor exceptions, accurate. There is little important difference, it might seem, between your eyes and a high-resolution digital video camera. For that matter, your ears seem like compact microphones that accurately record the sounds of the world, and your fingertips appear to detect the three-dimensional shape of objects in the outside world. What intuition suggests is dead wrong.[1]

If we think of our eyes as video cameras, then logically we would expect that light hits our retinas, and that the retinas pass on this information to the rest of the brain for processing. In their classic book The Tree of Knowledge, Humberto Maturana and Francisco Varela explain that this is not how vision works in the brain: they estimate that only 20% of what we see comes from information passed along by the retinas. The key part of the brain here is the lateral geniculate nucleus, or LGN. Maturana and Varela write:

This nucleus is the most prominent region of connections between the retina and the central nervous system.…As is clear, the retina does not affect the brain like a telephone line that encounters a relay station at the LGN, since more than 80 percent of the interconnections come together at the LGN at the same time. Consequently, the retina can modulate — but not specify — the state of the neurons in the geniculate nucleus, whose state will be given by all the connections it receives from many different parts of the brain.[2]

In other words: your brain is inventing 80% of what you see — just making it up, based on what it already believes to be true about the world.

Take a look at the figure below, which is similar to an example given by Maturana and Varela (p. 20). Position your face about 12 inches from the screen. Now, while keeping your left eye closed and your right eye fixed on the plus sign, slowly — very slowly — move your face closer to the screen. As your face moves, keep your right eye laser-focused on the plus sign. Don't waver. You’ll see that at one point the image to the right just…disappears. Keep moving closer and it appears again. What happened?

David Eagleman, who also uses this example in Incognito, explains:

How could brilliant minds like Michelangelo, Shakespeare, and Galileo have lived and died without ever detecting this basic fact of vision? One reason is because there are two eyes and the blind spots are in different, nonoverlapping locations; this means that with both eyes open you have full coverage of the scene. But more significantly, no one had noticed because the brain “fills in” the missing information from the blind spot. Notice what you see in the location of the dot when it’s in your blind spot. When the dot disappears, you do not perceive a hole of whiteness or blackness in its place; instead your brain invents a patch of the background pattern. Your brain, with no information from that particular spot in visual space, fills in with the patterns around it.[3]

In fact, the sense that the dot has disappeared could only result from the brain filling in the background pattern: “disappearing” means the same thing as “looking exactly the same as the background.”

I invite you to do this a few times. Try keeping the face invisible for as long as you can. Remember to stay focused on the plus sign. As you stare at the lonely plus sign, think about what’s going on. You know the face is right there, waiting to be seen. Can you feel it taunting you? Resist the temptation to shift your gaze. Keep focusing and contemplating. What is going on here? You just saw it. Why can’t you see it? And how did that background pattern sneak into its place?

Next, flick your right eye’s gaze to the face. There it is! Then, flick back. Gone again. Do this a few times.

It’s such a simple little experiment, yet it offers a profound opportunity to start to get, in a meaningful way, what a gigantic, fantastic illusion our brains and bodies have created for us.

Why does it matter? I invite you to think about it.

[1]

David Eagleman, 2011.

Incognito: The Secret Lives of the Brain

. New York: Pantheon Books. iBooks Enhanced Edition.

[2]

Humberto R. Maturana and Francisco Varela, 1987.

The Tree of Knowledge: The Biological Roots of Human Understanding

. Boston: New Science Library, p. 162.

[3]

Eagleman, 2011, iBooks enhanced edition.