Full transcript of the episode (it gives away every surprise)
No introduction, no lecture.
An experiment, right now. It takes thirty seconds.
Interactive experiment
The dot that disappears
Close your left eye, or cover it with your hand. Stare at the cross with your right eye, never looking away from it. Then slowly move your face toward the screen, or away from it.
A cross and a large black dot are shown side by side. With the left eye closed and the gaze fixed on the cross, you move your head closer or farther away: at a certain distance, the black dot disappears completely.
The dot never left the screen, not for a single second.
You have a hole in your eye.
In each eye, in fact. You always have.
The Hole in Your Eye
You don’t see the world. You see what your brain guesses about it.
At the back of the eye, the retina is carpeted with millions of light sensors. Except in one spot: where the fibers of the optic nerve gather to leave the eye and head for the brain.
In that spot, nothing picks up light. And the blind spot is big: you could line up about ten full moons inside it.
Yet you have never seen a hole. Close one eye and look around you: no black patch.
Your brain plugs the hole. With what? With whatever is around it. It makes things up.
Interactive experiment
The mended line
Same position: left eye closed, gaze fixed on the cross. This time, the line is broken in the middle. Find the distance at which the gap falls into your blind spot.
A horizontal line is interrupted where the dot used to be. When the gap falls into the blind spot, most people see an unbroken line.
For most people, the line looks unbroken.
Your brain drew in the missing piece.
What you “see” in your blind spot comes from no ray of light at all. It is a hypothesis, painted in a hurry.
The first to notice was a priest and physicist, Edme Mariotte. Around 1660, while dissecting eyes, he noticed that the optic nerve does not reach the retina at its center. He deduced that there must be a hole in our vision, went looking for it… and found it.
The story goes that King Charles II of England, once shown the experiment, loved to “behead” his courtiers by looking at them with one eye.
Question for the reader
At any given moment, how much of what surrounds you do you actually see sharply?
- About half
- About a quarter
- A tenth
- Less than a hundredth (correct answer)
If you answer “About half”:
Much, much less than that.
If you answer “Less than a hundredth”:
Exactly.
The zone of sharp vision, the fovea, covers about two degrees: the width of your thumbnail at arm’s length. Everything else is blurry, and almost colorless at the edges.
Try it yourself
- Stretch your arm out in front of you, thumb up.
- Stare at your thumbnail, without moving your eyes.
- Without looking away, try to read a word on this page, right next to your thumb.
Can you do it?
- Impossible
- Barely
- No problem
Two thumb-widths away from the point you are staring at, reading becomes impossible. That is the real size of your sharp vision.
Interactive experiment
What your eyes really pick up
Move your finger or your mouse over this text: only a small area is sharp, just as it is for your eye.
A paragraph is shown blurred and faded, except in a small sharp circle that follows the finger or the mouse: a simulation of foveal vision.
To make the world look sharp everywhere, your eyes jump constantly from one point to another: about three times a second. These leaps are called saccades.
And during each saccade, something very strange happens.
Try it yourself
- Go up close to a mirror.
- Look at your left eye, then your right eye, then your left eye again.
- Watch your eyes while they move.
Can you see them move?
- No, never
- Yes
- No mirror at hand
Nobody sees their own saccades. During each leap, the brain cuts the picture, then glues the pieces back together as if nothing had happened.
150,000 saccades a day, roughly
Put end to end, these moments of blindness add up to more than half an hour a day. You have no memory of any of them: your brain screens you a film without cuts.
So where does this sharp, colorful, stable picture spread out in front of you come from?
Neuroscience’s answer: from your brain. It receives blurry, patchy, choppy signals. It combines them with everything it already knows about the world, and it deduces the most likely scene.
You don’t see the world. You see your brain’s best guess about it.
The physicist and physician Hermann von Helmholtz understood this as early as 1867: perceiving means making an “unconscious inference.” The brain reasons, calculates and concludes, without ever showing you its calculations.
“We’re all hallucinating all the time. When we agree about our hallucinations, we call that reality.”
The brain doesn’t just add what is missing. It also erases what it deems useless. Let’s check.
Interactive experiment
The dots that switch off
Stare at the green dot in the center, without looking away, for about twenty seconds. Keep an eye on the three yellow dots out of the corner of your eye.
Three still yellow dots are surrounded by a swarm of rotating blue crosses. Staring at the center, you see the yellow dots disappear now and then, one at a time or all together, and come back: this is motion-induced blindness.
The yellow dots never disappeared.
They stayed on the screen, perfectly still, the whole time. Your brain decided they didn’t matter, and removed them from your awareness. Without asking you.
If the brain erases what it deems useless… what happens when you are focused on a task? One last test, for your attention.
Interactive experiment
Count the bounces
White and black shapes will bounce around for fifteen seconds. Count how many times the WHITE shapes touch an edge of the frame. Ignore the black ones.
Four white shapes and four black shapes bounce around a frame; the reader counts the bounces of the white ones. For five seconds, a dark red cross slowly moves across the frame. The reader is then asked how many bounces they counted, and whether they noticed anything else.
A dark red cross moved right across the frame.
Slowly, through the middle, for five seconds. Go back to the previous step: the experiment offers to show you the scene again.
In 1999, at Harvard, Daniel Simons and Christopher Chabris filmed students passing a ball to one another. The instruction for viewers: count the passes made by the team in white.
In the middle of the video, a student in a gorilla suit walks through the scene, thumps her chest and walks off. About half the viewers fail to see her.
In 2013, researchers hid a gorilla, 48 times the size of a nodule, in lung scans. They handed the scans to 24 experienced radiologists, whose job was to look for nodules.
20 radiologists out of 24 failed to see the gorilla. Yet eye tracking showed that most of them had looked right at it.
Inattentional blindness. The inability to notice a perfectly visible object when your attention is busy elsewhere. Looking is not enough to see: the brain also has to decide that the thing matters.
One last story. Mike May lost his sight at the age of three, in an explosion. He became the world speed-skiing record holder for blind skiers, started a company, raised a family.
In 2000, at 46, a transplant gave him back one working eye. The eye was repaired. One problem remained: his brain had never learned to see.
Colors and motion came back to him quickly. Faces and depth, much less so.
Years after the operation, he still struggled to recognize a familiar face or judge a distance. His eyes could see; his brain was deciphering a foreign language.
Seeing is something you learn.
You learned it as a baby, over thousands of hours. Then you forgot that you had ever learned it.
Your eyes invent the world. You still have your memory: at least it faithfully keeps what you have lived through. Really? In the next episode, you are going to remember something that never happened.
Sources
- Edme Mariotte, Nouvelle découverte touchant la veüe, 1668.
- Hermann von Helmholtz, Handbuch der physiologischen Optik, 1867.
- Yoram Bonneh, Alexander Cooperman and Dov Sagi, “Motion-induced blindness in normal observers,” Nature, 2001.
- Steven Most et al., “How not to be seen: the contribution of similarity and selective ignoring to sustained inattentional blindness,” Psychological Science, 2001.
- Daniel Simons and Christopher Chabris, “Gorillas in our midst,” Perception, 1999.
- Trafton Drew, Melissa Võ and Jeremy Wolfe, “The invisible gorilla strikes again,” Psychological Science, 2013.
- Ione Fine et al., “Long-term deprivation affects visual perception and cortex,” Nature Neuroscience, 2003.
- Anil Seth, Being You: A New Science of Consciousness, 2021.