What is an eye saccade?

0:46 Welcome to Science Is Not Boring by Kidopoly.com. I'm Mira! And I'm Finn. Before we begin, we have a huge shout-out today to Luke in Minnesota. Hey Luke, thanks for listening. Luke asked us to investigate how our eyes work as a team and why they jump across the page when we read. That is such a great question.

1:08 This whole episode is for you, Luke.

1:10 If you haven't already, tap follow wherever you're listening so you never miss an episode.

1:15 Okay, Mira, wait a second. Luke said our eyes jump when we read. That's right.

1:22 When you read a book, it feels like your eyes are sliding smoothly across the page, doesn't it?

1:28 Yeah, totally smooth. Like ice skating.

1:31 Well, prepare to have your mind blown. Your eyes do not slide smoothly at all. They are actually bouncing, hopping, and leaping across the text at incredible speeds.

1:44 No way!

1:45 Why don't I feel them jumping? Because your brain is tricking you! Today we are going deep into the high-speed team sport happening right inside your head. Let's go back to the year 1878 in Paris, France. A doctor named Émile Javal made a massive discovery.

2:04 Did he have a fancy machine to look at eyes?

2:06 Nope. He used experimental methods like studying after images and placing a microphone against the eyelid to hear eye jumps.

2:16 That sounds a little awkward. A microphone on their eyelids.

2:19 A little bit, but Javal noticed that their eyeballs didn't sweep across the lines. They stopped, then jerked forward, then stopped again.

2:28 Like an old typewriter.

2:30 Exactly. He named these sudden tiny jumps saccades, which is a French word that means the quick jerk of a horse's reins.

2:39 Sicards. How fast are these horse jerks happening?

2:43 Hold on to your seat, Finn. A single saccade takes only 20 to 200 milliseconds.

2:50 Wait, a millisecond is a thousandth of a second? That's faster than a blink.

2:55 Way faster. During a saccade, your eye moves at a speed of up to 900 degrees per second. It's the fastest movement the human body can make.

3:06 Whoa, the fastest movement in our whole body is just reading a book.

3:10 Yes, but here is the crazy part. When your eye is moving, it's actually completely blind. Wait, what? I go blind when I read. Only for tiny fractions of a second. You see, if your eye tried to process the light while it was moving that fast, everything would look like a giant blurry smear.

3:30 Oh, like when I wave a camera around while taking a video. It's totally dizzying.

3:36 Exactly. So your brain shuts off the vision during the jump. Then your eye lands on a word and stays perfectly still. How long does it stay still? That's called a fixation. Your eye stops for about a quarter of a second, just long enough to take a high-resolution snapshot.

3:54 So reading is really just jump, snap a picture, jump, snap a picture. All in milliseconds.

4:02 You've got it! Now, the reason we have to do this is because of a tiny, tiny spot at the back of your eye called the fovea. The fovea? How big is it? It is only 1.5 millimetres across. That is about the size of the tip of a crayon. That's microscopic! Why is it so important? Because it's the only part of your eye that sees perfectly clear, sharp details.

4:27 Everything else in your vision is a little bit blurry.

4:30 Wait, really? Let me test that. I'm staring straight ahead at a book. You're right. I can't read the words on the edges of the page.

4:39 See? To read, you have to aim that tiny 1.5mm fovea spotlight exactly at the letters.

4:46 But how do my eyes know exactly where to point? Especially since I have two of them.

4:52 That brings us to Luke's question about teamwork. You have six tiny muscles attached to the outside of each eyeball. Six muscles per eye? Twelve in total? Yes. Four pull up, down, left, and right. Two pull diagonally. And they are some of the most active muscles in your entire body.

5:12 So they are doing a massive workout just reading a comic book.

5:16 A massive workout! These twelve muscles perform something called binocular convergence. They angle your two eyes perfectly, so they focus on the exact same letter. And what if they mess up? If they were off by even a tiny millimeter, you would see double words. But they coordinate so well, your brain takes the two pictures and fuses them into one clear 3D image.

5:43 So my brain is photoshopping reality in real time.

5:47 Yes, it's called binocular fusion. To study psychardic exploration, a Russian scientist named Alfred Yarbus did some wild experiments in the 1950s.

5:59 Oh I love wild science experiments. What did he do?

6:02 He wanted to track exactly where people's eyes jumped, but he didn't have modern lasers or computers. So he built a tiny metal device with a mirror on it.

6:13 Okay. Where did he put the mirror?

6:16 He attached it to a tiny suction cup and stuck it directly onto the volunteer's eyeball. Ooh! On their actual eyeball! Yep, don't worry, he used numbing drops so it didn't hurt. Then he shone a beam of light at the mirror, and the light bounced onto photographic paper.

6:35 That is genius! So as the eyeball jumped, the light drew squiggly lines on the paper. Exactly!

6:42 Yabus proved that when we look at a picture or read a book, our eyes dart all over the place to gather the most important clues, while our brain stitches it into one smooth movie.

6:53 So every time we read, our twelve tiny eye muscles are performing a perfectly choreographed dance at hundreds of miles per hour.

7:02 That's exactly why reading is a high-speed team sport. Over a single day, your eye muscles move more than a hundred thousand times. A hundred thousand times? That is bonkers! It is! And here is a wait, what? Fun fact. You know how you snap a picture during a fixation?

7:21 Yeah, taking the high-rose snapshot of the word.

7:24 Your reading window isn't symmetrical. In English, your eyes actually take in about three to four letters to the left, but fourteen to fifteen letters to the right. Whoa. Why does it see more on the right? Because we read from left to right. Your brain is looking ahead to plan the next jump. If you read Hebrew or Arabic, which go right to left, your window is the exact opposite.

7:49 The brain literally rewires how our eyes take pictures, depending on what language we read.

7:54 Yes. And one more fun fact: want to see circadic masking in action? Go look in a mirror and stare at your left eye, then your right eye.

8:04 Wait, what happens?

8:05 You will never see your own eyes move. Your brain edits the motion out. But if a friend watches you, they'll see your eyes darting back and forth.

8:14 I am absolutely trying that as soon as this episode is over.

8:18 Alright, Finn, quiz time. Let's see how much you remember about our jumping eyes. Are you ready? Oh I am so ready. Bring it on. First question. What is the scientific name for the tiny rapid hops your eyes make when reading? Next one. How many tiny muscles control the movement of each eye? Question three.

8:50 What is the name of the tiny spot in your eye where vision is the sharpest? Here's number four. Exactly how wide is that tiny sharp vision spot? Last question. Who was the French doctor that first discovered eye jumps in 1878? Great job, let's see how you did. The first answer is they are called saccades.

9:27 For question two, there are six muscles per eye. Number three, the answer is it's called the fovea. For question four, the fovea is only 1.5 millimeters wide. And the final answer is the doctor was Emile Javal.

9:47 I can't believe how much work my brain is doing just to read a single page of a book.

9:52 It is incredible! Those twelve muscles and the fovea are an absolute dream team.

9:58 If you enjoyed learning about the high-speed sport inside your head, please leave us a five-star review. Just scroll down and tap the stars.

10:06 Yes, please. And make sure to follow the show and check out our sister shows. History and philosophy is not boring, and maths is not boring.

10:16 Oh, and if you want a shout out like Luke, or just want to say hi, email us at hello at kidopoly.com.

10:23 We love hearing from you. Thanks for exploring with us today.

10:27 See you next time on Science is Not Boring. Bye everyone!