We're going to do a little experiment. Make sure you watch this part of the tutorialin full screen. Close or cover your left eye, look at theplus sign. Be aware of the circle, but don't focus on it! Keep looking at the plus. You may need to move your head back and fortha little bit, or move your thing closer to your face. But at some point, the circle isgoing to disappear. Now close your right eye and look at the circle.Move your head back and forth until the plus sign disappears.
You've just found your naturally occurringblind spot in each eye. And of course daily practice we do not notice this. The human eye has what you might call a fundamentalflaw. Lightsensing cells in your retina send signals to your brain via nerves. And thosenerves are in front of the lightsensing cells. Eventually, those nerves have to pass throughthe back of your eye to get to your brain and in the part of your retina where theypass through, there aren't any lightsensing cells. That's your blind spot. Now this isn't normally a problem, becausethe blind spots are located at slightly different
points in each eye, and each of your eyeswork together to fill in a complete picture. But even with one eye closed, you're notseeing a big black hole. Instead, your brain fills in what it figures ought to be there.That's why, when the circle disappears, you see the color of the background. Yourbrain is guessing, and it's guessing wrong. Although! At least one very small study foundthat you might be able to shrink your blind spot with practice. Researchers showed ten participants an imagethat fell within the margins of their blind spots and asked them to describe it. By theend of the experiment, people got a little
better at describing those images. The researchers think it's because the lightsensingcells right around the edges of the blind spots became more sensitivebetter at pickingup and passing on light signals. That's the kind of skill that's probablynot going to ever make any kind of difference in a lifeordeath situation, and humans havehad blind spots in their eyes for as long as we've had eyes. But it's a neat wayto try and hone your brain, if you're into that sort of thing. There is a different kind of creature, though,that just completely avoids this problem.
Cephalopods, like octopuses and squids, havetheir nerves behind their lightsensing cells, so there's no need for them to have a blindspot. Why did we not do it that wayé Evolution.Well I, for one, welcome our tentacled overlords. Thank you for asking, and thank you especiallyto all of our patrons on Patreon who keep these answers coming. If you'd like to submitquestions to be answered, or get these Quick Questions a few days before everyone else,you can go to patreon scishow. And if you just want to keep getting smarter withus, you can go to scishow and subscribe! Just do it.
Why Do People Have Blind Spots
This episode was proudly made possible bythe allnew 2015 Subaru Legacy. It's not just a sedan, it's a Subaru. Can you see everything on the screen withoutmoving your eyesé Look at me did you see thaté Didn't think so.a Howdy lookers, thanks for tuning into DNews,I'm Trace. Eyes are little 1 inch diameter balls (2.54 cm) tucked in your head, and thoughthey're magnificent, human eyes aren't even CLOSE to the best in the animal kingdom. Infact, your brain is tricking you into thinking your eye is seeing things that aren't thereRIGHT NOW. We've all got blind spots that
we can't see, and huge sections of our visionare justâ€¦ wrong! Take a piece of paper with a dot and a pluson it. Close one eye, hold the paper close to your face and look straight at the dot,pulling it away slowly. Eventually, the plus should disappear! That's your blind spot.If you switch eyes, the dot should disappear. You might have to try it a couple of times.To understand WHY this happens, we have to go inside the eye. If you could fly into the pupil and insidethe eyeball to look around you'd be floating in vitreous humor of the inner eye. It's aclear, gellike material. On the walls of
the eye, you'd see blood vessels streamingeverywhere, and spread across the back of the eye you'd see the rod and cone cells ofthe retina. Rods are for low light, cones are for color and visual detail. If you got closer to the retina, you couldspot the macula right in the middle with only cone cells that's the only part of yourvision that you can see clearly. If you look at my face here, you can see it clearly, butwhile still looking at my face pay attention to the walls around me, or my shirt. It'sVERY difficult to comprehend the rest of your visual field. The macula is truly the onlypoint of visual clarity!
From there, you could follow the nerves ofthe retina, like following a bunch of extension cords to the wall. Eventually, you'd get tothe optic disk; a part of the eye with no rods or cones. That's where the optic nerveenters the eyeball! It's kind of like where your eye plugs in. And it causes a blind spot.There's nothing there for your eye to perceive since there are no rod or cone cells! My eye was nice enough to send me apicture of the inside of my own eye. You can see the macula here, and the optic nerve here.That is my point of clarity, and my blind spot!
If it seems stupid to have a blind spot, you'reright. Cephalapods don't have one, because their optic nerves run BEHIND the retina ratherthan in front of it. Think of it this way, cephalopods have their wiring inside the walls,but because of an unfortunate evolutionary mutation, mammals didn't and thus we all haveblind spots. The reason you don't see this spot all thetime is because your brain fills it in. Try doing the dot and plus experiment with a pieceof colorful paper and your brain will fill it in with color. That's your brain trickingyou into thinking your vision is perfect, when it's really not. Silly brain, tricksare for nonscientists.
Blind spots are very important to be awareof especially when you're driving! That's why we wanna thank Subaru for making thisepisode possible. And especially the allnew 2015 Subaru Legacy. Every sedan has its benefits,but only one combines them all. It's not just a sedan. It's a Subaru. If you could have the eyes of another animal,which would it beé Catsé Eaglesé Cephalopodsé Mantis Shrimpé!We're watching for your comments, so leave one. Thanks for watching DNews. Here's lookingat you, kid. Please subscribe.