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.
Eye Tests That Look Like Magic
blind spots. human eyes have a blind spot when the optic nerve exits the back in the eye. this area has no light receptors because the nerve is in the way you can find your blind spot with this test. look at the red cross on the left with your right eye closing your left eye, move your head slowly closer and further away quot;So weirdquot; at a certain distance the blue dot on the right will disappear
pause if you need to, you've just found your blind spot After images, stare at the white dot in the center of the image and try not to blink. your eyes have three types of cones or color receptors red, green and blue. after images appear when you stare at a particular color for too long and those color receptors get tired. so when you suddenly switch to looking at a black and white image
those tired receptors aren't working as well this leaves an afterimage of the reverse colors. See your veins. blood vessels and veins feed the photosensitive cells in your eye to keep them alive these blood vessels cast shadows on your retina here's how you can see those veins. poke a small hole in a post it note or piece of paper hold that against a bright white background like your computer screen
look to the hole while shaking the postit slightly you should see a shadowy network online sort of like the veins a leaf these are the veins in your eye quot;Oh, wowquot; Gross. dominant eyes most people have a dominant eyes, I'm meaning that one eye gives more priority information than the other eighty percent of people are right eye dominant see which is your dominant eye. make a triangle between your thumbs and
forefingers With both eyes open, look at an object several feet away Center the object inside the triangle, close one eye and then the other. your dominant eye will look like this and your nondominant eye it will look like this. it's almost visionary. see what I did there.