Weekly Optical Illusion: Blind Spot in the Optic Nerve

How many times did your driving instructor remind you to check your blind spot before switching lanes? Well, it turns out the old guy really knows what he’s talking about.

What is this nebulous blind spot? It turns out that the sheet of photoreceptors (little things that receive light) in our retina has a hole in it. Yes, you read that right: a hole. At one point, called the optic nerve head, neurons pass through the photoreceptor sheet to form the optic nerve, which transports the information the eye is receiving to the rest of the brain. It’s also the entry point for the blood vessels that supply the retina.
 

 
Obviously this nerve is pretty necessary, but there is one downside: due to the lack of photoreceptors at the optic nerve head, your brain doesn’t get any information from the area that’s missing. Now, as your probably know, brains are clever and they fill in this little spot with surrounding information so that we barely notice it. But, as this diagram will show you, it doesn’t disappear altogether.

Want proof? At a comfortable distance from your screen, cover your left eye and look at the crosshatch on the left. Now, slowly move your head towards the crosshatch and notice what happens to the black dot.
 

 
WHOA. Check that out! What is happening? At one point, the black dot disappears altogether. That is your blind spot! Just be glad it’s a black dot on a screen and not a car in the lane next to you.
 

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