How perception works: why what you see is your brain's best guess
The blind spot: why each of your eyes has a hole you never see
Each of your eyes has a hole in what it sees, and you have never noticed it. All it takes to find yours is one closed eye and your screen.
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1
This morning you and Rono are volunteers in Miva's vision lab, for a free lunch. Rono builds tables for a living and trusts his eyes completely. Miva just smiles and hands you each a card.
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2
Try it on this card, now. Hold the screen about a hand away from your face. Close your left eye and look at the cross with your right eye. Keep looking at it, and move the screen slowly closer or further.
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3
At some point, the dot is gone. Nothing covers it, and the screen still shows it. Move a little closer or further, and it comes back.
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4
Here is why. The back of your eye is lined with light cells. Their signals leave the eye along one thick cable to your brain, the optic nerve, and where it leaves there is no room for a single light cell. Light that lands there is never caught. This spot is the blind spot, and each eye has one.
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5
The hole sits a little to the side of where you look, and it is not small. Keep your left eye closed and look at Miva across the lab: Rono's head, a step to her right, can fall into it and vanish.
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6
Open both eyes, and each eye sees what falls in the hole of the other. But close one again and look at the wall: your eye sends nothing from that spot, and still you see no black patch, no edge, nothing missing.
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Quick check
You want to find the blind spot of your left eye. Should the dot be on the left or on the right of the cross?
Show the answer
On the left. The blind spot of each eye sits a little to its own side, so close your right eye, put the dot on the left of the cross, and look at the cross with your left eye.
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Go further
Two names to know, and an animal with no blind spot at all.