How perception works: why what you see is your brain's best guess
Filling in: how your brain paints over the hole in your eye
Every eye has a blind spot, and you never see a hole there. Put a broken line across it, and your brain shows you a piece that is not there. What does it paint in?
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1
Miva's second card has the same cross, and a line with a small gap in it. Rono spots the gap at once and says so, loudly.
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2
Same trick as with the dot. Left eye closed, eyes on the cross, and move the screen until the gap falls into your blind spot.
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3
When the gap falls into the hole, it closes. You see one whole line, while the screen still shows two pieces.
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4
Your eye sends the two pieces, and nothing from the hole between them. Your brain does not show you a hole. It looks at what lies around it, a line coming in from both sides, and draws the line through. This is called filling in.
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5
It fills the hole with whatever is around it: a color, a pattern, a wall. Remember Rono's head, gone in the first test? It left no black patch: in its place, you saw the tiles of the wall behind him.
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6
Miva writes it down: one patch of what you see is made by your brain. Then she looks up at the two of you.
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Quick check
Rono's shirt has stripes, with one small black dot in the middle of them. One eye closed, the dot falls into your blind spot. What do you see there?
Show the answer
Stripes. The dot is gone, and your brain fills the hole with what is around it, so the stripes run straight through.
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Go further
When the brain fills holes outside the lab.