The Lab · Try it yourself

Find your blind spot

Every eye has a spot where it truly cannot see. It sits where the optic nerve leaves the retina, so there are no light-sensing cells there and nothing that lands on it ever registers. This test finds yours in about a minute. Cover one eye, lock the other on the cross, and slide the dot outward until it drops into the gap your brain normally hides from you.

The optic disc

Your blind spot

Where the optic nerve leaves each eye there are no photoreceptors, so a small patch of your vision is truly blank. Cover one eye, lock the other on the +, and slide the dot outward until it drops into that hole. Then swap it for a line or a speckled field and watch your brain paper over the gap.

  1. 1 cover one eye
  2. 2 stare hard at the +
  3. 3 slide until the dot is gone
right eye open · stare at the +
the dot sits about 12° to the side keep sliding on-screen gap 9.6 cm · sitting about 45 cm away
What to show
Which eye is covered

What this can and can't do. You need two eyes with one covered, so it will not work with both open. It can't print your exact angle either. Where your blind spot lands depends on your screen and how far you sit, and it shifts a little from person to person, so the degrees here are a guide drawn from the on-screen geometry, not a measurement of your eye. And notice the screen never actually removes the gap. What fills it back in is you.

Based on Edme Mariotte's 1668 discovery of the blind spot, the point where the optic nerve exits the retina and there are no rods or cones. It sits roughly 15° to the side of where you look and spans about 5°. The seamless completion is filling-in by the visual brain. Your own single run, n = 1, not a clinical test.

The science of your blind spot

The blind spot is the optic disc, the roughly 1.5 millimetre patch at the back of each eye where about a million nerve fibers gather and leave as the optic nerve. Because that patch carries wiring instead of photoreceptors, any image that falls on it is simply not recorded. You have one in each eye, off to the outer side of center, and you go through life never noticing either of them.

The reason you never notice is that the brain quietly completes the missing patch with whatever surrounds it, an effect called filling-in. In the test above you can watch it happen. Slide a plain dot into the gap and it disappears, but replace it with a continuous line or a speckled field and the brain paints the texture straight across the hole, so the gap looks seamless even though the eye sent nothing. The French physicist Edme Mariotte first described the blind spot in 1668, and the filling-in that hides it has been studied carefully since.

Ramachandran, V. S., & Gregory, R. L. (1991). Perceptual filling in of artificially induced scotomas in human vision. Nature, 350(6320), 699–702. doi.org/10.1038/350699a0

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