The blind spot

How it works

The retina is the light-sensing sheet at the back of the eye. Its nerve fibres gather into one bundle, the optic nerve, and leave the eye through a small round area called the optic disc. The optic disc has no rods or cones. Light that lands there is not detected at all¹.

The optic disc sits about 12 to 15 degrees to the outer side of the centre of your vision, and slightly below the middle¹. It covers an oval about 5.5 degrees wide and 7.5 degrees high¹. The disc itself is about 1.5 millimetres across on the retina².

You do not see a black hole. The brain fills the gap with what it expects, based on the colour and pattern around it. This is called filling-in. The other eye also covers the gap, because the two blind spots point in opposite directions¹. Only when you close one eye and hold your head still does the missing patch become testable.

Octopus and squid eyes have no blind spot. Their nerve fibres run behind the retina instead of in front of it¹.

Try it

1. Draw a small cross on the left of a sheet of paper and a small dot about 10 centimetres to the right of it.
2. Close your left eye. Hold the paper at arm’s length and look at the cross with your right eye.
3. Keep looking at the cross. Slowly bring the paper towards your face.
4. At some point the dot disappears. It is now in your blind spot.
5. Keep moving closer and it comes back.
6. Draw a straight line through the dot and repeat. The line looks unbroken even when the dot is gone. That is filling-in.

History and culture

The blind spot was discovered in the 1660s by Edme Mariotte, a French physicist¹. He is said to have used it to make courtiers’ heads vanish at the court of Louis XIV. The XKCD comic “Visual Field” marks the blind spot on a map of everything the eye can and cannot see³.