In “Product Review: The Human Eye (Four Stars)”, we complained that every eye ships with a permanent blind spot, a patch with no light-sensing cells at all. You might reasonably not believe us. You’ve used your eyes your whole life and never seen a hole in anything. So let’s find it.
Here’s how. Close or cover your left eye. With your right eye, look straight at the plus sign above, and keep looking at it; don’t let your eye drift to the dot. Now slowly move your face towards the screen, or away from it. At some distance, usually somewhere around 25 to 40 centimetres on a laptop and a bit closer on a phone, the dot will simply vanish. Keep going and it comes back. That gap is your blind spot.
What’s happening is a matter of geometry. The light from the dot lands on one particular place at the back of your right eye, called the optic disc. That’s where all the nerve fibres from the retina, around a million of them, bundle together and leave the eye to travel to the brain, along with the blood vessels that feed the retina. To make room for this exit, there are no rods or cones there. Light that lands on the optic disc is not detected by anything. It’s a hole in the camera sensor, about the size of a dozen full moons side by side in the sky, which is much bigger than most people expect.
The interesting question is why you never notice. Part of the answer is that you have two eyes. The blind spot of each eye is off to the side, towards the ear, so the two holes fall on different parts of the scene, and each eye fills in what the other misses. But the test above still works with one eye closed, and even then you don’t see a black hole where the dot was. You see the background, smoothly continued.
That’s your brain at work. Rather than show you a gap, the visual system fills it in with whatever surrounds it: plain screen, a continued line, a repeated pattern. Researchers have tested this with grids, stripes and textures, and the brain’s guess is usually a seamless continuation. You don’t see nothing. You see a confident prediction of what’s probably there.
This is the “good guess” the review was joking about, when it said the brain fills the gap with a guess: “It is a good guess. It is still a guess.” The test lets you watch the guess happen. And the brain edits your vision like this all day. You blink around fifteen to twenty times a minute, and each blink cuts off your sight for about a tenth of a second, yet the world never flickers. When your eyes make the quick jumps the review described, from one point to the next, your vision is partly switched off during each movement, so you don’t see a blur. The seamless picture you experience is stitched together from gaps.
It’s a small trick, but a revealing one. What you experience as seeing isn’t a direct live feed from the eyes. It’s a picture your brain builds from incomplete information, patched with good guesses wherever the data runs out. Most of the time the guesses are right. The blind spot is just one of the few places where you can catch it guessing.