What do you think of this fact?
There is a hole in your vision. Right now, in each eye. You have never noticed it because your brain covers it up so well.
Where the cable leaves the eye
Your retina is covered with light-sensitive cells that turn light into nerve signals. Those signals have to get to the brain somehow, and they leave the eye through the optic nerve. According to Britannica, the fibers of over one million nerve cells in the retina come together at one spot to form it: the optic disc. It is about 1.5 millimeters across and has no photoreceptors at all. Whatever lands there is simply not detected.
The result is a blind spot in the field of view of each eye. Encyclopedia.com credits the French scientist Edme Mariotte with predicting it in the 17th century. While dissecting a human eye, he noticed the optic disc and concluded that every eye must have a small blind area.
Your brain fills the gap
So why aren’t you walking around with a dark blob in your view? Two reasons. With both eyes open, the fields of view overlap, so each eye covers the other’s gap, Britannica explains. And even with one eye closed, your brain fills in the missing patch using the surroundings. Encyclopedia.com describes how the blind spot normally appears in the same color as the background. The brain can even join lines across the gap.
You can find your own with a test from Neuroscience for Kids at the University of Washington: close one eye, look at a plus sign with the other and slowly bring the page closer. At a certain distance, the dot next to it simply disappears.
Bonus fact: Octopuses and squid don’t have this blind spot, because in their eyes the optic nerve reaches the receptors from behind instead of breaking through the retina.
The view you trust every day has a hole in it, and your brain patches it before you can notice.