The hole you don't see

In each of your eyes there is a spot where you see absolutely nothing. And yet you don't perceive any black hole in the world. Why?

That blind spot is where the optic nerve leaves the eye: there are no light-sensitive cells there. We all have one, in both eyes. The reason we don't notice it is that the brain fills the void by inventing, from what surrounds it, what it assumes should be there.

With a simple trick you can catch it: close one eye, fix your gaze on a point, and slide a finger along the side until, all of a sudden, it disappears. A good part of what you think you see is, in fact, being filled in for you by your brain.

Source: American Academy of Ophthalmology