What happens is that the brain expects to see something, based on what has happened before and how much it knows.

Create an image of what your eyes expect to see.

This information is sent from the brain to the eyes through intermediate stages.

If a discrepancy is detected between what the brain expects and the light reaching the eyes, only then will the neural circuits send signals to the brain. That is, from the eyes to the brain, the image of the observed environment does not travel, but only the news of any discrepancies with respect to what the brain expects.

The conceptual implications on the relationship between what we see and the world, however, are considerable.

When we look around we are not really "observing": we are rather dreaming of an image of the world based on what we knew ( including wrong prejudices) and unconsciously we scrutinize to detect  any discrepancies and, when necessary, try to correct.

What we see, in other words, is not a reproduction of the outside. This is what we expect, corrected by what we are able to grasp.

The relevant inputs are not the ones that confirm what we already knew. They are the ones who contradict our expectations.

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Carlo Rovelli, Helgoland