The Brain. David Eagleman

      Detailed expectations about the world – in other words, what the brain “guesses” will be out there – are being transmitted by the visual cortex to the thalamus. The thalamus then compares what’s coming in from the eyes. If that matches the expectations (“when I turn my head I should see a chair there”), then very little activity goes back to the visual system. The thalamus simply reports on differences between what the eyes are reporting, and what the brain’s internal model has predicted. In other words, what gets sent back to the visual cortex is what fell short in the expectation (also known as the “error”): the part that wasn’t predicted away.

      So at any moment, what we experience as seeing relies less on the light streaming into our eyes, and more on what’s already inside our heads.

      And that’s why Cold Blue Luke sat in a pitch-black cell having rich visual experiences. Locked in the Hole, his senses were providing his brain with no new input, so his internal model was able to run free, and he experienced vivid sights and sounds. Even when brains are unanchored from external data, they continue to generate their own imagery. Remove the world and the show still goes on.

      You don’t have to be locked up in the Hole to experience the internal model. Many people find great pleasure in sensory deprivation chambers – dark pods in which they float in salty water. By removing the anchor of the external world, they let the internal world fly free.

      And of course you don’t have to go far to find your own sensory deprivation chamber. Every night when you go to sleep you have full, rich, visual experiences. Your eyes are closed, but you enjoy the lavish and colorful world of your dreams, believing the reality of every bit of it.

       Seeing our expectations

      When you walk down a city street, you seem to automatically know what things are without having to work out the details. Your brain makes assumptions about what you’re seeing based on your internal model, built up from years of experience of walking other city streets. Every experience you’ve had contributes to the internal model in your brain.

      Instead of using your senses to constantly rebuild your reality from scratch every moment, you’re comparing sensory information with a model that the brain has already constructed: updating it, refining it, correcting it. Your brain is so expert at this task that you’re normally unaware of it. But sometimes, under certain conditions, you can see the process at work.

      Try taking a plastic mask of a face, the type you wear on Halloween. Now rotate around so you’re looking at the hollow backside. You know it’s hollow. But despite this knowledge, you often can’t help but see the face as though it’s coming out at you. What you experience is not the raw data hitting your eyes, but instead your internal model – a model which has been trained on a lifetime of faces that stick out. The hollow mask illusion reveals the strength of your expectations in what you see. (Here’s an easy way to demonstrate the hollow mask illusion to yourself: stick your face into fresh snow and take a photo of the impression. The resulting picture looks to your brain like a 3D snow sculpture that’s sticking out.)

       When you’re confronted with the hollow side of a mask (right), it still looks like it’s coming towards you. What we see is strongly influenced by our expectations.