The Brain: The Story of You

by David Eagleman

You become who you are not because of what grows in your brain, but because of what is removed.

Particular kinds of epilepsy make people more religious. Parkinson’s disease often makes people lose their faith, while the medication for Parkinson’s can often turn people into compulsive gamblers.

Our brains and bodies change so much during our life that – like a clock’s hour hand – it’s difficult to detect the changes. Every four months your red blood cells are entirely replaced, for instance, and your skin cells are replaced every few weeks. Within about seven years every atom in your body will be replaced by other atoms.

Perhaps memory can serve as the thread that makes you who you are. It sits at the core of your identity, providing a single, continuous sense of self.

Now let’s imagine that six months later you taste one of those little French cakes, just like the one you tasted at the birthday party. This very specific key can unlock the whole web of associations. The original constellation lights up, like the lights of a city switching on. And suddenly you’re back in that memory.

The enemy of memory isn’t time; it’s other memories. Each new event needs to establish new relationships among a finite number of neurons.

So a single event may be perceived somewhat differently by you at different stages in your life.

Clues to the malleability of our memory come from the pioneering work of Professor Elizabeth Loftus at University of California, Irvine. She transformed the field of memory research by showing how susceptible memories are.

So not only was it possible to implant false new memories in the brain, but people embraced and embellished them, unknowingly weaving fantasy into the fabric of their identity.

And that’s how Loftus had the opportunity to experience what it was like to possess her own false memory, richly detailed and deeply felt.

The more we keep our brains cognitively fit – typically by challenging them with difficult and novel tasks, including social interaction – the more the neural networks build new roadways to get from A to B.

Think of the brain like a toolbox. If it’s a good toolbox, it will contain all the tools you need to get a job done. If you need to disengage a bolt, you might fish out a ratchet; if you don’t have access to the ratchet, you’ll pull out a wrench; if the wrench is missing you might try a pair of pliers. It’s the same concept in a cognitively fit brain: even if many pathways degenerate because of disease, the brain can retrieve other solutions.

While it sounds strange to imagine that the movement of our bodies is required for vision, this concept was elegantly demonstrated with two kittens in 1963.

almost two tenths of a second. (In fact, if they move off the blocks before that duration, they’re disqualified – they’ve “jumped the gun”.) Athletes train to make this gap as small as possible, but their biology imposes fundamental limits: the brain has to register the sound, send signals to the motor cortex, and then down the spinal cord to the muscles of the body. In a sport where thousandths of a second can be the difference between winning and losing, that response seems surprisingly slow.

Visual data goes through more complex processing than auditory data. It takes longer for signals carrying flash information to work their way through the visual system than for bang signals to work through the auditory system.

In fact, the brain generates its own reality, even before it receives information coming in from the eyes and the other senses. This is known as the internal model.

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.

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.

It’s also your internal model that allows the world out there to remain stable – even when you’re moving.

your internal model operates under the assumption that the world outside is stable.