The Brain. David Eagleman

assumed that an immaterial soul exists separately from the brain. His speculation, depicted in the figure, was that sensory input feeds into the pineal gland, which serves as the gateway to the immaterial spirit. (He most likely chose the pineal gland simply because it sits on the brain’s midline, while most other brain features are doubled, one on each hemisphere.)

      The idea of an immaterial soul is easy to imagine; however, it’s difficult to reconcile with neuroscientific evidence. Descartes never got to wander a neurology ward. If he had, he would have seen that when brains change, people’s personalities change. Some kinds of brain damage make people depressed. Other changes make them manic. Others adjust a person’s religiosity, sense of humor, or appetite for gambling. Others make a person indecisive, delusional, or aggressive. Hence the difficulty in the framework that the mental is separable from the physical.

      As we’ll see, modern neuroscience works to tease out the relationship of detailed neural activity to specific states of consciousness. It’s likely that a full understanding of consciousness will require new discoveries and theories; our field is still quite young.

      So who you are depends on what your neurons are up to, moment by moment.

       Brains are like snowflakes

      After I finished graduate school, I had the opportunity to work with one of my scientific heroes, Francis Crick. By the time I met him, he had turned his efforts to addressing the problem of consciousness. The chalkboard in his office contained a great deal of writing; what always struck me was that one word was written in the middle much larger than the rest. That word was “meaning”. We know a lot about the mechanics of neurons and networks and brain regions – but we don’t know why all those signals coursing around in there mean anything to us. How can the matter of our brains cause us to care about anything?

      The meaning problem is not yet solved. But here’s what I think we can say: the meaning of something to you is all about your webs of associations, based on the whole history of your life experiences.

      Imagine I were to take a piece of cloth, put some colored pigments on it, and display it to your visual system. Is that likely to trigger memories and fire up your imagination? Well, probably not, because it’s just a piece of cloth, right?

      But now imagine that those pigments on a cloth are arranged into a pattern of a national flag. Almost certainly that sight will trigger something for you – but the specific meaning is unique to your history of experiences. You don’t perceive objects as they are. You perceive them as you are.

      Each of us is on our own trajectory – steered by our genes and our experiences – and as a result every brain has a different internal life. Brains are as unique as snowflakes.

      As your trillions of new connections continually form and re-form, the distinctive pattern means that no one like you has ever existed, or will ever exist again. The experience of your conscious awareness, right now, is unique to you.

      And because the physical stuff is constantly changing, we are too. We’re not fixed. From cradle to grave, we are works in progress.

       Your interpretation of physical objects has everything to do with the historical trajectory of your brain – and little to do with the objects themselves. These two rectangles contain nothing but arrangements of color. A dog would appreciate no meaningful difference between them. Whatever reaction you have to these is all about you, not them.

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      WHAT IS REALITY?

       How does the biological wetware of the brain give rise to our experience: the sight of emerald green, the taste of cinnamon, the smell of wet soil? What if I told you that the world around you, with its rich colors, textures, sounds, and scents is an illusion, a show put on for you by your brain? If you could perceive reality as it really is, you would be shocked by its colorless, odorless, tasteless silence. Outside your brain, there is just energy and matter. Over millions of years of evolution the human brain has become adept at turning this energy and matter into a rich sensory experience of being in the world. How?

       The illusion of reality

      From the moment you awaken in the morning, you’re surrounded with a rush of light and sounds and smells. Your senses are flooded. All you have to do is show up every day, and without thought or effort, you are immersed in the irrefutable reality of the world.

      But how much of this reality is a construction of your brain, taking place only inside your head?

      Consider the rotating snakes, below. Although nothing is actually moving on the page, the snakes appear to be slithering. How can your brain perceive motion when you know that the figure is fixed in place?

       Nothing moves on the page, but you perceive motion. Rotating Snakes illusion by Akiyoshi Kitaoka.

       Compare the color of the squares marked A and B. Checkerboard illusion by Edward Adelson.

      Or consider the checkerboard above.

      Although it doesn’t look like it, the square marked A is exactly the same color as the square marked B. Prove this to yourself by covering up the rest of the picture. How can the squares look so different, even though they’re physically identical?

      Illusions like these give us the first hints that our picture of the external world isn’t necessarily an accurate representation. Our perception of reality has less to do with what’s happening out there, and more to do with what’s happening inside our brain.

       Your experience of reality

      It feels as though you have direct access to the world through your senses. You can reach out and touch the material of the physical world – like this book or the chair you’re sitting on. But this sense of touch is not a direct experience. Although it feels like the touch is happening in your fingers, in fact it’s all happening in the mission control center of the brain. It’s the same across all your sensory experiences. Seeing isn’t happening in your eyes; hearing isn’t taking place in your ears; smell isn’t happening in your nose. All of your sensory experiences are taking place in storms of activity within the computational material of your brain.

      Here’s the key: the brain has no access to the world outside. Sealed within the dark, silent chamber of your skull, your brain has never directly experienced the external world, and it never will.

      Instead, there’s only one way that information from out there gets into the brain. Your sensory organs – your eyes, ears, nose, mouth, and skin – act as interpreters. They detect a motley crew of information sources (including photons, air compression waves, molecular concentrations, pressure, texture, temperature) and translate them into the common currency of the brain: electrochemical signals.

      These electrochemical signals dash through dense networks of neurons, the main signaling cells of the brain. There are a hundred billion neurons in the human brain, and each neuron sends tens or hundreds of electrical pulses to thousands of other neurons every second of your life.

       Neurons communicate with one another via chemical signals called neurotransmitters. Their membranes carry electrical signals rapidly along their length. Although artistic renditions like this one show empty space, in fact there is no room between cells in the brain – they are packed tightly against one another.

      Everything you experience – every sight, sound, smell – rather than being a direct experience, is an electrochemical rendition in