one aspect above all that can’t be ignored: I am a sentient being. I experience my existence. I feel like I’m here, looking out on the world through these eyes, perceiving this Technicolor show from my own center stage. Let’s call this feeling consciousness or awareness.
Scientists often debate the detailed definition of consciousness, but it’s easy enough to pin down what we’re talking about with the help of a simple comparison: when you’re awake you have consciousness, and when you’re in deep sleep you don’t. That distinction gives us an inroad for a simple question: what is the difference in brain activity between those two states?
One way to measure that is with electroencephalography (EEG), which captures a summary of billions of neurons firing by picking up weak electrical signals on the outside of the skull. It’s a bit of a crude technique; sometimes it’s compared to trying to understand the rules of baseball by holding a microphone against the outside of a baseball stadium. Nonetheless, EEG can offer immediate insights into the differences between the waking and sleeping states.
When you’re awake, your brain waves reveal that your billions of neurons are engaged in complex exchanges with one another: think of it like thousands of individual conversations in the ballgame crowd.
When you go to sleep, your body seems to shut down. So it’s a natural assumption that the neuronal stadium quiets down. But in 1953 it was discovered that such an assumption is incorrect: the brain is just as active at night as during the day. During sleep, neurons simply coordinate with one another differently, entering a more synchronized, rhythmic state. Imagine the crowd at the stadium doing an incessant Mexican wave, around and around.
Consciousness emerges when neurons are coordinating with one another in complex, subtle, mostly independent rhythms. In slow-wave sleep, neurons are more synchronized with one another, and consciousness is absent.
As you can imagine, the complexity of the discussion in a stadium is much richer when thousands of discrete conversations are playing out. In contrast, when the crowd is entrained in a bellowing wave, it’s a less intellectual time.
So who you are at any given moment depends on the detailed rhythms of your neuronal firing. During the day, the conscious you emerges from that integrated neural complexity. At night, when the interaction of your neurons changes just a bit, you disappear. Your loved ones have to wait until the next morning, when your neurons let the wave die and work themselves back into their complex rhythm. Only then do you return.
THE MIND–BODY PROBLEM
Conscious awareness is one of the most baffling puzzles of modern neuroscience. What is the relationship between our mental experience and our physical brains?
The philosopher René Descartes 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
