The neuroscience of you, p.8
The Neuroscience of You, page 8
In the process of conducting these investigations, Michael Gazzaniga, who began this research as a graduate student working with Sperry, noticed something fascinating: Sometimes, when a patient saw what their left hand was doing (with both hemispheres), they would spontaneously make up a story to help bridge the inconsistency between what they said they saw (nothing) and what they drew. In one example that Gazzaniga captured on video, two different pictures were flashed on the screen at the same time: A sun was presented on the right side of the screen, and an hourglass timer on the left. “What did you see?” he asks the patient. “The sun,” the patient responds, based on the information that is available to his left, speaking hemisphere. “Can you draw it?” Gazzaniga asks, putting a pencil in the patient’s left hand. He draws an hourglass, because that is what his right hemisphere, which controls his left hand, has seen. Now the speaking left hemisphere can see what his left hand has drawn, and it starts to make up a story about why it did that. “What did you see?” Gazzaniga asks again. “A sun,” the patient responds. “But I drew a timer because I was thinking about a sundial,” the patient says, confabulating a plausible connection between what his left hemisphere knows and what it observed. And voilà, the patient’s brain was caught red-handed in its storytelling process.[*]
Over the course of these experiments, Gazzaniga inadvertently discovered that the speaking part of our brain also seems to generate causal explanations that relate events to one another. Since then, Gazzaniga and other scientists, including myself, have gone on to experimentally investigate differences in these “inferential” processes in both hemispheres of intact participants and callosotomy patients alike. The general consensus is that in most people, the left hemisphere generates hypotheses about what might link two events together based on details it deems relevant. And because of its ability to do so, Gazzaniga named the left hemisphere “the interpreter.” From that observation, subsequent journalists and researchers created the idea of an analytical hemisphere.
As you might suspect, this specific kind of analysis—the kind that reverse-engineers causality from observed events, is really important for predicting the future. And let me make it quite clear that your healthy brain does this all the time. Much like the left hemisphere of a split-brain patient makes up stories to fill in the gaps when it observes a behavior it isn’t controlling, your brain is constantly creating a personal narrative that weaves a causal explanation through the actions it observes you doing. Though your two hemispheres are probably well connected, your interpreter still needs to fill in the gaps about why you do the vast majority of things that are, in all actuality, driven by subconscious brain processes.[*] But this happens so frequently, and so fluidly, that we are largely totally unaware of our brain’s storytelling process.
If you’re still not convinced that your brain is making stuff up, try to remember a time when you woke up after a lapse in consciousness. The most striking memory I have of such an event happened during graduate school.[*] The shortest version of my story begins with me “waking up” with my head sticking out the front door of my apartment. My first conscious experience was hearing my inner voice saying something like “I must have taken a nap.”[*] Almost immediately afterward, my brain ran a reality check on that interpretation. The idea was followed by something like “I must have been really tired to have taken a nap in the doorway!” And then, when faced with conflicting information like “Wait—I don’t take naps in doorways!” my brain started searching its memory bank for different solutions. When it did, it was able to pull up the memory of burning skin and talking to a nurse on the phone, and my left hemisphere was able to use that new data to construct the new, more plausible, story that I had fainted![*]
Although this process becomes noticeable in unusual situations like mine, even sleep can produce a lapse of consciousness that lets you catch your interpreter in the act. You’re more likely to notice when things are unexpected—like when you wake up in a different place than your bedroom, and your sleepy brain has to process why the things you’re seeing and hearing are not the things it expected. When this happens, you can sometimes “hear” your interpreter trying to figure out where the heck you are. In those fleeting moments, where the dots aren’t easily connected, you might become more aware of your brain’s storytelling processes. And although this has mostly been studied in the domain of reading, there are hints that people with more balanced brains rely more on the broader context to interpret what is going on, while more lopsided brains may focus in on the individual details first.
I completely understand that this might make you feel weird, but trust me—if your brain didn’t tell you stories, you’d be in big trouble. At the pace that natural conversations occur, for example, if you took even five seconds to wonder what else a person might mean when they say “They are cooking apples,” you’d miss the next ten words they said. Good luck trying to figure out what’s going on after that!
One thing I’ve always wondered about this, which hasn’t yet been studied systematically, is how tightly related our conscious storytelling processes are to speech. According to a tweet that went viral in January 2020, the experience I described, in which you can “hear” your thoughts expressed verbally in your mind, is not universal. In fact, a significant number of people (including my husband, Andrea) don’t experience their internal thoughts as words at all. This makes me wonder whether, in some balanced brains, the functions of speaking and interpreting get assigned to different hemispheres. If that happens, does it fundamentally change the nature of your “personal narrative”? What might the nonnarrative version of this narrative be?[*]
We have hints that come from observing patients whose two hemispheres have been severed. Many of them seem to “identify” more with whatever is going on in their left hemisphere than they do with their right. In the days that followed her callosotomy surgery, a patient named Vicki described her frustrating experience with everyday tasks, like grocery shopping or picking out her clothes for the day. “I’d reach with my right [hand] for the thing I wanted, but the left would come in and they’d kind of fight . . . almost like repelling magnets.” In these anecdotes, two things are clear. First, when the two hemispheres are surgically separated from each other, they can come up with different ideas about how to behave, based on their unique ways of understanding the world. The second is that the subjective experience these patients report verbally is consistent with how their left hemispheres behave.
Fortunately for most of us, approximately 150 million high-speed neurons, collectively referred to as the corpus callosum, connect our two hemispheres, allowing them to rapidly share their views of the world with each other. And so, while we may still have difficulty deciding what to wear, we experience these decisions from the perspective of a single, unified “self” that has access to the integrated outputs of both hemispheres. The principles of neural engineering that allow us to control the flow of information from one brain area to the next will be the focus of the next two chapters.
Summary: Different degrees of forest- and tree-level brain computations shape our understanding
Before we move on, let’s take a minute to review some of the key concepts that were covered in this chapter. There were a lot of them, and the coming chapters will build on them further to give you a better idea of how your brain works. One critical topic we discussed is the relation between the structure of your two hemispheres, and the different computations they perform. In most people, the left hemisphere seems to be optimally structured for a divide-and-conquer approach—one that uses modules to execute specialized computations that don’t interact with one another. This is kind of like building an understanding of a forest one tree at a time. The right hemisphere, on the other hand, takes a big-picture approach, integrating as much information as possible from different processing centers into a coherent story about the event or scenario unfolding around it. This is kind of like saying, “I know I’m in the forest, so that vertical thing in front of me must be a tree!”
Though these structural asymmetries may be reversed in a very small percentage of the population, the biggest difference between brains is the degree of specialization between hemispheres. And though there are clearly advantages that pushed us to have two hemispheres that can see the world from two different perspectives at one time, the disadvantages of having extremely lopsided brains include increased vulnerability to injury, as well as potentially weaker functioning in things that require seeing the big picture.
We also discussed the fact that the assignment of a particular function, like speech or sentence reading, to one hemisphere or the other depends not only on how different the two hemispheres are from each other but also on how much experience a person has performing a particular task. But there are even subtler differences we haven’t talked about yet that can dynamically influence how much one or the other of your hemispheres contributes to any function.
For instance, one remarkable study by Casagrande and Bertini measured patterns of brain activity and relative hand skill in a small group of 16 healthy, right-handed volunteers during various parts of their wake/sleep cycles. They showed that all participants had greater activity in the left hemisphere and better skill in their right hands during the waking hours, but immediately before falling asleep, and immediately after waking up, their right hemispheres were more active, and their left hands were more skilled! This means that each of us has an opportunity at the beginning and end of the day to catch a glimpse of what the other half of our brains might be “thinking,” although we may not be as adept at “talking” about it.
And if you think that’s bizarre, multiple experiments have demonstrated that something as simple as clenching one of your hands into a fist for a prolonged amount of time can influence your pattern of thinking, feeling, and behaving by changing activation levels in one of your hemispheres. For instance, some of these studies have shown that if you clench your left hand (activating your right motor cortex), you can increase the relative degree of “avoid” feelings, or amount of dislike you report for any given stimulus, while clenching your right hand and activating your left motor cortex increases your “approach” motivation, or the degree to which you report liking something. These research findings remind us that while some proportion of our differences in lopsidedness is relatively stable, changes do occur within us, both slowly as we become more experienced with certain processes across our life span, and more quickly as we move through various states of wakefulness or respond to different environmental factors that activate one hemisphere more than the other. And so, if you feel like a different person when you wake up in the morning or go to sleep at night, it might help to know that there are fundamental differences at play in how your brain works. In the next chapter, we’ll turn our discussion to some more nuanced aspects of your brain design, discussing the way its chemical ingredients shape the types of information it shares, both within and between hemispheres.
CHAPTER 2
MIXOLOGY
The Chemical Languages of the Brain
In this chapter, we’re going to zoom in to discuss the smallest of your brain’s design features—your neurotransmitters. To put it simply, your neurotransmitters are the chemicals that your neurons rely on to communicate with one another. Though all brains use them, the human brain has hundreds[*] of different kinds at its disposal. And at any given point in time, your brain is floating in a cocktail made up of its own unique mixture of these ingredients.
If you’ve ever smoked marijuana with a group of friends,[*] or enjoyed an alcoholic beverage or three at a social event, you probably already understand some of the most important things about your brain’s mixology. The first is that substances that alter your brain chemistry can change the way you think, feel, and behave—sometimes in dramatic ways. And the second is that these changes don’t look the same in everyone. Both phenomena relate to the ways different brains use chemicals in their communication systems. In this chapter, you’ll learn why the littlest parts of you can make such a big difference in how you work!
Take caffeine, the most popular drug in the world,[*] as an example. Drinking a cup of coffee, tea, or other caffeinated beverage affects your chemical mix in multiple ways. The most wonderful of these, in my opinion, is that it increases the availability of a neurotransmitter called dopamine in your brain. Dopamine is one of the most important ingredients in your neural cocktail, because it is the chemical the pleasure circuits in your brain use to communicate. And because all brains are motivated to feel good, your brain’s dopamine circuits are heavily involved in learning and decision-making. Their goal is to shape the decisions, big and small, that move you through the world in a way that earns you the most pleasure points. This makes the popularity of caffeinated beverages a no-brainer.
And now imagine the possibility that the differences in baseline levels of dopamine between two brains might exceed the difference you feel before and after your morning cup of caffeine. One person’s baseline mental state could feel like you after a shot of espresso, while for another, your morning brain before coffee or tea might be their personal high.
To get a better idea about how the levels of the different ingredients in your neural cocktail influence how you think, feel, and behave, let’s take a closer look at the relation between structure, computation, and function that we began discussing in the last chapter. For starters, consider the fact that the computational differences we talked about in “Lopsided” arise because of the way that networks of millions, or even hundreds of millions of neurons work together. But as we touched on in “Introductions,” when it comes to individual neurons, they all perform essentially the same job. Their computations involve listening to the “gossip” of other neurons around them and deciding whether they have enough evidence to pass their own signal down the line.
In fact, the extent to which any individual neuron will contribute to some function, like language, depends largely on where it is in the brain. This is because location is one of the main factors that determine what gossip a neuron is listening to. In other words, the function a neuron performs depends almost entirely on the inputs it performs its computations on.
In 1988, a group of neuroscientists made this point very clearly by surgically rewiring a newborn ferret brain so that the neurons carrying signals from its eyes were connected to the neurons that usually process information coming in from the ears. When they did, they created a ferret that learned to “see” using its auditory cortex—the part of the brain usually assigned the job of hearing. Eventually, the auditory cortex in the ferret was able to take over the function of the seeing when it was fed the same inputs.[*]
But we can also observe fascinating naturally occurring evidence of what happens when neural signals get crossed, in humans who exhibit synesthesia. This condition, which occurs in an estimated 2 to 4 percent of the population, involves the merging of two unrelated streams of sensory information in the mind and brain. The outcome can range from people who experience some sense of shape (for example, square or round) when presented with different food tastes to the more frequent case of seeing colors when presented with particular letters or words. The moral of the story is, when you’ve got a thundering 86 billion neurons gossiping in your brain, it needs systems for organizing who is talking (and listening) to whom.
True to the spirit of this book, this engineering problem—the need to keep track of overlapping signals between neurons to organize their functions—creates a brain-design space that has several different solutions. Ironically, this relatively large problem space occurs in a very tiny physical space—in the synapse between neurons—a 0.02-micron gap, 1/2,000th the diameter of a piece of hair, that separates them from one another. It’s here that your mixology plays a pivotal role in shaping the function of your neurons, by determining how well they can communicate with others.
To get a handle on how this works, let’s use as a model for neural communication the game called Telephone that we used to play when I was a kid. In Telephone, one person comes up with some secret message and then whispers it to the child next to them. That child then whispers it to the next child in the chain, and the process is repeated until the message gets passed all the way around a circle back to the sender. The funny part of the game is that when your original message comes back to you, it’s usually been changed into something totally different. At each transition between people, the conditions caused by the combination of a soft signal (a whisper) and a noisy room (usually full of giggling children) drive some level of interpretation or improvising on the part of the listener. The result is that a message like “Do you want banana pancakes?” can easily be transformed into something like “Cthulhu haunts bandana manscapes.”
Though it might be hard to believe, your brain works something like this: In its version of Telephone, the whispering between neurons is accomplished by the release of neurotransmitters. Much like a message whispered between humans temporarily assumes the physical form of a sound wave traveling from mouth to ear, the messages exchanged in the space between neurons must temporarily take the physical form of chemical packages. And this is where your brain’s smallest design features, the chemical ingredients of you, start to shape the way you work.
