The neuroscience of you, p.3
The Neuroscience of You, page 3
HOW NEURONS WORK
In fact, there are enough shared features between the physiology of human and nematode neurons that hundreds of millions of national funding dollars are devoted to research on C. elegans. The things we have learned from this work fill dozens of books, with titles like Neurobiology of the Caenorhabditis Elegans Genome, Ageing: Lessons from C. Elegans, and my favorite, WormBook. Of course, if you consider the differences between human brains against the backdrop of how much we have in common with a roundworm, it seems easy to view them as insignificant.
But now consider the other end of the spectrum—the differences between the mental lives of humans and chimpanzees, our closest living relatives. As you might imagine, our brains are remarkably similar to chimp brains. This makes pretty good sense, if you consider the fact that the DNA blueprints that build human and chimp brains overlap by about 95 percent. But the functional implications of that 5 percent difference allow me to write a book, in a shared symbolic language that you can understand, while the wild chimpanzees still spend a good chunk of their day finding food and grooming one another to maintain their social bonds.
In this comparison, you can start to see that when it comes to the relation between minds and brains, a little difference can go a long way. But since you’ve never been a chimpanzee, here are a few examples that are closer to home. Remember the way you thought, felt, and behaved when you were a teenager?[*] Though the brain that guides you through life now still bears the scars of that time, the neural changes that happen across your life span can also have big consequences on your mental life. For an even subtler difference, consider how you feel first thing in the morning versus late at night. Within a twenty-four-hour cycle, changes in the neurochemical signaling of your brain’s pacemaker, the suprachiasmatic nucleus, can have pretty dramatic effects on your inner workings. Hopefully, reflecting on the range of spaces your own brain and mind can occupy will help you start to appreciate the relevance of small differences. But before you decide whether these differences are important, let me talk a bit about their scientific implications.
Take, as an example, my early research investigating how the two halves, or hemispheres, of the brain collaborate to help you understand the stories you read or listen to. To get a better handle on the work your brain does for you in these situations, consider the following sentence:
The haystack was important because the cloth ripped.
Though this is a perfectly legal English sentence, you probably feel a bit disoriented after reading it. It’s not that you don’t understand the sentence, per se. You likely know the meanings of all of the words. And you can use your linguistic knowledge to figure out how the meanings of the words relate to one another. For instance, based on the order these words occur in, you know that it was the haystack, not the cloth, that was important. You also know that this importance is somehow causally related to the action of the cloth ripping. But you still don’t really understand what the heck is going on.
This is because when we read, or even listen, to language, we have different levels of understanding it. The first is the one we’ve been discussing, based purely on the linguistic information contained in a sentence. But the second involves interpreting this information in the broader context of what you know about the world, and what’s going on around you at the time.
The reason the haystack sentence feels weird is that it has been plucked out of its context. If I were to tell you that the sentence was part of a story about parachuting, how would your understanding of it change? Hopefully, things would click into place, shifting your comprehension of the sentence from a place of disconnected ideas to a scenario you can imagine, like a little movie clip unfolding in your mind. If it did, your brain connected a bunch of dots between things you already knew about the real world, like how gravity and parachutes work, and what was written on the page. From there, the reason a haystack might be important becomes clearer.
What’s interesting about these two ways of understanding is that research on people with brain damage seems to suggest that different parts of the brain are involved in computing them. Prior to my research, it was generally believed that the left side of the brain, which is typically involved in processing linguistic information,[*] was responsible for understanding the ideas printed on the page; while the right side, which is typically implicated in more visual or spatial thinking, constructed the scenario. But these ideas, like most of what we know about how brains work, were based on findings that were averaged across groups of participants.
But we also know, from pioneers in the field of reading research like my graduate adviser, Debra Long, that not all people understand what they read in the same way. And I wondered whether this variance had anything to do with the way the jobs were divided up between the two sides of their brains. To explore this possibility, I conducted a study looking at differences in what each hemisphere remembered about a story, in more than 200 readers with different skill levels.
Here’s how the experiments worked in a nutshell: Participants were told to read, and try to remember, short, two-sentence vignettes that were presented in the center of their computer screens. After reading a few stories, they would see a series of words flashed either in the center of their screens or just to the left or the right of the place they were told to focus their eyes. Their task was simple—to indicate with a button press as quickly as possible whether the word presented had been used in one of the stories. For instance, if I gave you the word “important” after reading the haystack story, you would say yes because that word was in the sentence.
Based on our participants’ patterns of responding, we were able to reverse-engineer something about the way each of their hemispheres processed the stories. For instance, sometimes we presented words like “parachute” that weren’t actually in any of the stories but were thematically related to them. If participants were slow to reject those words, or mistakenly said they had seen them, we had pretty good evidence that they were understanding the broader scenario of the stories. And we measured the linguistic type of understanding by checking to see whether people were faster to recognize words like “important” when they were presented after linguistically related words, like “haystack,” than when they followed words from different grammatical clauses in the sentence, like “cloth.”
And we used one last trick to figure out how each hemisphere might be involved in these different ways of understanding. Because of the way information flows from our eyes to our brains, everything coming from the left side of any focal point goes into the right hemisphere first, and vice versa. Though both hemispheres in a healthy brain eventually share this information, differences in the speed and pattern of responding to words presented to the left or right sides of the screen provide critical clues about how each hemisphere processed the sentences.
Although all participants in our study were college students with no diagnosed reading disabilities (in other words, they were all in the typical bucket), differences in their reading skill corresponded to brains that were doing different things—particularly in the right hemisphere. As predicted by the patient data, we showed that the left hemisphere of all of our readers did seem to understand the linguistic aspects of texts (that is, their left hemispheres understood that the haystack was important, not the cloth). However, the right hemisphere of the less-skilled readers in our group was also sensitive to these linguistic relations. So much for language being a left-hemisphere-only function! And when it came to the scenario-based way of understanding stories, both hemispheres of the less-skilled readers got tripped up by words like “parachute,” showing that they were sensitive to both the scenarios as well as linguistic features. On the other hand, only the left hemisphere of the skilled readers seemed sensitive to the scenarios. Ironically, the most-skilled readers had right hemispheres that were like Jon Snow from Game of Thrones—they knew nothing. They didn’t respond differently when words like “important” were presented after “parachute” or “cloth,” or “crow” for that matter. And they were no more tripped up by words like “parachute” that were thematically related to the stories than they were by words that were totally unrelated to the sentences.
At the end of the day, no individual in my experiment showed the specific pattern of results that was predicted based on the data you get when you average groups of people with different reading skill. It’s kind of like taking a room full of people and saying that their average age is forty-two, even if no one in the room is actually forty-two. But in this case, the failure to understand how brains differed led not only to incomplete data but to incorrect conclusions about how the two hemispheres contribute to reading comprehension.
If you’re still wondering why to care about this, imagine what might happen if you found yourself with an injury in the right hemisphere of your brain. What changes might the doctor tell you to expect? How might they evaluate the risks and benefits of some optional surgery?
Throughout my career, I have argued that although focusing on group averages has allowed the field to learn more quickly about the things we do have in common (like many of the mechanisms that underlie our sensory processes), it has also slowed our ability to understand the things that make us unique (like how we go about understanding stories, or jokes, or one another for that matter). One implication of this “one-size-fits-all” approach is that the vast majority of what we know about how the human brain gives rise to the human mind either ignores or glosses over the things that make us different.[*] For instance, many neuroscientists and even physicians still consider language comprehension to be the left hemisphere’s job. And as a result, when it comes to understanding how, and in whom, the right hemisphere contributes to different ways of understanding language, there is little consensus in the field, even though descriptions of language difficulties following right-hemisphere damage have been reported for more than 150 years.
But before I ride off into the sunset on my “differences matter” high horse, please allow me to make a confession: There are good, practical, reasons that people who are interested in human neuroscience don’t study individual differences. The first relates to that whole “brains trying to understand brains” conundrum. The human brain is so incredibly complicated that we will definitely not fully understand it in my lifetime,[*] even if we do gloss over all the things that make us different and focus on our commonalities. The truth is that we still don’t even fully understand C. elegans! Even though we have a perfect map of each of their neurons and what they are connected to, we cannot predict with 100 percent accuracy what a C. elegans will do in any given situation. We can get close, but we don’t understand everything.[*] Imagine how that scales up from a map of 302 neurons to one of 86 billion, and you will have the appropriate mindset for appreciating how much we still don’t know about your brain.
Which brings me to the second reason that studying individual differences in human brains is challenging. Many of the interesting variables can’t be ethically manipulated in the laboratory. Instead, when a person walks in for testing, they bring all of their brain’s design features with them—those that they were born with, as well as those that were shaped by their experience. But as you’ll learn in this book, these things are often related to one another. Trying to tease differences apart to figure out why someone is the way they are is very challenging in the best of circumstances. The task always takes us back to one of the oldest questions in psychology—how much of what makes you you is inherent in your DNA, and how much has been shaped by your experiences?
The misunderstood battle of nature versus nurture
So what came first—the linguistically ignorant right hemisphere, or skilled reading ability? By now, most people who study human behavior understand that our biology and our experiences are so intertwined that it hardly makes sense to “blame” one or the other when trying to figure out what makes you you. The answer is always a combination of both. For one thing, every single life experience changes your brain. Some of the changes are inconsequential and others are incremental. But on rare occasions, for better or for worse, a single event can change the way we work, forever.
This is an important note to take, before diving deeper into the neuroscience of you. The fact that something about your brain causes you to think, feel, or behave in a certain way does not necessarily mean either that you were born that way or that it can’t change. The truth is that your brain is a moving target. And most research linking brains to behaviors, like my work on the two hemispheres and reading skill, only looks at a single point in time—a snapshot, so to speak. With this kind of experiment, it’s simply impossible to tell how much of a particular brain design is stable or has been shaped by your experiences.
One way to separate the influences of our genetic blueprints (or “nature”) from our environments (or “nurture”) is to do a longitudinal study. With this type of design, researchers measure the same brain at different points in time, to see how general maturation, or a specific experience, might change it. For example, in yet another clever follow-up experiment conducted with London’s taxi drivers, Katherine Woollett and Eleanor Maguire did exactly this. Their goal was to figure out whether people who are able to pass the Knowledge test were born with bigger hippocampal tails, or whether it was the act of studying for the test itself that caused this area to grow.
To do this, they took images of the brains of 110 people on two occasions, three to four years apart. The majority of them (79) were taxi-driving hopefuls, first scanned while they began training to become taxi drivers but had not yet passed their tests, and the rest (31) were “control” participants, selected to match them on factors like age and IQ that might relate to the sizes and shapes of their brains. And since more than half of the trainees fail to pass the Knowledge, the researchers planned to make two comparisons with their data. First, they wanted to compare the brains of people who eventually passed the test to those who didn’t, to see if there were observable brain design features that separated the groups. Second, they wanted to see if there were any notable changes that happened as a result of studying for the Knowledge and stuffing one’s brain full of maps.
The results from Woollett and Maguire’s longitudinal study provided pretty clear evidence about the causal relation between the taxi drivers’ brains and what they’d been asked to do. Before training, there was no way to identify who would or wouldn’t eventually pass the Knowledge. There were no reliable brain differences between the eventual “pass” and “fail” groups when they signed up for training—not in the size of their hippocampi, or of any other brain region for that matter. In fact, the only difference between those who passed the test and those who didn’t was the amount of time they spent training every week. The group who passed spent an average of 34.5 hours each week training, while those who didn’t typically spent less than 17 hours a week studying! And over three years, that intense training schedule left its mark, but only on the brains of the group that passed. After squeezing all of that knowledge into their brains, their hippocampal tails grew.[*] In other words, the exceptional brains of the London taxi drivers were created by the demands placed on them. Case closed.
For researchers who don’t have the time, money, or desire to follow their participants through life and take repeated measurements of their brains, twin studies provide another option for disentangling the influences of nature and nurture. The field of behavioral genetics has largely progressed this way—attempting to disentangle nature from nurture by looking at people who share different proportions of each. For instance, monozygotic, or “identical,” twins are created from the same egg and sperm and are nearly genetically identical at birth;[*] whereas dizygotic, or fraternal, twins come from two different sperm and two different eggs, and thus share the same amount of genetic overlap as any two nonidentical siblings. Many studies estimate heritability, or the extent to which some measurable characteristic is genetically driven, by comparing how similar it is in pairs of monozygotic twins to pairs of dizygotic twins. If the monozygotic twins are more similar to each other with respect to the characteristic of interest (say, their ability to remember the location of landmarks) than dizygotic twins are, the difference between twins is assumed to be related to genetics. This type of analysis relies on the assumption that monozygotic twins and dizygotic twins share roughly the same degree of overlapping environments.
The problem with this assumption is that some characteristics that have genetic influences, like extraversion (which you’ll read about in the “Mixology” chapter), also influence the kinds of environments and experiences people will seek out. And other genetic factors, like how tall or attractive you are, can influence your experiences by shaping the way others treat you. To further muddy the nature-versus-nurture waters, the rapidly evolving field of epigenetics is showing that environmental experiences can create chemical changes in our DNA! As a result, a single gene can have different effects on the proteins it creates in the brain (or body) when placed in different environments. Through these mechanisms our experiences can become “biologically encoded.”[*] In other words, if you put the same strand of DNA into different environments, it might build different people.
But sometimes the outcomes aren’t that different.
The documentary Three Identical Strangers does a fantastic job capturing this. The movie is based on the remarkable true story of identical triplets who were adopted into different families at birth, and only discovered one another accidentally at the age of nineteen. In case you haven’t seen it, I won’t ruin the surprise (and sometimes scandalous) twists—but suffice to say that the ways in which these boys were similar to one another might go beyond what your mind conjures up when you think of how your biology makes you you. Of course they look, walk, and talk alike—but smoke the same brand of cigarettes? That’s just wild. Or is it?
