Read an Excerpt
Chapter 1
Lopsided
The Two Sides of Your Brain's Story
If I were to show you a picture of your brain, the first thing you'd probably notice is that it looks like a big walnut (no offense), with two largely independent halves, or hemispheres, connected by a high-speed core. As strange as this might sound, it isn't a very unique brain design. In fact, all vertebrate animals have brains that are divided down the middle, and they have probably been engineered this way for hundreds of millions of years.
What makes human brains remarkable in this design space is how lopsided we are, on average. Differences in the size, shape, and patterns of connectivity in our left and right hemispheres leave us far from symmetrical. And as you'll learn in this chapter, these structural differences shape the way each side processes the information it receives.
However, contrary to the popular notion of the "left-brained" analytical person and the "right-brained" creative type, the most striking distinction between human brains isn't which hemisphere is "in charge" of things. Instead, differences in our characteristic ways of thinking, feeling, and behaving are driven by our degree of lopsidedness, or how big the differences between our two hemispheres are. And so, this book about the differences between brains will begin with a discussion of the fundamental divide within them. But before we get into the nitty-gritty details about how your brain looks, let's talk about why evolution might have landed on the different options in the first place. The idea, in essence, boils down to specialization.
The costs and benefits of brain specialization
To better understand the pros and cons of having a more lopsided or balanced brain design, let's imagine that your brain is a team made up of two people. If both members of your team are well rounded, and have comparable skill sets, it would be easiest, and most equitable, to distribute tasks between them randomly. On the other hand, if one member of your team has incredibly strong verbal skills, while the other is an excellent graphic designer, your team would perform better as a whole if the tasks were systematically assigned to the individual best qualified for the job.
Job assignment in the brain works a bit like this. If the two hemispheres were truly identical to each other, there would be no rhyme or reason to which functions they might come to perform. But as soon as they start to differ-even a little bit-an opportunity is created for one hemisphere to be better suited for certain types of jobs than for others. When this happens, the assignment of jobs across hemispheres becomes more systematic. And as the jobs that a particular brain region is asked to do become more similar to one another, that region can adapt, developing a more specialized structure that allows it to perform the particular type of tasks it's involved in even better.
I assume that the benefits of specialization are somewhat self-explanatory. If everything else were equal, many people would rather have an extremely talented graphic designer on their team than one with average skills. But what if that graphic designer was bad at everything else? If your whole team were made up of people with non-overlapping skills, what would happen if someone needed help or called in sick? One of the measurable costs to specialization in the brain is that the refinement process by which an area becomes specialized makes it better and better suited for doing fewer and fewer things.
Stefan Knecht and colleagues demonstrated this increased vulnerability associated with lopsidedness in a study that looked at language laterality-a term neuroscientists use to describe the extent to which any of your brain's functions come to depend more on one hemisphere than the other. To do so, they first measured changes in blood flow in the two hemispheres when 324 volunteers named pictures in the lab. Then they selected 20 participants who had different patterns of laterality for speech, with approximately equal numbers of people who relied on their left or right hemispheres uniquely, or on both, for speech production.
Next, to study their vulnerability to brain injury, the research team used a tool called transcranial magnetic stimulation, or TMS for short. TMS uses magnetic fields to safely, and temporarily, stimulate different regions of the brain noninvasively. And if you stimulate one area over and over for a long enough time, it runs out of gas-creating an effect called a "virtual lesion." If you've ever gotten a blind spot after seeing a bright light, you've experienced a similar phenomenon.
As expected, when Knecht and colleagues created virtual lesions in the hemisphere that a person's speech was dependent on, their participants got significantly slower at the language task they were asked to perform. However, the more balanced a person's speaking profile was-that is, the more both of their hemispheres were involved in the act of speaking-the less their behavior was affected when only one side of the brain was fatigued using TMS. The effect is kind of like benching different members of your team and measuring the dip in productivity that results. The more balanced brain designs, like the well-rounded teams, were more resilient to the injury of any single player.
But even for the majority of us who are lucky enough to make it through life without damaging too many brain cells, there are still prices to pay for brain specialization. One of them relates to how our hemispheres become different in the first place. Though I spent a decent amount of time in "Introductions" explaining how evolution has worked to cram as much brainpower as possible into our heads, the mechanisms that cause our hemispheres to become specialized may be an exception to this rule. According to the Right-Shift theory proposed by Marian Annett, the human propensity to be lopsided may be driven by a genetic variation that shrinks parts of the right hemisphere. According to Annett, our brains evolved this type of handicapping system as a way of refining job assignment in the brain. Consistent with her theory, Annett's results suggest that people who have more "balanced" brains might not be as skilled at the more newly evolved human functions-like language-but they are also using more of the real estate in the right halves of their skulls, which you'll learn is important for many other things, like visuospatial skills. On the other hand, she argues that highly lopsided people are less likely to have deficits in language-related skills but are more likely to struggle with the types of jobs that typically get assigned to the right hemisphere, like visuospatial tasks.
And there's one other thing I'd like you to keep in mind when considering the costs and benefits of the specialization of our two hemispheres. As you'll learn in this chapter, one of the ways your brain becomes specialized is by using highly experienced processing centers called modules. These modules are singularly focused on the task they've been given, and don't consider input from other brain areas while they are doing their jobs. The result of this is that a more specialized brain tends to process the world by piecing together specific details rather than taking the whole picture into account. In other words, as a brain moves from being more balanced to more lopsided, its processing shifts from focusing on the more global, "forest-level" features to focusing on more specific, "tree-level" details. We'll talk more about the specifics of this in the second half of the chapter. First, let's get to work figuring out how lopsided you are.
Assessing laterality
One of the best ways to determine how lopsided your brain is, is to measure a bunch of different functions in each hemisphere separately. If your left and right hemispheres do them equally well, your brain is likely more balanced, but if one hemisphere tends to take the lead on these functions, your brain is probably more lopsided.
We'll start with one of the most obvious asymmetries to observe in most people-our hand preference. Those of you who work with your hands for a living, or have suffered an injury that prevents you from doing so easily, are likely already aware of how much skill goes into precision hand movements. The rest of you might be largely oblivious to one of the most important benefits that our genetic differences from chimps created-our long thumbs. The fact that we can press our thumbs to the tips of each finger with precision levels of force allows us to execute movements ranging from removing an eyelash from someone's cheek to hitting a nail on the head with a hammer. And these common tasks probably require a lot more brainpower than you think.
In fact, the neural circuitry that controls the movement of your hands is so large that it creates a U-shaped bulge in your brain called the hand knob. With a bit of training, you'd be able to identify your hand knob when looking at a picture of your walnut-shaped brain. It sits near the top of your motor cortex, a strip of brain that runs from temple to temple (about where a pair of glasses would fall if you rested them atop your head), and controls the movement of all of your body parts. In most people, you can even figure out whether they're left- or right-handed by comparing the size of the two knobs in each hemisphere. And this is how we're going to start the process of reverse-engineering your brain.
Though most people identify as either right- or left-handed, handedness is not a binary category. Instead, we each fall on a continuum ranging from extremely right-handed to extremely left-handed. Figuring out where you fall along this axis is the first step to understanding how lopsided your brain is. To start, I'll give you a questionnaire I adapted based on the Edinburgh Handedness Inventory. This simple checklist, which asks about how you use your two hands for everyday tasks, is by far the most common tool used by neuroscientists to measure handedness.
To get an idea of where you fall along the handedness axis, answer each of the ten items below based on everyday activities that you might engage in with either your left or right hand. For each action, answer on a scale ranging from +2 to -2: If your preference for this activity is so strongly right-handed that you wouldn't ever use your left hand, answer +2; if you prefer to use your right hand for this activity, but may occasionally use the left as well, answer with a +1; if you are truly indifferent, and use both hands equally well and equally frequently to accomplish this task, answer with 0; if you prefer to use your left hand for this activity, but may occasionally use the right as well, answer with a 1; and finally if your preference for this activity is so strongly left-handed that you wouldn't ever use your right hand, answer with -2. The only time you should leave a question blank is if you have no experience with the activity in question (and if you've never held a broom, or a toothbrush, I'll do my best not to judge you, since it's antithetical to my goals for writing this book).
Handedeness Assessment
1. Writing with a pen or pencil.
2. Hammering.
3. Throwing (most commonly a ball but any object will suffice).
4. Holding the match when striking a match.
5. Holding a toothbrush when brushing your teeth.
6. Using scissors to cut.
7. Cutting with a knife (without a fork, such as when chopping food for cooking).
8. Eating with a spoon.
9. The upper hand when holding a broom to sweep. (If it's been a while, grab a broom-sweep for science!)
10. Opening the lid of a box.
Now, let's calculate your handedness index. To figure out your "average" response, add the answers to each of the ten questions together and divide their sum by ten. To check your math, the result should fall within the -2 (strongly and consistently left-handed) to +2 (strongly and consistently right-handed) range. The closer you are to the extreme ends of this distribution, the more lopsided your brain is. Those of you who scored closer to the middle (between -1 and +1), the mixed-handers, likely have more balance in the capabilities of your two hemispheres. Still, you probably identify as right- or left-handed based on your answers to the first few questions. As you move from the top to the bottom of the scale, the precision required to execute the movements generally decreases, opening up the possibility for a less-skilled hemisphere to do a "good enough" job.
So, what does your degree of handedness tell me about how lopsided your brain is? The first thing to note is that the motor cortex in the left hemisphere of your brain controls the right half of your body, and vice versa. If you are strongly right-handed, it is likely that the motor cortex in your left hemisphere, particularly around the hand knob, is bigger. The reverse is true for the much smaller percentage of the population that is extremely left-handed. We'll talk about the broader implications of what this means for how you work in a bit. For now, let's check out some other functions, to see whether your brain is consistently more balanced or lopsided in its job assignments.
For starters, let's check in with your feet. Although our feet are much less skilled than our hands, most lopsided people will also exhibit a preference for using one foot over the other when executing skilled movements. Which foot do you usually kick with? When going up stairs, which foot do you typically lead with? What if I asked you to put the tip of your toe on a quarter? Would you instinctively pick one foot over the other? Most people will find these foot skills more interchangeable than the hands, but if you answered each of these questions consistently with one foot, it provides further evidence that skills are unevenly distributed in your two hemispheres.
Now, let's switch to an even more subtle function-the difference between how you use your two eyes. Though both eyes carry information about the world to the brain, some of us rely more on information coming in from one eye than the other. And here's a fun fact-most people have a preference for information coming in from their right eye! We might go about assessing your eye dominance like we did handedness-by asking questions like which eye would you use to look into a microscope, or the viewfinder of a camera? But we can also measure this a bit more objectively with the following "sighting" experiment: Find an object about eight to twelve feet away from you and hold one of your index fingers up in front of it. With both eyes open, you might have the experience that you can "see through" your finger, or you might feel like you see two fingers (depending on where you're focusing), but do your best to focus on the object and position your finger so that it is in a straight line between you and the object. Now, close your left eye. What happened? If your finger now looks like it is solidly blocking the object, you are right-eye dominant. If your finger now looks like it is off to the side of the object, try closing your right eye. Is it lined up now? If so, you are left-eye dominant. As long as you've picked something sufficiently far away, if your finger doesn't line up when you close either eye, you've got mixed-eye dominance.
