Excerpted from The Tale of the Dueling Neurosurgeons: The History of the Human Brain as Revealed by True Stories of Trauma, Madness and Recovery, by Sam Kean. Copyright © 2014 by Sam Kean. Reprinted with permission of Little, Brown and Co.; all rights reserved.

It is possible to take the idea of left/right differences within the brain too far: it’s not like one side of the brain talks or emotes or recognizes faces all by itself while the other one just sits there twiddling its neurons. But the left and right hemispheres of the human brain do show striking differences in some areas, especially with regard to language, the trait that best defines us as human beings.
Scientists suspect that left-right specialization first evolved many millions of years ago, since many other animals show subtle hemispheric differences: they prefer to use one claw or paw to eat, for instance, or they strike at prey more often in one direction than another. Before this time, the left brain and right brain probably monitored sensory data and recorded details about the world to an equal degree. But there’s no good reason for both hemispheres to do the same basic job, not if the corpus callosum—a huge bundle of fibers that connects the left and right brain—can transmit data between them. So the brain eliminated the redundancy, and the left brain took on new tasks. This process accelerated in human beings, and we humans show far greater left/right differences than any other animal.

In the course of its evolution the left brain also took on the crucial role of master interpreter. Neuroscientists have long debated whether certain people have two independent minds running in parallel inside their skulls. That sounds spooky, but some evidence suggests yes. For example, there are split-brain patients, who had their corpus callosums surgically severed to help control epilepsy and whose left and right brain cannot communicate as a result. Split-brain patients have little trouble drawing two different geometric figures at the same time, one with each hand. Normal people bomb this test. (Try it, and you’ll see how mind-bendingly hard it is.) Some neuroscientists scoff at these anecdotes, saying the claims for two separate minds are exaggerated. But one thing is certain: two minds or no, split-brain people feel mentally unified; they never feel the two hemispheres fighting for control, or feel their consciousness flipping back and forth. That’s because one hemisphere, usually the left, takes charge. And many neuroscientists argue that the same thing happens in normal brains. One hemisphere probably always dominates the mind, a role that neuroscientist Michael Gazzaniga called the interpreter. (Per George W. Bush, you could also call it “the decider.”)

Normally, having an interpreter/decider benefits people: we avoid cognitive dissonance. But in split-brain patients, the know-it-allness of the left brain can skew their thinking. In one famous experiment University of California, Santa Barbara psychologist Michael Gazzaniga flashed, for a split-second, two pictures to a split-brain teenager named P.S.—a snowscape to his right brain and a chicken claw to his left brain. Next, Gazzaniga showed P.S. an array of objects and had him pick two. P.S.’s left hand grabbed a snow shovel, his right hand a rubber chicken. So far, so good. Gazzaniga then asked him why he’d picked those things. P.S.’s linguistic left brain knew all about the chicken, of course, but remained ignorant of the snowscape. And, unable to accept that it might not know something, his left-brain interpreter devised its own reason. “That’s simple,” P.S. said. “The chicken claw goes with the chicken, and you need a shovel to clean out the chicken shed.” He was completely convinced of the truth of what he’d said. Less euphemistically, you could call the left-brain interpreter a part- time confabulator.

Split-brain patients confabulate in other circumstances, too. As we’ve seen, thoughts and sensory data cannot cross over from the left hemisphere to the right hemisphere, or vice versa. But it turns out that raw emotions can cross over: emotions are more primitive, and can bypass the corpus callosum by taking an ancient back alley in the temporal lobe. In one experiment scientists flashed a picture of Hitler to a split-brain woman’s left side. Her right brain got upset and (the right brain being dominant for emotions) imposed this discomfort onto her left brain. But her linguistic left brain hadn’t seen Hitler, so when asked why she seemed upset, she confabulated: “I was thinking about a time when someone made me angry.” This trick works with pictures of funeral corteges and smiley faces and Playboy bunnies, too: people frown, beam, or titter, then point to some nearby object or claim that some old memory bubbled up. This result seems to reverse neurological cause and effect, since the emotion came first and the conscious brain had to scramble to explain it. Makes you wonder how much we actually grasp about our emotions in everyday life.

Along those lines, split-brain people can help illuminate certain emotional struggles we face. Consider P.S., the teenager who confabulated about chickens and shovels. In another experiment scientists flashed “girlfriend” to his right hemisphere. In classic split-brain fashion, he claimed he saw nothing; but in classic teenage fashion, he giggled and blushed. His left hand then used some nearby Scrabble tiles to spell L-I-Z. When asked why he’d done that, he said he didn’t know. He certainly wouldn’t do anything as stupid as like a girl. Tests also revealed conflicting desires in his right and left brain. P.S. attended a fancy finishing school in Vermont, and when asked what he wanted to do for a living, his left brain bid him say “Draftsman,” a respectable career. Meanwhile his left hand spelled out “automobile race[r]” with tiles. His brain even betrayed a red/blue political divide: post-Watergate, his left brain expressed sympathy for President Nixon, while his right brain hinted it was glad to see Tricky Dick go. When facing a crisis or controversy, we often talk about feeling torn or being of two minds. Perhaps those aren’t just metaphors.

This left-right asymmetry within the brain affects how we read emotions in other people as well. Imagine simple line drawings of two half-smiley, half-frowny faces, one with the smile on the left side of the face, one with the frown on the left. In a literal sense, these faces are equal parts sad and happy. But to most people the emotion on the left side (from the viewer’s point of view) dominates, and determines the overall emotional tenor. That’s because whatever’s in your left visual field taps into the emotion-dominant and face-dominant right brain. Along those lines, if you bisect a person’s photograph and view each half independently, people usually think he “looks like” the left half more than the right half.

Artists have long exploited this left-right asymmetry to make their portraits more dynamic. Generally, the left half of someone’s face (the side controlled by the emotive right brain) is more expressive, and surveys in European and American art museums have found that some- thing like 56 percent of men and 68 percent of women in portraits face the left side of the canvas and thereby show more of the left side of the face. Crucifixion scenes of Jesus suffering on the cross showed an even stronger bias, with over 90 percent facing left. (By chance alone, you’d expect closer to 33 percent, since subjects could face left, right, or straight ahead.) And this bias held no matter whether the artists themselves were left- or right-handed. Whether this happens because the sitters prefer to display their more expressive left side or because the artists themselves find that side more interesting isn’t clear. But the bias seems universal: it shows up even in high school yearbook photos. A leftward pose also allows the artist to center the sitter’s left eye on the canvas. In this position most of her face appears on the canvas’s left side, where the face-hungry right hemisphere can study it.

There are exceptions to this leftward bias in portraiture, but even these are telling. The highly ambidextrous Leonardo often broke convention and drew right-facing profiles. But perhaps his most classic piece, the Mona Lisa, faces left. Another exception is that self-portraits often face right. Artists tend to paint self-portraits in a mirror, how- ever, which makes the left half of the face appear on the right side of the canvas. So this “exception” might actually confirm the bias. Finally, one study found that prominent scientists, at least in their official portraits for the Royal Society in England, usually face right. Perhaps they simply preferred to seem cooler and less emotional— more the stereotypical rationalist.

In contrast to portraits, art in general doesn’t show a leftward bias, not in all cultures. In Western paintings, the so-called glance curve — the line the eye naturally follows—does often travel left to right. In art from east Asia, the glance curve more often runs right to left, more in line with reading habits there. A similar bias exists in theater: in Western theaters, as soon as the curtain rises, audiences look left in anticipation; in Chinese theaters, audiences swivel right.

The reason we show a left-right preference for some things (portraits) but not others (landscapes) probably traces back to our evolutionary heritage as animals. Animals can safely ignore most left-right differences in the environment: a scene and its mirror image are more or less identical with regard to food, sex, and shelter. Even smart and discriminating animals—such as rats, who can distinguish squares from rectangles pretty easily—struggle in telling mirror images apart. And human beings, being more animal than not, can be similarly oblivious about left/right differences, even with our own bodies. Russian drill sergeants in the 1800s got so fed up with illiterate peasants not knowing left from right that they’d tie straw to one leg of recruits, hay to the other, then bark, “Straw, hay, straw, hay!” to get them to march in step. Even brainiacs like Sigmund Freud and Richard Feynman admitted to having trouble telling right and left apart. (As a mnemonic, Freud made a quick writing motion with his right hand; Feynman peeked at a mole on his left.) There’s also a famous (right-facing) portrait of Goethe showing him with two left feet, and Picasso apparently shrugged at (mis)printed reversals of his works, even when his signature ran the wrong way.

So why then do humans notice any left-right differences? In part because of faces. We’re social creatures, and because of our lateralized brains, a right half-grin doesn’t quite come off the same as a left half-grin. But the real answer lies in reading and writing. Preliterate children often reverse asymmetric letters like S and N because their brains can’t tell the difference. Illiterate artisans who made woodblocks for books in medieval times were bedeviled by the same problem, and their misprinted letters added a clownish levity to dry Latin manuscripts. Only the continual practice we get when reading and writing allows us to remember that these letters slant the way they do. In fact, in all likelihood only the advent of written scripts a few millennia ago forced the human mind to pay much attention to left versus right. It’s one more way that literacy changed our brains.