by Sam Kean
Neuroscientists could name only one thing the corpus callosum definitely did: spread seizures. By using a helmet, they could monitor the electrical patterns of a seizure within the brain. For whatever reason, small epileptic storms seemed to gather momentum after reaching the corpus callosum, and would soon overrun the whole globe. That danger did suggest a way to head off seizures, though—sever the corpus callosum. The two L.A. surgeons began practicing this procedure on cadavers, and they finally convinced W.J. to submit in 1962. They drilled two holes in his skull, one forward, one aft, then slid spatulas inside to hoist up his lobes. You might think this type of surgery would go quickly—just stick a knife inside and start carving—but it actually required ten hours of work: whereas upper brain tissue can be scooped like tapioca, the corpus callosum is tough as gristle.
Recovery was slow, but W.J. started talking a month later and walking three months after that; doctors also monitored his fine motor skills and rejoiced to see him do coordinated, bimanual tasks like lighting cigarettes. (It was a different era.) Best of all, W.J.’s seizures vanished. The surgery had aimed to confine his fits to one hemisphere, but for unknown reasons it all but eradicated them. For the first time in a decade he started sleeping through the night, and he gained forty much-needed pounds. Just as important, W.J. didn’t suffer any H.M.-like crisis: his personality and speech and memories remained intact. Encouraged, the L.A. surgeons started performing more callosectomies. And aside from the short-term pains of surgery—one patient woke up, he quipped, with a “splitting headache”—patients showed no ill effects. They could still read, reason, and remember; they could talk, walk, and emote. Their minds worked exactly the same as before.
And yet—could that possibly be true? Could hacking through 200 million fibers produce zero side effects? Neuroscientist Roger Sperry didn’t buy it, and set out to prove otherwise.
Proving otherwise was a habit with Sperry. He had an atypical background for a scientist, having focused as much on sports as academics when young. He set the state javelin record in high school in Connecticut and lettered in baseball, basketball, and track at Oberlin College. When not training, he spent his undergraduate hours mooning over seventeenth-century poetry. But his Psych 101 class intrigued him, and after he hung up his jock at Oberlin he stuck around and got a master’s degree in psychology. He then pursued a doctorate in zoology at the University of Chicago, where—in an impolitic but no doubt satisfying move—he demolished his thesis advisor’s lifework.
Paul Weiss promoted the trendy “blank slate” theory of brain function. He argued that any neuron could do the job of any other neuron, and that brain circuits could be rewired to an infinite degree. Sperry thought this superplasticity sounded pretty cool, so in 1941 he started a series of diabolical experiments on rats to test the idea. These experiments involved, among other things, opening up the rats’ two back legs, finding the nerves that carried pain signals to the brain, and switching them, so that the left pain nerve was now located in the right leg, and vice versa.
Once a rat recovered from surgery, Sperry placed it on an electrified grille where, if it stepped on a certain spot, it got a shock. The result was black comedy. If the rat shocked its back left leg, its brain (thanks to the switched nerves) felt the sting of pain in the right leg. So the rat jerked the right leg up and began limping. Unfortunately, this put more weight on the left leg, which is where the wound actually was. Worse, as soon as the rat wandered near that electrified spot again, its left leg got another shock. Which because of the switched nerves made it seem like the right leg was hurting even worse. Which made the rat favor the damaged left leg all the more. Which led to more pain and more shocks the next time around, and so on, a vicious cycle. Crucially, and contra Weiss, the rat’s neurons never learned better. Month after month passed, but no matter how many times the poor booby shocked one leg, it always hiked the other one up.
Sperry did even more Dr. Moreauvian things to fish. He’d pop out their eyeballs, sever the optic nerves, rotate the eyeballs 180 degrees in the socket, then sew them back in. Fish nerves can regenerate themselves no problem, so the fish learned to see again. But because the eyeballs had been rotated, the nerves rewired themselves backward—forcing the fish to see the world upside down. Wriggle a worm below its jaw, and it snapped upward; dangle a morsel above, and it lurched downward. And once again, the fish never ever unlearned this behavior.
Sperry determined from the rat and fish stunts that all creatures have some hardwired neural circuits: certain neurons are born to do certain jobs and cannot learn other tasks. That’s not to say there’s no plasticity within the brain (especially in humans). But Sperry demolished the idea that we’re born with blank neurological slates.
Not satisfied with ruining Weiss’s work, Sperry took a postdoc at Harvard University and slew his advisor there, too. Starting in the 1920s Karl Lashley had helped popularize that all-time-classic psychology experiment, running rats through mazes. In his case, after the rats had learned a maze Lashley would anesthetize them, make lesions in their brains, then test them again. To his shock, no matter where he inflicted damage, the rats could usually still negotiate the labyrinth, provided he didn’t damage too much tissue overall. In other words, he claimed, the location of the lesions didn’t matter, only their size. Based on this work Lashley developed a theory of antilocalization. He admitted that brains must have some specialized components. But for advanced tasks, such as learning mazes, Lashley argued that creatures utilized all parts of the brain simultaneously. As a corollary, Lashley promoted the idea, prominent before H.M., that all parts of the brain contribute equally to forming and storing memories.
For Lashley’s theory to work, distant regions in the brain—even regions not connected by axon wires—had to communicate almost instantly. So he downplayed the idea that neurons sent messages only to direct neighbors, bucket-brigade-style. Instead, he envisioned neurons emitting long-distance electrical waves—similar to the then-new medium of radio. Sperry once again thought this sounded awesome, but he once again proved too good a scientist. Starting in the mid-1940s, he opened up the skulls of cats and inlaid their brains either with strips of mica (to insulate) or with tantalum wires (to short-circuit). Either addition should have disrupted the electrical waves propagating through the brain and thereby shut down higher thinking. Nope. Sperry ran the felines through every neurological test he knew, and they acted exactly the way cats always have and always will. This killed Lashley’s theory* of long-distance electrical communication and reinforced the belief in neuron-to-neuron chemical communication.
To the relief of advisors everywhere, Sperry opened up his own lab at the California Institute of Technology in 1954. After settling in, he decided to expand some earlier work he’d done on the corpus callosum. These experiments involved slicing through this bundle in cats and monkeys and monitoring their behavior.
All in all, the split-brain animals seemed normal—at least mostly. Every so often they’d do something funny, something off. For instance, if he taught a split-brain cat to navigate a maze while wearing an eye patch, then switched the patch to the other eye and put the cat back in the maze, it would start getting lost again.* That didn’t happen to controls. Sperry saw enough quirks like this to doubt that humans who had their corpus callosums severed would escape with no side effects. So when the L.A. surgeons asked Sperry to test W.J. and other callosectomy patients, Sperry agreed—and once again proved otherwise.
The tests—run by Sperry and his graduate student, Michael Gazzaniga—took place in three-hour blocks each week. At first W.J. seemed normal. He handled everyday interactions just fine, and even extensive psychological testing turned up no oddities. Then came the tachistoscope. The tachistoscope was basically a mechanical shutter hooked up to a projector. It snapped open and shut quickly, allowing scientists to flash images onto a screen for a tenth of a second. Before the 1950s, the tachistoscope was best known for helping train American fighter pilots during World War II. Psychol
ogists would flash silhouettes of planes—both Allied and enemy aircraft—to the flyboys, who, after suitable training, learned to distinguish good guys from bad guys in an instant.
Rather than planes, W.J. saw flashes of words or objects. He sat at a table six feet away from a white screen and fixed his gaze on the center. Beneath the table sat a telegraph key, which he pressed to signal that he’d seen an image. Tap. After each round, as a backup, Sperry and Gazzaniga also asked W.J. to state whether he’d seen any image, yes or no. The crucial aspect of the experiment was this: Sperry and Gazzaniga would flash the word or object only on one side of the screen—far to the left or far to the right of the center line. As a result, the image entered only one side of W.J.’s brain. His response to these fleeting images made the scientists’ skin tingle.
With images flashed to his right side, W.J. responded as you’d expect. Those images entered his left brain, which controls both language and his right hand. So his right hand pressed the telegraph key and he answered yes, he’d seen an image. Images flashed to his left side were a different story. These images entered his right brain, which can’t produce language; nor could the right brain signal the left brain to speak up, because of the split corpus callosum. So W.J. denied seeing anything. But his left hand still pressed the telegraph key. His left hand knew, even if his left brain didn’t. This happened over and over. W.J. would insist he’d seen nothing—nothing—nothing. Meanwhile he was practically tapping Morse code beneath the table.
Other split-brain patients showed a similar disconnect between right and left. In one test, Sperry and Gazzaniga blindfolded patients and placed pencils, cigarettes, hats, pistols, and other objects in their left palms. The patients could use these objects just fine—scribble, puff, doff the hat, pull the trigger—but they could never name them. In another test the scientists used the tachistoscope to flash “hot” and “dog” simultaneously on opposite sides of the screen, then asked people to draw a picture* of what they’d seen. When normal people took this test, they drew a Nathan’s Famous—wiener, bun, maybe some mustard. The split-brain people drew two pictures: a pooch with their right hand and a withering sun with their left. (They also botched head/stone and sky/scraper.) In sum, they failed any test that required the right brain and left brain to share information. Minus a corpus callosum, each hemisphere remained isolated.
Sperry and Gazzaniga didn’t just scour for deficits, though. Split-brain patients also helped them tease out the unique talents of each hemisphere—what we now call left brain versus right brain thinking. Again, scientists at the time considered the left brain superior in pretty much every skill that mattered. But split-brain patients revealed that the right brain recognized faces better: when split-brain people saw an Arcimboldo portrait, the left brain saw the component fruits and vegetables, while the right brain saw the “person.” The right brain also did a better job at spatial tasks such as rotating objects mentally, or determining how big a circle was after seeing a small arc. Perhaps most interesting, the right brain outgambled the smarty-pants left brain. Imagine a game in which you draw marbles from a giant tub. Eighty percent are blue marbles, 20 percent red, and if you guess the right color before each draw, you get a dollar. In tasks like this, whole-brain people generally guessed blue 80 percent of the time, red 20 percent—a moronic strategy. If you do the math, you’ll get only 68 percent correct this way. Better to guess blue every single time, since you’re guaranteed 80 percent success. Rats and goldfish (in animal-appropriate versions of the game) got this right: they always guessed the same color. The left brain of split-brain people guessed like normal people. The right brain didn’t. It guessed the way rats and goldfish did, and cashed in.
A “portrait” by Giuseppe Arcimboldo. Depending on the brain damage, some victims see only the constituent fruits and vegetables, some see only the overall face. The right brain also tends to notice the face alone, while the left zeroes in on the comestibles.
Building on this work, other neuroscientists discovered other right-brain talents, further eroding the left brain’s hegemony. The right brain proved a better musician in split-brain folks, and its superior spatial skills allowed it to read maps more fluently. The right brain even dominated certain aspects of language. If the right-brain equivalent of Broca’s speech area gets damaged, people end up with a condition called aprosodia. They understand the literal meaning of words but remain oblivious to the rhythms and emotional nuances of actual conversation—the things that make language sizzle. The right brain tends to dominate anything we think of as “arty.” In fact, if the domineering left brain suffers damage, the right brain’s artistic instincts often come to the fore. There are well-documented cases of people suffering left-brain trauma and suddenly becoming obsessed with painting or poetry, things they never cared a whistle about before. Similarly, many idiot savants suffered prenatal left-brain damage, and their uncanny talents (like musical mimicry) might actually be normal right-brain talents that found an outlet.
Nevertheless, despite these distinct talents, Sperry and Gazzaniga cautioned against making too much of right-brain/left-brain differences. It’s not as if one hemisphere speaks or paints all by itself while the other just sits there, twiddling its axons. The left-brain/right-brain relationship is more complementary, more like the relationship between the left and right hands. Most people have a dominant right hand, but the left still helps tie shoes, type, pour drinks, and scratch certain places. In the same way, the brain cannot complete most tasks without both hemispheres working in concert. One great example here is scientific reasoning. Split-brain patients have demonstrated that the right brain does a better job determining whether two events are causally linked (that is, determining whether A actually caused B, or whether the connection was spurious); it also keeps a better, more faithful record of what we see, hear, and feel. The left brain does a superior job culling patterns from data, and only it can take basic information and leap to something new, to a law or principle. In sum, both sides of the brain sense reality but do so in different ways, and without their unique perspectives, we’d have gaps in our scientific understanding.
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 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 split-brain folks have two independent minds running in parallel inside their skulls. That sounds spooky, but some evidence suggests yes. For example, split-brain people have little trouble drawing two different geometric figures (like ⊔ and ⊏) 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 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 thei
r thinking. In one famous experiment Gazzaniga flashed 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.
