by Sam Kean
* in 1931. Along the way—always confident in his own abilities—he also operated on his family members a few times, removing two of his children’s appendixes and a tubercular growth from a daughter’s neck.
Decades of nonstop work finally wore Cushing down, and his health deteriorated further after he himself was diagnosed with a small brain tumor in the late 1930s; he eventually died of complications from a heart attack in 1939. Again, not all of Cushing’s early work has held up. But more than anyone else of his era, he illuminated how the brain’s glands work (or fail to), and he laid crucial groundwork for understanding how the brain influences the body. Even as Cushing lay dying, in fact, other scientists were expanding on his insights and linking his master gland to another major brain-body system, the system that produces human emotions.
Appropriately enough, the modern study of emotions began with one man’s rage. In 1937 Cornell neuroscientist James Wenceslaus Papez (rhymes with “drapes”) got wind of a new research grant being offered; the money would help scientists study how emotions work inside the brain. Papez thought the underlying implication of the grant—that scientists knew nothing about the neuroscience of emotions—insulted some colleagues of his who’d already illuminated key aspects of the field. So in what he later described as “a fit of spleen,” Papez wrote a paper outlining the current state of knowledge. But the whole of his paper far exceeded the sum of its parts.
Papez drew on cases of brain damage where people’s emotions went into under- and overdrive. For instance, lesions in the thalamus, a cluster of gray matter deep inside the brain, can cause spontaneous laughing or crying. In contrast, when another internal structure, the cingulate gyrus, is destroyed, people become emotionally vacant. Perhaps most powerfully, Papez cited cases of rabies. Because swallowing can cause painful neck spasms, many rabies victims actually fear water—it terrifies them. (The inability to swallow also contributes to foaming at the mouth because of excessive saliva.) Rabies produces aggression as well, by attacking certain gray-matter clusters in the brain. The resulting aggression makes dogs, raccoons, and other animals more volatile and therefore more likely to bite and pass on the virus. Human victims lash out in similar rages, and hospital staff often have to lash patients to a bed.
In all these cases, brain damage led to either exaggerated or blunted emotions, and Papez soon realized—his big insight—that these damaged structures must work together in some sort of “emotion circuit.” Scientists later named this circuit the limbic system. The word “limbic” comes from the same Latin word that gave us limbo, and the limbic system does indeed serve as a transition between the brain’s upper and lower regions. As one neuroscientist put it, “like the limbo of Christian mythology, the limbic system is the link between the cortical sky and the reptilian hell.”
Scientists have bickered about what structures do and don’t belong in the limbic system practically since Papez finished the first draft of his paper. This confusion arises in part because different people mean different things by “emotion”—a subjective feeling, a flood of hormones, a physical response or action. Things also get confusing because different emotions activate different brain structures. Finally, the limbic system interacts with so many other brain regions that it’s hard to draw a boundary around it.
Still, the limbic system does have a few core components, many housed in and around the temporal lobes. In general they work like this. Say you see something scary, like a tiger. Sights and sounds get filtered through the thalamus, a double-lobed structure smack in the center of the brain. The thalamus takes a first pass at this sensory input—claws, teeth, growling, hmmm—and splits the data into multiple streams for further processing. One stream flows into the hippocampus, which helps form and access memories. Another stream splits into two branches, one of which flows straight into the amygdala, the other of which detours into the frontal lobes before also arriving at the amygdala. (More on the amygdala in a moment.) By now the brain has a good grasp on the tiger, so it’s time to alert the body. To do so, the amygdala pings the hypothalamus,
* one of the busiest beavers in the body: it takes full or partial responsibility for all sorts of bold-faced Biology 101 terms like metabolism, homeostasis, appetite, and libido, among other things. (Biologists summarize these hypothalamic duties as the “four F’s” of animal behavior—feeding, fleeing, fighting, and, well, sexual congress.) Finally, neurons in the hypothalamus rouse Cushing’s pituitary gland, which in turn releases hormones into the bloodstream that cause us to sprint, shiver, wet ourselves, or otherwise feel emotions viscerally.
The limbic system reaches far and wide, and interacts with many other parts of the brain and body. It pings our facial muscles to produce blushes, growls, grins, and grimaces. It can take feedback from the face as well. The mere act of smiling, for instance, can send uplifting hormones surging through us, brightening our moods. (The brain associates smiling with a good time, so once the smile circuits flash on, the good-mood circuits often flash on, too. Something similar happens with frowns and feeling crummy. Wire together, fire together. And on the flip side, hindering people’s facial expressions—with Botox, for instance, which paralyzes facial muscles—can actually dampen emotions like rage.) As for mental phenomena, limbic structures work with the frontal lobes to produce rich emotional experiences such as euphoria and melancholy and lust, the swells and highs that make us feel fully alive. Part of the horror of lobotomies was that they severed the links between the frontal lobe and limbic system, blunting or even destroying emotional experiences.
Ironically enough, Papez’s paper on the limbic system—written to make the grant providers look foolish—provided exactly what the grant was seeking, an outline of how emotions work. But for all his brilliance, Papez missed one crucial aspect of the limbic system: his 1937 paper skipped right over the amygdala. Named for their shape (from the Greek for “almond”), the two amygdalae sit deep inside the temporal lobes. They jump-start the startle reflex; process smell, the one sense that skips the thalamus; and help determine which things around us are worth paying attention to. Indeed, some neuroscientists, riffing on the what and where streams in vision research, have declared the amygdala and other nearby structures the so what stream. I see this, your brain says, but should I bother caring? The amygdala helps make that call.
If we should care, the amygdala also takes the next step and helps mount an appropriate response, especially if that response involves fear. In fact, the amygdala is often called the fear spot in the brain. That’s simplistic—the amygdala processes lots of emotions, including happy ones—but there is some truth to it. Having a structure to scan for frightening things is mostly good, since it steers us clear of fanged animals, dark spaces, clowns, and so on. But like any other brain part, the amygdala can malfunction, making people feel fearful all the time. They see threats where none exist, and might go postal if pushed too far.
Conversely, as research on a woman named S.M. shows, amygdala damage can also lead to the opposite problem—an alarming lack of fear. As a child S.M. responded normally to scary things. One midnight she followed her brother into a graveyard, and when he leapt out from behind a tree, she screamed. She also got cornered by a Doberman once and felt her heart quake and her gut seize up—typical fear responses. But around age ten she began suffering from Urbach-Wiethe disease, a rare disorder that petrifies and kills amygdala cells. Within a few years she had two “black holes” where her amygdalae should have been. She hasn’t felt a lick of fear since.
Studies involving S.M. are actually a hoot to read, since they basically consist of scientists dreaming up ever-more-elaborate ways to scare her. For instance, doctors once drove her to an exotic pet store that carried snakes. On the ride over she claimed to hate snakes, but as soon as she arrived she practically yanked the serpents out of the store clerk’s hands to play with them. She even tried stroking the snakes’ tongues (snakes do not like this), and asked fifteen separate times to pet some of the ve
nomous snakes on hand. Her doctors also ran her through a haunted house—an old lunatic asylum with plenty of creaking doors and shadowy corners for monsters to leap out of. Five strangers, all women, toured with S.M. and served as effective controls. They screamed every few seconds, but S.M. was always dashing ahead to find out what was next. At one point she jabbed a monster—an actor on duty—in the head because she wanted to know what his mask felt like. She ended up scaring him.
To test whether S.M. was simply blunted (i.e., dead to all emotion), her doctors had her examine photographs of people making various faces. She could read most emotions just fine, but fear didn’t register. Similarly, when viewing a range of movie clips, she reported feeling sad, surprised, mirthful, and disgusted at appropriate times but barely blinked during The Shining and The Silence of the Lambs. What’s more, family members report that, if anything, S.M. gets overly emotional sometimes, excessively sad and lonely. This pattern makes sense, because while other emotions can sidestep the amygdala, fear cannot: to feel fear, the amygdala needs to rouse us.
Lest you think these experiments sound contrived—S.M. didn’t face any real danger inside the haunted house, after all—consider the “knife incident.” While walking home alone one night, she cut behind a church and into a public park. A man she described as “drugged-out” yelled at her, and yet she walked over to him without hesitating. He grabbed her, whipped out a knife, and held it to her throat, hissing, “I’m going to cut you, bitch!” She didn’t struggle. Instead, she listened to the church choir wrapping up rehearsal, then murmured something about God’s angels protecting her. The man, weirded out, let go. At this point, rather than spring and sprint for her life, S.M. simply walked away. She even returned to the park the next day. S.M. has also been held at gunpoint and once almost died during an episode of domestic abuse. But despite feeling upset, the words she used to describe these incidents made them sound like inconveniences or annoyances. Fear never came up.
Critics of S.M.’s case have argued that her behavior sounds less like a lack of fear and more like a lack of common sense—that the black holes in her amygdalae were really just holes in her head. But knowledge of the limbic system negates that criticism. Notice that when S.M. saw a snake or some other danger, she didn’t just shrug—she was dying to get her hands on it. This makes sense biologically. If you saw a viper in the wild, you wouldn’t want to get distracted; best to pay close attention. On some level, then, S.M.’s brain did recognize frightening things, because she fixated on them. Her brain simply couldn’t mount the subsequent emotional response to get her the heck outta there. So saying that S.M. lacked common sense misses the point. When it comes to detecting danger around us, fear is common sense. Fear bestows that common sense in the first place, and you can’t have one without the other.
Although processed within the limbic system, emotions often spill over into other areas of the brain, in ways subtle and surprising. Some blind people with visual cortex damage—who have no conscious visual awareness of anything around them—can still read emotions on other people’s faces. That’s because the optic nerves, in addition to routing data to the conscious mind, also route data to the limbic system along secondary, subliminal tracts. So if the conscious pathway gets damaged but the unconscious, limbic one remains intact, blind people will still respond to smiles, scowls, or quivering lips, all without realizing why. They can even catch yawns
* from other people.
Similarly, the limbic system can bypass certain kinds of paralysis. People who suffer a stroke in the brain’s voluntary movement centers often have trouble smiling on command: the right side of their mouths might perk up, while the left side droops pitifully. Tell them a joke, though—something that engages a genuine emotion—and they’ll often brighten right up with full, radiant, symmetric smiles. That’s because the limbic system connects to the face through different axon channels than our voluntary motor centers do; the limbic brain can therefore still move the facial muscles whenever we ourselves feel moved.
(What’s more, the limbic system and voluntary motor centers actually move different sets of facial muscles—and therefore produce different-looking smiles. This divergence explains the difference between genuine smiles and fakey, say-cheese smiles in photographs. People have trouble faking other genuine expressions, too, like fear, surprise, or an interest in someone’s pet stories. To overcome this limitation actors either drill with a mirror and practice conjuring up facial expressions à la Laurence Olivier, or, à la Constantin Stanislavsky, they inhabit the role and replicate the character’s internal feelings so closely that the right expressions emerge naturally.)
The limbic system, and the temporal lobes generally, are also closely tied up with sex. Scientists discovered this connection in a roundabout way. In the mid-1930s a rogue biologist named Heinrich Klüver started some experiments with mescaline (a.k.a. peyote), a hallucinogenic plant. He kicked these experiments off during a summer holiday in New Hampshire when—bored out of his skull and lacking any lab animals—he solved both problems in one go by giving mescaline to a farmer’s cow. Whether Klüver injected the beast with a syringe or let it nibble dried peyote buttons from his hand isn’t known. What is known is that the cow croaked, and the farmer got hopping mad. Despite this inauspicious start, Klüver decided to sample mescaline himself, and nearly died. Undeterred, he began still more experiments, on monkeys, when he returned to his University of Chicago lab.
Around 1936 Klüver developed a theory that all hallucinations originate inside the temporal lobes. To test this idea he had a colleague, neurosurgeon Paul Bucy, remove the temporal lobes of a few monkeys. (This excavation also tore out key limbic structures.) The experiments failed—the monkeys still got high—but the duo did notice some odd side effects. For one, the monkeys lost the ability to recognize objects, even food. The monkeys also developed an oral fixation. The scientists determined this by scattering peppermints, sunflower seeds, and banana slices on the floor; they also scattered nails, lint, combs, eggshells, tin foil, cigarette ash, and seemingly whatever else they could scrape from their desk drawers. Instead of going straight for the treats, the monkeys methodically picked up every last item and licked or bit it, a trait now called hyperorality. The monkeys even subjected baby rats and feces to the taste test. Equally disturbing, the monkeys turned into sex fiends. They masturbated themselves raw and rubbed their genitals on any animate being in sight. One poor fellow, who combined the worst of hyperorality and nymphomania, had to be put down because, unable to recognize it, he kept biting his own penis.
Nowadays a neuroscientist would blame the monkeys’ inability to recognize food on the destruction of the what stream within their temporal lobes. But their lack of a functioning limbic system also contributed to their bizarre behavior, since one purpose of emotions is to help animals appreciate and react appropriately to objects. In short, in a brain with a functioning limbic system, the amygdala’s so what circuit will “tag” different objects with good or bad emotions. When we encounter those same objects later, the tag tells us whether to run, smile, fight, or approach. Monkeys respond appropriately to bananas, for instance, because bananas assuaged their hunger once and gave them a shot of sugar. This in turn flooded their brains with dopamine, a neurotransmitter linked to rewards. So upon seeing bananas again, the monkeys repeat the steps—approaching and eating—that led to that good feeling before. Conversely, they shy away from fire and snakes because those things are tagged as frightening, and they shun feces because they’re tagged as disgusting.
Now, imagine that those tags have vanished. No one thing would seem any more desirable or repulsive or terrifying than any other thing—which is exactly what happened to Klüver and Bucy’s monkeys. Minus a limbic system, bananas and lint and poop nuggets all seemed like potential food, and no matter how many times they grabbed lit matches or humped a technician’s leg, they never hesitated to do it again. And if you think that all this failure and futility frustrated the m
onkeys no end, you’d be wrong. Because they lacked a limbic system, they never got upset about repeating the same damn mistake over and over. In fact they never really betrayed any emotions, ever. No joy, no resentment, no anger, no nothing. Even when a rival monkey nearly bit through one de-lobed fellow’s hand—something no self-respecting primate would tolerate without a brawl—the bitten one just yanked it away and sauntered off.
Klüver and Bucy studied monkeys, but human beings with limbic damage display many of the same traits, a disorder now called Klüver-Bucy syndrome. Like the bitten monkey, one symptom of Klüver-Bucy is “pain asymbolia,” which leaves people indifferent to physical pain. They can recognize intellectually that getting a hand crushed or having a needle break off beneath their skin should hurt, but because the pain lacks any emotional impact they don’t get worked up about it. Klüver-Bucy victims also become hyperoral, and doctors have caught them chomping down on soap, catheters, blankets, flowers, cards, pillows, glass thermometers, and everything else in their hospital rooms. One victim suffocated while trying to swallow an Ace bandage.
