Until the past few decades, neuroscientists had one way to plumb the human brain: wait for disaster to strike people and, if the victims pulled through, see how their minds worked differently afterward.

These poor men and women endured strokes, seizures, saber gashes, botched surgeries, and accidents so horrific that their survivals seemed little short of miracles. To say these people “survived,” though, doesn’t quite capture the truth. Their bodies survived, but their minds didn’t quite; their minds were warped into something new.

Some people lost all fear of death; others started lying incessantly; a few became pedophiles. But however startling, in one way these transformations proved predictable, since people with the same deficit tended to have damage in the same area of the brain—offering vital clues about what those areas did.

There are a thousand and one such stories in neuroscience. These tales expand our notions of what the brain is capable of, and show that when one part of the mind shuts down, something new and unpredictable and sometimes even beautiful roars to life.

A Bike Accident, Epilepsy, and, Eventually, a Partial Lobotomy

In the early 1930s a bicyclist in Connecticut struck a boy, who tumbled and cracked his skull. He started having seizures. Each lasted around forty seconds, during which time his mouth flopped open, his eyes slipped shut, and his arms and legs crossed and uncrossed as if curled by an invisible puppeteer. He suffered his first grand mal on his fifteenth birthday. So at an age when most people are struggling to find an identity, he was saddled with one he didn’t want: the kid who shook, who bit his tongue, who slumped over and blacked out and pissed himself.

Excerpted from The Tale of the Dueling Neurosurgeons.

Finally the desperate young man — soon immortalized as H.M. — decided to try surgery. He started seeing Dr. William Scoville around 1943. A noted daredevil — before a medical conference in Spain once, he’d stripped off his jacket and mixed it up with the toros in the bullring — Scoville liked risky surgeries, too, and had jumped onto the American lobotomy bandwagon early.

But he disliked the drastic changes in his patients’ personalities, so he began experimenting with “fractional” lobotomies, which destroyed less tissue. Over the years he basically worked his way around the brain, carving out this piece or that and checking the results, until he finally reached the hippocampus. Because it was part of the limbic system, scientists at the time believed that the hippocampus helped process emotions, but its exact function remained unknown.

In the early 1950s he started removing the hippocampi (you have one in each hemisphere) from a few psychotics. Although it was hard to be sure in people with such disturbed minds, they seemed to suffer no ill effects, and two women in particular showed a marked reduction in seizures. Unfortunately Scoville neglected to do careful followup tests until November 1953 — after he’d convinced H.M. to try the surgery.

>Scoville peeled back the scalp, then used a hand crank and a drill saw from a local hardware store to remove a bottle cap’s worth of bone from above each eye.

H.M.’s operation took place on September 1, 1953. Scoville peeled back his patient’s scalp, then used a hand crank and one-dollar drill saw from a local hardware store to remove a bottle cap’s worth of bone from above each eye. As cerebrospinal fluid drained away, the brain settled down in its cavity, giving Scoville more room to work. With what looked like an elongated shoehorn, he nudged aside H.M.’s frontal and temporal lobes and peered inside.

The hippocampus sits at ear level and has the rough shape and diameter of a curled thumb. Scoville grabbed a long metal tube and began cutting and sucking out tissue gram by gram; he eventually removed three inches’ worth of hippocampus on each side. (Two nubs of hippocampal tissue remained behind, but because Scoville also removed the connections between those nubs and other parts of the brain, the nubs were useless, like unplugged computers.) For good measure, Scoville removed H.M.’s amygdalae and other nearby structures as well. Given how deeply all these structures are embedded in the brain, only a neurosurgeon could have destroyed them with such precision.

The Seizures Stop—But at What Cost?

By many measures, the operation succeeded. The seizures all but disappeared (two attacks per year at most); and when the fog of epilepsy lifted, his IQ jumped from 104 to 117. Just one problem: His memory was shot.

Aside from a few small islands of recollection — like the fact that Dr. Scoville had operated on him — an entire decade’s worth of memories from before the surgery had vanished. Equally terrible, he couldn’t form new memories. Names escaped him now, as did the day of the week. He repeated the same comments over and over, verbatim, and while he might remember directions to the bathroom long enough to get there, he always had to ask again later. He’d even consume multiple lunches or breakfasts if no one stopped him, as if his appetite had no memory, either. His mind had become a sieve.

>He’d even consume multiple lunches or breakfasts if no one stopped him, as if his appetite had no memory, either. His mind had become a sieve.

In light of modern knowledge, H.M.’s deficit makes sense. Memory formation involves several steps. First, neurons in the cortex jot down what our sensory neurons see and feel and hear. This ability to record first impressions still worked in H.M. But like messages scrawled on the beach, these impressions erode quickly.

It’s the next step, involving neurons in the hippocampus, that makes memories last. These neurons produce special proteins that encourage axon bulbs to swell in size. As a result, the axons can stream more neurotransmitter bubbles toward their neighbors. This in turn strengthens the synapse connections between those neurons before the memory decays.

Over months and years — provided the first impression was strong enough, or we think about the event from time to time — the hippocampus then transfers the memory to the cortex for permanent storage. In short, the hippocampus orchestrates both the recording and the storage of memories, and without it, this “memory consolidation” cannot occur.

After his memory vanished, H.M. lost his job and had no choice but to keep living with his parents. He spoke in a monotone now and had no interest in sex, but otherwise seemed normal. He took a part-time job packing rubber balloons into plastic bags, and did odd chores around the house. (Although his parents had to remind him where they kept the lawn mower every single time, he could actually mow just fine, since he could see what grass he hadn’t cut.)

He whiled away most days peacefully, either doing crossword puzzles — working through the clues methodically, in order — or flopping in front of the television and watching either Sunday Mass or the old movies that, to him, would never become classics. It was like early retirement, except for the days a Ph.D student named Brenda Milner arrived to test him.

The Test That Redefined What Memory Even Means

Her battery of tests confirmed Scoville’s basic observations pretty quickly: H.M. had little memory of the past and no ability to form new memories going forward. This was already a big advance — proof that some parts of the brain, namely the hippocampus, contribute more to forming and storing memories than other parts. And what Milner discovered next redefined what “memory” even meant.

She gave him a piece of paper with two five- pointed stars on it, one nested inside the other: The outer star was about six inches wide, and there was a half- inch or so gap between them. The test required H.M. to trace a third star between the two with a pencil. The catch was, he couldn’t see the stars directly: Milner had shielded the diagram, and he had to look at them in a mirror instead. Left was right, right was left, and every natural instinct about where to move his pencil was wrong. Anyone taking this mirror test for the first time makes a mess — the pencil line looks like an EKG — and H.M. proved no exception.

Somehow, though, H.M. got better. He didn’t remember any of the 30 training sessions Milner ran him through. But his unconscious motor centers did remember, and after three days he could trace the star in the mirror fluently. He even commented near the end, “This is funny ... I would have thought it would be rather difficult, but it seems I’ve done pretty well.”

>This distinction between procedural and declarative memories (sometimes called “knowing how” versus “knowing that”) now undergirds all memory research.

Milner remembers the star test as a eureka. Before this, neuroscientists thought of memory as monolithic: the brain stored memories all over, and all memory was essentially the same. But Milner had now teased apart two distinct types of memory. There’s declarative memory, which allows people to remember names, dates, facts; this is what most of us mean by “memory.” But there’s also procedural memory — unconscious memories of how to pedal a bicycle or sign your name.

Tracing the stars proved that H.M., despite his amnesia, could form new procedural memories. Procedural memories must therefore rely on distinct structures within the brain. This distinction between procedural and declarative memories (sometimes called “knowing how” versus “knowing that”) now undergirds all memory research.

Scientists also discovered that time worked differently for H.M. Up to about 20 seconds, he reckoned time as accurately as any normal person. After that, things veered wildly. Five minutes lasted, subjectively, just 40 seconds for him; one hour lasted three minutes; one day 15 minutes. This implies that the brain uses two different timekeepers — one for the short term and one for everything beyond 20 seconds, with only the latter suffering damage in H.M. Eventually more than one hundred neuroscientists examined H.M., making his probably the most studied mind in history.

A Brain That Can Never Remember, Preserved Forever

In 1980, after H.M.’s father died and his mother got too sick to care for him, he moved into a nursing home. He got pretty portly after too many forgotten second helpings of cake and pudding. But overall he was a fairly normal patient and lived a (mostly) placid life.

He loafed through the nontesting days reading poems or gun magazines, watching trains rumble by, and petting the dogs, cats, and rabbits the facility owned. When he dreamed at night, he often dreamed of hills — not of struggling up them, but cresting them and being at the top.

He finally died in 2008, aged 82, of respiratory failure — at which point scientists revealed him to the world as Henry Gustav Molaison.

His brain is still providing insight: Before his death, his nursing home had started stockpiling ice packs in preparation; when he passed, employees ringed his skull with them to keep his brain cool. Doctors soon arrived to claim the body, and that night they scanned his brain in situ and then liberated it.

After two months hardening in formalin, it was flown cross-country in a cooler to a brain institute in San Diego. Scientists there soaked it in sugar solutions to draw out excess water, then froze it to solidify it. Finally, they used the medical equivalent of a deli slicer to shave Molaison’s brain into 2,401 slices, each of which they mounted on a glass plate and photographed at 20x magnification, to form a digital, zoomable map down to the level of individual neurons. The slicing process was broadcast live online, and 400,000 people tuned in to say goodbye to H.M.

Excerpted from the book 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 Company.*