The brain is a dark, mostly featureless planet, and it helps to have a map. A simple one will do, to start. The sketch below shows several areas that are central to learning: the entorhinal cortex, which acts as a kind of filter for incoming information; the hippocampus, where memory formation begins; and the neocortex, where conscious memories are stored once they’re flagged as keepers.
This diagram is more than a snapshot. It hints at how the brain operates. The brain has modules, specialized components that divide the labor. The entorhinal cortex does one thing, and the hippocampus does another. The right hemisphere performs different functions from the left one. There are dedicated sensory areas, too, processing what you see, hear, and feel. Each does its own job and together they generate a coherent whole, a continually updating record of past, present, and possible future.
In a way, the brain’s modules are like specialists in a movie production crew. The cinematographer is framing shots, zooming in tight, dropping back, stockpiling footage. The sound engineer is recording, fiddling with volume, filtering background noise. There are editors and writers, a graphics person, a prop stylist, a composer working to supply tone, feeling—the emotional content—as well as someone keeping the books, tracking invoices, the facts and figures. And there’s a director, deciding which pieces go where, braiding all these elements together to tell a story that holds up. Not just any story, of course, but the one that best explains the “material” pouring through the senses. The brain interprets scenes in the instants after they happen, inserting judgments, meaning, and context on the fly. It also reconstructs them later on—what exactly did the boss mean by that comment?—scrutinizing the original footage to see how and where it fits into the larger movie.
It’s a story of a life—our own private documentary—and the film “crew” serves as an animating metaphor for what’s happening behind the scenes. How a memory forms. How it’s retrieved. Why it seems to fade, change, or grow more lucid over time. And how we might manipulate each step, to make the details richer, more vivid, clearer.
Remember, the director of this documentary is not some film school graduate, or a Hollywood prince with an entourage. It’s you.
• • •
Before wading into brain biology, I want to say a word about metaphors. They are imprecise, practically by definition. They obscure as much as they reveal. And they’re often self-serving,* crafted to serve some pet purpose—in the way that the “chemical imbalance” theory of depression supports the use of antidepressant medication. (No one knows what causes depression or why the drugs have the effects they do.)
Fair enough, all around. Our film crew metaphor is a loose one, to be sure—but then so is scientists’ understanding of the biology of memory, to put it mildly. The best we can do is dramatize what matters most to learning, and the film crew does that just fine.
To see how, let’s track down a specific memory in our own brain.
Let’s make it an interesting one, too, not the capital of Ohio or a friend’s phone number or the name of the actor who played Frodo. No, let’s make it the first day of high school. Those tentative steps into the main hallway, the leering presence of the older kids, the gunmetal thump of slamming lockers. Everyone over age fourteen remembers some detail from that day, and usually an entire video clip.
That memory exists in the brain as a network of linked cells. Those cells activate—or “fire”—together, like a net of lights in a department store Christmas display. When the blue lights blink on, the image of a sleigh appears; when the reds come on, it’s a snowflake. In much the same way, our neural networks produce patterns that the brain reads as images, thoughts, and feelings.
The cells that link to form these networks are called neurons. A neuron is essentially a biological switch. It receives signals from one side and—when it “flips” or fires—sends a signal out the other, to the neurons to which it’s linked.
The neuron network that forms a specific memory is not a random collection. It includes many of the same cells that flared when a specific memory was first formed—when we first heard that gunmetal thump of lockers. It’s as if these cells are bound in collective witness of that experience. The connections between the cells, called synapses, thicken with repeated use, facilitating faster transmission of signals.
Intuitively, this makes some sense; many remembered experiences feel like mental reenactments. But not until 2008 did scientists capture memory formation and retrieval directly, in individual human brain cells. In an experiment, doctors at the University of California, Los Angeles, threaded filament-like electrodes deep into the brains of thirteen people with epilepsy who were awaiting surgery.
This is routine practice. Epilepsy is not well understood; the tiny hurricanes of electrical activity that cause seizures seem to come out of the blue. These squalls often originate in the same neighborhood of the brain for any one individual, yet the location varies from person to person. Surgeons can remove these small epicenters of activity but first they have to find them, by witnessing and recording a seizure. That’s what the electrodes are for, pinpointing location. And it takes time. Patients may lie in the hospital with electrode implants for days on end before a seizure strikes. The UCLA team took advantage of this waiting period to answer a fundamental question.
Each patient watched a series of five- to ten-second video clips of well-known shows like Seinfeld and The Simpsons, celebrities like Elvis, or familiar landmarks. After a short break, the researchers asked each person to freely recall as many of the videos as possible, calling them out as they came to mind. During the initial viewing of the videos, a computer had recorded the firing of about one hundred neurons. The firing pattern was different for each clip; some neurons fired furiously and others were quiet. When a patient later recalled one of the clips, say of Homer Simpson, the brain showed exactly the same pattern as it had originally, as if replaying the experience.
“It’s astounding to see this in a single trial; the phenomenon is strong, and we knew we were listening in the right place,” the senior author of the study, Itzhak Fried, a professor of neurosurgery at UCLA and Tel Aviv University, told me.
There the experiment ended, and it’s not clear what happened to the memory of those brief clips over time. If a person had seen hundreds of Simpsons episodes, then this five-second clip of Homer might not stand out for long. But it could. If some element of participating in the experiment was especially striking—for example, the sight of a man in a white coat fiddling with wires coming out of your exposed brain as Homer belly-laughed—then that memory could leap to mind easily, for life.
My first day of high school was in September 1974. I can still see the face of the teacher I approached in the hallway when the bell rang for the first class. I was lost, the hallway was swarmed, my head racing with the idea that I might be late, might miss something. I can still see streams of dusty morning light in that hallway, the ugly teal walls, an older kid at his locker, stashing a pack of Winstons. I swerved beside the teacher and said, “Excuse me” in a voice that was louder than I wanted. He stopped, looked down at my schedule: a kind face, wire-rimmed glasses, wispy red hair.
“You can follow me,” he said, with a half smile. “You’re in my class.”
Saved.
I have not thought about that for more than thirty-five years, and yet there it is. Not only does it come back but it does so in rich detail, and it keeps filling itself out the longer I inhabit the moment: here’s the sensation of my backpack slipping off my shoulder as I held out my schedule; now the hesitation in my step, not wanting to walk with a teacher. I trailed a few steps behind.
This kind of time travel is what scientists call episodic, or autobiographical memory, for obvious reasons. It has some of the same sensual texture as the original experience, the same narrative structure. Not so with the capital of Ohio, or a friend’s phone number: We don’t remember exactly when or where we learned those things. Those are what researchers call seman
tic memories, embedded not in narrative scenes but in a web of associations. The capital of Ohio, Columbus, may bring to mind images from a visit there, the face of a friend who moved to Ohio, or the grade school riddle, “What’s round on both sides and high in the middle?” This network is factual, not scenic. Yet it, too, “fills in” as the brain retrieves “Columbus” from memory.
In a universe full of wonders, this has to be on the short list: Some molecular bookmark keeps those neuron networks available for life and gives us nothing less than our history, our identity.
Scientists do not yet know how such a bookmark could work. It’s nothing like a digital link on a computer screen. Neural networks are continually in flux, and the one that formed back in 1974 is far different from the one I have now. I’ve lost some detail and color, and I have undoubtedly done a little editing in retrospect, maybe a lot.
It’s like writing about a terrifying summer camp adventure in eighth grade, the morning after it happened, and then writing about it again, six years later, in college. The second essay is much different. You have changed, so has your brain, and the biology of this change is shrouded in mystery and colored by personal experience. Still, the scene itself—the plot—is fundamentally intact, and researchers do have an idea of where that memory must live and why. It’s strangely reassuring, too. If that first day of high school feels like it’s right there on the top of your head, it’s a nice coincidence of language. Because, in a sense, that’s exactly where it is.
• • •
For much of the twentieth century scientists believed that memories were diffuse, distributed through the areas of the brain that support thinking, like pulp in an orange. Any two neurons look more or less the same, for one thing; and they either fire or they don’t. No single brain area looked essential for memory formation.
Scientists had known since the nineteenth century that some skills, like language, are concentrated in specific brain regions. Yet those seemed to be exceptions. In the 1940s, the neuroscientist Karl Lashley showed that rats that learned to navigate a maze were largely unfazed when given surgical injuries in a variety of brain areas. If there was some single memory center, then at least one of those incisions should have caused severe deficits. Lashley concluded that virtually any area of the thinking brain was capable of supporting memory; if one area was injured, another could pick up the slack.
In the 1950s, however, this theory began to fall apart. Brain scientists began to discover, first, that developing nerve cells—baby neurons, so to speak—are coded to congregate in specific locations in the brain, as if preassigned a job. “You’re a visual cell, go to the back of the brain.” “You, over there, you’re a motor neuron, go straight to the motor area.” This discovery undermined the “interchangeable parts” hypothesis.
The knockout punch fell when an English psychologist named Brenda Milner met a Hartford, Connecticut, man named Henry Molaison. Molaison was a tinkerer and machine repairman who had trouble keeping a job because he suffered devastating seizures, as many as two or three a day, which came with little warning and often knocked him down, out cold. Life had become impossible to manage, a daily minefield. In 1953, at the age of twenty-seven, he arrived at the office of William Beecher Scoville, a neurosurgeon at Hartford Hospital, hoping for relief.
Molaison probably had a form of epilepsy, but he did not do well on antiseizure drugs, the only standard treatment available at the time. Scoville, a well-known and highly skilled surgeon, suspected that whatever their cause the seizures originated in the medial temporal lobes. Each of these lobes—there’s one in each hemisphere, mirroring one another, like the core of a split apple—contains a structure called the hippocampus, which was implicated in many seizure disorders.
Scoville decided that the best option was to surgically remove from Molaison’s brain two finger-shaped slivers of tissue, each including the hippocampus. It was a gamble; it was also an era when many doctors, Scoville prominent among them, considered brain surgery a promising treatment for a wide variety of mental disorders, including schizophrenia and severe depression. And sure enough, postop, Molaison had far fewer seizures.
He also lost his ability to form new memories.
Every time he had breakfast, every time he met a friend, every time he walked the dog in the park, it was as if he was doing so for the first time. He still had some memories from before the surgery, of his parents, his childhood home, of hikes in the woods as a kid. He had excellent short-term memory, the ability to keep a phone number or name in mind for thirty seconds or so by rehearsing it, and he could make small talk. He was as alert and sensitive as any other young man, despite his loss. Yet he could not hold a job and lived, more so than any mystic, in the moment.
In 1953, Scoville described his patient’s struggles to a pair of doctors in Montreal, Wilder Penfield and Brenda Milner, a young researcher who worked with him. Milner soon began taking the night train down to Hartford every few months to spend time with Molaison and explore his memory. It was the start of a most unusual, decade-long partnership, with Milner continually introducing Molaison to novel experiments and he cooperating, nodding his head and fully understanding their purpose—for as long as his short-term memory could hold on. In those fleeting moments they were collaborators, Milner said, and that collaboration would quickly and forever alter the understanding of learning and memory.
In her first experiment, conducted in Scoville’s office, Milner had Molaison try to remember the numbers 5, 8, and 4. She then left the office to have coffee and returned twenty minutes later, asking “What were the numbers?” He’d remembered them by mentally rehearsing while she was gone.
“Well, that’s very good,” Milner said. “And do you remember my name?”
“No, I’m sorry,” he said. “My trouble is my memory.”
“I’m Dr. Milner, and I come from Montreal.”
“Oh, Montreal, Canada—I was in Canada once, I went to Toronto.”
“Oh. Do you still remember the number?”
“Number?” Molaison said. “Was there a number?”
“He was a very gracious man, very patient, always willing to try the tasks I would give him,” Milner, now a professor of cognitive neuroscience at the Montreal Neurological Institute and McGill University, told me. “And yet every time I walked in the room, it was like we’d never met.”
In 1962, Milner presented a landmark study in which she and Molaison—now known as H.M. to protect his privacy—demonstrated that a part of his memory was fully intact. In a series of trials, she had him draw a five-point star on a piece of paper while he watched his drawing hand in a mirror. This is awkward, and Milner made it more so. She had him practice tracing the star between borders, as if working his way through a star-shaped maze. Every time H.M. tried this, it struck him as an entirely new experience. He had no memory of doing it before. Yet with practice he became proficient. “At one point after many of these trials, he said to me, ‘Huh, this was easier than I thought it would be,’ ” Milner said.
The implications of Milner’s research took some time to sink in. Molaison could not remember new names, faces, facts, or experiences. His brain could register the new information but, without a hippocampus, could not hold on to it. This structure and others nearby—which had been removed in the surgery—are clearly necessary to form such memories.
He could develop new physical skills, however, like tracing the star and later, in his old age, using a walker. This ability, called motor learning, is not dependent on the hippocampus. Milner’s work showed that there were at least two systems in the brain to handle memory, one conscious and the other subconscious. We can track and write down what we learned today in history class, or in geometry, but not in soccer practice or gymnastics, not in anything like the same way. Those kinds of physical skills accumulate without our having to think much about them. We may be able to name the day of the week when we first rode a bike at age six, but we cannot point to the exact physical abiliti
es that led up to that accomplishment. Those skills—the balance, the steering, the pedal motion—refined themselves and came together suddenly, without our having to track or “study” them.
The theory that memory was uniformly distributed, then, was wrong. The brain had specific areas that handled different types of memory formation.
Henry Molaison’s story didn’t end there. One of Milner’s students, Suzanne Corkin, later carried on the work with him at the Massachusetts Institute of Technology. In the course of hundreds of studies spanning more than forty years, she showed that he had many presurgery memories, of the war, of FDR, of the layout of his childhood house. “Gist memories, we call them,” Dr. Corkin told me. “He had the memories, but he couldn’t place them in time exactly; he couldn’t give you a narrative.”
Studies done in others with injuries in the same areas of the brain showed a similar before/after pattern. Without a functioning hippocampus, people cannot form new, conscious memories. Virtually all of the names, facts, faces, and experiences they do remember predate their injury. Those memories, once formed, must therefore reside elsewhere, outside the hippocampus.
The only viable candidate, scientists knew, was the brain’s thin outer layer, the neocortex. The neocortex is the seat of human consciousness, an intricate quilt of tissue in which each patch has a specialized purpose. Visual patches are in the back. Motor control areas are on the side, near the ears. One patch on the left side helps interpret language; another nearby handles spoken language, as well as written.
This layer—the “top” of the brain, as it were—is the only area with the tools capable of re-creating the rich sensory texture of an autobiographical memory, or the assortment of factual associations for the word “Ohio” or the number 12. The first-day-of-high-school network (or networks; there likely are many) must be contained there, largely if not entirely. My first-day memory is predominantly visual (the red hair, the glasses, the teal walls) and auditory (the hallway noise, the slamming lockers, the teacher’s voice)—so the network has plenty of neurons in the visual and audio cortex. Yours may include the smell of the cafeteria, the deadweight feel of your backpack, with plenty of cells in those cortical patches.
How We Learn Page 2