How the Brain Makes Memories
How the Brain Makes MemoriesMemory: The Human Talent
Memory Studies from the Past
The Stem of the Matter
Where Memories are Made
The Power of the Mind
Telegrams to the Brain
The Memory Mystery
The Case of HM
Brain Experiments: Few and Far Between
The human brain may be the most complicated thing in the known world. It is the organ that controls every other part of the body. It makes the heart beat, reminds us to breathe even when we are asleep, and controls the systems that help us to grow from infants to adults. The brain pulls information from everything around us, letting us see, hear, taste, smell, and touch. Perhaps most importantly, the brain allows us to remember.
A powerful memory is what sets humans apart from the other animals that share our world. Most creatures have brains that do all the same basic things a human brain can do—feel pain and heat and cold, see and hear things, and sense danger. But only the smartest animals on our planet form memories of these experiences. Only the smartest can learn.
A housefly might land on a hot fireplace, and its brain will tell it to move. But a housefly is not very smart. It might buzz away when it feels the heat, but a moment later, it is likely to come back and land on the same hot fireplace again. A mouse, on the other hand, a more intelligent creature, may run across the fireplace, and his brain, too, will tell him to move. But when the mouse comes through the room again, he will remember that the fireplace was hot. He will take a different path to avoid the fireplace altogether.
The mouse’s brain created a memory. It gathered many details from the environment around the mouse—not just that the fireplace was hot, but what the fireplace looked like, where it was in the room, and even where the mouse was headed when he scampered across the fireplace to begin with. The mouse’s brain stored these details, something the fly’s brain could not do. Because of memory, the mouse can learn to run around the fireplace, not across it.
The mouse’s ability to learn is remarkable. It proves that even a small brain, in an animal that has relatively little intelligence, can store information. The human brain, by comparison, is capable of everything the mouse brain can do, and endlessly more. Humans do not need to touch a fireplace to know they should avoid it. They have already learned that if something is glowing red, it will probably be hot, and touching it would not be wise. Unlike mice, humans are also curious. Mice do not care about why the fire is hot, but the human brain cares. So humans study fire. They remember what they learn. They share their knowledge. They teach their children.
“The brain is a complex biological organ of great computational ability,” says Eric R. Kandel, author of In Search of Memory: The Emergence of a New Science of Mind. “The brain is responsible not only for relatively simple motor behaviors, such as running and eating, but also for the complex acts that we consider quintessentially human, such as thinking, speaking, and creating works of art.”1
For most animals, picking up information from the five senses—touch, smell, sight, sound, and taste—is the brain’s main job, but in people, the five senses are the tip of the iceberg compared to what the human brain can do.
The Brain’s Filing System
Scientists think memory works like a filing cabinet. Every time a sensory experience—a sight, smell, taste, sound, or feeling—passes into the brain through the brain stem, the experience is processed by the brain. If it is important, the brain shuffles the detail to a certain part of the brain, where it is classified with other, similar experiences the brain has processed before.
Finding a memory means the brain must look for it in the right place, with bits of information that are similar. When you struggle to remember a fact or detail, you try to think of things that are linked to it. If you are trying to recall a person’s name, you may try to remember where you met. Eventually, you may hit on a detail that was stored in the same “drawer” of your brain’s filing cabinet as the person’s name, and the forgotten detail will come to you.
“Receiving incoming sensory signals is just the beginning of the brain’s work,” says Lisa Schoenbrodt in her book Children with Traumatic Brain Injury: A Parent’s Guide. “What the brain does with the information it receives is what determines the quality of thought and behavior.”2
Just how the brain remembers is something that continues to baffle scientists. It all has to do with the structure of the brain, right down to the tiny cells that create it.
Since the late 1700s, scientists have known that electricity powers the brain. Early researchers showed that electrical pulses to a frog’s brain could move its legs. As for how electricity forms memories and leads to learning, however, scientists still have much to figure out.
For thousands of years, leading thinkers such as Aristotle believed that the heart, not the brain, was responsible for feelings, learning, and intelligence. Galileo, in the 1600s, thought that human personality and emotions were spiritual and had nothing to do with science. Even the philosopher René Descartes, famous for his saying I think, therefore I am, had trouble imagining thoughts and memories as part of the science of the brain. He did not think of the brain as doing both things— reacting to the physical world outside the body and creating feelings and new ideas from within.
In the centuries that have passed since these philosophers wrote about human thought and memory, science has discovered that the brain does do both. New technology creates pictures and electrical maps of living, working brains, proving that different parts of the brain work together to do very complicated things. But this organ’s amazing abilities still are not completely understood. People with brain disorders such as amnesia are depending on science to get an even better picture of what is happening inside the skull.
“Understanding the human mind in biological terms has emerged as the central challenge for science in the twenty- first century,”3 says Kandel. The search for this understanding begins at the bottom of the issue—at the base of the brain.
The place where the neck meets the head is also where the spinal cord dead-ends into the human brain. This is called the brain stem. In very simple creatures, such as the fly, the brain stem is nearly all there is to the brain. Smarter creatures, such as reptiles, birds, and mammals, have much larger brains with many more parts.
The human brain is more complicated than the brain of any other animal that has ever lived on Earth. Humans have enormous brains. An adult’s brain weighs about 3 pounds (1.4kg)—as much as a dozen uncooked hamburger patties. This is much, much larger than the nut-sized brains that sat in the massive skulls of dinosaurs. Human beings are, therefore, much smarter, with brains and thought processes that are much harder to understand. Human thought, says Kandel, is “a set of operations carried out by the brain, much as walking is a set of operations carried out by the legs, except dramatically more complex.”4
Every human brain has three main parts. The lower section, called the hindbrain, surrounds the brain stem. This area of the brain mostly handles the things people do not have to think about. It reminds the body to breathe in and out every day (and night). It makes the heart beat. After a meal, this part of the brain makes sure the stomach digests the food. It controls reflexes and muscle movements, too. If the hindbrain is injured, the result may be paralysis of the muscles or even death. After all, if the body’s control center can no longer tell it to breathe or pump blood, the body will die.
“You can think of the hindbrain as a kind of base camp,” says author James S. Nairne. “Not surprisingly, damage to these lower regions of the brain seriously affects the ability of the organism to survive.”5
The hindbrain also works like a doorway between the body and the rest of the brain. All messages from the body to the brain and from the brain to the body go through the brain stem. The hindbrain is, in some ways, the most important part of the brain. Without it, the body and the rest of the brain will not work as they should. But the hindbrain is also the primitive part of the brain, handling only the simplest jobs, the things that do not require thinking, concentration, and creativity.
More complicated tasks, such as remembering, are done in the midbrain and the forebrain. These activities take a lot of brain power, and they happen somewhere deep in the cerebrum, the biggest part of the brain.
The cerebrum, the largest part of the brain, has two halves, called hemispheres. Each hemisphere, the right and the left, has four smaller sections called lobes. The lobes help gather information and signals from the body and its environment.
The occipital lobes help make sense of what the eyes see. The parietal lobes take in touching and feeling messages from the skin and joints, giving the body information about where it is, what it is touching, and how it is moving. The temporal lobes receive messages from the ears, important for understanding sounds and for remembering them. Each hemisphere of the cerebrum also has a frontal lobe that makes choices about what the body will do. Unlike the brain stem and hindbrain, which control actions that are not by choice, the frontal lobes are in charge of what a person chooses to do. Eating a third candy bar or pulling someone’s hair are actions that come out of the frontal lobe.
The midbrain and the forebrain have other, very vital jobs. One of these is the all-important ability to remember. Scientists think that one brain structure in particular, called the hippocampus, is especially important for human memory.
“A number of psychologists and neurobiologists have argued that the hippocampus is a key structure—perhaps the key structure—underlying explicit memory for recent experiences in monkeys, humans, and other animals,”6 says Harvard University psychology professor Daniel L. Schacter in his book, Searching for Memory: The Brain, the Mind, and the Past.
Unlike damage to the brain stem, which can cause death, damage to a part of the brain such as a temporal lobe or the hippocampus is usually not deadly—but it can have serious effects on how the brain works. Personality and the way someone thinks, speaks, and understands can be changed forever if any of these parts of the brain are damaged. The way the person remembers, too, can be affected. Damage to the brain can cause changes to the way someone learns and stores information. It can also wipe out memories altogether if the brain cannot conduct the all-important thing that makes memory—electricity.
Like any organ of the body, the brain is made of cells, tiny building blocks so small that they cannot be seen with the naked eye. In the brain, these cells are designed for a single job: to carry electricity. They work like millions of tiny wires to carry messages back and forth to other parts of the brain and to the rest of the body. These are the nerve cells, also called neurons, and they are specially shaped for the task of passing electrical messages. At its center, every neuron has a round cell body that produces the neuron’s energy, helps it reproduce, and keeps it alive, but branching out from the cell body are the crucial parts of the neuron: its dendrites and axons.
Dendrites and axons look like plant roots. Under a microscope, in fact, neurons look very much like trees that have lost their leaves during winter. A dendrite is a branch that brings electrical signals into the neuron, and an axon is a branch that sends electrical signals out to other nerve cells. Neurons are like trees in a forest, says Diane Ackerman in her book, An Alchemy of Mind: The Marvel and Mystery of the Brain—and they talk among themselves.
“For that purpose,” Ackerman says, “they have two kinds of limbs, dendrites and axons: the former to listen, the latter to speak.” Dendrites, she says, “hear what neighboring neurons signal through their axons.”7
Each neuron is arranged so that its “branches”—its dendrites and axons—touch the branches of other neurons. The place where branches touch is called a synapse. Electricity passes through synapses, jumping from the axon of one neuron to the dendrite of another, at 200 miles per hour (322kmph). Because there are trails of neurons throughout the body, connecting the brain to fingers and toes, eyes and ears, nose and tongue, the brain can quickly send messages anywhere in the body.
Every part of the body can also send speedy messages back to the brain. These messages do not go just anywhere. They are instantly shuffled to the part of the brain that will know what to do with them.
“This system allows us to link the varied components of everyday episodes into integrated records of experiences: what we hear, what we think, and how we feel,”8 says Schacter.
Many scientists think of the brain like a computer, gathering input, coding it for storage, and filing it away in the proper place. Like a computer, the brain has different kinds of “software,” and these all have different jobs. Some of these jobs are to take in signals, such as a hot fireplace, and help the body to respond by moving away. Other jobs, however, seem to involve making, not just receiving, electrical signals. This is what sets a brain apart from a computer and baffles scientists who study the nervous system. The brain invents new thoughts and can imagine false ones. It lets humans feel physical pain, but also emotions such as love and hate.
Scientists are still learning how the brain does its most complicated work—forming memories—and where in the brain this process happens. All human beings are born with a complete set of neurons. Scientists believe that as experiences and senses send electrical pulses through the cells of the brain, these cells move around to make it easier for the same kinds of pulses to come through the next time. The brain gets faster at moving along that particular message.
Neurons actually rearrange themselves with each new sensory experience. They line up differently every time a new electrical charge passes along their branches. This way, the experience that caused the charge will be more familiar to the neurons when it happens again. It is as though the neurons say to one another, “We have seen a pulse like this before. Step aside to let it through.” When this happens, a “memory” of that particular experience is formed.
“The brain’s billions of cells are engaged in a continuous, frenetic dance of activity,” says Nairne. “From what seem to be chaotic patterns of cellular activity arise the intricacies of human behavior, thought, emotion, and creativity.”9
It is impossible to overestimate the value of this ability to remember. Memory lets people recognize their loved ones. Memory tells people what is dangerous and what is safe. Memory lets people sing along with their favorite songs. Memory even makes it possible to read words like the ones on this page. A reader without a memory would have forgotten the beginning of this sentence by now.
The neurons in the brain are constantly changing positions every time a person smells, hears, tastes, sees, and touches. Neurons are constantly replacing the old experiences with the new. No two people ever have the same experiences, in the same order, from birth. Therefore, no two brains are the same. In a healthy brain, new memories are being made every moment, and many of these memories are stored by the brain for life. It is a remarkable ability. Very old people still recall things from early childhood, even after their brains have spent decades recording new details, nonstop, every single day.
The brain’s method of remembering is incredibly complicated. Like a computer with its many file folders and destinations for information, there seems to be no single area of the brain that is responsible for storing all memories.
“Memory is a process rather than a place in the brain,” says Schoenbrodt. “There is not a single, unitary memory ‘storage vault,’ located in a specific area of the brain.”10
Just the same, scientists have learned that certain parts of the brain, such as the temporal lobes and the hippocampus, seem to be especially important areas for making and storing memories. This knowledge comes from studying people whose memory has somehow gone terribly wrong.
In the 1950s, one patient, now famous to anyone who has studied memory and the brain, showed scientists just how important
Flashbulb Memory, or Déjà Vu?
A memory “illusion” is when the brain is tricked into thinking it has seen or done something before. The result is the strange feeling of déjà vu—a sudden sense that something is very familiar, even though you cannot remember having experienced it before. Scientists think the phenomenon happens when you have had a prior experience that is very similar, but your brain never made a concrete memory of it. You are left with a “shadow” of a memory, and the false sense that you have been in the exact same situation before.
The opposite situation may be what brain experts call “flashbulb” memories—sharper than average memories of emotional times in life. For example, many people claim they can remember exactly where they were, what they were doing, what they were wearing, or even what they were eating when they heard that terrorists had flown planes into the World Trade Center on 9/11. These sharp, emotionally charged memories are called flashbulb memories, and they usually do not fade away with time.
certain parts of the brain are to making and keeping memories. The patient became known by his initials, HM, and his amnesia was so extreme that he has since lived his life without being able to make any new memories at all.
HM had epilepsy, a condition that causes seizures—episodes of jerking, shaking, and loss of consciousness that come on suddenly and without warning. HM’s seizures were so bad that in 1953, when he was twenty-seven years old, he had surgery. Doctors took out the parts of HM’s brain that they believed were causing his seizures. Parts of HM’s hippocampus and temporal lobes were removed. While he recovered, it seemed the surgery was a success. HM’s seizures were not as bad. He was just as smart he had been before the operation. The surgery did not seem to have caused any problems, except one: HM could not remember anything after his operation.
“If his doctors went out of his sight for even a few minutes and then returned, H. M. couldn’t recall having ever seen them before and had to be reintroduced,” says Richard M. Restak, author of The Modular Brain. “He could still recall those things he learned before the operation, but he could not retain any new information for more than a few seconds.”11
For HM, life after the surgery has been very different than it was before the surgery. As soon as his lunch plate is cleared from the table, he forgets not only what he ate, but also that he has eaten lunch at all. He promptly forgets every person he meets. He greets the doctors and nurses he sees several times a day as if he were meeting them for the first time.
Now in his eighties, HM can remember what year he was born, but for most of his life, he has had no idea of his age. When someone tells him his mother has passed away, he cries and grieves as if it is the first time he has heard the news—even if he was told the same thing the day before.
There has never been any doubt that HM is still intelligent. After his surgery, he could still carry on a perfectly normal conversation with anyone. He just could not remember it five minutes later.
Because of an operation to solve one problem, HM has lived for decades with a different problem. He will never again form a new memory that lasts longer than a few moments. Missing the parts of his brain that are necessary for normal memory, he has lived a life that probably makes very little sense to him, and he must often feel very confused and lonely.
Despite having no memory himself, HM will always be remembered. As sad as his story is, HM is famous to scientists because, for the first time, doctors had a chance to see what happened to a living person when specific parts of his brain were removed. This is not the kind of experiment researchers normally perform on living people. Taking out chunks of someone’s brain just to see what would happen is not ethical. But HM taught scientists a great deal about how the brain makes and stores memory, in what parts of the brain this happens, and how injury to the brain can affect a person’s ability to remember things and make new memories. HM gave scientists their first good look at the phenomenon of memory loss, known as amnesia.