NEW KIND OF FITNESS REGIMENYour brain’s health may be the most powerful indicator of how long you will live. It is crucial to whether that life will be rich and satisfying from youth well into old age, or something substantially less rewarding, and for less time.
A car driven wisely, fueled with high-quality gasoline, given regular oil changes, and repaired with new parts as old ones wear out is likely to last longer than one that’s abused or neglected. Likewise, the easiest way to have a healthy brain in middle age and beyond is to start with one as a youth and to follow good physical and mental habits. Exercise it. Feed it. Challenge it. Then enjoy the rewards.
But what of the person who comes late to repairs, like the owner of a car that rusts for years on blocks or runs too long on dirty oil? The car owner can always swap out the engine. You, on the other hand, have only one brain, basically composed of the same neurons you were born with, plus a few added to some narrowly specific areas. Once they’ve begun to deteriorate, can they be saved—or even made stronger?
Brain researcher Marian Diamond is certain they can.
In the 1960s, Diamond compared two groups of lab rats. The first group was confined to the equivalent of a gray isolation cell in a maximum-security prison. They ate simple rations to keep them alive from day to day, but their brains received little stimulation. No rat games, no rat puzzles, no rat get-togethers to break the boredom. She enrolled the second group in a version of rat school, complete with recess. They had toys and balls for play, challenging mazes to explore, exercise equipment to get blood pumping to their muscles and their neurons, and best of all, other rats to share their experiences. When she pitted the two in timed contests in which they ran the same mazes, the rats that had lived in the mentally and physically invigorating environment performed much better.
NEW KIND OF FITNESS REGIMENDiamond then did what she could not do to humans in a similar experiment. She put both winners and losers under the knife to examine their brains. (Life’s not fair, especially for rodents.) Rats that had enjoyed the richer learning environment and had won the maze races exhibited markedly different brains from those in the control group. Their cerebral cortices—the outer, wrinkled shells that are home to neural pathways that make sense of the world—were thicker than those of the unstimulated rats. The enriched-brain rats had more neural connections, a sign of greater mental activity. And they had more blood vessels to carry vital oxygen to keep those connections firing at peak efficiency. Diamond had gathered concrete evidence that what goes on in the mind manifests itself in the physical state of the brain. Learning strengthens the organ of the brain just as exercise strengthens muscles in the legs, arms, and abdomen.
As revealing as Diamond’s research was, it had a twist: She didn’t experiment on young rats. She chose to work with rats in middle age and older, equal to ages between 60 and 90 in humans. Old rats had brains they could reshape in response to new experiences, a condition known as plasticity.
That revelation changed widespread beliefs about the plasticity of older brains. Studies with young rats, cats, and other mammals had suggested that the brain opened a crucial window for learning during youth, and then closed it. For instance, in a series of famous experiments in the 1960s, neurobiologists David Hubel and Torsten Wiesel took a group of kittens and sewed one of each pair of eyes shut at birth but left the other untouched. At six months, they opened the closed eye. Although the eye was physically sound, the kitten never learned to see through it.
The experiment demonstrated that the kitten’s brain had been wired to expect, and process, visual information at a crucial time in development. When that time passed, those abilities were gone forever. Scientists call this type of brain development “experience-expectant,” meaning the brain awaits the stimulus of a particular experience, such as sight or sound, to develop the means to process that information.
But there’s a second category of brain-developing experiences. These are called “experience-dependent.” They prompt brain growth in response not to stimuli common to the species, such as light and sound, but rather to the individual’s unique environment. A child raised in the Amazon jungle learns a lot about plants and animals of the rain forest. A child growing up in the suburbs figures out how to play on the jungle gym and swings, or to swim in a pool or kick a soccer ball.
This second type of brain development can occur at any time. Some types of learning, such as mastery of a second language, are easier before the age of puberty, but on the other hand, vocabulary building occurs throughout life. In general, there is no single, crucial time window for this kind of learning. Your brain can learn experience-dependent knowledge at any time.
THE UNIVERSE INSIDE YOUR HEADThe brain’s plasticity reveals much about its amazing structure. It is the most complicated object in the universe, composed of billions of independent units that work together in remarkably complex symphonies that manage to comprehend the world; process, store, and retrieve information; and use that information to decide how to interact with the world. Each new experience changes the brain’s physical makeup, so that by the time you finish reading this page, your brain will be slightly different from your brain at the time you began with the page’s first word.
At the cellular level, the human brain is a collection of as many as 100 billion nerve cells called neurons and about 50 trillion neuroglia cells. The latter sometimes are simply called glial cells, from the Greek word for “glue.” Their role is similar to that of hordes of servants in a castle: They serve their comparatively few masters, the neurons. Glial cells help neurons make connections and promote their health and steady functioning. Some take an active role in physical health by attacking microbes. Others, called oligodendrocytes, produce an insulating substance called myelin that speeds communication from neuron to neuron.
Neurons are the brain’s key players. Each begins as its own little orb. Once the neuron has fixed itself into its particular cubbyhole in the brain during fetal development, two types of projections sprout from its central core: a single, whiplike axon, some as short as a fraction of an inch and others several feet long, and from one to as many as 100,000 dendrites branching out like the knobby ends of a cat-o’-nine-tails. Dendrites reach out to other neurons, some near and some far. They receive information from the axons of their neighbors and pass it to their neuron for processing; their input allows a neuron to gather data—to learn. The neuron initiates or passes information via its axon to the dendrites of other neurons, like a teacher speaking to a classroom of students, with each of those students channeling the information to other students, parents, and friends. The web of axons and dendrites pointing in every direction makes the brain’s interior wiring resemble the chaos of a mangrove swamp.
BRAIN INSIGHT
From Chaos to Cognition
As Emily Dickinson once wrote, “The brain is wider than the sky”
Your brain is astonishingly, incomprehensibly, jaw-droppingly complicated.
Assume that each of your brain’s roughly 100 billion neurons (nobody has counted them, but that’s a pretty good estimate) has the capability to connect with one to as many as 10,000 other neurons, thanks to its arrangement of axons and dendrites. If that’s the case, then the number of theoretical connection patterns in your brain is 40 quadrillion: 40,000,000,000,000,000. If you factor in the variable power of how strongly neurotransmitters send a signal from one neuron to the next, hypothesizing that each neuron has ten different signal strengths, then the number of electrochemical configurations in the brain runs to ten to the one-trillionth power. That’s the number 1, followed by a trillion zeros. Compare this with the estimates of the number of atoms in the observable universe: 10 to the 80th power, or 1 followed by 80 zeros.
Out of this mind-blowingly vast maze of neural connections of varying intensities comes the ability to comprehend and interact with the universe. At some point in the brain’s development, consciousness arises within a three-pound, tofu-like mass of wrinkled matter. The universe becomes aware of itself—and can marvel at its own complexity.
IT STARTS WITH A SPARKIf only you could watch as information passes along circuits of neurons, it might look like flocks of birds darting, converging, and scattering against the sky. Like birds in flight, reeling and turning as if by magic, neurons communicate without the need to touch one another. A tiny space, called a synapse, separates the would-be embrace of axons and dendrites.
The neuron’s language of communication within its own cell body is electricity. A spark received via a dendrite travels as electric energy until it reaches the end of the neuron’s axon. There, the information it contains is translated into a variety of chemicals known as neurotransmitters. Each neurotransmitter has its own particular job, ranging from energizing the receiving neuron to fostering positive feelings of rewarding behaviors to suppressing particular actions. The neurotransmitters traverse the synaptic gap and dock in matching receptor sites like keys in a lock. Their joining with the cell wall of the receptor neuron initiates a new electrical charge, which travels the length of that neuron until it is converted to chemical energy at the far side.
A neuron requires stimulation to fire. That stimulus could begin outside the body, as when you look at the sea and electromagnetic waves reflecting off the surface activate the light-sensitive rod and cone cells in your eyes’ retinas. That sensation—blue or green, flat, rolling, or choppy—knocks down the first domino in the line. From the retina, the signal passes along a neural chain to the visual cortex at the back of the brain and then back to the front for further processing. A stimulus could also originate internally, as when you feel hungry, or when your conscious mind remembers the face of your fifth-grade teacher and activates the first neuron in another domino-like pattern. Some neurons fire consciously, and some fire below the level of conscious thought. Some even fire in ways to mimic the world outside.
When a particular bit of information travels throughout a circuit of neurons, it changes from electrical to chemical, back and forth, propagating a signal at speeds that can reach more than two hundred miles an hour. As it travels, it may prompt the addition or subtraction of more information, or set off a flood of new signals with new information. All this motion requires energy. The brain accounts for only about 2 percent of a body’s weight, but it uses about 25 percent of the body’s blood sugar and oxygen.
Because neurons aren’t bound to each other like bricks in a wall, they remain free to make new connections and break old ones. That’s exactly what happens when the brain learns something new: The information physically alters the connections.
BRAIN INSIGHT
These chemicals are some of the most important messengers in the brain
ACETYLCHOLINE. Causes muscles to contract; also linked to memory, sleep, and attention.
DOPAMINE. Crucial for movement of the body, as well as the brain’s reward system, associated both with pleasure and addiction. Patients with Parkinson’s disease have lowered dopamine levels, causing characteristic shaking of limbs and head.
ENDORPHIN. Released following stress or pain, acting like a natural opiate by binding to opiate receptors on neurons.
GAMMA-AMINOBUTYRIC ACID, or GABA. Quiets, rather than excites, neurons, because the brain needs to decelerate as well as accelerate a multitude of functions.
GLUTAMINE. Excites neurons, except in high concentrations fatal to neurons; required for learning and memory.
NOREPINEPHRINE. Plays a key role in regulating mood, as well as blood pressure, heartbeat, and arousal.
SEROTONIN. Prompts sleep and appetite; also plays a role in mood, related to everything from depression to anxiety to sexual arousal.
PIECES OF A PUZZLEAt the micro level, neurons in their billions form an intricate electrical net. At the macro level, they are organized into the discrete structures of the brain, with four main parts: cerebrum, diencephalon, cerebellum, and brain stem.
The outer surface of the cerebrum, nearest the skull, is the cerebral cortex. It’s the wrinkly, gray, walnut-shaped covering that most people think of when they visualize the brain. The cerebral cortex is home to the functions of information processing that separate humans from other animals.
The cerebrum exists in two hemispheres, the left and right, connected by a band of neural tissue called the corpus callosum, which allows information to pass between the two. The left hemisphere has long been considered dominant because it typically is the site of language processing. Strokes in the left hemisphere sometimes impair speech. Injuries to the right hemisphere, on the other hand, sometimes result in reductions in or loss of the ability to integrate information—to see the forest for the trees. The right hemisphere apparently plays a crucial role in emotional and spatial recognition, such as seeing raised eyebrows and upturned corners of the mouth and realizing that a face expresses joy.
The two hemispheres duplicate many anatomical structures. Brain experts speak of folds and fissures dividing the hemispheres into four lobes, but each lobe has a separate left and right half. As a result, neurologists sometimes refer to a particular lobe in the singular or plural, but may be talking about the same thing.
THE LOWDOWN ON YOUR LOBESIn general, the back portion of the brain takes in information about the world and begins to process it. The front portion decides what to do with that information.
The foremost part of the brain, appropriately enough, is called the frontal lobe. It lies in front of a major divide called the central fissure. A frontal lobe region called the precentral gyrus controls movement. A quirk of evolution has caused the left half of the frontal lobe to control the right side of the body, and vice versa. Related areas of the frontal lobe oversee complex motion and inhibit motion, giving the brain the power to override the desire to run away from danger or shout with happiness.
In the very foremost part of the frontal lobe, right behind the forehead and eyes, lies the prefrontal cortex (PFC). This part of the brain developed last on the evolutionary time line, and is the last to become fully myelinated. The PFC is the region that most separates humans from all other animals, including apes. It comprises 30 percent of the human brain, compared with only 11 percent for a chimpanzee and 3 percent for a cat. It is the home of decision making and thus is sometimes called the brain’s seat of “executive function”—it is the boss.
Clinical neuroscientist Daniel G. Amen considers the boss metaphor apt. When the boss is absent in an office or factory, sometimes little serious work gets done, Amen says. And when the PFC boss works very hard, micromanaging the rest of the conscious brain, it sometimes promotes anxiety and worry—and again, little useful work gets done. Amen also likens the PFC to the Disney cartoon character Jiminy Cricket, who acted as the conscience of the puppet-boy Pinocchio. The cricket’s still, small voice suggested the best ways to behave. When the guiding voice grows too quiet, the result is what Amen calls “Jiminy Cricket deficiency syndrome.” Negative behaviors multiply, including impulsive action, confusion, short attention span, bad judgment, low empathy, poor time management, and diminished conscience.
Behind the frontal lobe lie the parietal and temporal lobes. The parietal lobe processes sensations from the body, including pain and the pressure of touch. The temporal lobe processes sounds, including speech. It is also associated with memory and the emotional content of experiences. Each half of the temporal lobe includes a small, seahorse-shaped form called the hippocampus, which comes from the Greek words for “horse” and “sea monster.” The hippocampus is part of the limbic system, a collection of structures on the inside of the cerebral cortex associated with emotions, motivation, and behavior.
THE MIRACLE OF MEMORYNeuroscientists once believed that the mature brain was incapable of producing new neurons—that the neurons you had at birth, or shortly afterward, were the only ones you would have for your entire life. Research in the 1990s laid that idea to rest. New neurons have been found growing in the hippocampus, a region that is crucial in the formation and storage of memories. In a famous study in 2000, University College London neuroscientist Eleanor Maguire demonstrated enlarged hippocampi in the brains of London cabdrivers. Cabbies spend two to four years memorizing London’s intricate street grids, including the shortest distance between any two points. Using a magnetic resonance imaging (MRI) scanner, Maguire found that cabbies’ right posterior hippocampus, a region devoted to spatial navigation, measured 7 percent larger than the norm. Evidently, neuroplasticity had reshaped the cabbies’ brains as they learned more and more about navigating through London.
At the back of the brain is the occipital lobe. Although it is at the opposite end of the brain from the eyeballs, the occipital lobe processes visual information. Much of the lobe interprets shape, color, motion, and other qualities of objects sensed by the retinas of the eyes, which are extensions of the brain. This information-processing set of neural circuits is called the visual cortex.
BRAIN INSIGHT
A famous accident illustrated the connections between personality and parts of the brain
A gruesome accident in 1848 linked the prefrontal cortex to moral behavior. Vermont railroad worker Phineas Gage was jamming a tamping iron into a hole to pack gunpowder and sand when the iron sparked on a rock. The powder exploded, sending the 13-pound rod through Gage’s cheek, prefrontal cortex, and skull crown. Gage initially seemed barely fazed. “Here is business enough for you,” he told a doctor.
Before the accident, co-workers liked Gage and considered him reliable and temperate. After the accident, he became rude, impatient, profane, and unable to stick to plans. Gage drifted from job to job and died 12 years later, at 36, after a series of seizures.
In 2012, a team of researchers at UCLA’s Laboratory of Neuro Imaging (LONI) published a paper tracing the path of the tamping iron through Gage’s brain. They managed to produce a striking new image of the damage, while tracing the probable severed connections to other parts of his brain. They concluded that Gage’s behavioral changes came not only from harm to his prefrontal cortex, but also from the disrupted pathways to other areas that involve emotional response—showing once again that the brain is an immensely interconnected organ.
ALWAYS ON DUTYMoving out of the cerebrum, the diencephalon lies between the left and right hemispheres. Its structures regulate body rhythms such as sleeping and wakefulness, as well as body temperature, digestion, perspiration, and other body functions that usually occur below the level of consciousness. A portion of the diencephalon known as the thalamus relays sensory information from other brain regions and plays a role in emotion and memory.
Below the occipital lobe is the cerebellum. Although it contributes to emotional life and action, the cerebellum’s most obvious task is to coordinate movement and balance. The cerebellum automatically processes the neural signals required to perform practiced tasks. For example, when you learn to type, you concentrate to find and hit the right keys. That action, which initially requires focused attention and choices, takes place mostly in your frontal lobes. But as you grow comfortable with the location of the letters, you type without consciously thinking of where to find each key. Your cerebellum houses the autopilot that keeps the right keys clicking. Other brain regions jump into action to pay attention and recognize when you hit the wrong ones.
The fourth major brain region is the brain stem. It comprises the area where the brain meets the spinal cord, which is merely an extension of neurons into the body. Key brain stem regions include the medulla oblongata, which controls heartbeat and respiration, and the pons, which controls reflexes such as the startled jump you make when a door slams shut. Largely beyond conscious control, this lowermost portion of the brain is nevertheless crucial to survival.
BRAIN INSIGHT
The Man Who Forgot
In 1953, time stopped passing for Henry Molaison, known to history as HM
Henry Molaison’s troubles began early. He began having epileptic seizures as a child after a being hit by a bicycle in his native Connecticut. Whole-brain seizures increased in frequency until, by age 27, in 1953, he suffered nearly a dozen a week.
His doctor reasoned they would stop if he eliminated their point of origin, so he surgically removed much of Molaison’s temporal lobes, including the left and right hippocampus. The operation indeed halted the seizures. But Molaison awoke without the ability to make new memories. The hippocampus turns out to be crucial to the transference of short-term memories into longer storage.
For the next 55 years, until he died in 2008, Molaison had the most-studied brain on the planet. Researchers repeatedly tested aspects of his memory, including the conscious and unconscious, the short and long term. Academic papers referred only to patient “HM” to preserve Molaison’s privacy while he lived.
Molaison cheerfully submitted to every test; he never realized their endless repetition and could never get bored. If someone erased a crossword puzzle after he completed it, he happily did it—again and again. He lived in a permanent “now,” thinking he was still 27, until the day he died.
THE BIRDS AND THE BEES AND THE BRAINThe entire structure of the adult human brain lies hidden in the genetic code that begins to find expression at conception. When sperm slams into egg, uniting a father’s and mother’s DNA, the reaction causes the fertilized egg to begin dividing. At four weeks, the first brain structures begin to appear. A spoon-shaped neural plate takes shape at the head end of the developing body. A groove later appears in the center of the plate, and hemispheres begin to form on either side shortly after that. The spoon’s bowl becomes the brain itself, and the handle transforms into the spinal cord. Major brain regions start to develop, with the cerebral cortex witnessing the most explosive growth.
About a quarter million neurons form every minute in the early months of fetal development, and migrate to particular regions of the brain to take up specialized tasks. Axons and dendrites sprout when the neuron arrives at its destination. Neuroscientists once believed neurons chose their favored sites because each new neuron already had a predetermined function and sought the site associated with that function. Now, scientists believe that the journey and the destination shape the neuron to perform one task or another.
Although the brain reacts to its environment at all times, the period of so-called “neuron migration” in the womb marks a high state of sensitivity. Serious consequences can result from toxins or anything else interfering with neurons as they move through the prenatal brain, causing them to fall short of or overshoot their destination. Dyslexia and autism, for example, have been linked, in part, to less-than-optimal neural migration. The brain’s in utero sensitivity underscores the need for pregnant women to eat well, exercise, and avoid alcohol, tobacco, and other substances that could hinder the neural development of their children.
At eight months after conception, the fetal brain contains twice as many neurons as an adult’s, even though the younger brain weighs only about one-third of the adult brain. The brain cannot sustain that many neurons and neural connections, so it begins to cut back. About half of the brain’s neurons die in the final weeks of fetal development. In addition, many of the neural connections grow weak and dissolve, a process known as pruning. The result, at birth, is that the brain contains virtually all of the neurons it will have for life. Brains get bigger as the child ages into an adult for two reasons. First, the brain’s neurons grow physically larger by sending out more dendrites. And second, supporting cells around the neurons, particularly the glial cells, multiply to increase the brain’s total volume.
After birth, the newborn’s brain undergoes a kind of neural Darwinism. Neural networks compete to have the strongest links, while weak links receiving little or no stimulation undergo rigorous pruning. A widely repeated adage about the brain says, “Use it or lose it.” That appears to be literally true, especially when a child’s brain, primed to react to virtually any stimulation, receives some kinds but not others.
BRAIN INSIGHT
Maximum Capacity
Sorry, but you’re already using all of your brain
In the 2011 movie Limitless, a character played by actor Bradley Cooper takes a pill that supposedly allows him to call on the cognitive power of his entire brain, rather than the 20 percent that the movie says humans normally access.
University of Minnesota physics professor James Kakalios, author of The Amazing Story of Quantum Mechanics, concedes that taking certain drugs can boost brainpower in the short run. However, he considers taking a pill to become a genius to be “crazy,” far, far beyond current neurochemistry’s boundaries.
The movie also perpetuates the myth that the human brain uses only a small percentage of its neurons. “We use all of our brains,” Kakalios told NBC. “We don’t understand a lot about how the brain works, but evolutionarily, everything in the three-pound hunk of meat on the top of your head is there for a reason.”
Brain-imaging technology reveals no dead spots in a healthy brain. Although neural plasticity allows the brain to reroute the pathways for some functions around dead or damaged cells, demonstrating that some neural regions sometimes can be circumvented without complication, no neurons take a free ride inside the skull. Every brain cell has its function.
HEAD STARTThe younger the brain, the more plasticity it has. Not just language but virtually any skill is learned most easily when taught to a young brain. Thanks to his father’s instruction, Tiger Woods, perhaps the greatest golfer in the history of the game, already had learned the fundamentals of the game at age two. Plasticity also makes its virtues evident when a child suffers a brain injury. Young brains have a greater capacity to rewire themselves to minimize or even eliminate the impact of a serious injury. The reason, according to University of Wisconsin neuroscientist Ronald E. Kalil, may be that youthful brains are bathed in growth-enhancing chemicals that assist the brain in reconstruction and reorganization. Kalil found these factors in young cats, whose brains repaired themselves efficiently, but found far less of them in less-responsive adult cat brains.
The brain goes through a rocky phase in adolescence. It directs a physical body that resembles an adult’s, but it has yet to complete its development. In particular, the foremost parts of an adolescent brain—the portion that controls behavior—usually have yet to complete myelination. That can lead to moody behavior (surprise!) as well as imperfect impulse control and other negative actions. A fully developed teenage cerebellum and motor cortex can direct a teen to smack a tennis serve into the opponent’s court at 90 miles an hour. The impartially myelinated prefrontal cortex can also contribute to a temper tantrum if the teen double-faults on match point.
THE TICKING CLOCKIn the years from adolescence to old age, the brain continues to make new connections and prune underused ones. Aging brains shrink a bit as they lose some neurons and neural connections. A 90-year-old’s brain typically weighs about 10 percent less than it did at its peak. The aging brain also begins to show changes in at least four important areas: speed of information processing, memory, neurons’ inhibitory function, and sensation. Much of the decline occurs in the prefrontal cortex, as if the last part of the brain to be complete were the first to decline—a case of last in, first out. Variation in the species means that the brain can show signs of cognitive decline, affecting thinking and memory, at virtually any age. However, for many people, the first signs of slowing appear around age 50. The aging brain takes more time to learn new things and store them in memory. Meanwhile, the prefrontal cortex loses a measure of its ability to hold information in working memory, a sort of computer desktop where the brain keeps information at hand for immediate use. With the decline in long-term and working memory, the brain requires more time to store memories, retrieve them, and then make decisions based on them.
The brain’s inhibitory function includes filtering out distractions. Too much information can make it difficult for the brains of typical people in their late 60s to figure out what’s important and what’s not. That can make driving difficult, as it requires discriminating between important traffic signs and unimportant ones. Or, elderly brains may experience sensory overload on busy metropolitan streets, but be OK on rural dirt roads.
Mature and elderly adults can still retain full plasticity to learn new tasks. Late in the 19th century, for example, people of all ages learned to ride bicycles when they made their appearance in the United States. Today, grandfathers of 70 and their granddaughters aged 2 can both learn how to use a tablet computer or smartphone.
