Including sections: mapping--the top-down approach, cells--the bottom-up approach, connections--the dynamic approach
Since a human brain weighs on average some three pounds, it is easy to hold one in your hands. This simple fact somehow makes it even harder to imagine how such a small mass of tissue can be the source of all that we think of as human. Yet that is what the brain is, and how that can possibly be is one of the most fundamental questions in brain science.
What is the link between the anatomy of a brain and the workings of a human mind? The big challenge is that there are no obvious moving parts within the brain—it does not operate mechanically as our hearts and lungs do. If we simply look at the brain, our only remote clue about how it works is that it seems to be made up of different parts, easily discernible to the naked eye. In addition to the cerebral hemispheres (resembling a pair of large walnuts pressed together), the smaller structure that sits behind them (the cerebellum) is visible, as is the stalk that connects to the spinal cord (the brain stem). But there are many more regions than these three.
One easy way to think about the brain would be to view each of these different regions as having a clear function. Every part would be a sort of independent minibrain, controlling one aspect of our mental and behavioral repertoire: movement, emotion, ethics, balance, mathematical thinking, and so on. Simple and attractive though this idea is, it quickly runs into problems. After all, such a scenario would merely be miniaturizing the problem, not solving it; we would still have to figure out how each of those minibrains operates. And, as neuroscientists have learned through extensive observations and experiments, the brain just doesn’t work that neatly.
Let’s start with a straightforward way of trying to match up the brain’s physical structures with specific functions. We know that within the animal kingdom, each species has a very different range of abilities and behavior patterns. If the brains of different animals diverge in form, that would give us significant clues about what structures are important for what kinds of functions.
For instance, no animal has a language function anywhere near as sophisticated as ours. If there is a particular structure for language, it should be especially well developed in human brains, and small or nonexistent in the brains of other species. However, the brains of very different creatures, such as a reptile, a bird, and a mammal, differ mainly in size. In all cases we can make out the same big features: the hemispheres, the brain stem, and the cerebellum. So whatever makes one species so different from another—and above all makes the human species so different even from other primates—is not some new, clearly conspicuous structure in their brains that no other animal has.
If, however, we look at various animals’ brains for a difference not in quality but in quantity, then one clue about the physical basis of mental differences becomes apparent. The biggest discrepancy appears in the surface of the outer layer of the hemispheres. This layer is called the cortex, after the Latin for “bark,” because it wraps around the brain the way its namesake wraps around a tree. In a rat or rabbit, for example, the surface of the cortex is completely smooth. In a cat it has clear convolutions. By the time we look at monkeys and apes, and eventually humans, the cortex takes on an ever more wrinkled appearance. Why?
Imagine trying to hold a sheet of paper in one fist. The more you crumple the paper, the more the sheet will fit inside your fingers. In a way, this is what has happened to the cortex within the skull. As species have become more sophisticated, the surface of their cortices has increased faster than the limited confines of their heads. The only way to develop more “working surface” in the cortex was to fold and wrinkle it. We can see this same evolutionary trend in the development of an individual human. The brain of the six-month-old fetus has a completely smooth cortex. But in the final three months of pregnancy, the baby’s neurons proliferate at an astonishing 250,000 a minute. The cortex expands enormously so that by birth it has become as walnutlike as we know it. (See our section on the brain’s prenatal development for more on this topic.)
Mapping the Regions—The Top-Down Approach
We can call this method of thinking about the brain—looking at its physical regions and their traits—the top-down approach. The surface area of the cortex and the degree to which it is wrinkled seem to hold a clue about how a species’ brain relates to its mental abilities. Small wonder, then, that the cortex has fascinated many brain researchers. But how might it accommodate the uniqueness of our human traits?
The top-down approach has given us some valuable insights into how the cortex is organized and how it plays a part in brain function. We know, for example, that despite the way its surface looks the same everywhere, different parts of the cortex participate in different processes. Certain areas, along with many deep brain structures below the cortex, seem to relate directly to the processing of each of the senses: vision, hearing, smell, and so on.
As an example, let’s take one thin strip of cortex that straddles the brain a little like a hair band. This region is called the somatosensory (that is, body-sensing) cortex. The cells in this strip collect signals from other brain structures, which in turn are activated by impulses buzzed up the spinal cord that report on touch, pain, or temperature felt in certain parts of the body. Clearly, this strip of cortex must contain some sort of representation of the body. How else would you know that a pain was in your toe as opposed to your hand?
So far, so good. The most logical way of thinking about your body being “mapped” in the brain would be in direct relation to size. A large part of the body like the back would have a large allocation of brain territory, and a small area like the fingertips or the tongue would be represented by a correspondingly meager area of cortex. But here is a simple experiment you can do at home to prove that this “obvious” scenario is wrong. All you need are a pair of sharp pencils, or unbent paper clips, and a willing friend.
Ask the friend to close his or her eyes and turn away. Hold the pencils so their points are close together—three-eighths inch or so. Gently touch both pencil points to your friend’s skin in different parts of the body, and ask if you are applying one or both points. (You can also try touching just one point at a time to see if your friend feels a clear difference between one and two points.) When you touch both pencils to your friend’s back, he or she will almost always report feeling a single point. Now position the points much closer, only one-sixteenth inch apart, and apply them to your friend’s fingertip or (with permission) tongue. Surprisingly, this time your friend will be able to feel two distinct points.
Even though the fingertips and tongue represent only small fractions of our bodies, they are extremely sensitive to physical detail. Despite their small size, the fingers and the tongue have the lion’s share of territory in the relevant strip of brain. That’s because our brains are organized according to the functional needs of our bodies rather than simple physical size. Our fingers and tongue have to be more sensitive to touch than our back—so they have more brain territory allocated to them.
Thus we can start to see that the structures of our brains are in tune with our daily lives. But testing such primitive processes as touch doesn’t help with the question of how our cortex works differently from those of other species. So let’s go back to what is arguably the monopoly of us humans, language. Surely if we understood how our brains process language, we would have a route into understanding the physical basis of what makes us so special.
Paul Broca was a physician working in Paris during the mid-nineteenth century. He has earned his place in neurological history thanks to one of his patients, a Monsieur Leborgne. Everyone knew this unfortunate man by his nickname, “Tan,” because that was all he could say. Leborgne had a severe speech problem, an aphasia, which meant he could not articulate words. When Tan died, Broca examined his brain and discovered a clear hole in the side of its left hemisphere. Tan’s aphasia was obviously related to the damage in this region, henceforth known as Broca’s area.
But does this mean that Broca had discovered the mind’s center for speech? Far from it. Within a decade, a German physician, Carl Wernicke, identified a second site, also on the left-hand side of the brain but clearly well behind Broca’s area, where damage gave rise to a different type of speech problem. Wernicke’s aphasia is also referred to as jargon aphasia because although a person with this problem can articulate words perfectly well, all that comes out of his or her mouth is a string of gibberish.
By the end of the twentieth century, scientists had come to realize that there are still more brain regions involved in speech. Imaging techniques have made it possible for us to see the brain at work in conscious humans without causing any pain or harm. Positron-emission tomography (PET) and functional magnetic resonance imaging (fMRI) exploit the facts that the brain is very greedy for oxygen or glucose and that the hardest-working brain regions are hungriest of all. (See more on these technologies.)
Studies have now revealed that, during such seemingly simple behaviors as using language, many brain regions are working together, rather like the instruments in an orchestra. Each region will be making a specialized contribution, but the whole is somehow more than the sum of its parts. What we don’t know yet is how all the different brain regions involved in any one task, be it language or vision or memory, somehow come together.
But what might we learn about the particular contribution of one brain region? Let’s look at an area toward the front of our brains, the prefrontal cortex. This area is twice the size it should be for a primate of our body weight. Could this contain the secret of our awesome mental abilities?
Again, as long ago as the mid-nineteenth century people realized that there was something special about the prefrontal cortex. This was demonstrated in 1848 in a most dramatic way by Phineas Gage, a railway worker in Vermont. One day, Gage was working to clear the path for a new railroad when the gunpowder he was using exploded prematurely. As a consequence, the bar with which he had been tamping down the explosive shot right through his prefrontal cortex. In effect, he had speared himself through the head. Surprisingly, Gage lost sight in one eye but otherwise appeared to be unaffected by this horrific accident. His movements and senses all were as before, and he actually went back to work. Only then did his workmates start to notice a difference.
Gage was not badly affected in how he walked, pronounced words, ate, or did other normal human activities, but a far more subtle change had occurred. He had become very unpleasant and antisocial, cursing in inappropriate situations. So could the prefrontal cortex be the brain’s center for character (or good character)?
In fact, in the decades since Phineas Gage had his accident, scientists have studied many other patients suffering damage to the prefrontal cortex. More recently, they have observed the region’s activity in healthy people. The prefrontal cortex has now been implicated in a welter of seemingly disparate functions, ranging from “forward planning” to “working memory.” In people suffering from clinical depression this area appears overactive, and in those with schizophrenia it can be underactive. There is clearly no single common, easily identifiable theme to these findings. So where does that leave us in our effort to localize functions within the brain, to match up different functions with different structures?
The emerging picture is certainly not a brain composed of autonomous minibrains. Rather, every function is divided among many brain regions, and every brain region participates in the many functions that make up the human behavioral repertoire. We can say that certain regions of the brain are more active than others when it comes to certain functions, but we can’t say those functions are confined to particular areas. And we are still a long way from knowing how to assemble the different structures of the brain to make up a human mind.
Studying the Cells—The Bottom-Up Approach
We can also think about the brain in the opposite way, or “bottom up.” This alternative approach involves starting with the brain’s most basic components and then figuring out how they connect with each other.
The most basic working unit of the brain is a special type of cell, the neuron. You have approximately 100 billion neurons—as many trees as there are in the Amazon rain forest. But neurons are not the only cells in the brain: they are actually outnumbered ten to one by another type, glial cells. These cells maintain a healthy and nurturing microenvironment within our heads for the neurons to operate at their best.
So what do neurons actually do? Since the 1920s, neurologists have known that neurons generate minute electrical signals. Each neuron alive in your brain at this moment is producing a tiny voltage, a potential difference between the charge inside the cell and the charge outside. Under certain conditions, such as when a signal comes in from a neighboring cell, tiny channels open in the wall of the neuron so that there is a sudden, brief interchange of ions (atoms with an electrical charge, in particular sodium and potassium). This ion interchange causes a temporary shift in the neuron’s charge—an electrical blip called an action potential.
Action potentials last for only about one thousandth of a second. Yet a neuron will typically fire off a hundred or so every second. This traffic in charges represents the “moving parts” of the brain, the actions that make it work. Ultimately all that we are—all our memories, hopes, and feelings—can be boiled down to the banal transfer of a few ions across the membrane wall of our brain cells.
Using the right sensors, we can read those electrical signals through the bone of the skull; the result is the valuable diagnostic tool called the electroencephalogram. Over the last few decades, furthermore, technology has enabled neurophysiologists to record the activity of a single neuron. Therefore, we have a fairly good idea of what makes those little building blocks of our brains work.
What happens once a neuron has generated an action potential? This tiny blip, some eighty thousandths of a volt in amplitude, buzzes away at speeds up to 250 miles per hour along the biological equivalent of a wire: an axon. But unlike any household electrical circuit, the brain isn’t wired so that all neurons form one single continuous network. Instead, in most cases, there is a gap between the axon of one neuron and the next neuron. This gap is called a synapse. It is as impossible for the action potential to cross a synapse as it is for a car to screech down a road and then float across a river. This might seem to be a cumbersome weakness in our wiring, but it is actually a powerful advantage.
The brain has an alternative way to send a signal across the fluid-filled gap. When the electrical impulse invades the end of the axon, it triggers the release of a chemical that can spread across the synapse and activate the target neuron. This chemical, because it transmits a signal, is known as a transmitter (or a “neurotransmitter,” if we want to make absolutely clear that it is at work in the brain). Once the transmitter hits the target cell, it enters into a kind of molecular handshake with a custom-made protein, a receptor, on the outside of that cell. This molecular handshake then causes the opening of the tiny channels into that neuron so that ions can cross over, once again generating the electrical signal. The brain therefore is not like a computer or any other electrical device, because it operates by means of a cascade of alternating electrical and chemical events.
Furthermore, there are many different transmitters in the brain, each with several different subtypes of receptors. So unlike a standard electronic circuit within a computer, which can only be on or off, the brain has a powerful spectrum of functions. Different chemicals will trigger different states within the brain. To appreciate just how important chemical signaling is to brain function, and hence to our mental abilities, let’s take a look at drugs.
All drugs that modify moods and feelings, whether prescribed or proscribed, do so by changing the availability or the efficacy of different transmitters within the brain. For example, some 30 years ago scientists discovered that the drug morphine worked by imitating a naturally occurring neurotransmitter called enkephalin (literally, “in the head”).
But that does not mean that it is natural or safe to take the most abused derivative of morphine, heroin. Enkephalin is released in minute amounts as and when it’s needed in the brain; then, even more important, it is disposed of very rapidly. Not so with heroin. First, it is not released in a small quantity exactly where it is needed; a heroin user effectively marinates his or her whole brain, setting the drug free to act wherever there are appropriate receptors. Second, when heroin does encounter a receptor and enters into a molecular handshake, the drug can’t be removed as readily as its natural counterpart. Because heroin is a different chemical, it will remain stubbornly in place. The result is like a handshake with an excessively strong grip. And just as the hand being gripped quickly starts to turn numb, the brain’s special receptors become less sensitive. The heroin user needs increasing amounts of the drug to obtain the same effect, one sign of an addiction.
The powerful effects of drugs on the brain surely demonstrate the importance of transmitters and, above all, of the connections over which they operate. Even the awesome number of neurons in our brains is dwarfed by the number of connections between them. There can be as many as 10,000 inputs to any one neuron. One estimate has it that counting each connection in your cortex alone, one a second around the clock, would take you 32 million years!
Making Connections—The Dynamic Approach
Looking at the connections our brain cells forge is a sort of middle approach, halfway between studying large brain regions and examining single cells. It is this aspect of the brain that will most likely allow us to discover what it is about these squishy organs that makes humans such an intelligent species, and makes each one of us unique.
As you’ll see in more detail in other parts of this reference, we are born with pretty much all the neurons we will ever have. (In fact, many brain cells die off during childhood.) But the marvelous feature of being human is that many of the connections among those neurons are laid down after we are born. This forging of connections in the most basic and broadest sense underpins what we refer to as learning. We have highly adaptable brains that reflect and benefit from our experiences. In contrast, simpler organisms like bugs operate at the dictates of their genes, following preprogrammed instincts.
We call the adaptability of our human brains plasticity. Our brains reflect each new experience. As a consequence, we become individuals. Everyone undergoes different experiences, and everyone’s brain develops differently. Of course genes play an important part in constructing the molecular machinery at work on each side of your synapses. But there are about 1 billion more connections in your brain than genes in your chromosomes; it is impossible for each connection to be programmed by a gene. Instead, the connections are shaped by your experiences.
The basis of this adaptability is the growth of connections between cells, strengthened and promoted by the activation of the relevant neurons. An axon coming from one neuron makes contact with the next neuron along the circuit by means of what are called dendrites on the receiving neuron. The more dendrites a neuron has, the more connections it will be able to make and the greater the circuitry underpinning a particular process or function. Just as a muscle grows with appropriate exercise, so selective circuits in the brain branch out and expand as they are worked.
We can see this change even at the level of a single neuron. In one study with adult rats, half were housed in humane but isolated cages while the other half were housed collectively and exposed to interactive objects, such as ladders. The neurons from the group in the temporarily “enriched” environment showed more dendrites emanating from a single cell than those in the nonenriched group.
The more sophisticated a species, the longer it takes for an individual to grow to adulthood. We humans are the most sophisticated animals of all, so we take many years to develop. Our brains need that much time to collect and store the experiences that shape our minds. Childhood is usually a time of exploration, of making the mental connections that go along with the growing connections among our brain cells. That is why an individual’s circumstances in youth help mold that person’s personality, skills, and other qualities. (See more on brain development in childhood and adolescence.)
But learning doesn’t have to stop in childhood. The plasticity of our brains means that they can usually adapt to further challenges. A recent study showed that London taxi drivers, who have to memorize all the street names and routes of that huge city, have a larger part of the brain relating to memory than do other adults of a similar age.
Another striking example of our brain’s ability to learn entails not a lifetime at a profession but merely five days spent practicing a piano exercise—this study showed that over such a short time the brain territory allocated to the fingers became enhanced. Even more amazing, mere mental practice has a similar effect on the brain.
But is the brain’s power to learn limited as we get older? We have already briefly explored the physical basis of “blowing the mind” with drugs. Sadly, old age can bring the horror of “losing one’s mind” because of degenerative diseases like Alzheimer’s. In this disorder, still not fully understood, the connections that a person’s brain has so painstakingly accumulated throughout life gradually become dismantled: increasingly, everything around the person comes to “mean” less.
Imaging techniques have now revealed that certain brain regions in Alzheimer’s patients shrink far faster than in healthy individuals of a similar age. This finding has an encouraging implication: the symptoms of senile dementia that come with this disease are not a natural consequence of aging but are due to some special factor or factors that are as yet a matter of conjecture.
In fact, healthy older brains retain their plasticity. We can see this in the often remarkable recoveries of people who have had strokes. Parts of their brains suffered severe damage, having been deprived of blood and oxygen for significant periods. Nevertheless, many of these people are able to offset the damage and regain functions they initially lost. Their brains create new neural pathways, or start to use old ones, to bypass the damaged areas. Once again, the brain responds to experience by creating new connections and new functionality. (See more about the brains of older adults.) In the near future the brain sciences will be shedding more light on these mechanisms of plasticity, as well as giving us insight into the specific losses that characterize Alzheimer’s disease and other disorders.
Because of plasticity, as we go through life, our brains become increasingly personalized. Everything we encounter will be interpreted in the light of all that we have seen before. It is this personalization of the brain that gives rise to the mind. Viewed in this way, the mind is not some airy, whimsical alternative to the physical brain, but the aspect of it that makes each of us unique.
The Director of the Royal Institution of Great Britain, Dr. Susan Greenfield, has a memory that captures nicely the mystique of the physical human brain and its relationship to mind. Here is how she tells it:
“Once upon a time, over 25 years ago, I was undertaking a dissection of the human brain as part of a college class. Each pair of us students had our own plastic bucket, containing in preservative liquid the organ that had once defined a unique person. I stared down at this odd object in my fingers, resembling two compacted giant walnuts with a smaller walnut on the back. A macabre thought struck me: What if I weren’t wearing protective gloves and got a piece of this brain stuff stuck under my fingernail? Would that be a thought or a memory, a habit or a feeling? Exactly what part of the individual would be nestling on top of my finger?”
Evocative, fundamental questions remain to tantalize. We are at an exciting time, when we no longer need to think about the brain as a collection of static anatomical regions, nor as a mere mass of generic cells and chemicals. We can now peek into the brain and see it shaping and reshaping every moment of our lives. It is truly the most dynamic and the most personal part of our bodies.
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