The Brain

by Gary L Wenk

  Meanwhile, back in the synapse, after interacting with the receptor, the actions of the neurotransmitter must be terminated by means of its reabsorption back into the neuron that originally released it. This vacuuming-up process is called reuptake. Alternatively, the neurotransmitter also might be acted upon by local enzymes and converted into a chemical that can no longer interact with your brain. Once the neurotransmitter is inactivated, it is removed from the brain into the bloodstream. Such byproducts of the ordinary hustle and bustle of the brain can be monitored easily in many of our body fluids, and this information can be used to determine whether our brains are functioning normally. One thing that does not happen: Neurotransmitters produced in the brain do not leave the brain intact. They either are metabolized or their escape is blocked by the blood–brain barrier. This blockade is crucial because if these neurotransmitters escaped from the brain they would change, possibly with lethal or unpleasant consequences.

  In summary, at the level of individual neurons communicating with other neurons, the process is both electrical, via the passage of an electrical disturbance called an action potential traveling down the axon, and chemical, via the release of a neurotransmitter onto the next neuron. Imagine that when your phone rings the electrical signal traveling via the telephone lines outside your house that brought the call into your telephone is similar to an action potential traveling down an axon. Someone has sent you a signal. Now imagine that you pick up the telephone receiver, you hold it to your ear, and the phone spits some chemicals into your ear. Your ear is the receptor for the chemical. This is how the brain works at the level of one neuron communicating with another neuron. Everywhere in your brain, one neuron is being electrically induced to spit chemicals at its neighbor. The chemicals released are obtained from your diet; this offers some insight into how important your diet is to normal brain function. This topic will be discussed in greater detail later. Now that you are familiar with the individual components of the brain, neurons and glia, and the chemicals that they use to communicate with each other, let us put it all together. Now would be a good time to examine closely the drawing of the brain included at the front of this book.

  How is my brain organized?

  During the past few decades, with the introduction of noninvasive techniques to examine brain function, neuroscientists have resurrected, rather inadvertently, a description of brain function that resembles a discredited idea from 1796. In that year a German physician, Franz Gall, developed an approach to understanding the brain by focusing on measurements of the human skull based on the concept that certain brain areas, or modules, have specific localized functions. The idea was intuitively attractive and became quite popular; in addition, the approach rather crudely described how the brain actually functioned. Today, over 200 years later, thanks to the invention of some highly sophisticated and expensive scanning machines, we have returned to the concept of a compartmentalized brain. Some parts of the brain clearly are dedicated to specific functions, such as vision, hearing, or touch; thus, the idea of modules is not entirely erroneous. A better analogy, however, is to view the brain as an orchestra that requires many unique instruments (brain regions) to work together simultaneously to produce a complex pattern of activity leading to the emergence of a beautiful piece of music (e.g., a perception of a blue bird flying across one’s field of vision). Please keep this concept of an orchestra in mind as we now introduce the functions of various brain regions.

  What happens in the front half of my brain?

  Just behind each eye, in the front half of your brain, are the frontal lobes. Let us begin this discussion with the frontal lobes because they control such fascinating behaviors. Much of what is known about the function of these frontal lobes was learned by examining the behavior of people who suffered traumatic brain injury, stroke, cancer, or infection within that area of the brain. During the past few decades the use of noninvasive technologies, such as the magnetic resonance imaging (MRI) scanner, has provided the opportunity to monitor the activity of the frontal lobes in conscious humans performing specific tasks. This is what scientists have learned.

  The frontal lobes allow you to make decisions, plan your actions, organize your thoughts about specific goals, and control your behavior around others. You inherit numerous personality traits from your parents, and these traits are controlled by selected regions within the frontal lobes. It is probably not too surprising to learn that variances in the size of some regions of your frontal lobes correlate with specific personality features. For example, introspection is correlated with the size of a region that lies on the top part of the frontal lobes called the prefrontal cortex. If you move your focus to a region just lateral to the part of the frontal lobe (just behind your eyebrows) responsible for introspection, you will find a region that becomes active when we perform complex behaviors such as paying attention or lying.

  Lying is apparently a complex task that requires considerable attentional abilities and significant participation by this frontal brain region. The demand for such energetically costly cortical activation might explain why most of us are not very good liars; we may simply lack the piece of cortex necessary to pull it off successfully. Some individuals are born with cortical proclivities that allow them to be very good liars and also allow them to be successful in certain publicly supported professions.

  If you now move your focus to the most lateral aspect of the frontal lobes, just inside of your temples and behind your eyes, you will find a region, the inferior frontal cortex, that is responsible for controlling low-risk behaviors. This region is in constant communication, and competition, with a region of the brain called the nucleus accumbens that induces you to participate in high-risk behaviors. These two regions might be seen as competing for control when you are trying to decide whether to have another piece of chocolate cake: the inferior frontal cortex is saying “No, you do not need another piece, it will just make you fat,” while the nucleus accumbens is saying “Eat it! It will taste so good.” Once your frontal lobes have made a decision, there is only one thing they can do—instruct some muscles to contract and move a part of your body. In spite of the complexity of our frontal lobes and the sophistication of their neural processes, our brain has a limited number of options—it can contract a muscle to move a limb or finger to pick up that second piece of cake. That is about all they can do in response to our wonderfully complex thoughts—move you from here to there, and back again.

  You live in the frontal lobe of your brain. Yes, the you who is thinking about the meaning of the last sentence; the you who is feeling hot or cold right now, or hungry, or angry, or absolutely anything at all—that you. This particular region of cortex is part of a small circuit of brain systems that turn on as soon as you wake up in the morning. Think of that very familiar feeling you have immediately upon awakening that you know who you are and where you are in relation to familiar objects and other people. The frontal lobes are also one of the brain regions that become active when your mind is wandering or when you are simply recalling episodes from your past or contemplating actions in the future. Thanks to recent studies using sophisticated scanning machines, we now understand that the region of cortex on the medial (deep inside between both hemispheres) surface of the frontal lobes is responsible for allowing us to have a “Theory of Mind,” or the ability to recognize that other people have thoughts and feelings that are independent of the you that lives just behind your eyes.

  How does my brain produce speech?

  Now that you have generated some thoughts, it is time to tell your friends what you are thinking. Speaking begins with the frontal lobes. Neurons in the frontal lobes control all of your muscles, including the muscles of the mouth necessary for speech. On the left side of your frontal lobe is a brain region, called Broca’s area, which controls the actions of the muscles in your face and throat that allow you to produce sounds that have meaning to the people listening to you, that is, to produce speech. This small brain region wa
s named after the French physician Paul Broca (1824–1880), who first identified a patient with damage to this part of the brain. Unfortunately, Broca’s area lies in a region of the brain that is highly vulnerable to strokes. When Broca’s area is damaged, speech becomes difficult, incoherent, or completely nonfluent. In contrast, people with damage to Broca’s area can understand what is being said to them; this is because the part of the brain that is responsible for understanding language is in the back half of the brain. People with damage to Broca’s area cannot generate normal speech. Speech is a complicated process that is not entirely understood. Neuroscientists have discovered that people who are deaf and use sign language to communicate also are impaired by damage to Broca’s area. Overall, the job of the frontal lobes is to decide what you wish to say and then plan the actions of the muscles in the throat and mouth, with the help of neurons in Broca’s area, to produce the sounds that can be interpreted by another person as something meaningful, that is, as speech. An important general rule for the brain is that the amount of cortex devoted to a particular function is related to the complexity of the function. Broca’s area is, therefore, quite large in humans because speech is a complicated thing to produce. Obviously, because the size of the cortex is finite, there are limits to how much processing the cortex can simultaneously perform. The generation of speech is so demanding for our frontal lobes that talking to someone while driving draws mental resources away from driving and leads to a noticeable deterioration in driving performance. Similarly, focusing our attention on visual imagery reduces our ability to detect sounds.

  What happens in the back half of my brain?

  The remaining parts of the cortex are devoted to receiving, interpreting, and storing sensory information received from the eyes, ears, nose, tongue, skin, muscles, and joints. Let us begin with your eyes. How do you see? The occipital cortex lies at the back of your head. It is dedicated to processing information coming from the eyes. Both eyes send information to both halves, or hemispheres, of your brain. The occipital cortex “sees” lines; that is all, just lines. Amazingly, that is enough. After all, every object in your field of view is essentially lots of very short lines connected together to make letters, faces, and shapes. Within the occipital lobe, the world “looks” like a 1950s-era television show; that is, it is in black and white. At this level, the information on color that was provided by the retina is still being processed separately from the information about lines that make up shapes and faces. Currently, no one knows with certainty where in the brain the color of an object and its shape are merged to produce the typical visual image of the world around us.

  The occipital lobe, along with some nearby regions, is responsible for some truly amazing visual gymnastics to make sure that your visual world behaves as you expect it to behave. For example, your eyes are constantly moving as you survey your environment. Take a moment and watch someone else’s eyes as they dart back and forth: This is called saccadic eye movement. The other person is not even aware that his eyes are doing this; of course, your eyes were doing the same thing while you were looking at him. A saccadic eye movement shifts the image of the object you are looking at from one part of the retina to another. As you read this sentence, your eyes are bouncing around very rapidly. So, why doesn’t your visual world appear to bounce around as quickly as your eyes are jumping? How does the brain manage to make the world appear so smooth and quiet? The solution is quite simple: The brain does not pay attention to any signals coming from the eyes when they are making these fast saccadic movements. During each saccadic jump, your brain stops processing visual images for just an instant and makes you believe that nothing happened. Your brain preserves your personal instant-by-instant visual representation by merging information obtained before and after each saccade. By doing this, the brain fills in your visual reality with a continuation of what you were seeing just before the visual image on your retina jumped. This sleight of hand by the brain gives us all the impression that the world is a smooth, flawless, and continuous flow of images. This deception is identical to what happens when you are watching a movie. You are aware that the movie is a sequence of photographic stills being flashed quickly up on to the screen in front of you, and yet you experience the movie as a smooth flow of images. Saccadic eye movements are occurring constantly; indeed, they occur so often that you are essentially not “seeing” anything at all for 60% to 70% of the time! Yes, two thirds of the time while you are awake and your eyes are wide open, you are effectively blind; you just never notice it. Your brain is incredibly good at fooling your mind into thinking that nothing happened and that the world is an uninterrupted, endless flow of images.

  In contrast, you are aware that you are not seeing anything for a small fraction of a second when you blink. Blinks last longer than saccades; thus, the brain has time to take advantage of this short pause of incoming electrical signals and activate a unique pattern of brain regions called the “default network.” These same brain regions also become engaged when you are by yourself, undisturbed and bored, and your mind begins to wander. It is hard to believe that during the time it takes you to blink, your brain attempts to go off-line and daydream, but it does. Sometimes your mind wanders for much longer periods of time.

  Why does your mind wander? Why does your brain find it so easy to reconsider distracting thoughts from the day or to speculate on potential conversations you might have tomorrow? Why can’t your brain just lie still and be quiet? Anyone who has ever tried meditation has discovered how incredibly difficult it is to quiet the mind—it takes lots of practice to be successful. Simply stated, your brain did not evolve in a world that rewarded you for being completely still and without thought. Any organism with a brain that completely disengages itself regularly would soon find itself being digested by a bigger organism not distracted by self-reflection. I could be wrong, but I doubt that Tyrannosaurus rex was a very pensive creature.

  Why do I daydream?

  Surprisingly, the answer has everything to do with why humans enjoy ingesting coffee and cocaine. Brains really like stimulation—thoughts or drugs, it really does not seem to matter. When you do not provide your brain with input from the external world, such as TV, music, or exploring social media sites, your brain actively disengages and starts to produce its own stimulation, namely, daydreaming. So when you are sitting through a boring lecture or listening for the hundredth time to your uncle tell that story about the big fish that got away, your mind has a tendency, sometimes an urgency, to go offline and entertain itself with other thoughts that it finds more interesting, such as “What will you have for dinner tonight?” or “When did your sister grow that mustache?” Neuroscientists now have identified the brain regions that selectively turn on, as well as those that turn off, when people are daydreaming; these include regions in the frontal and parietal lobes. These studies also have determined that daydreaming is important for normal brain function; daydreaming helps you to sort out important thoughts and discard nonsense and worries. Recently, scientists have estimated that while you are awake you spend approximately 60% to 70% of the time daydreaming! So, overall, most of the time that we are awake during the day we are either blind (due to saccadic eye movements) or completely offline (daydreaming). It is amazing that we ever get anything accomplished.

  Why does the brain devote so much time to daydreaming?

  Your brain evolved in a sensory rich world and rewards you for exposing it to ever more complex sensory experiences. Every time you experience something new, your brain releases a jolt of dopamine in the frontal lobe; dopamine is the major reward-related neurotransmitter in the brain. You can stimulate the release of dopamine in your brain artificially by ingesting coffee and cocaine or you can turn on the TV, listen to music, have sexual relations, or simply communicate with someone else important to you. The brain has evolved a system that rewards you for obtaining new information and having lots of thoughts because doing so might have survival value. The more you know, the mo
re likely you are to survive and pass on your inquisitive genes to the next generation. Thus, you are burdened, or possibly blessed, with a brain that demands constant entertainment, via its own thoughts or exogenous chemicals, and that powerfully rewards you, by releasing dopamine, for providing it.

  What is the function of the temporal lobe?

  Moving your attention forward, the parts of your brain that lie directly underneath your ears are the left and right temporal lobes. As discussed in the first chapter, different parts of the temporal lobe are devoted to processing memories, the recognition of objects such as chairs and faces, and responding to these memories and objects with emotion. Damage to the bottom half of the temporal lobe can prevent someone from recognizing familiar faces and objects. For these unlucky people everything appears to be unfamiliar, even though they are certain the situation is familiar. This is called jamais vu from French meaning “never seen.” The condition often involves a sense of eeriness; the patient feels as though she is seeing the situation for the first time even though she has been in the situation before.