We’ll begin our tour of the brain by examining the big picture: the brain’s overall structure and major subparts. Next, we’ll “zoom in” a bit to look at the communications networks that connect these subparts to one another. We then will zoom in even more, to examine the brain’s individual cells, or neurons, and how they communicate with one another.

Scholars studying the brain have long noted that its overall structure consists of three main parts. The first such brain model is more than 2000 years old, yet looks remarkably modern.

Aristotle, the greatest scientist of ancient Athens, suggested that the mind and brain have a three-part structure, with some parts being conceptually “high-level” structures that contribute to distinctly human thinking processes. In Aristotle’s model, the lowest level is a vegetative mind responsible for growth and reproduction. One level up is an animal mind responsible for feelings of pleasure and pain. Finally, on top, a rational mind enables people to engage in logical thought (Aristotle, trans. 2010).

When formulating his model, Aristotle completely lacked contemporary scientific knowledge of brain structures. Nonetheless, his work has a contemporary ring to it. The twentieth century’s most renowned conceptual model of the brain, proposed by the neuroscientist Paul MacLean (1990), similarly identifies a three-part, bottom-to-top organization of brain structures.

MacLean’s model is known as the triune brain. “Triune” means “three in one.” The triune brain model, then, suggests that the overall human brain consists of three main parts, each of which is a distinct functioning brain that carries out its own unique activities. You have, in essence, three brains in one. As in Aristotle’s model, some parts of the brain are more advanced, and at a higher level, than others.

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MacLean contended that the three levels of brain emerged at different points in the evolution of Earth’s species (Figure 3.3):

According to MacLean, then, you possess not only the unique brain systems of a modern human, but also, tucked underneath, the brain of a nonhuman mammal and, beneath that, the brain of a reptile.

At first, “three brains in one” sounds weird. But consider the following three facts:

MacLean’s triune brain model explains that these different experiences reflect the actions of different levels of brain. When you put experiences into words, you are using your neomammalian brain. Emotions, by contrast, are produced by the paleomammalian brain. It is not capable of producing language (so nonhuman animals aren’t speaking to one another), which helps to explain why the emotions it produces sometimes can’t easily be put into words. Finally, the reptilian brain executes simple functions like regulating body temperature and breathing. By itself, this brain system cannot produce consciously experienced feelings, so you don’t feel your brain at work on the task of regulating internal physiological states. The three parts of the triune brain are connected to one another, and their activities thus can be coordinated. Nonetheless, their functions are distinct.

MacLean first proposed the triune brain model in 1969 (Newman & Harris, 2009). Although enormous scientific advances have been made since that time, MacLean’s three-part, bottom-to-top organization remains a valuable overview of brain structures. So let’s now look at these “three brains” in detail. In doing this, we’ll use their standard contemporary biological names: (1) the brain stem (MacLean’s reptilian brain), (2) the limbic system (the paleomammalian brain), and (3) the cerebrum (the neomammalian brain) and its outermost layer, the cerebral cortex.

BRAIN STEM. The lowest region of the brain is the brain stem, which sits at the top of the spinal cord. Three main structures of the brain stem—the medulla, the pons, and the midbrain—regulate bodily activities critical to survival (Figure 3.4).

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The medulla plays a major role in homeostasis, which is the body’s maintenance of a stable, consistent inner physical state. It contributes to homeostasis by regulating rates of physiological activity, such as heart rate and blood pressure. The medulla also is the key pathway from the brain to the rest of the body; communications between the spinal cord and higher regions of the brain run through the medulla. In addition, the medulla controls the “gag reflex,” the contraction of the throat that prevents choking (Urban & Caplan, 2011).

The pons is a region of the brain stem located just above the medulla. It performs a number of functions and contains structures that control your rate of breathing. It also generates a distinctive stage of sleeping (REM sleep; see Chapter 9) in which the brain is highly active, generating dreams, but the body is essentially paralyzed. Studies show that if the pons is damaged, an animal, while asleep and dreaming, will move around and attack imaginary prey (Siegel, 2005). The pons also functions as a relay station, conveying signals among other brain regions (which you’ll learn about below).

The third main brain stem structure is the midbrain, a small yet complex structure that contributes to an organism’s survival in a number of ways. One region of the midbrain protects the organism by generating defensive reactions to threatening events. Evidence of this comes from studies in which researchers activate the key region of the midbrain artificially. When the midbrain is activated, animals display defensive responses even when no threat is present. For example, when researchers stimulated the midbrain of a rat, it became highly alert, its heart rate increased, and it began to flee—even though there was nothing in the environment to flee from (Brandaõ et al., 2005).

In addition to these three main structures (the medulla, pons, and midbrain), the brain stem houses a network of brain cells that you’re relying on right now to keep you awake and alert as you read this chapter. This network is the reticular formation, a brain system that influences an organism’s overall level of arousal (Gupta et al., 2010). This function of the reticular formation was discovered in studies with cats (Moruzzi & Magoun, 1949). Researchers stimulated the reticular formation, a low-level brain system, while recording activity in the cortex (discussed below), a high-level brain region. When they stimulated the reticular formation, activity in the cortex increased. The reticular formation, then, regulated the arousal level of other parts of the brain. Damage to the reticular formation can cause a coma, a state in which a person is alive but motionless, unaware of events, and unable to be awakened (Gupta et al., 2010).

Before we move up from the brain’s lowest-level region, the brain stem, we note a significant brain structure located just behind it. The cerebellum, which looks like a miniature brain tucked under the back of the brain, regulates motor movement. With a damaged cerebellum, you would still be able to move your body, but those movements wouldn’t be as coordinated and precise. Your posture might be altered and your stride less smooth, and you wouldn’t be able to accurately perform tasks such as tracing an image on a piece of paper (Daum et al., 1993). The cerebellum is also needed to perform a task that a doctor may have asked you to do during a physical exam: Close your eyes and quickly touch your index finger to your nose (Ito, 2002). The doctor’s test assesses your cerebellar functioning.

Controlling motor movements is not the cerebellum’s only job. Like most parts of the brain, the cerebellum is connected to many other brain regions. Thanks to these connections, the cerebellum also is active in the control of emotion and thinking, including the accurate perception of passages of time (Strick, Dum, & Fiez, 2009). Compared to others, people with cerebellar damage are less accurate in perceiving small variations in time intervals and in tapping out timed musical rhythms (Ivry & Keele, 1989).

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LIMBIC SYSTEM. If all you had were a brain stem, you wouldn’t have much psychological life. The brain stem would maintain your basic bodily functions, but you would lack feelings and emotions. For this, you need more brain.

Fortunately, evolution has provided you with more brain. All mammals possess a limbic system, which is a set of brain structures that resides above the brain stem but below higher brain regions (Figure 3.5). The limbic system enables mammals to have emotional lives.

MacLean first recognized that these different structures, located in different parts of the brain, should be thought of as a “system,” that is, as a set of parts that work together (Newman & Harris, 2009). Later work confirmed MacLean’s intuition; research shows that the limbic system’s different parts are highly interconnected. Mammals thus possess an interconnected system of brain structures that substantially expands their mental abilities, as compared to evolutionarily older organisms (Reep, Finlay, & Darlington, 2007). Let’s look at the most significant structures in this system.

The hypothalamus is a limbic system structure that is small yet critical to survival. It plays a key role in maintaining internal bodily states, such as body temperature. The hypothalamus also triggers behaviors that have been important throughout evolution, such as eating, drinking, and sexual response (King, 2006). The hypothalamus can perform these tasks thanks, in part, to its connections to the nearby pituitary gland, which is part of the body’s endocrine system (discussed later in this chapter). The hypothalamus sends signals directly to the pituitary, which in turn communicates to the rest of the body. The hypothalamus is located just underneath the thalamus, a brain structure we’ll revisit later. (Hypo means “under,” so the name hypothalamus indicates its location.)

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Startling research results obtained in the 1950s showed that the hypothalamus also is key to motivation (Olds, 1958; Olds & Milner, 1954). Researchers surgically implanted electrodes directly into the hypothalamus of rats. They connected the electrode to a lever that the rat could press, thereby delivering current to the electrode. By pressing the lever, then, the rat could stimulate its own hypothalamus (Figure 3.6). Would a rat really want to do that? Sure it would! Rats pressed the lever more than 5000 times an hour (Olds, 1958). In later research, rats consistently chose the stimulation even over food, starving themselves to death (Bozarth, 1994).

The brain stimulation was rewarding because the hypothalamus is part of a reward circuit, that is, a set of interconnected brain structures that normally becomes active when an organism pursues a rewarding stimulus or experience, such as food or sex (Haber & Knutson, 2010). Amazingly, even when there is no rewarding stimulus in the environment, activation of the circuit is highly rewarding (see Chapter 7). Although the anatomical details differ across species, contemporary neuroimaging research shows that a reward circuit similar to the one first identified in rats also exists in primates, including humans (Haber & Knutson, 2010).

The hippocampus is a curved, roughly banana-shaped structure in the limbic system that participates in two major tasks of everyday life. One of those tasks is remembering. The creation of permanent memories is carried out, in part, in the hippocampus (Bliss & Collingridge, 1993; Nadel & Moscovitch, 1997). If this biological tool is damaged, permanent memories of experiences cannot be formed (also see Chapter 6). Evidence of the hippocampus’s role in memory comes from cases of Alzheimer’s disease, a medical condition that generally strikes people in older adulthood. The hippocampus atrophies (i.e., becomes reduced in size) in Alzheimer’s patients (Barnes et al., 2009), who lose their normal ability to remember events.

A second task the hippocampus helps to accomplish is spatial memory, the recall of geographic layouts and the location of items within them (Nadel, 1991). Have you ever parked your car in a large parking lot (e.g., at a shopping center or sports stadium) and later wandered around trying to find it? It’s good you didn’t leave your hippocampus in the car—you need it to remember where you parked. Evidence that the hippocampus contributes to spatial memory comes from research on a group of people who rely heavily on spatial skills: taxi drivers in London (Woollett, Spiers, & Maguire, 2009).

Prior to obtaining their taxi driver’s license, London cabbies undergo extensive training to learn the location of thousands of city streets and how they interconnect. Researchers hypothesized that, as a result, cabbies’ brains would differ from those of ordinary drivers. To test that idea, they asked taxi drivers and a control group of regular drivers to play a virtual reality video game in which players navigate a car through London streets. While participants played, their brain activity was recorded using fMRI (see Research Toolkit). When researchers compared the brains of the two groups of participants, they found that the hippocampus of taxi drivers was more highly developed. Taxi drivers had a greater volume of brain cells in the rear region of their hippocampus (Woollett et al., 2009).

This finding should remind you of the lesson about plasticity from the beginning of this chapter. The brain has specialized parts that strengthen with use. Just as athletes who exercise their muscles develop more muscle, taxi drivers who exercise their hippocampus develop more hippocampus.

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The amygdala is a small structure shaped roughly like an almond (from which it gets its name: amygdala is the Greek word for “almond”). Like most brain structures, the amygdala is connected to numerous other structures in the brain; as a result, it is active during a variety of psychological processes (Labar, 2007). Yet one psychological process in which its role is particularly central is the detection of threat. Suppose you’re walking in the woods and notice a snake slithering on the ground (Figure 3.7). Before you know it—that is, before you even say to yourself, “Geez, look at that, a snake!”—your body responds: You instantly experience fear and move out of harm’s way. That quick response was generated by the amygdala, which receives inputs from your eyes and rapidly signals other brain mechanisms that, in turn, generate emotional arousal and halt your walking toward the snake (LeDoux, 1994). The amygdala response is so rapid that your body reacts before the rest of your brain has time to create a conscious experience of the snake.

The amygdala is not limited to the processing of information about threats that involve physical harm, however. Another type of threat that it processes is financial. In one study, two research participants with damaged amygdalas played a gambling game (De Martino, Camerer, & Adolphs, 2010). They chose whether to bet on coin flips with varying monetary payoffs. Most players are loss averse, that is, they avoid gambles that pose a good chance of losing (Kahneman & Tversky, 1982). For example, you would probably refuse if someone offered to flip a coin, saying, “Heads, you win $50; tails, you lose—you give me $50.” The thought of losing $50 is aversive, so you avoid the game. But if your amygdala were damaged, you might take the bet. The two participants with amygdala damage accepted bets that most other people avoided; specifically, they accepted bets that posed a significant chance of losing. They understood the gambling game and played it intelligently, but felt much less threatened by possible monetary loss (De Martino et al., 2010).

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Let’s conclude our discussion of the limbic system by noting three other structures. The fornix (Latin for “arch”) is a curving structure that provides a connection between the hippocampus and the hypothalamus, thus helping these structures of the brain to interact and work as a system. The olfactory bulbs are brain structures required for a sense of smell. They receive information from the nasal cavity and transmit it to other parts of the brain. The cingulate gyrus, which wraps around the top of the limbic system, contributes to people’s ability to stop themselves from doing one thing and switch to doing something else. For example, suppose you are asked to name the color of the letters in the following word: ORANGE. You have to stop yourself from saying “orange,” which springs to mind immediately, and switch to saying “blue.” Brain imaging studies show that a region of the cingulate gyrus is particularly active when people perform such switching tasks (Carter et al., 1998).

RESEARCH TOOLKIT

CEREBRAL CORTEX. The last step up in our bottom-to-top voyage through the brain takes us all the way to the top, to the cerebral cortex, a layer of cells on the outer, top surface of the brain that is only a few millimeters thick. Although thin, it is powerful. This network of brain cells is the biological tool that enables people to have human thinking powers: to contemplate ourselves, our past, and our future; to communicate using language; to create works of art; and to gain some control over the impulses and emotions generated in lower regions of our brains.

When looking at the outer surface of a brain—at its cerebral cortex—the first thing you notice is that it’s folded. The brain contains numerous ridges and grooves, and the cerebral cortex wraps its way around them. These folds in the brain’s outer surface have a big advantage: They allow room for more brain—or, more specifically, for more cortical surface area. Just as folding a large item of clothing helps fit it into a small piece of luggage, the folds in the brain’s surface allow a relatively large cerebral cortex to fit into your relatively small skull. (By way of comparison, you have a lot more brain power than an elephant, yet a much smaller skull.) The surface area of your cerebral cortex is about 2.5 square feet (Kolb & Whishaw, 1990), but thanks to the folds, your head does not have to be greater than 1.5 feet wide and 1.5 feet long to fit it all in there.

The next thing you would notice is that some of the grooves in the brain’s surface are particularly deep. These are fissures that divide the cortex into a number of distinct parts, known as cerebral lobes. Let’s now look at the brain’s various lobes and the psychological activities in which they take part (Figure 3.9). But first, a reminder. As you learned in this chapter’s opening, the parts of the brain are highly interconnected. When you perform almost any complex task, the brain’s communication networks automatically coordinate activity in multiple lobes of the brain (Sporns, 2011).

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Let’s start our survey of the lobes of the cerebral cortex at the back of the brain. At the brain’s rear is the occipital lobe. This region is heavily involved in the processing of visual information—in fact, it is commonly called the visual cortex. To see clearly, then, you need not only your eyes, but also your occipital lobe. Evidence of this comes from medical cases in which patients experience seizures (periods of abnormal brain activity) in their visual cortex. Although their eyes are working properly, their visual experience is distorted; they see things that aren’t there and, in some cases, experience temporary blindness (Panayiotopoulos, 1999).

In addition to processing visual information coming in from the eyes, the occipital lobe is active when you generate visual information in your head—in other words, when you engage in mental imagery (see Chapter 8). Researchers find that when people are asked to close their eyes and think of specific images, their visual cortex becomes highly active (Kosslyn et al., 1996). Furthermore, patients whose occipital cortex is damaged may have difficulty generating mental imagery (Farah, 1984). Thus, even though seeing and imagining are two different types of activity—one being a detection of stimuli out in the world, the other an act of fantasy in your head—both activities employ the same biological tool: the brain’s occipital lobe.

Moving from the occipital lobe toward the front and top of the brain, we next reach the parietal lobes. The parietal lobes contain brain matter needed for somatosensory information processing (Andersen et al., 1997), that is, the processing of information that relates features of your body (or soma) to features of the environment (which you detect through sensory systems). When do you use your somatosensory system? All the time! Imagine a simple activity: picking up a paper cup filled with water. This action seems so routine that you don’t even give it a thought. Yet it’s actually quite complex. To pick up the cup, there’s a lot of information to take into account: (1) the exact location of the cup, (2) the amount of force required to pick it up without crushing it, (3) the location of your arm and hand, and (4) the amount of force exerted by your hand when you grasp the cup. The first two pieces of information come into the brain from a sensory system—your visual system, which lets you see where the cup is and what it’s made of. The second two pieces of information come from your soma, or body—your nervous system provides your brain with information about the location and motions of your body. These two streams of information, sensory and somatic, have to be integrated somewhere. This happens in the parietal lobe (Andersen et al., 1997).

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At the front of the parietal cortex is the sensory cortex, a strip of brain matter that receives information from all parts of the body. By processing this information, the sensory cortex lets you know that your back is itchy, your foot’s “fallen asleep,” or your arm is extended straight up in the air (something you can feel, without having to look to see where it is).

Nature has given the sensory cortex a remarkably systematic design. The sensory cortex represents each of the various parts of the body; body parts are “mapped” into the cortex (Figure 3.10). Sensory input from your foot goes to one part of the cortex, input from your knee goes to another, and so forth. This body-to-brain mapping has two notable features:

Normally, the sensory cortex is activated by input from parts of the body. But what would happen if it was activated directly—that is, if scientists reached right into the brain and electrically stimulated it? Wilder Penfield, a physician performing surgery on a patient with epilepsy, was the first person to do this (Costandi, 2008; Penfield & Boldrey, 1937). During surgery, the patient was awake. This is possible because there are no sensory receptors in the brain itself, so patients do not feel pain during brain surgery. When Penfield applied a mild electric stimulus to one region of the sensory cortex, the patient felt numbness in his tongue. When a nearby region was stimulated, he felt numbness in a different part of his tongue. When a different region was stimulated, he felt a tingling in his knee (Figure 3.11). By stimulating various regions and noting the sensations produced, Penfield was able to discover the mapping between the brain’s sensory cortex and the rest of the body (Penfield & Boldrey, 1937).

Below the parietal lobes, its location and shape akin to a thumb on a mitten or baseball catcher’s glove, is the temporal lobe (see Figure 3.9). Two psychological tasks that rely on the temporal lobe are hearing and remembering. Hearing is accomplished thanks to a region in the upper surface of the temporal lobe known as the auditory cortex. This part of the temporal lobe is active whenever you listen to sounds, detecting their pitch, volume, and timing in relation to one another. Listening to both spoken words and music requires use of your auditory cortex (Zatorre, Belin, & Penhune, 2002). As for memory, certain regions in the temporal lobe are key to organizing the multiple brain systems that become active when you remember facts and experiences (McClelland & Rogers, 2003). Damage to the temporal lobe causes people to forget even the names of common objects. For example, one patient with temporal lobe damage, when shown pictures of 24 different animals, was able to name only three correctly: cat, dog, and horse (McClelland & Rogers, 2003).

Finally, moving forward from the parietal lobes, you reach the frontal lobes. In humans, as well as in our evolutionary cousins, the great apes, the frontal lobes are the largest region of the brain, comprising about 35% of its total volume (Semendeferi et al., 2002). Yet human frontal lobes have a number of unique features. These include especially rich interconnections among regions of the frontal lobes and similarly rich connections between the frontal lobes and other parts of the brain. These unique biological features underlie humans’ most distinctive mental abilities (Semendeferi et al., 2002; Wood & Grafman, 2003): to think about ourselves; to set goals for ourselves and stick to them; to control our emotions; to recognize ourselves as social beings who are evaluated by others; in short, to live as members of civilized society. As you saw in the case of Phineas Gage, frontal lobe damage reduces the ability to control one’s behavior according to the rules of society.

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The frontal lobes also contain brain matter that is needed to control body movements. At the rear of the frontal lobes, in the area closest to the parietal lobe, is a region of the brain known as the motor cortex. This cortical region sends out signals that move the body’s muscles; as we discuss in more detail below, the signals are sent to the spinal cord, where they are relayed to muscles in the body’s extremities.

Studies by Penfield and colleagues provided early evidence of the functions carried out by the motor cortex (just as they did for the sensory cortex, as described above). Penfield found that electrical stimulation in different areas of the motor cortex triggers different types of motor movement. If you stimulate one area of the motor cortex, the patient’s hand moves; stimulate another area, and an arm moves; and so forth (Penfield & Boldrey, 1937).

Penfield’s findings seemed to suggest that the motor cortex, like the sensory cortex, contains a “map” of body parts, with different parts each controlling a different muscle of the body. Recent research, however, has shown that this idea is inaccurate (Graziano, 2010). Individual parts of the motor cortex do not control individual muscles. Instead, a given part of the motor cortex commonly triggers coordinated activity in a number of different muscles. The activity generally is a meaningful action that, over the course of evolution, was adaptive for organisms of the given species. Stimulating different parts of a monkey’s motor cortex, for example, produces movements such as reaching out to grasp an object, placing the hand to the mouth, or climbing or leaping movements (Graziano et al., 2002; Figure 3.12).

In front of the motor cortex are regions of the frontal lobe containing association areas of the cerebral cortex. Association areas receive sensory input that has been processed by other regions of the brain. They connect these inputs to memories and stored knowledge of the world (Pandya & Seltzer, 1982; Schmitz & Johnson, 2007). These connections between sensory input and stored knowledge enable people to have experiences that are psychologically meaningful. For example, you perceive not just a slowly moving human figure with a hand extended, but an old acquaintance who appears ready to apologize for a past wrongdoing. Your sensory system delivers the sound of a human voice coming through a small speaker, but association areas enable you to hear the loving tones of a parent calling to see how you’re doing at college.

In front of the association areas is the prefrontal cortex. In terms of location, this is the part of the brain that resides immediately behind your forehead. The prefrontal cortex is a complex piece of biological machinery that contains many specialized subsections that contribute to a variety of mental functions. Two types of mental activities, however, stand out as particular specialties of the prefrontal cortex.

One is the ability to keep information in mind—to concentrate on facts, focus your attention, and manipulate information in your mind (Levy & Goldman-Rakic, 2000). The prefrontal cortex gives you a mental “workspace” (Dehaene & Naccache, 2001) where you can combine and manipulate information, whether it is coming in through your sensory systems or stored in memory. This ability enormously increases your mental powers. Without it, the flow of your thoughts would be determined almost entirely by stimuli in the environment. Every sight, smell, and sound would pull your thoughts in one direction or another. The prefrontal cortex, however, gives you the “executive” abilities (Posner & Rothbart, 2007) to concentrate your thoughts on people, objects, and events that are not present in your current environment. You can even think about things that happened long ago or that might happen in the distant future; the prefrontal cortex enables humans, unlike other species, to engage in “mental time travel” (Suddendorf & Corballis, 2007).

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The second function of the prefrontal cortex is the one lost by Phineas Gage: the ability to align your behavior with social rules and conventions. Why does frontal lobe damage impair this ability? The explanation involves both thoughts and emotions. Frontal lobe damage breaks the normal connections between (1) thoughts about the social world, which are generated in the cortex, and (2) feelings about the social world, which are generated in lower regions of the brain. As a result, people with frontal lobe damage may fail to experience emotions that normally keep our behavior in line with others’ expectations. These include feelings of embarrassment over not fitting in with a crowd and anxiety about the possibility that others will think poorly of them (Bechara, Damasio, & Damasio, 2000). Frontal lobe damage also reduces empathy (Stuss, 2011; Wood & Williams, 2008), which is the ability to personally feel the emotions experienced by others with whom you interact (Agosta, 2010). This lack of empathy may explain the behavior of psychopathic criminals, that is, people who commit violent criminal acts without experiencing guilt, remorse, or empathy for their victims (Shamay-Tsoory et al., 2010).

In summary, you’ve seen that the brain has a bottom-to-top organization. Low-level structures in the brain stem regulate bodily states and serve basic survival-related needs. Middle-level structures of the limbic system enable organisms to have feelings and to form memories. And the high-level structures of the cerebrum, especially the cerebral cortex, enable people to engage in complex, creative, rational thought.

This might already seem like a lot of organization for one bodily organ. Yet there’s more: The brain also has a left/right organization.

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You’ve got two legs, one on the left and one on the right. Same with your arms, hands, eyes, ears, kidneys, and many other body parts—you’ve got two of them, left and right.

CEREBRAL HEMISPHERES. The same is true, in a sense, for the brain. Although humans have only one brain, it has two parts, on the left and right, which are separated by a deep groove. The two sides are known as the brain’s two cerebral hemispheres (Figure 3.13).

The relation between the left/right organization of the brain and the left/right organization of the body is surprising. When going from body to brain, signals cross. Information from the left side of the body reaches the right side of the brain, and information from the body’s right side reaches the brain’s left. Similarly, commands sent out from the brain to the body switch sides. The right side of the motor cortex, for example, controls the left side of the body. Even signals from parts of the body located very close to the brain switch sides. Information from your left eye, for example, reaches the right side of your brain. Research suggests that this crossing benefits the complex “wiring” of the brain that occurs early in the development of an organism (Shinbrot & Young, 2008).

The left and right hemispheres are connected through the corpus callosum, a structure containing more than 200 million cells that transmit signals from one side of the brain to the other. The corpus callosum extends from the front to the rear of the brain and thus connects the left and right sides of the frontal, parietal, and occipital lobes. Thanks to these connections, your left and right generally are in synchrony. Your two-handed tennis backhand is a coordinated movement, despite the fact that different sides of your brain control your left and right arms. You experience a coherent stream of music from the sounds coming out of your headphones, even though different sides of your brain process sounds from the earphones on the left and right.

SPECIALIZATION OF THE HEMISPHERES. The left and right hemispheres of a person’s brain look quite similar, but “you can’t tell a brain by its cover.” Despite similar appearance, the hemispheres are specialized to perform different types of psychological activities. Some tasks primarily draw on the left side of the brain, whereas others primarily use the right.

The left hemisphere specializes in language. About 97% of right-handed individuals understand and produce language by using their left hemisphere; among left-handers, the percentage is still high, about 70% (Toga & Thompson, 2003). Two nineteenth-century physicians—Frenchman Paul Broca and German Karl Wernicke—discovered this in studies of people with impaired language ability, or aphasia.

Broca’s insight came from a patient who was nicknamed “Tan” because “tan” was the only word he could speak. After a disease in early adulthood, he experienced an aphasia in which he could understand other people when they spoke, but could not produce words himself—other than, mysteriously, “tan” (Schiller, 1992). After the patient died, Broca examined the man’s brain and found damage in a region of the left hemisphere that has become known as Broca’s area (Figure 3.14). Normal functioning in this area is required for a person to produce words.

Some years later, Wernicke saw patients who had lost the ability to understand language. They could hear the sound of words, but could not determine what those sounds meant (Geschwind, 1970). Again, post-mortem examinations revealed brain damage. But this time, the damage was found in a different area of the brain than was seen in Tan’s brain. This new area, which is needed for the comprehension of spoken language, is now known as Wernicke’s area (Figure 3.14).

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Both Broca’s and Wernicke’s areas are in the left hemisphere. Although later research (e.g., Sperry, 1982) showed that the right hemisphere has more involvement in language comprehension than Wernicke had thought, contemporary findings nonetheless confirm that, among the large majority of people, the predominant hemisphere in language production and understanding is the left (Josse & Tzourio-Mazoyer, 2004).

Language isn’t the left hemisphere’s only specialty. Another one is arithmetic. Images of people’s brain activity while they multiply numbers “in their head” reveal that the left hemisphere is significantly more active than the right (Chochon et al., 1999).

Language and arithmetic have a lot in common. In both, individual symbols (words, numbers) are combined in a specific order according to various rules (of grammar or arithmetic). If the symbols are out of order, the result is meaningless; neither “2 2 4 + =” nor “Dog her ran Jane spot after” make sense. Activities that require one to combine symbols or objects in a step-by-step manner, according to specified rules, are called analytical tasks. On analytical tasks, the left hemisphere predominates. Interestingly, the two analytical tasks we just discussed, language and arithmetic, are so similar that the regions of the left hemisphere active during the tasks overlap (Baldo & Dronkers, 2007).

What is not considered an analytical task? Spatial activities differ from analytical ones. In spatial thinking, you create images in your mind. Suppose you were asked how many windows were in the front of the home in which you grew up. You would first picture your home in your mind and then mentally count the windows you see. Mentally picturing the home is an example of spatial thinking.

Can you think of another mental activity that involves spatial thinking? You worked on one in this chapter’s Try This! experiment.

TRY THIS!

Our Try This! activity was a mental rotation experiment. In mental rotation, people form an image in their minds and then try to imagine what that image would look like from a different visual angle. In our experiment, you saw the letter R printed at odd angles and had to rotate it in your mind until it was upright (see Figure 3.15). Mental rotation tasks of this sort are a form of spatial thinking because they involve thinking about images rather than logical relations among words or numbers.

Mental rotation experiments yield two fascinating results. You experienced one of them for yourself: It takes longer to rotate an image through a larger visual angle than a smaller one. Rotating a mental image thus is somewhat like rotating a physical object; just as it takes longer to rotate an object 180 degrees than 90 degrees, it takes longer to rotate an image in your head 180 degrees than 90 degrees (also see Chapter 8).

The second result involves the brain. Findings indicate that spatial thinking such as mental rotation is a specialty of the right hemisphere. Evidence comes from a study in which brain images were taken while participants performed the same task you encountered in our Try This! activity. While people performed the task, the right hemisphere of their brains was most active (Milivojevic, Hamm, & Corballis, 2009a). A big advantage of the right hemisphere during mental rotation tasks is speed; regions in the right hemisphere become active more quickly than do left-hemisphere regions when people try to rotate mental images as fast as they can (Milivojevic, Hamm, & Corballis, 2009b).

SPLIT BRAIN SYNDROME. The brain’s two hemispheres, then, are like two teams of workers who specialize in different jobs. Thanks to the corpus callosum, they’re in constant communication, so their overall activities are coordinated.

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What if the lines of communication were broken? What would happen if the corpus callosum were cut? You wouldn’t have “two brains” functioning in ignorance of each other, would you?

Incredibly, you would. Without the corpus callosum, the left and right hemispheres do, in fact, function as if they are “two separate brains” (Sperry, 1961, p. 1749)—like two groups of workers laboring on their specialized tasks unaware of the other’s activities. Evidence of this comes from split brain experiments, which are studies of people or animals whose corpus callosum is cut, rendering it unable to transmit information between the hemispheres. Split brain research was pioneered by the neuroscientist Roger Sperry, who won a Nobel Prize in 1981 for his work.

Early research by Sperry and his students was done with cats. Like most animals, cats learn new responses in reaction to specific environmental stimuli (see Chapter 7). Sperry and colleagues taught cats a new response that was triggered by a stimulus displayed to only one eye of the cat; thus, that reached only one side of the cat’s brain. Later, researchers displayed the information to the cat’s other eye, to see whether the response the cat had learned would generalize from one eye—and from one side of the brain—to the other. Among normal cats (i.e., those whose corpus callosum was intact), the learning transferred. However, among cats whose corpus callosum was cut surgically, it did not. Split brain cats could perform the task using one side of their brain, but not the other (Sperry, 1961). The hemispheres therefore were ignorant of each other’s learning if the corpus callosum had been cut.

Later research showed how splitting the corpus callosum affects the psychological experience of humans (Sperry, 1982; Sperry, Gazzaniga, & Bogen, 1969). Some people have their corpus callosum cut for medical reasons; the surgical procedure stops epileptic seizures from spreading from one side of the brain to the other. After surgery, patients seem remarkably normal; based on their everyday behavior, cutting the corpus callosum seems to have little effect. In everyday life, however, any given piece of information reaches both of the person’s eyes or ears and thus is sent directly to both sides of the brain. What happens when information reaches only one side of the human brain?

Sperry and colleagues devised a clever procedure to find out. They placed patients in front of a projection screen, asked them to concentrate on a dot in the middle of the screen, and then briefly flashed words, simultaneously, on the screen’s left and right sides (Figure 3.16). The word on the left thus reached only the brain’s right hemisphere, while the word on the right reached only the brain’s left side. When split brain patients were asked what word or words they saw, they inevitably named only the word on the right (“Ring” in the example shown in the image). You might think, then, that the patient was completely unaware of the fact that the other word (“Key” in the example) had even been shown. This, however, is not the case (Sperry et al., 1969).

Patients were unable to name “Key,” but that’s merely because the word reached their right hemisphere, which is unable to produce language. When given an opportunity, with their left hand (which is controlled by the right hemisphere), to reach behind the screen and pick up an object corresponding to the word that had been displayed, the patient did pick up a key. Thus, the right hemisphere saw and understood the word “key,” but was unable to put the idea of “Key” into words. If, before revealing the key from behind the screen, the person was asked what he or she picked up, the patient would say “a ring.” The answer to the question was being given by the left hemisphere, which was completely unaware of what the right hemisphere was doing.

Specialized testing therefore reveals the unique psychological experience of the split brain patient. Without such testing, the patient appears quite normal. The left hemisphere has such a range of capabilities that it “does not miss” (Gazzaniga & Miller, 2009, p. 261) the right hemisphere from which it has been disconnected; patients seem not to be much bothered by occasional experiences in which they cannot name an object appearing briefly in their left visual field. In fact, people can live a relatively normal life with just the left side of their brain. In 2007, surgeons removed the entire right hemisphere of 6-year-old Cameron Mott, to stop violent seizures that she had been experiencing daily (Celizic, 2010). A few years after the surgery, Cameron was back in school and a good student. When a journalist asked “if she had any lingering effects from the surgery,” she replied, “No. None at all” (Celizic, 2010). (People have more difficulty living with removal of the brain’s left hemisphere, which is required for some complex language abilities; Bayard & Lassonde, 2001.)

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In addition to its bottom-to-top and left/right arrangement, the brain has yet a third type of organization. We’ll introduce it with an analogy. Imagine a large apartment building. Its organization is both bottom-to-top (e.g., 40 floors atop one another) and left-to-right (e.g., apartments on the east and west sides of each floor might have different floor plans). Yet that’s not all. A third type of organization connects people living in different parts of the building. Communications networks link residents on different floors and different sides of the building. For example, the tenants in apartments 38B, 19D, 21C, 4A, and 6E may be “friends” on a social networking site—as might the residents of 38C, 19A, 17C, 3B, and 8E, and numerous other combinations of residences. The network enables each group of friends to exchange information frequently and rapidly among themselves, despite their residing in different locations. These communications networks, then, are a third form of organization—one that cuts across floors and sides of the apartment building.

The brain works similarly. It has a third type of organization based on communications links that cut across the higher, lower, left, and right sides of the brain (Figure 3.13). These “networks in the brain” connect different regions to one another, enabling rapid communication among them (Bullmore & Sporns, 2009; Sporns, 2011, 2012). Thanks to these communications, brain activity in the different regions is coordinated. Like friends in a social network, the brain regions can share information, integrate knowledge, and synchronize their activities.

SYNCHRONIZING REGIONS OF THE BRAIN. Almost any complex thinking task draws on multiple brain regions that are networked. Consider an everyday example, such as a recent argument. In your mind, you can “relive” the event: You can picture where you were, feel the emotions you experienced when arguing, and remember words and sentences that were said. Importantly, you can do all of this simultaneously; you can bring the images, emotions, and words into your mind all at once. How—at the level of analysis of the brain—can we explain the ability of the mind to relive the argument?

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To explain it, we need to refer to two aspects of the brain’s biology:

What, biologically, are these networks that enable different parts of the brain to work in synchrony? The networks consist of large numbers of nerve cells in the brain. As we’ll discuss in more detail below, many of these cells are thin and long—so long that they can transmit information from one part of the brain to another.

Many of these transmissions run through a brain structure known as the thalamus, which is located near the center of the brain. The thalamus serves as a kind of “relay station” for connections among brain regions (Figure 3.17; Izhikevich & Edelman, 2008). Connections into and out of the thalamus are made so rapidly that the brain as a whole can integrate activity in its higher and lower regions, as well as its left and right sides.

VISUALIZING BRAIN NETWORKS. Researchers recently have developed novel methods for visualizing the nerve fibers that connect regions of the brain (Le Bihan et al., 2001). Unlike fMRI methods that reveal activation in a specific brain region (see Research Toolkit), these new methods yield three-dimensional portraits of information networks that cross from one area of the brain to another. Color codings in these images indicate bundles of nerve fibers that travel in the same direction, taking information from one brain region to another.

These methods yield remarkable images of networks in the brain (Figure 3.18). The images immediately transform our understanding of the biological machinery that underlies our ability to think and feel. Beneath the cerebral cortex is an immensely complex web of interconnections that provide much of the brain’s power.

The brain’s power, then, is analogous to that of the World Wide Web. Your computer or smart phone enables you to do a lot of things: communicate with friends, get travel directions, shop, get weather reports, watch live sports events. These abilities are based not just on the machine in your hand, but on the fact that it’s networked, that is, linked into a system that contains hundreds of millions of other computers. Similarly, most of your mental abilities are based not on activity in just one part of your brain, but on the coordinated activity of numerous brain regions that are networked.

Research on brain networks is still at an early stage of development. The major effort in the United States to study these networks was launched only in 2009 (National Institutes of Health, 2009). The long-term goal of this research is to map the entire connectome, the complete network of neural connections in the brain and overall nervous system of an organism (Sporns, Tononi, & Kötter, 2005). (The term connectome, a map of an organism’s neural connections, is analogous to the word genome, a map of an organism’s genetic information.)

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Our discussion of brain networks should remind you of this chapter’s opening story. You learned there that Capgras syndrome results from a breakdown in connections among different regions of the brain (also see Thiel et al., 2014). When this breakdown occurs, people recognize that the face of a loved one is a face, but lack the feelings required to judge that the face is familiar. Your own ability to recognize that, for example, Mom is actually Mom, and not some imposter, thus rests on the complex communications systems that are the “networks” in your brain (Figure 3.19).

CULTURAL OPPORTUNITIES

Let’s continue zooming in on the brain. So far, we’ve discussed parts of the brain that you could see with the naked eye if you were to open up someone’s skull and start looking around. But what would you see if, instead of the naked eye, you looked through a microscope?

What you’d see are individual cells. Like every other structure in the body, the brain is made up of cells. In the case of the brain, there are about 100 billion cells, and they are called neurons. Neurons—also called nerve cells—are the building blocks of the brain.

In many respects, neurons are like any other cell of the body. Each has a cell wall that separates the inside of the cell from its outside environment, a nucleus that contains genetic material, and additional structures that perform basic functions of life, such as providing energy that powers the cell. However, two features of neurons’ anatomy (i.e., their biological structure) distinguish them from the body’s other cells (Kuffler & Nicholls, 1976). The first is their shape, which is unique due to the presence

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DENDRITES AND AXONS. If you were asked to imagine a biological cell, you would probably picture something with a round or oval shape. Many cells indeed are shaped like that, but neurons are not. Two specialized structures give neurons a shape that is unique.

The first is dendrites, which are projections that branch out from the main body of the neuron, known as the soma. Like branches on a tree, the dendrites become thinner as they reach out farther from the soma (Figure 3.20).

The dendrites receive incoming signals from other neurons. They are like microphones, listening for signals from the neurons in their vicinity. Large numbers of dendrites can project out from the soma of any neuron, which means that any one neuron listens for, and can receive, incoming signals from many other neurons.

Neurons’ second specialized structure is a thin and long projection known as an axon. Every neuron has one axon, which sends information out to other neurons. You could think of the axon as being analogous to a loudspeaker, broadcasting messages that might be heard by the “microphones”—the dendrites of nearby neurons.

Axons can be very long—as long as about 1 meter in human beings (Maday & Holzbaur, 2012). This means that a neuron can send information across relatively long distances, from one part of the brain to another or from the brain to a different part of the body. The “networks” that connect one brain region to another, which you read about earlier in this chapter, consist of axons that reach across brain regions.

At its far end, the axon branches into a large number of axon terminals. It is at the axon terminals that a neuron transmits its signals to other neurons. We discuss these transmissions below, in the section on synapses and neurotransmitters.

ACTION POTENTIALS. Sometimes neurons are at rest, “just sitting around,” engaging in the same internal biochemical processes that you might see in any other cell of the body. But, periodically, neurons spring to life. They generate action potentials (also known as nerve impulses or spikes), which are electrochemical events in which an electrical current travels down the length of the axon, from the soma to the axon terminals (Bean, 2007). The word “electrochemical” indicates that the electricity generated during the action potential comes from chemical substances that have an electric charge.

Action potentials follow an “all or none” principle (Rieke et al., 1997). The neuron is either “firing” (generating an action potential) or not. There is no “in between,” no small firings or half firings.

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Action potentials are remarkably powerful. The neurons of electric eels, for example, generate hundreds of volts of electricity—enough to shock a human being to death (Ornstein & Thompson, 1984). In humans, the electrical power of action potentials is so large that scientists can record brain activity by using electrodes attached to the outside of the head (see Chapter 2); in other words, the brain’s electrical current is easily detectable through the skull.

The basis of this power is the electrical charges of chemical substances on the inside and outside of the neuron. When at rest, the neuron’s interior mostly contains substances that are charged negatively. On the outside of the cell are sodium ions, which are charged positively. During an action potential, the sodium ions briefly enter the neuron through channels in the neuron’s cell wall. This flow of charged particles generates an electric impulse in the vicinity of the channel (Figure 3.21).

Like lightning shooting from a thundercloud to Earth, the action potential shoots its way from the neuron’s soma to its axon terminals. The electrical impulse moves down the length of the axon because electrical activity at one channel in the cell membrane acts as a switch that causes the next channel to pop open. When sodium ions rush in there, this electrical activity causes the next set of gates to open. In this way, the nerve impulse rushes down the length of the axon.

And it does rush! Although the process is complex, it’s also quick; nerve impulses can travel down neurons at speeds in excess of 100 meters per second. They are speeded along by the myelin sheath, a fatty substance that surrounds the axon and acts as an electrical insulator. Myelin happens to differ in color from neurons; neurons are grey, whereas myelin is white. Areas of the brain that contain mostly neuron cell bodies and dendrites, which are not covered by myelin, thus are called the brain’s grey matter. Bundles of myelin-covered axons traveling from one region to another are referred to as the brain’s white matter.

This high speed of movement of any one action potential enables the neuron to generate a large number of action potentials in a small amount of time. Neurons can fire more than 100 times a second. The neuron’s firing rate is the key piece of information in the signals that it sends to other neurons (Gabbiani & Midtgaard, 2001).

SYNAPSES AND NEUROTRANSMITTERS. Exactly how does a neuron send signals to other neurons? Scientists once believed that neuron-to-neuron communications were direct. They thought the axon terminals of one neuron were connected to the dendrites of another, like strings that are tied together. Activating one neuron thus would activate the other directly.

But then they discovered synapses. A synapse is a small gap that separates any two neurons. Although it is very small—about 20 nanometers, that is, 20 billionths of a meter (Thompson, 2000)—neurons still must communicate across the synaptic gap; neurons are not connected to one another directly. How do they communicate?

Neurons communicate chemically. A sending neuron—that is, a neuron transmitting a signal to a second neuron—releases neurotransmitters, which are chemical substances that travel across synapses. When neurotransmitter molecules are released into a synapse, some make their way across the synaptic gap and reach a receiving neuron, that is, a neuron that is receiving a signal from the sending neuron. This chemical connection from sending neuron to receiving neuron is the primary way that neurons communicate. (However, it is not the only way; see This Just In.) Let’s examine the neurotransmitter’s voyage across the synapse in detail (Figure 3.22).

The sending neuron stores neurotransmitters in small sacs known as synaptic vesicles. Synaptic vesicles are like tiny bubbles, each of which contains a small amount of neurotransmitter. The synaptic vesicles move within the neuron, down the length of the axon. When they reach the end of the axon and “dock” with the outer edge of the axon terminal (Hammarlund et al., 2007), they are able to release their contents, the neurotransmitters, into the synaptic gap.

Some of the neurotransmitter molecules released from the sending neuron reach receptors on the dendrites of receiving neurons. Neurotransmitter receptors are sites to which neurotransmitters can attach. Chemically, the receptors are molecules to which neurotransmitter molecules can bind; the molecular shape of the neurotransmitter molecule determines whether it can bind to a given receptor. When a neurotransmitter from the sending neuron binds to the receptor of a receiving neuron, one bit of communication between neurons is complete.

The brain contains a number of different neurotransmitters. A variety of molecules, in other words, take part in the chemical communications between neurons. Some of them are listed in Table 3.1. As shown, different neurotransmitters are found in high concentrations in different parts of the brain and body.

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What is the purpose of these chemical communications—that is to say, what is accomplished by the neurotransmitters that travel from the sending neuron to the receiving neuron? Their key function is to affect the receiving neuron’s firing rate. Some neurotransmitters bind to excitatory receptors, which increase the likelihood that the receiving neuron will generate an action potential. Others bind to inhibitory receptors, which decrease the likelihood that the receiving neuron will fire. The receiving neuron integrates inputs from its various receptors, and the integrated information determines the receiving neuron’s firing rate (Gabbiani & Midtgaard, 2001).

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GLIAL CELLS. Neurons are not the only cells in the brain. They have neighbors, called glial cells—and lots of them. The brain contains about as many glial cells as neurons (Azevado et al., 2009).

Glial cells support the biological functioning of neurons, by supplying nutrients and disposing of the brain’s biological waste matter. They also hold neurons in place, which is what gives them their name; glia is the Greek word for “glue.”

Glia differ from neurons anatomically. Unlike neurons, they do not have axons or dendrites, and do not generate action potentials. Thus, people had thought that glia do nothing other than provide support to neurons. However, recent research suggests that glia may have been underestimated. For example, glia change anatomically as a result of an organism’s experiences, just as interconnections among neurons do (Fields, 2008), and they appear to influence the amount of communication that occurs in networks of connected neurons (Araque & Navarette, 2010; Werner & Mitterauer, 2013). Many scientists expect that future research will reveal unexpected ways in which glia contribute to mental life.

THIS JUST IN