When you think about your brain, you’re thinking with your brain—sending billions of neurotransmitter molecules across trillions of synapses. Indeed, say neuroscientists, “the mind is what the brain does” (Minsky, 1986).

For most of human history, we had no device high-powered yet gentle enough to reveal a living brain’s activity. In the past, brain injuries provided clues to brain-mind connections. For example, physicians had noted that damage to one side of the brain often caused paralysis on the body’s opposite side, and they correctly guessed that the body’s right side is wired to the brain’s left side, and vice versa. Other early observers linked vision problems with damage to the back of the brain, and speech problems with damage to the left-front brain. Gradually, a map of the brain began to emerge.

Now a new generation of mapmakers is at work charting formerly unknown territory, stimulating various brain parts and watching the results. Some use microelectrodes to snoop on the messages of individual neurons. Some attach larger electrodes to the scalp to eavesdrop with an EEG (electroencephalograph) on the chatter of billions of neurons. Others use scans that peer into the thinking, feeling brain and give us a Superman-like ability to see what’s happening.

The PET (positron emission tomography) scan tracks a temporarily radioactive form of the sugar glucose. Your brain accounts for only about 2 percent of your body weight. But this control center—the heart of your smarts—uses 20 percent of your body’s energy. Because active neurons gobble glucose, a PET scan can track the radioactivity and detect where this “food for thought” goes. Rather like weather radar showing rain activity, PET-scan “hot spots” show which brain areas are most active as the person solves math problems, looks at images of faces, or daydreams (FIGURE 2.9).

MRI (magnetic resonance imaging) scans capture images of brain structures by briefly disrupting activity in brain molecules. Researchers first position the person’s head in a strong magnetic field, which aligns the spinning atoms of brain molecules. Then, with a brief pulse of radio waves, they disrupt the spinning. When the atoms return to their normal spin, they give off signals that provide a detailed picture of soft tissues, including the brain. MRI scans have revealed, for example, that some people with schizophrenia, a disabling psychological disorder, have enlarged fluid-filled brain areas (FIGURE 2.10).

A special application of MRI, fMRI (functional MRI), also reveals the brain’s functions. Where the brain is especially active, blood goes. By comparing MRI scans taken less than a second apart, researchers can watch parts of the brain activate as a person thinks or acts in certain ways. As the person looks at a photo, for example, the fMRI shows blood rushing to the back of the brain, which processes visual information (see FIGURE 2.19). This technology enables a very crude sort of mind reading. Neuroscientists scanned 129 people’s brains as they did eight different mental tasks (such as reading, gambling, and rhyming). Later, viewing another person’s brain images, they were able, with 80 percent accuracy, to identify which of these mental tasks the person was doing (Poldrack et al., 2009).

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What the telescope did for astronomy, these brain-snooping tools are doing for psychology. By revealing how the living, working brain divides its labor, these tools have taught us more about the brain in the past 30 years than we had learned in the prior 30,000 years.

Brain structures determine our abilities. In sharks and other primitive vertebrates (animals with backbones), a not-so-complex brain mainly handles basic survival functions: breathing, resting, and feeding. In lower mammals, such as rodents, a more complex brain enables emotion and greater memory. In advanced mammals, such as humans, a brain that processes more information also enables the ability to plan ahead.

The brain’s increasing complexity arises from new systems built on top of the old, much as new layers cover old ones in Earth’s landscape. Digging down, one discovers the fossil remnants of the past—brainstem components performing for us much as they did for our distant ancestors. Let’s start with the brain’s base and work up.

The Brainstem

The brainstem is the brain’s oldest and innermost region. Its base is the medulla, the slight swelling in the spinal cord just after it enters the skull (FIGURE 2.11). Here lie the controls for your heartbeat and breathing. Just above the medulla sits the pons, which helps coordinate movements and control sleep. As some severely brain-damaged patients illustrate, we need no higher brain or conscious mind to orchestrate our heart’s pumping and our lungs’ breathing. The brainstem handles those tasks. If a cat’s brainstem is severed from the rest of the brain above it, the animal will still live and breathe. It will even run, climb, and groom (Klemm, 1990). But cut off from the brain’s higher regions, it won’t purposefully run or climb to get food.

The brainstem is a crossover point. Here, you’ll find a peculiar sort of cross-wiring, with most nerves to and from each side of the brain connecting to the body’s opposite side. Thus, the right brain controls the left side of the body, and vice versa (FIGURE 2.12). This cross-wiring is one of the brain’s many surprises.

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The Thalamus

Sitting at the top of the brainstem is the thalamus, which acts as the brain’s sensory control center. This joined pair of egg-shaped structures receives information from all your senses except smell. It then forwards those messages to their final destination in other brain regions that deal with seeing, hearing, tasting, and touching. Your thalamus also receives some higher brain regions’ replies, which it forwards to your medulla and cerebellum for processing. For sensory information, your thalamus is something like Chicago’s O’Hare International Airport, a hub through which traffic flows in and out on its way to various locations.

The Reticular Formation

Inside the brainstem, between your ears, lies your reticular (“netlike”) formation. This neuron network extends upward from your spinal cord, through your brainstem, and into your thalamus (see FIGURE 2.11). As sensory messages travel from your spinal cord to your thalamus, this long structure acts as a filter, relaying important information to other brain areas.

The reticular formation also controls arousal, as researchers discovered in 1949. Electrically stimulating the reticular formation of a sleeping cat almost instantly produced an awake, alert animal (Moruzzi & Magoun, 1949). When a cat’s reticular formation was cut off from higher brain regions, without damaging nearby sensory pathways, the effect was equally dramatic. The cat lapsed into a coma and never woke up.

The Cerebellum

At the rear of the brainstem is the cerebellum, meaning “little brain,” which is what its two wrinkled halves resemble (FIGURE 2.13). This baseball-sized structure plays an important role in a lot that happens just outside your awareness. Quickly answer these questions. How long have you been reading this text? Does music sound better on your TV or on your phone? How’s your mood today?

If you answered those questions easily, thank your cerebellum. It helps you judge time, discriminate textures and sounds, and control your emotions (Bower & Parsons, 2003). Your cerebellum also coordinates voluntary movement. When a soccer player masterfully controls the ball, give the cerebellum some credit. If you injured your cerebellum or drugged it with alcohol, you would have trouble walking, keeping your balance, or shaking hands. The cerebellum also helps process and store memories for things you cannot consciously recall, such as how to ride a bicycle. (Stay tuned for more about this in Chapter 7.)

* * *

Note: These older brain functions all occur without any conscious effort. Once again, we see one of our Big Ideas at work: Our two-track brain processes most information outside of our awareness. We are aware of the results of our brain’s labor (say, our current visual experience) but not of how we construct the visual image. Likewise, whether we are asleep or awake, our brainstem manages its life-sustaining functions, freeing our newer brain regions to think, talk, dream, or savor a memory.

The Limbic System

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We’ve traveled through the brain’s oldest parts, but we’ve not yet reached its newest and highest regions, the cerebral hemispheres (the two halves of the brain). Before we do, we must pass the limbic system, which lies between the oldest and newest brain areas (limbus means “border”). The limbic system contains the amygdala, the hypothalamus, and the hippocampus (FIGURE 2.14).

THE AMYGDALA The amygdala—two lima-bean-sized neural clusters—enable aggression and fear. In 1939, researchers surgically removed a rhesus monkey’s amygdala, turning the normally ill-tempered animal into the most mellow of creatures (Klüver & Bucy, 1939).

What, then, might happen if we electrically stimulated the amygdala of a normally mellow domestic animal, such as a cat? Do so in one spot and the cat prepares to attack, hissing with its back arched, its pupils wide, its hair on end. Move the electrode only slightly within the amygdala, cage the cat with a small mouse, and now it cowers in terror.

Many experiments have confirmed the amygdala’s role in processing emotions and perceiving rage and fear. Monkeys and humans with amygdala damage become less fearful of strangers (Harrison et al., 2015). After her amygdala was destroyed by a rare genetic disease, one woman no longer experienced fear. Facing a snake, speaking in public, even being threatened with a gun—she’s not afraid (Feinstein et al., 2013).

But a critical thinker should be careful here. The brain is not neatly organized into structures that reflect specific behaviors and feelings. When we feel afraid or act aggressively, many areas of our brain become active, not just the amygdala. If you destroy a car’s battery, you won’t be able to start the engine. Yet the battery is merely one link in the whole working system.

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THE HYPOTHALAMUS Just below (hypo) your thalamus is your hypothalamus, an important link in the command chain that helps your body maintain a steady internal state. Some neural clusters in the hypothalamus influence hunger. Others regulate thirst, body temperature, and sexual behavior.

To monitor your body state, the hypothalamus tunes in to your blood chemistry and any incoming orders from other brain parts. For example, picking up signals from your brain’s information-processing center, the cerebral cortex, that you are thinking about sex, your hypothalamus will secrete hormones. These hormones will in turn trigger the nearby “master” gland of the endocrine system, your pituitary (see FIGURE 2.14), to influence your sex glands to release their hormones. These hormones will intensify the thoughts of sex in your cerebral cortex. (Once again, we see the interplay between the nervous and endocrine systems. The brain influences the endocrine system, which in turn influences the brain.)

A remarkable discovery about the hypothalamus illustrates how progress in science often occurs—when curious, smart-thinking investigators keep an open mind. Two young psychologists, James Olds and Peter Milner (1954), were trying to implant an electrode in a rat’s reticular formation when they made a magnificent mistake. They placed the electrode incorrectly (Olds, 1975). Curiously, the rat, as though seeking more stimulation, kept returning to the location where it had been stimulated by this misplaced electrode. When Olds and Milner discovered that they had actually placed the device in a region of the hypothalamus, they realized they had stumbled upon a brain center that provides pleasurable rewards. In later studies, rats allowed to control their own stimulation in this and other reward centers in the brain did so at a feverish pace—pressing a pedal up to 1000 times an hour, until they dropped from exhaustion.

Animal researchers have discovered similar reward centers in or near the hypothalamus in goldfish, dolphins, monkeys, and other species. One general reward system triggers the release of the neurotransmitter dopamine. Specific centers help us enjoy the pleasures of eating, drinking, and sex. Animals, it seems, come equipped with built-in systems that reward activities essential to survival.

Do we humans also have limbic centers for pleasure? Some evidence indicates we do. To calm violent patients, one neurosurgeon implanted electrodes in such limbic system areas. Although those patients reported mild pleasure, they—unlike Olds and Milner’s rats—were not driven to a frenzy (Deutsch, 1972; Hooper & Teresi, 1986). And some studies reveal that stimulating the human brain’s reward circuits may trigger more desire than pure enjoyment (Kringelbach & Berridge, 2012).

THE HIPPOCAMPUS The hippocampus—a seahorse-shaped brain structure—processes conscious, explicit memories of facts and events. Surgery or injury that removes the hippocampus also removes the ability to form or retrieve these conscious memories. Children who survive a hippocampal brain tumor will later struggle to remember new information (Jayakar et al., 2015). Birds with a damaged hippocampus will be unable to recall where they buried seeds (Kamil & Cheng, 2001; Sherry & Vaccarino, 1989). National Football League players who experience a concussion may later have a shrunken hippocampus and poor memory (Strain et al., 2015).

Later in this chapter, we’ll see how the hippocampus helps store the day’s experiences while we sleep. In Chapter 7, we’ll explore how the hippocampus interacts with our frontal lobes to create our conscious memory.

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FIGURE 2.15 locates the brain areas we’ve discussed—as well as the cerebral cortex, our next topic and the final stop on our journey through the brain.

Older brain networks sustain basic life functions and enable memory, emotions, and basic drives. High above these older structures are the cerebrum—two large hemispheres that contribute 85 percent of the brain’s weight. Covering those hemispheres, like bark on a tree, is the cerebral cortex, a thin surface layer of interconnected neurons. In our brain’s evolutionary history, the cerebral cortex is a relative newcomer. Its newer neural networks form specialized work teams that enable your thinking, sensing, and speaking. The cerebral cortex is your brain’s thinking crown, your body’s ultimate control and information-processing center.

Structure of the Cortex

If you opened a human skull, exposing the brain, you would see a wrinkled organ, shaped somewhat like an oversized walnut. Without these wrinkles, a flattened cerebral cortex would require triple the area—roughly that of a large pizza. The brain’s left and right hemispheres are filled mainly with axons connecting the cortex to the brain’s other regions. The cerebral cortex—that thin surface layer—contains some 20 to 23 billion nerve cells and 300 trillion synaptic connections (de Courten-Myers, 2005). Being human takes a lot of nerve.

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Each hemisphere’s cortex is subdivided into four lobes, separated by deep folds (FIGURE 2.16). You can roughly trace the four lobes, starting with both hands on your forehead. The frontal lobes lie directly behind your forehead. As you move your hands over the top of your head, toward the rear, you’re sliding over your parietal lobes. Continuing to move down, toward the back of your head, you’ll slide over your occipital lobes. Now move each hand forward, to the sides of your head, and just above each ear you’ll find your temporal lobes. Each hemisphere has these four lobes. Each lobe carries out many functions. And many functions require the cooperation of several lobes.

Functions of the Cortex

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More than a century ago, surgeons found damaged areas of the cerebral cortex during autopsies of people who had been partially paralyzed or speechless. This rather crude evidence was interesting, but it did not prove that specific parts of the cortex control complex functions like movement or speech. A laptop with a broken power cord might go dead, but we would be fooling ourselves if we thought we had “localized” the Internet in the cord.

MOTOR FUNCTIONS Early scientists had better luck showing simple brain-behavior links. In 1870, for example, German physicians Gustav Fritsch and Eduard Hitzig made an important discovery. By electrically stimulating parts of an animal’s cortex, they could make other parts of its body move. The movement happened only when they stimulated an arch-shaped region at the back of the frontal lobe, running roughly ear-to-ear across the top of the brain. Moreover, if they stimulated this region in the left hemisphere, the right leg would move. And if they stimulated part of the right hemisphere, the opposite leg—on the left—reacted. Fritsch and Hitzig had discovered what is now called the motor cortex.

Lucky for brain surgeons and their patients, the brain has no sensory receptors. Knowing this, in the 1930s, Otfrid Foerster and Wilder Penfield were able to map the motor cortex in hundreds of wide-awake patients by stimulating different cortical areas and observing the body’s responses. They discovered that body areas requiring precise control, such as the fingers and mouth, occupied the greatest amount of cortical space (FIGURE 2.17).

As is so often the case in science, new answers have triggered new questions. Might electrodes implanted in the motor cortex identify what neurons control specific activities? If so, could people learn to mentally control these implanted devices, perhaps to direct a robotic limb? Clinical trials are now under way with people who have severe paralysis or have lost a limb (Andersen et al., 2010; Nurmikko et al., 2010). Some who received implants have indeed learned to control robotic limbs (Collinger et al., 2013; Hochberg et al., 2012).

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SENSORY FUNCTIONS The motor cortex sends messages out to the body. What parts of the cortex receive incoming messages from our senses of touch and movement? Penfield supplied the answer. An area now called the somatosensory cortex receives this sensory input. It runs parallel to the motor cortex and just behind it, at the front of the parietal lobes (see FIGURE 2.17). Stimulate a point on the top of this band of tissue, and a person may report being touched on the shoulder. Stimulate some point on the side, and the person may feel something on the face.

The more sensitive a body region, the larger the somatosensory area devoted to it. Why do we kiss with our lips rather than rub elbows? Our supersensitive lips project to a larger brain area than do our arms (see FIGURE 2.17). Similarly, rats have a large brain area devoted to their whisker sensations, and owls to their hearing sensations.

Your somatosensory cortex is a very powerful tool for processing information from your skin senses—such as touch and temperature—and from movements of your body parts. But this parietal lobe area isn’t the only part of your cortex that receives input from your senses. After surgeons removed a large tumor from his right occipital lobe, in the back of his brain, a friend of mine [DM’s] became blind to the left half of his field of vision. Why? Because in an intact brain, visual information travels from the eyes to the visual cortex, in the occipital lobes (FIGURE 2.18). From your occipital lobes, visual information travels to other areas that specialize in tasks such as identifying words, detecting emotions, and recognizing faces (FIGURE 2.19).

If you have normal vision, you might see flashes of light or dashes of color if stimulated in your occipital lobes. (In a sense, we do have eyes in the back of our head!)

Any sound you now hear is processed by your auditory cortex in your temporal lobes (just above your ears; see FIGURE 2.18). Most of this auditory information travels a roundabout route from one ear to the auditory receiving area above your opposite ear. If stimulated in your auditory cortex, you alone might hear a sound. People with schizophrenia sometimes have auditory hallucinations (false sensory experiences). MRI scans taken during these hallucinations show active auditory areas in the temporal lobes (Lennox et al., 1999).

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ASSOCIATION AREAS So far, we have pointed out small areas of the cortex that receive messages from our senses, and other small areas that send messages to our muscles. Together, these areas occupy about one-fourth of the human brain’s thin, wrinkled cover. What, then, goes on in the remaining vast regions of the cortex? In these association areas, neurons are busy with higher mental functions—many of the tasks that make us human. Electrically probing an association area won’t trigger any observable response. So, unlike the somatosensory and motor areas, association area functions can’t be neatly mapped. Does this mean we don’t use them? See Thinking Critically About: Using More Than 10 Percent of Our Brain.

Association areas are found in all four lobes. In the forward part of the frontal lobes, the prefrontal cortex enables judgment, planning, and processing of new memories. People with damaged frontal lobes may have intact memories, high intelligence test scores, and great cake-baking skills. Yet they would not be able to plan ahead to begin baking a cake for a loved one’s birthday (Huey et al., 2006).

Frontal lobe damage can alter personality, as it did in the famous case study of railroad worker Phineas Gage. One afternoon in 1848, Gage, then 25 years old, was using an iron rod to pack gunpowder into a rock. A spark ignited the gunpowder, shooting the rod up through his left cheek and out the top of his skull, causing massive damage to his frontal lobes (FIGURE 2.20a). To everyone’s amazement, he was immediately able to sit up and speak. After the wound healed, he returned to work. But friendly, soft-spoken Gage was now irritable, profane, and dishonest. The accident had destroyed frontal lobe areas that enable control over emotions (Van Horn et al., 2012). This person, said his friends, was “no longer Gage.” His mental abilities and memories were unharmed, but his personality was not. (Gage later lost his railroad job, but, over time, he adapted to his injury and found work as a stagecoach driver [Macmillan & Lena, 2010].)

Without the frontal lobe brakes on his impulses, Gage became less inhibited. When his frontal lobes ruptured, his moral compass seemed to disconnect from his behavior. Studies of others with damaged frontal lobes reveal similar losses—their moral judgments seem untouched by normal emotions. Would you agree with the idea of pushing someone in front of a runaway train to save five others? Most people would not, but those with damage to the prefrontal cortex often do (Koenigs et al., 2007). The frontal lobes help steer us away from violent actions (Molenberghs et al., 2015; Yang & Raine, 2009). In 1972, Cecil Clayton lost 20 percent of his left frontal lobe in a sawmill accident. His intelligence test score dropped to an elementary school level. He became increasingly impulsive. In 1996, he shot and killed a deputy sheriff. In 2015, when he was 74, the State of Missouri executed him (Williams, 2015).

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Damage to association areas in other lobes would result in different losses. If a stroke or head injury destroyed part of your parietal lobes, you might lose mathematical and spatial reasoning (Ibos & Freedman, 2014). If the damaged area was on the underside of the right temporal lobe, which lets you recognize faces, you would still be able to describe facial features and to recognize someone’s sex and approximate age. Yet you would be strangely unable to identify the person as, say, Taylor Swift or even your grandmother.

Nevertheless, brain scans show that complex mental functions don’t reside in any one spot in your brain. Performing simple tasks may activate tiny patches of your brain, far less than 10 percent. During a complex task, a scan would show many islands of brain activity working together—some running automatically in the background, and others under conscious control (Chein & Schneider, 2012). Memory, language, and attention are the products of interaction among distinct brain areas (Knight, 2007). Ditto for religious experience—there is no simple “God spot.” More than 40 brain regions become active in different religious states, such as prayer and meditation (Fingelkurts & Fingelkurts, 2009).

Earlier, we learned about the brain’s plasticity—how our brain adapts to new situations. What happens when we experience mishaps, big and little? Let’s explore the brain’s ability to modify itself after damage.

Brain-damage effects were discussed in several places in this chapter. Most can be traced to two hard facts. (1) Severed brain and spinal cord neurons, unlike cut skin, usually do not repair themselves. If a spinal cord is severed, the person will probably be paralyzed permanently. (2) Some brain functions seem forever linked to specific areas. A newborn with damage to facial recognition areas on both temporal lobes was never able to recognize faces (Farah et al., 2000).

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But there is good news: The brain often attempts self-repair by reorganizing existing tissue. Some brain tissue—especially in a young child’s brain—can reorganize even after serious damage (Kolb, 1989) (FIGURE 2.21). If a slow-growing left-hemisphere tumor disrupts language, the right hemisphere may take over the task (Thiel et al., 2006). If a finger is lost, the somatosensory cortex that received its input will begin to pick up signals from the neighboring fingers, which then become more sensitive (Oelschläger et al., 2014). Blindness or deafness makes unused brain areas available for other uses (Amedi et al., 2005). This plasticity helps explain why deaf people who learned sign language before another language may have better-than-average peripheral and motion detection (Bosworth & Dobkins, 1999; Shiell et al., 2014). An area of the temporal lobe is normally dedicated to hearing. But without stimulation from sounds, it is free to process other signals, such as those from the visual system.

Although self-repair by reorganizing is more common, the brain sometimes tries to mend itself through neurogenesis—producing new neurons. Researchers have found baby neurons deep in the brains of adult mice, birds, monkeys, and humans (Jessberger et al., 2008). These neurons may then migrate elsewhere and form connections with neighboring neurons (Aimone et al., 2010; Egeland et al., 2015; Gould, 2007).

Might new drugs spur the production of new nerve cells? Right now, companies are working on such possibilities, so stay tuned. In the meantime, we can all benefit from natural aids to neurogenesis, such as exercise, sex, and sleep (Iso et al., 2007; Leuner et al., 2010; Pereira et al., 2007; Stranahan et al., 2006).

Our brain’s look-alike left and right hemispheres serve different functions. This lateralization is clear after some types of brain damage. Language processing, for example, seems to reside mostly in your left hemisphere. A left-hemisphere accident, stroke, or tumor could leave you unable to read, write, or speak. You might be unable to reason, do arithmetic, or understand others. Similar right hemisphere damage seldom has such dramatic effects.

Does this mean that the right hemisphere is just along for the ride—a silent junior partner or “minor” hemisphere? Many believed this was the case until 1960, when researchers found that the “minor” right hemisphere was not so limited after all. The unfolding of this discovery is a fascinating chapter in psychology’s history.

Splitting the Brain: One Skull, Two Minds

In 1961, two neurosurgeons believed that the uncontrollable seizures of some patients with severe epilepsy was caused by abnormal brain activity bouncing back and forth between the two cerebral hemispheres. If so, they wondered, could they end this biological tennis game by cutting through the corpus callosum, the wide band of axon fibers connecting the two hemispheres and carrying messages between them (FIGURE 2.22)? The neurosurgeons knew that psychologists Roger Sperry, Ronald Myers, and Michael Gazzaniga had divided cats’ and monkeys’ brains in this manner, with no serious ill effects.

So the surgeons operated. The result? The seizures all but disappeared. The patients with these split brains were surprisingly normal, their personality and intellect hardly affected. Waking from surgery, one even joked that he had a “splitting headache” (Gazzaniga, 1967). By sharing their experiences, these patients have greatly expanded our understanding of interactions between the intact brain’s two hemispheres.

To appreciate these studies, we need to focus for a minute on the peculiar nature of our visual wiring, illustrated in FIGURE 2.23. Note that each eye receives sensory information from the entire visual field. But information from the left half of your field of vision goes to your right hemisphere, and information from the right half of your visual field goes to your left hemisphere. In an intact brain, data received by either hemisphere are quickly transmitted to the other side, across the corpus callosum.

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In a person with a severed corpus callosum, this information sharing does not take place. Because the surgery had cut the communication lines between the patient’s two hemispheres, Sperry and Gazzaniga were able to quiz each hemisphere separately. They could send information to the patient’s left hemisphere by having the person stare at a dot and by then flashing a stimulus (a word or photo) to the right of the dot. To send a message to the right hemisphere, they would flash the item to the left of the dot. (If they tried to do this with you, the hemisphere receiving the information would instantly pass the news to the other side of your intact brain.)

In an early experiment, Gazzaniga (1967, 2016) flashed the word HEART across the screen in such a way that HE appeared to the left of the dot, and ART appeared to the right (FIGURE 2.24b). Asked to say what they had seen, the patients reported the letters sent to the left hemisphere, which usually controls speech—“ART.” Asked to point with their left hand to what they had seen, they were startled when that hand (controlled by the right hemisphere) pointed to “HE” (FIGURE 2.24c). One skull was housing two minds.

A few people who have had split-brain surgery have for a time been bothered by the unruly independence of their left hand. It seems the left hand truly didn’t know what the right hand was doing. One hand might unbutton a shirt while the other buttoned it, or put grocery store items back on the shelf after the other hand put them in the cart. It was as if each hemisphere was thinking, “I’ve half a mind to wear my green (blue) shirt today.” Indeed, said Sperry (1964), split-brain surgery leaves people “with two separate minds.” (Reading these reports, can you imagine a patient playing a solitary game of “rock, paper, scissors”—left hand versus right?)

What happens when the “two minds” disagree? If a split-brain patient follows an order (“Walk”) sent to the right hemisphere, the left hemisphere won’t know why the legs start walking. But if asked, the left hemisphere doesn’t say “I don’t know.” Instead, it instantly invents—and apparently believes—an explanation (“I’m going into the house to get a Coke”). Thus, Gazzaniga (1988), who has called split-brain patients “the most fascinating people on Earth,” concluded that the conscious left hemisphere resembles an interpreter that instantly constructs theories to explain our behavior. There goes our brain, running on autopilot as usual. It feels and acts and later explains itself (Kahneman, 2011).

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Right-Left Differences in Intact Brains

So, what about the 99.99+ percent of us with undivided brains? Does each of our hemispheres also perform distinct functions? The short answer is Yes. If you were performing a perceptual task, a brain scan would show increased activity (brain waves, bloodflow, and glucose consumption) in your right hemisphere. If you were speaking or doing math calculations, the scan would show increased activity in your left hemisphere.

A dramatic demonstration of lateralization happens before some types of brain surgery. To locate the patient’s language centers, the surgeon injects a sedative into the neck artery feeding blood to the left hemisphere, which usually controls speech. Before the injection, the patient is lying down, arms in the air, chatting with the doctor. Can you predict what probably happens when the drug puts the left hemisphere to sleep? Within seconds, the patient’s right arm falls limp. If the left hemisphere is controlling language, the patient will be speechless until the drug wears off.

To the brain, language is language, whether spoken or signed. Just as hearing people usually use the left hemisphere to process spoken language, deaf people usually use the left hemisphere to process sign language (Corina et al., 1992; Hickok et al., 2001). Thus, a left-hemisphere stroke disrupts a deaf person’s signing, much as it disrupts a hearing person’s speaking (Corina, 1998). The same brain area is involved in both. (For more on how the brain enables language, see Chapter 8.)

Let’s not forget that our left and right brain hemispheres work together. The left hemisphere is good at making quick, exact interpretations of language. But the right hemisphere excels in making inferences (reasoned conclusions) (Beeman & Chiarello, 1998; Bowden & Beeman, 1998; Mason & Just, 2004). It also helps fine-tune our speech to make meaning clear—as when we say “Let’s eat, Grandpa,” instead of “Let’s eat Grandpa!” (Heller, 1990). And it helps orchestrate our self-awareness. People with partial paralysis sometimes stubbornly deny their condition. They may claim they can move a paralyzed limb—if the damage causing the paralysis is in the right hemisphere (Berti et al., 2005).

Simply looking at the two hemispheres, so alike to the naked eye, who would suppose they each contribute uniquely to the harmony of the whole? Yet a variety of observations—of people with split brains and those with intact brains, and even of other species’ brains—leaves little doubt. We have unified brains with specialized parts (Hopkins & Cantalupo, 2008; MacNeilage et al., 2009). And one product of all that brain activity is consciousness, our next topic.