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LO 9 Describe the functions of the two brain hemispheres and how they communicate.

If you look at a photo or an illustration of the brain, you will see a walnut-shaped wrinkled structure—this is the cerebrum (Latin for “brain”), the largest and most conspicuous part of the brain. The cerebrum includes virtually all parts of the brain except the brainstem structures, which you will learn about later. Like a walnut, the cerebrum has two distinct halves, or hemispheres. Looking at the brain from above, you can see a deep groove running from the front of the head to the back, dividing it into the right cerebral hemisphere and the left cerebral hemisphere. Although the hemispheres look like mirror images of one another, with similar structures on the left and right, they do not have identical jobs nor are they perfectly symmetrical. Generally speaking, the right hemisphere controls the left side of the body, and the left hemisphere controls the right. This explains why Brandon, who was shot on the left side of his head, suffered paralysis and loss of sensation on the right half of his body. Christina’s situation is roughly the opposite. Rasmussen’s encephalitis struck the right side of her brain, which explains why her left ankle started twitching at the pool and why all of her subsequent seizures affected the left side of her body. This is why Dr. Freeman recommended the removal of her right hemisphere.

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LO 10 Explain lateralization and how split-brain operations affect it.

Removing nearly half of a brain may sound barbaric, but hemispherectomies have proven to be effective for eliminating and reducing seizures. In a study of the 111 children who had hemispherectomies at Johns Hopkins between 1975 and 2001, 65% no longer suffered seizures at all, and 21% experienced infrequent, “nonhandicapping” seizures. The remaining 14% still had seizures described as “troublesome” (Kossoff et al., 2003).

Hemispherectomies are exceptionally rare, used only when seizures occur many times a day, cannot be tempered with drugs, and stem from problems in one hemisphere (Choi, 2007, May 24). A less extreme, last-resort surgery for drug-resistant seizures is the split-brain operation, which essentially disconnects the right and left hemispheres. Normally, the two hemispheres are linked by a bundle of nerve fibers known as the corpus callosum (kȯr-pəs ka-lō-səm). Through the corpus callosum, the left and right sides of the brain communicate and work together to process information. But this same band of nerve fibers can also serve as a passageway for the electrical storms responsible for seizures. With the split-brain operation, the corpus callosum is severed so that these storms can no longer pass freely between the hemispheres (Wolman, 2012, March 15).

STUDYING THE SPLIT BRAIN In addition to helping many patients with severe, drug-resistant epilepsy (Abou-Khalil, 2010), the consequences of split-brain operations have provided researchers with an excellent opportunity to explore the specialization of the hemispheres. Before we start to look at this research, you need to understand how visual information is processed. Each eye receives visual sensations, but that information is sent to the opposite hemisphere, and shared between the hemispheres via the corpus callosum. Specifically, information presented in the right visual field is processed in the left hemisphere, and information presented in the left visual field is processed in the right hemisphere.

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Equipped with this knowledge, American neuropsychologist Roger Sperry (1913–1994) and his student Michael Gazzaniga (1939–) conducted groundbreaking research on epilepsy patients who had undergone split-brain operations to alleviate their seizures. Not only did Sperry and Gazzaniga’s “split-brain” participants experience fewer seizures, they had surprisingly normal cognitive abilities and showed no obvious changes in “temperament, personality, or general intelligence” as a result of their surgeries (Gazzaniga, 1967, p. 24). But under certain circumstances, the researchers observed, they behaved as though they had two separate brains (Gazzaniga, 1967, 1998; Figure 2.7).

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Because the hemispheres are disconnected through the surgery, researchers can study each hemisphere separately to explore its own unique capabilities (or specializations). Imagine that researchers flashed an image (let’s say an apple) on the right side of a screen, ensuring that it would be processed by the brain’s left hemisphere. The split-brain participant could articulate what she had seen (I saw an apple). If, however, the apple appeared on the left side of the screen (processed by the right hemisphere), she would claim she saw nothing. But when asked to identify the image in a nonverbal way (pointing or touching with her left hand), she could do this without a problem (Gazzaniga, 1967, 1998).

LATERALIZATION The split-brain experiments offered an elegant demonstration of lateralization, the tendency for the left and right hemispheres to excel in certain activities. When images are flashed in the right visual field, the information is sent to the left side of the brain, which excels in language processing. This explains why the split-brain participants were able to articulate the image they had seen on the right side of the screen. Images appearing in the left visual field are sent to the right side of the brain, which excels at visual-spatial tasks but is generally not responsible for processing language. Thus, the participants were tongue-tied when asked to report what they had seen on the left side of the screen. They could, however, reach out and point to it using their left hand, which is controlled by the right hemisphere (Gazzaniga, 1998; Gazzaniga, Bogen, & Sperry, 1965).

The split-brain studies revealed that the left hemisphere plays a crucial role in language processing and the right hemisphere in managing visual-spatial tasks. These are only generalizations, however. While there are clear differences in the way the hemispheres process information (and the speed at which they do it), they can also process the same types of information. In a split-brain individual, communication between the hemispheres is limited. This is not the case for someone with an intact corpus callosum. The hemispheres are constantly integrating and sharing all types of information (Lilienfeld, Lynn, Ruscio, & Beyerstein, 2011). Next time you hear someone claim that some people are more “left-brained” and others are more “right-brained,” ask him to identify the research that backs up such a claim. Similarly, beware of catchy sales pitches for products designed to increase your “logical and analytical” left-brain thinking or to help you tap into the “creative” right brain. This way of thinking is oversimplified. Keep this in mind while reading the upcoming sections on specialization in the left and right sides of the brain. The two hemispheres may have certain areas of expertise, but they work as a team to create your experience of the world.

Armed with this new knowledge of the split-brain experiments, let’s return our focus to Brandon. Brandon’s injury occurred on the left side of his brain, devastating his ability to use language. Before the battle of Fallujah, he had breezed through Western novels at breakneck speeds. After his injury, even the simplest sentence baffled him. Words on a page looked like nothing more than black lines and curls. Brandon remembers, “It was like a puzzle that I couldn’t figure out.”

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HANDEDNESS AND LANGUAGE DOMINANCE Brandon’s difficulties with language are fairly typical for someone with a brain injury to the left hemisphere, because regions on the left side of the brain tend to predominate in language. This is not true for everyone, however. In a study examining handedness and language dominance, researchers measured participants’ degree of handedness (righty or lefty) and used brain scan technology to determine their predominant side for language processing. They found that around 27% of strongly left-handed participants and 4% of strongly right-handed participants had language dominance in the right hemisphere (Knecht et al., 2000). What does this suggest? The left hemisphere controls language in most but not all people, though it doesn’t always correspond to right- or left-handedness.

LO 11 Identify areas in the brain responsible for language production and comprehension.

BROCA’S AREA Evidence for the “language on the left” notion appeared as early as 1861, when a French surgeon by the name of Pierre Paul Broca (1824 –1880) encountered two patients who had, for all practical purposes, lost the ability to talk. One of the patients could only say the word “tan,” and the other had an oral vocabulary of five words. When Broca later performed autopsies on the men, he found that both had sustained damage to the same area on the side of the left frontal lobe (right around the temple; Figure 2.8). Over the years, Broca identified several other speech-impaired patients with damage to the same area, a region now called Broca’s area (brō-kəz) (Greenblatt, Dagi, & Epstein, 1997), which is involved in speech production. However, some researchers propose that other parts of the brain may also be involved in generating speech (Tate, Herbet, Moritz-Gasser, Tate, & Duffau, 2014).

WERNICKE’S AREA Around the same time Broca was doing his research, a German doctor named Karl Wernicke (1848–1905) pinpointed a different place in the left hemisphere that seemed to control speech comprehension. Wernicke noticed that patients suffering damage to a small tract of tissue in the left temporal lobe, now called Wernicke’s area (ver-nə-kəz), struggled to make sense of what others were saying. Wernicke’s area is the brain’s headquarters for language comprehension.

Broca’s and Wernicke’s work, along with other early findings, highlighted the left hemisphere’s critical role in language. Scientists initially suspected that Broca’s area was responsible for speech creation and Wernicke’s area for comprehension, but it is now clear the use of language is far more complicated. These areas may perform additional functions, such as processing music and interpreting hand gestures (Koelsch et al., 2002; Xu, Gannon, Emmorey, Smith, & Braun, 2009), and they cooperate with multiple brain regions to allow us to produce and understand language (Tate et al., 2014). Furthermore, some speech processing appears to occur in the right hemisphere.

THE ROLE OF THE RIGHT Research involving two split-brain individuals suggests the right hemisphere is more proficient than the left in some visual tasks (Corballis, 2003), such as determining whether two objects are identical as opposed to mirror images of one another (Funnell, Corballis, & Gazzaniga, 1999), or judging if lines are oriented in the same direction (Corballis, Funnell, & Gazzaniga, 2002). Other findings suggest the right hemisphere is crucial for understanding abstract and humorous use of language (Coulson & Van Petten, 2007); somewhat better than the left for following conversations that change topic (Dapretto, Lee, & Caplan, 2005); and important for our ability to recognize faces (Kanwisher, McDermott, & Chun, 1997; Rossion, 2014).

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When Christina was wheeled out of surgery, her mother approached, grabbed hold of her right hand, and asked her to squeeze. Christina squeezed, demonstrating that she could understand and respond to language. Remember, she still had her left hemisphere.

Losing the right side of her brain did come at a cost, however. We know that Christina suffers partial paralysis on the left side of her body; this makes sense, because the right hemisphere controls movement and sensation on the left. We also know that it took Christina extra time to do her schoolwork. But if you ask Christina whether she has significant difficulty with any of the “right-brain” tasks described earlier, her answer will be no.

In addition to making the honor roll and leading the bowling team, Christina managed to get her driver’s license (even though some of her doctors said she never would), graduate from high school, and go to college. These accomplishments are the result of Christina’s steadfast determination, but also a testament to the brain’s amazing ability to heal and regenerate.

LO 12 Define neuroplasticity and recognize when it is evident in the brain.

The brain undergoes constant alteration in response to experiences and is capable of some degree of physical adaptation and repair. Its ability to heal, grow new connections, and make do with what is available is a characteristic we refer to as neuroplasticity. New connections are constantly forming between neurons, and unused ones are fading away. Vast networks of neurons have the ability to reorganize in order to adapt to the environment and an organism’s ever-changing needs, a quality particularly evident in the young. After brain injuries, younger children have better outcomes than do adults; their brains show more plasticity (Johnston, 2009).

In one study, researchers removed the eyes of newborn opossums and found that brain tissues normally destined to become visual processing centers took a developmental turn. Instead, they became areas that specialized in processing other types of sensory stimuli, such as sounds and touch (Karlen, Kahn, & Krubitzer, 2006). The same appears to happen in humans. Brain scans reveal that when visually impaired individuals learn to read Braille early in life, a region of the brain that normally specializes in handling visual information becomes activated, suggesting it is used instead for processing touch sensations (Burton, 2003; Liu et al., 2007).

Remarkably, this plasticity is evident even with the loss of an entire hemisphere. Researchers report that the younger the patient is when she has a hemispherectomy, the better her chances are for recovery. In fact, even with the loss of an entire left hemisphere (the primary location for language processing), speech is less severely impacted (though some impact is inevitable) in young patients. The younger the person undergoing the procedure, the less disability is evident in speech (Choi, 2008).

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STEM CELLS Scientists once thought that people were born with all the neurons they would ever have. Brain cells might die, but no new ones would crop up to replace them. Thanks to research beginning in the 1990s, that dismal notion has been turned on its head. In the last few decades, studies with animals and humans have shown that some areas of the brain are constantly generating new neurons, a process known as neurogenesis, which might be tied to learning and creating new memories (Eriksson et al., 1998; Gould, Beylin, Tanapat, Reeves, & Shors, 1999; Jessberger & Gage, 2014; Reynolds & Weiss, 1992).

The cells responsible for churning out these new neurons are known as stem cells, and they are quite a hot topic in biomedical research. Scientists hope to harness these little cell factories to repair tissue that has been damaged or destroyed. Imagine that you could use stem cells to bring back all the neurons that Brandon lost from his injury or replace those that Christina lost to surgery. Cultivating new brain tissue is just one potential application of stem cell science. These cellular cure-alls might also be used to alleviate the symptoms of Parkinson’s disease, or replenish neurons of the spine, enabling people with spinal cord injuries to regain movement. Both have already been accomplished in mice (Keirstead et al., 2005; Wernig et al., 2008). Although embryonic stem cells can be used for such purposes, some stem cells can be found in tissues of the adult body, such as the brain and bone marrow.

Imagine you were one of the surgeons performing Christina’s hemispherectomy. What exactly would you see when you peeled away the scalp and cut an opening into the skull? Before seeing the brain, you would come upon a layer of three thin membranes, the meninges, which envelop and protect the brain and spinal cord (Infographic 2.4 below). Perhaps you have heard of meningitis, a potentially life-threatening condition in which the meninges become inflamed as a result of an infection. The meninges are bathed in a clear watery substance called cerebrospinal fluid, which offers additional cushioning and helps in the transport of nutrients and waste in and out of the brain and the spinal cord. Once you peeled back the meninges, you would behold the pink cerebrum.

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As Christina’s surgeon, your main task would be to remove part of the cerebrum’s outermost layer, the cerebral cortex (sə-rē-brəl). The cerebral cortex processes information and is the layer of cells surrounding nearly all the other brain structures. You’ll remember our earlier comment that the cerebrum looks like a wrinkled walnut. This is because the cortex is scrunched up and folded onto itself to fit inside a small space (the skull). This outermost section of the brain is also the part that is “newest,” or most recently evolved compared to the “older” structures closer to its core. We know this because researchers have compared the brains of humans with other primates. The structures we share with our primate relatives are considered more primitive, or less evolved, than the structures that are unique to humans.

LO 13 Identify the lobes of the cortex and explain their functions.

The cortex overlying each hemisphere is separated into different sections, or lobes. The major function of the frontal lobes is to organize information among the other lobes of the brain. The frontal lobes are also responsible for higher-level cognitive functions, such as thinking, perception, and impulse control. The parietal lobes (pə-rī-ə-təl) receive and process sensory information like touch, pressure, temperature, and spatial orientation. Visual information goes to the occipital lobes (äk-si-pə-təl) for processing, and hearing and language comprehension are largely handled by the temporal lobes. We’ll have more to say about the lobes as we discuss them each in turn below.

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Prior to her hemispherectomy, Christina was extroverted, easygoing, and full of energy. “I had absolutely no worries,” she says, recalling her pre-Rasmussen’s days. After her operation, Christina became more introverted and passive. She felt more emotionally unsettled. “You go into surgery one person,” she says, “and you come out another.”

The transformation of Christina’s personality may be a result of many factors, including the stress of dealing with a serious disease, undergoing a major surgery, and readjusting to life with disabilities. But it could also have something to do with the fact that she lost a considerable amount of brain tissue, including her right frontal lobe. Networks of neurons in the frontal lobes are involved in processing emotions, making plans, controlling impulses, and carrying out a vast array of mental tasks that each person does in a unique way (Williams, Suchy, & Kraybill, 2010). The frontal lobes play a key role in the development of personality (Stuss & Alexander, 2000). A striking illustration of this phenomenon involves an unlucky railroad foreman, Phineas Gage.

PHINEAS GAGE AND THE FRONTAL LOBES The year was 1848, and Phineas Gage was working on the railroad. An accidental explosion sent a 3-foot iron tamping rod clear through his frontal lobes (Infographic 2.4). The rod, about as thick as a broom handle, drove straight into Gage’s left cheek, through his brain, and out the top of his skull (Macmillan, 2000). What’s peculiar about Gage’s accident (besides the fact that he was walking and talking just hours later) is the extreme transformation it caused. Before the accident, Gage was a well-balanced, diligent worker whom his supervisors referred to as their “most efficient and capable foreman” (Harlow, 1848, as cited in Neylan, 1999, p. 280). After the accident, he was unreliable, unpleasant, and downright vulgar. His character was so altered that people acquainted with him before and after the accident claimed he was “no longer Gage” (Harlow, 1848, as cited in Neylan, 1999, p. 280). However, there is evidence that Gage recovered to some degree. He spent almost 8 years working as a horse caretaker and stagecoach driver (Harlow, 1968, 1969, as cited in Macmillan, 2000).

Modern scientists have revisited Gage’s case, using measurements from his fractured skull and brain-imaging data to estimate exactly where the damage occurred. Their studies suggest that the metal rod caused destruction in both the left and right frontal lobes (Damasio, Grabowski, Frank, Galaburda, & Damasio, 1994), although, researchers now believe the rod did not pierce the right hemisphere (Ratiu et al., 2004; Van Horn et al., 2012). The only good thing about Gage’s horrible accident, it seems, is that it illuminated the importance of the frontal lobes in defining personality characteristics.

DOGS, CARTOONS, AND THE MOTOR CORTEX Toward the rear of the frontal lobes is a strip of the brain known as the motor cortex, which works with other areas to plan and execute voluntary movements (Infographic 2.4). Evidence for this region’s involvement in muscle movement first came from a study of dogs by Gustav Fritsch (1838–1927) and Edvard Hitzig (1838–1907). Working from a makeshift lab, the two doctors discovered they could make the animals move by electrically stimulating their brains (Gross, 2007). A mild shock to the right side of the cortex might cause a twitch in the left forepaw or the left side of the face, whereas stimulating the left would spur movement on the right (Finger, 2001).

North American neurosurgeon Wilder Penfield (1891–1976) used a method similar to Fritsch and Hitzig with humans to create a map showing which points along the motor cortex corresponded to the various parts of the body (Penfield & Boldrey, 1937). Penfield’s map is often represented by the “homunculus” (hō-məŋ-kyə-ləs; Latin for “little man”) cartoon, a distorted image of a human being with huge lips and hands and a tiny torso (Figure 2.9). The size of each body part in the figure roughly reflects the amount of cortex devoted to it, which explains why parts requiring extremely fine-tuned motor control (the mouth and hands) are gigantic in comparison to other body parts.

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ALBERT EINSTEIN AND THE PARIETAL LOBES Directly behind the frontal lobes on the crown of your head are the parietal lobes (Infographic 2.4). The parietal lobes help orient the body in space, are involved in tactile processing (for example, interpreting sensations related to touch, such as pain and pressure), and may play a role in mathematical reasoning associated with spatial cognition (Desco et al., 2011; Grabner et al., 2007; Wolpert, Goodbody, & Husain, 1998). A study published in 1999 compared the brain of Albert Einstein to a control group of 35 brain specimens from men who had donated their bodies for use in research. Prior to their deaths, these men had normal cognitive functioning, average intelligence, and no mental health issues. The researchers reported that Einstein’s brain did not weigh more than the average brain of the control group, but a region of his parietal lobe believed to be important for visual–spatial processing was 15% larger than those of the control group. They proposed that the differences in that specific region of the parietal lobe in Einstein’s brain may have been linked to his “exceptional intellect” in areas of visual-spatial cognition and mathematical thinking (Witelson, Kigar, & Harvey, 1999). Of course, the link between the size of Einstein’s parietal region and his “exceptional intellect” is correlational in nature, so we should be cautious in our interpretations.

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PENFIELD AND THE SOMATOSENSORY CORTEX The parietal lobes are home to the somatosensory cortex, a strip of brain running parallel to the motor cortex, which receives and integrates sensory information from all over the body (pain and temperature, for example). Penfield, the neurosurgeon who created the homunculus for the motor cortex, mapped the somatosensory cortex in the same way (Penfield & Boldrey, 1937; Figure 2.9). As you might expect, the most sensitive areas of the body like the face and tongue are oversized on the homunculus, whereas areas less sensitive to stimulation, such as the forearm and the calf, are smaller.

THE TEMPORAL LOBES AND THE AUDITORY CORTEX Below the parietal lobes, on the sides of your head, are the temporal lobes, which process auditory stimuli, recognize visual objects, especially faces, and play a key role in language comprehension and memory (Hickok & Poeppel, 2000; Infographic 2.4 on page 74). The temporal lobes are home to the auditory cortex, which receives information from the ears and allows us to “hear” sounds. In particular, researchers have supported the notion, based on studies of primate vocalizations, that the ability to recognize language has evolved over time and is processed within the temporal lobes (Scott, Blank, Rosen, & Wise, 2000; Squire, Stark, & Clark, 2004).

THE OCCIPITAL LOBES AND THE PRIMARY VISUAL CORTEX Visual information is initially processed in the occipital lobes, in the lower back of the head (Infographic 2.4). If you have ever suffered a severe blow to the rear of your head, you may remember “seeing stars,” probably because activity in the occipital lobes was disrupted (hopefully only for a few seconds). It is here, where the optic nerve connects to the primary visual cortex, that visual information is received and interpreted, and where we process that information (for example, about color, shape, and motion). (See Chapter 3 for more on the visual process.) Even for a person with healthy eyes, if this area were damaged, severe visual impairment could occur. At the same time, the individual would still be able to “see” vivid mental images (Bridge, Harrold, Holmes, Stokes, & Kennard, 2012).

LO 14 Describe the association areas and identify their functions.

THE ASSOCIATION AREAS In addition to the specialized functions of the different lobes described above, the cortex contains association areas whose role is to integrate information from all over the brain. The association areas are located in all four lobes; however, they are much harder to pinpoint than are the motor and sensory areas. The association areas allow us to learn (just as you’re doing now), to have abstract thoughts (for example, 2 + 2 = 4), and to carry out complex behaviors like texting and tweeting. The language-processing hubs we learned about earlier, Broca’s area and Wernicke’s area, are association areas that play a role in the production and comprehension of speech. In humans, the vast majority of the cortical surface is dedicated to the association areas.

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Question 1

1. split-brain.

2. callosotomy.

3. hemispherectomy.

4. lateralization.

Question 2

1. Wernicke’s area.

2. Broca’s area.

3. the visual field.

4. the corpus callosum.

Question 3

Question 4

1. neuroplasticity.

2. phrenology.

3. ablation.

4. lateralization.

Question 5

1. parietal lobes

2. frontal lobes

3. corpus callosum

4. temporal lobes

Question 6