2.3 The Cerebral Cortex and Our Divided Brain

56

The Cerebral Cortex

2-10 What are the functions of the various cerebral cortex regions?

cerebral [seh-REE-bruhl] cortex the intricate fabric of interconnected neural cells covering the cerebral hemispheres; the body’s ultimate control and information-processing center.

Older brain networks sustain basic life functions and enable memory, emotions, and basic drives. Newer neural networks within the cerebrum—the two cerebral hemispheres contributing 85 percent of the brain’s weight—form specialized work teams that enable our perceiving, thinking, and speaking. Like other structures above the brainstem (including the thalamus, hippocampus, and amygdala), the cerebral hemispheres come as a pair. Covering those hemispheres, like bark on a tree, is the cerebral cortex, a thin surface layer of interconnected neural cells. It is your brain’s thinking crown, your body’s ultimate control and information-processing center.

As we move up the ladder of animal life, the cerebral cortex expands, tight genetic controls relax, and the organism’s adaptability increases. Frogs and other small-cortex amphibians operate extensively on preprogrammed genetic instructions. The larger cortex of mammals offers increased capacities for learning and thinking, enabling them to be more adaptable. What makes us distinctively human mostly arises from the complex functions of our cerebral cortex.

The people who first dissected and labeled the brain used the language of scholars—Latin and Greek. Their words are actually attempts at graphic description: For example, cortex means “bark,” cerebellum is “little brain,” and thalamus is “inner chamber.”

RETRIEVE IT

Question

Xhr5vZOFvZ6BhZvsbpZ3cHAGenmSf6KQktp7DQw5jNlIcqgfUHrxmSz8nW5QnTn+c9gb76MXR/kp8anhVw+El8Jpr4InS7uFfN+navb1VHA+zJIXBdZ2FiIXI6EUNpER+0oaq+gOxk4H4Q8pS1fmgqStSviXYkKA7zovzCZQy7N25vJw78wUy+9aPVM8BYbUgj1ZpvwEm2bBpQBrpuZq7pE2o1W246LqBg2I3AqIJpcZ1+iyZMvdQ/Y/Hq4lAHq993hnTMReYmR79Q2p
ANSWERS: The brainstem; the cerebral cortex

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 of the brain’s nerve cells and 300 trillion synaptic connections (de Courten-Myers, 2005). Being human takes a lot of nerve.

frontal lobes portion of the cerebral cortex lying just behind the forehead; involved in speaking and muscle movements and in making plans and judgments.

parietal [puh-RYE-uh-tuhl] lobes portion of the cerebral cortex lying at the top of the head and toward the rear; receives sensory input for touch and body position.

occipital [ahk-SIP-uh-tuhl] lobes portion of the cerebral cortex lying at the back of the head; includes areas that receive information from the visual fields.

temporal lobes portion of the cerebral cortex lying roughly above the ears; includes the auditory areas, each receiving information primarily from the opposite ear.

Each hemisphere’s cortex is subdivided into four lobes, separated by prominent fissures, or folds (FIGURE 2.18). Starting at the front of your brain and moving over the top, there are the frontal lobes (behind your forehead), the parietal lobes (at the top and to the rear), and the occipital lobes (at the back of your head). Reversing direction and moving forward, just above your ears, you find the temporal lobes. Each of the four lobes carries out many functions, and many functions require the interplay of several lobes.

image
Figure 2.18: FIGURE 2.18 The cortex and its basic subdivisions

Functions of the Cortex

57

More than a century ago, surgeons found damaged cortical areas during autopsies of people who had been partially paralyzed or speechless. This rather crude evidence did not prove that specific parts of the cortex control complex functions like movement or speech. After all, if the entire cortex controlled speech and movement, damage to almost any area might produce the same effect. A TV with its power cord cut would go dead, but we would be fooling ourselves if we thought we had “localized” the picture in the cord.

motor cortex an area at the rear of the frontal lobes that controls voluntary movements.

MOTOR FUNCTIONS Scientists had better luck in localizing simpler brain functions. For example, in 1870, German physicians Gustav Fritsch and Eduard Hitzig made an important discovery: Mild electrical stimulation to parts of an animal’s cortex made parts of its body move. The effects were selective: Stimulation caused movement only when applied to an arch-shaped region at the back of the frontal lobe, running roughly ear-to-ear across the top of the brain. Moreover, stimulating parts of this region in the left or right hemisphere caused movements of specific body parts on the opposite side of the body. Fritsch and Hitzig had discovered what is now called the motor cortex.

MAPPING 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, occupy the greatest amount of cortical space (FIGURE 2.19). In one of his many demonstrations of motor behavior mechanics, Spanish neuroscientist José Delgado stimulated a spot on a patient’s left motor cortex, triggering the right hand to make a fist. Asked to keep the fingers open during the next stimulation, the patient, whose fingers closed despite his best efforts, remarked, “I guess, Doctor, that your electricity is stronger than my will” (Delgado, 1969, p. 114).

image
Figure 2.19: FIGURE 2.19 Left hemisphere tissue devoted to each body part in the motor cortex and the somatosensory cortex As you can see from this classic though inexact representation, the amount of cortex devoted to a body part in the motor cortex (in the frontal lobes) or in the somatosensory cortex (in the parietal lobes) is not proportional to that body part’s size. Rather, the brain devotes more tissue to sensitive areas and to areas requiring precise control. Thus, the fingers have a greater representation in the cortex than does the upper arm.

58

RETRIEVE IT

Question

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
ANSWERS: 1. The right limbs' opposed activities interfere with each other because both are controlled by the same (left) side of your brain. 2. Opposite sides of your brain control your left and right limbs, so the reversed motion causes less interference.

More recently, scientists were able to predict a monkey’s arm motion a tenth of a second before it moved—by repeatedly measuring motor cortex activity preceding specific arm movements (Gibbs, 1996). Such findings have opened the door to research on brain-controlled computers.

What might happen, some researchers are asking, if we implant a device to detect motor cortex activity in humans? Could such devices help severely paralyzed people learn to command a cursor to write e-mail or work online? Clinical trials are now under way with people who have suffered paralysis or amputation (Andersen et al., 2010; Nurmikko et al., 2010). The first patient, a paralyzed 25-year-old man, was able to mentally control a TV, draw shapes on a computer screen, and play video games—all thanks to an aspirin-sized chip with 100 microelectrodes recording activity in his motor cortex (Hochberg et al., 2006). Since then, others with paralysis who have been given implants have learned to direct robotic arms with their thoughts (Collinger et al., 2013; Hochberg et al., 2012).

somatosensory cortex area at the front of the parietal lobes that registers and processes body touch and movement sensations.

image
Figure 2.20: FIGURE 2.20 The brain in action This fMRI (functional MRI) scan shows the visual cortex in the occipital lobes activated (color represents increased bloodflow) as a research participant looks at a photo. When the person stops looking, the region instantly calms down.
NeuroImage, Vol. 4, V.P. Clark, K. Keill, J. Ma. Maisog, S. Courtney, L. G. Ungerleider, and J. V. Haxby, Functional Magnetic Resonance Imaging of Human Visual Cortex during Face Matching: A Comparison with Positron Emission Tomography, August 1996, with permission from Elsevier.
image
Figure 2.21: FIGURE 2.21 The visual cortex and auditory cortex The visual cortex in the occipital lobes at the rear of your brain receives input from your eyes. The auditory cortex, in your temporal lobes—above your ears—receives information from your ears.

SENSORY FUNCTIONS If the motor cortex sends messages out to the body, where does the cortex receive incoming messages? Penfield identified a cortical area—at the front of the parietal lobes, parallel to and just behind the motor cortex—that specializes in receiving information from the skin senses and from the movement of body parts. We now call this area the somatosensory cortex (FIGURE 2.19). 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 the body region, the larger the somatosensory cortex area devoted to it (FIGURE 2.19). Your supersensitive lips project to a larger brain area than do your toes, which is one reason we kiss rather than touch toes. Rats have a large area of the brain devoted to their whisker sensations, and owls to their hearing sensations.

Scientists have identified additional areas where the cortex receives input from senses other than touch. Any visual information you are receiving now is going to the visual cortex in your occipital lobes, at the back of your brain (FIGURES 2.20 and 2.21). Stimulated in the occipital lobes, you might see flashes of light or dashes of color. (In a sense, we do have eyes in the back of our head!) Having lost much of his right occipital lobe to a tumor removal, a friend of mine [DM’s] was blind to the left half of his field of vision. Visual information travels from the occipital lobes to other areas that specialize in tasks such as identifying words, detecting emotions, and recognizing faces.

Any sound you now hear is processed by your auditory cortex in your temporal lobes (just above your ears; see FIGURE 2.21). Most of this auditory information travels a circuitous route from one ear to the auditory receiving area above your opposite ear. If stimulated in your auditory cortex, you might hear a sound. MRI scans of people with schizophrenia have revealed active auditory areas in the temporal lobes during the false sensory experience of auditory hallucinations (Lennox et al., 1999). Even the phantom ringing sound experienced by people with hearing loss is—if heard in one ear—associated with activity in the temporal lobe on the brain’s opposite side (Muhlnickel, 1998).

59

RETRIEVE IT

Question

Our brain's R4qmLlFH1vCsIXGvMq7h7xQVi7Q= cortex registers and processes body touch and movement sensations. The g87dwysVOp/+0qnv cortex controls our voluntary movements.

association areas areas of the cerebral cortex that are not involved in primary motor or sensory functions; rather, they are involved in higher mental functions such as learning, remembering, thinking, and speaking

ASSOCIATION AREAS So far, we have pointed out small cortical areas that either receive sensory input or direct muscular output. Together, these 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 (the peach-colored areas in FIGURE 2.22), neurons are busy with higher mental functions—many of the tasks that make us human.

image
Figure 2.22: FIGURE 2.22 Areas of the cortex in four mammals More intelligent animals have increased “uncommitted” or association areas of the cortex. These vast areas of the brain are responsible for interpreting, integrating, and acting on sensory information and linking it with stored memories.

Electrically probing an association area won’t trigger any observable response. So, unlike the somatosensory and motor areas, association area functions cannot be neatly mapped. Their silence has led to what Donald McBurney (1996, p. 44) called “one of the hardiest weeds in the garden of psychology”: the claim that we ordinarily use only 10 percent of our brain. (If true, wouldn’t this imply a 90 percent chance that a bullet to your brain would land in an unused area?) Surgically lesioned animals and brain-damaged humans bear witness that association areas are not dormant. Rather, these areas interpret, integrate, and act on sensory information and link it with stored memories—a very important part of thinking. Simple tasks often increase activity in small brain patches, involving far less than 10 percent of the brain. Yet complex tasks integrate many islands of brain activity, some performing automatic tasks and others requiring conscious control (Chein & Schneider, 2012). The brain is a whole system, with no dead spot for a stray bullet.

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

image See LaunchPad’s Video: Case Studies for a helpful tutorial animation.

Frontal lobe damage also can alter personality and remove a person’s inhibitions. Consider the classic case study of railroad worker Phineas Gage. One afternoon in 1848, Gage, then 25 years old, was using a tamping iron 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, leaving his frontal lobes damaged (FIGURE 2.23). To everyone’s amazement, Gage was immediately able to sit up and speak, and after the wound healed he returned to work. But having lost some of the neural tracts that enabled his frontal lobes to control his emotions (Van Horn et al., 2012), the affable, soft-spoken man was now irritable, profane, and dishonest. This person, said his friends, was “no longer Gage.” His mental abilities and memories were intact, but his personality was not. (Although Gage lost his railroad job, he did, over time, adapt to his injury and find work as a stage coach driver [Macmillan & Lena, 2010].)

image
Figure 2.23: FIGURE 2.23 A blast from the past (a) Phineas Gage’s skull was kept as a medical record. Using measurements and modern neuroimaging techniques, researchers have reconstructed the probable path of the rod through Gage’s brain (Van Horn et al., 2012). (b) This photo shows Gage after his accident. (The image has been reversed to show the features correctly. Early photos, including this one, were actually mirror images.)
Collection of Jack and Beverly Wilgus

60

image
Measuring frontal lobe brakes With part of his left frontal lobe (in this downward-facing brain scan) lost to injury, Cecil Clayton became more impulsive and killed a deputy sheriff, for which, nine years later, his state executed him.
Cecil Clayton's brain scan, included with request for stay of execution filed with the Supreme Court, showing a missing portion of his frontal lobe.

Studies of others with damaged frontal lobes have revealed similar impairments. Not only may they become less inhibited (without the frontal lobe brakes on their impulses), but their moral judgments may seem unrestrained. Cecil Clayton lost 20 percent of his left frontal lobe in a 1972 sawmill accident. Thereafter, his intelligence test score dropped to an elementary school level and he displayed increased impulsivity. In 1996, he fatally shot a deputy sheriff. In 2015, when he was 74, the State of Missouri executed him (Williams, 2015).

Would you advocate pushing one person in front of a runaway trolley to save five others? Most people would not, but those with damage to a brain area behind the eyes are often untroubled by such ethical dilemmas (Koenigs et al., 2007). The frontal lobes help steer us away from violent actions (Molenberghs et al., 2015; Yang & Raine, 2009).

Association areas also perform other mental functions. The parietal lobes, parts of which were large and unusually shaped in Einstein’s normal-weight brain, enable mathematical and spatial reasoning (Ibos & Freedman, 2014; Witelson et al., 1999). On the underside of the right temporal lobe, another association area enables us to recognize faces. If a stroke or head injury destroyed this area of your brain, you would still be able to describe facial features and to recognize someone’s gender and approximate age, yet be strangely unable to identify the person as, say, Taylor Swift, or even your grandmother.

Nevertheless, complex mental functions don’t reside in any one place. There is no one spot in a rat’s small association cortex that, when damaged, will obliterate its ability to learn or remember a maze. Your memory, language, and attention result from synchronized activity among distinct brain areas and neural networks (Knight, 2007). Ditto for religious experience. More than 40 distinct brain regions become active in different religious states, such as prayer and meditation, indicating that there is no simple “God spot” (Fingelkurts & Fingelkurts, 2009). The point to remember: Our mental experiences arise from coordinated brain activity.

RETRIEVE IT

Question

J4W32RWOB+MgdirSRxC40RBmEKipymMiY+XMAucYbt8AqLtZsZ1xCZ/Fna4tem0XYdrzyiaS5CgZRTuh+4X0T3NlqUu1mFVSjyL4NDl3/rKvAUt4
ANSWER: Association areas are involved in higher mental functions—interpreting, integrating, and acting on information processed in other areas.

The Brain’s Plasticity

2-11 To what extent can a damaged brain reorganize itself, and what is neurogenesis?

plasticity the brain’s ability to change, especially during childhood, by reorganizing after damage or by building new pathways based on experience.

Our brains are sculpted not only by our genes but also by our experiences. In Chapter 4, we’ll focus more on how experience molds the brain. For now, let’s turn to another aspect of the brain’s plasticity: its ability to modify itself after damage.

image
Figure 2.24: FIGURE 2.24 Brain plasticity This 6-year-old had surgery to end her life-threatening seizures. Although most of an entire hemisphere was removed (see MRI of hemispherectomy above), her remaining hemisphere compensated by putting other areas to work. One Johns Hopkins medical team reflected on the child hemispherectomies they had performed. Although use of the opposite arm was compromised, the team reported being “awed” by how well the children had retained their memory, personality, and humor (Vining et al., 1997). The younger the child, the greater the chance that the remaining hemisphere can take over the functions of the one that was surgically removed (Choi, 2008; Danelli et al., 2013).
Joe McNally/Joe McNally Photography/Getty Images

Some brain-damage effects described earlier can be traced to two hard facts: (1) Severed brain and spinal cord neurons, unlike cut skin, usually do not regenerate. (If your spinal cord were severed, you would probably be permanently paralyzed.) And (2) some brain functions seem preassigned to specific areas. One newborn who suffered damage to temporal lobe facial recognition areas later remained unable to recognize faces (Farah et al., 2000). But there is good news: Some neural tissue can reorganize in response to damage. Under the surface of our awareness, the brain is constantly changing, building new pathways as it adjusts to little mishaps and new experiences.

61

Plasticity may also occur after serious damage, especially in young children (Kolb, 1989; see also FIGURE 2.24). The brain’s plasticity is good news for those with vision or hearing loss. Blindness or deafness makes unused brain areas available for other uses (Amedi et al., 2005). If a blind person uses one finger to read Braille, the brain area dedicated to that finger expands as the sense of touch invades the visual cortex that normally helps people see (Barinaga, 1992a; Sadato et al., 1996).

Plasticity also helps explain why some studies have found that deaf people have enhanced peripheral and motion-detection vision (Bosworth & Dobkins, 1999; Shiell et al., 2014). In deaf people whose native language is sign, the temporal lobe area normally dedicated to hearing waits in vain for stimulation. Finally, it looks for other signals to process, such as those from the visual system.

Similar reassignment may occur when disease or damage frees up other brain areas normally dedicated to specific functions. If a slow-growing left hemisphere tumor disrupts language (which resides mostly in the left hemisphere), the right hemisphere may compensate (Thiel et al., 2006). If a finger is amputated, the somatosensory cortex that received its input will begin to receive input from the adjacent fingers, which then become more sensitive (Fox, 1984). So what do you suppose was the sexual intercourse experience of one patient whose lower leg had been amputated? “I actually experience my orgasm in my [phantom] foot. [Note that in FIGURE 2.19, the toes region is adjacent to the genitals.] And there it’s much bigger than it used to be because it’s no longer just confined to my genitals” (Ramachandran & Blakeslee, 1998, p. 36).

neurogenesis the formation of new neurons.

Although the brain often attempts self-repair by reorganizing existing tissue, it sometimes attempts to mend itself by producing new brain cells. This process, known as neurogenesis, has been found in adult mice, birds, monkeys, and humans (Jessberger et al., 2008). These baby neurons originate deep in the brain and may then migrate elsewhere and form connections with neighboring neurons (Aimone et al., 2010; Gould, 2007).

Master stem cells that can develop into any type of brain cell have also been discovered in the human embryo. If mass-produced in a lab and injected into a damaged brain, might neural stem cells turn themselves into replacements for lost brain cells? Might surgeons someday be able to rebuild damaged brains, much as landscapers reseed damaged lawns? Stay tuned. Today’s biotech companies are exploring such possibilities. In the meantime, we can all benefit from natural promoters of neurogenesis, such as exercise, sleep, and nonstressful but stimulating environments (Iso et al., 2007; Pereira et al., 2007; Stranahan et al., 2006).

Our Divided Brain

2-12 What do split brains reveal about the functions of our two brain hemispheres?

Our brain’s look-alike left and right hemispheres serve differing functions. This lateralization is apparent after brain damage. Research spanning more than a century has shown that left hemisphere accidents, strokes, and tumors can impair reading, writing, speaking, arithmetic reasoning, and understanding. Similar right hemisphere damage has less visibly dramatic effects. Does this mean that the right hemisphere is just along for the ride? Many believed this was the case until the 1960s, when a fascinating chapter in psychology’s history began to unfold: Researchers found that the “minor” right hemisphere was not so limited after all.

Splitting the Brain

62

corpus callosum [KOR-pus kah-LOW-sum] the large band of neural fibers connecting the two brain hemispheres and carrying messages between them.

In 1961, Los Angeles neurosurgeons Philip Vogel and Joseph Bogen speculated that major epileptic seizures were caused by an amplification of abnormal brain activity bouncing back and forth between the two cerebral hemispheres, which work together as a whole system. If so, they wondered, could they end this biological tennis match by severing the corpus callosum, the wide band of axon fibers connecting the two hemispheres and carrying messages between them (FIGURE 2.25)? Vogel and Bogen knew that psychologists Roger Sperry, Ronald Myers, and Michael Gazzaniga had divided cats’ and monkeys’ brains in this manner, with no serious ill effects.

image
Figure 2.25: FIGURE 2.25 The corpus callosum This large band of neural fibers connects the two brain hemispheres. To photograph the half brain above left, a surgeon separated the hemispheres by cutting through the corpus callosum (see blue arrow) and lower brain regions. The high-resolution diffusion spectrum image above right, showing a top-facing brain from above, reveals brain neural networks within the two hemispheres, and the corpus callosum neural bridge between them.
Martin M. Rotker/Science Source
Dr. Patric Hagmann/CHUV, UNIL, Lausanne, Switzerland

split brain a condition resulting from surgery that isolates the brain’s two hemispheres by cutting the fibers (mainly those of the corpus callosum) connecting them.

image
Figure 2.26: FIGURE 2.26 The information highway from eye to brain

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 findings, we need to focus for a minute on the peculiar nature of our visual wiring, illustrated in FIGURE 2.26. Note that each eye receives sensory information from the entire visual field. But in each eye, 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, which usually controls speech. Information received by either hemisphere is quickly transmitted to the other across the corpus callosum. In a person with a severed corpus callosum, this information-sharing does not take place.

Knowing these facts, Sperry and Gazzaniga could send information to a patient’s left or right hemisphere. As the person stared at a spot, they flashed a stimulus to its right or left. They could do this with you, too, but in your intact brain, the hemisphere receiving the information would instantly pass the news to the other side. Because the split-brain surgery had cut the communication lines between the hemispheres, the researchers could, with these patients, quiz each hemisphere separately.

In an early experiment, Gazzaniga (1967) asked split-brain patients to stare at a dot as he flashed HE·ART on a screen (FIGURE 2.27). Thus, HE appeared in their left visual field (which transmits to the right hemisphere) and ART in the right field (which transmits to the left hemisphere). When he then asked them to say what they had seen, the patients reported that they had seen ART. But when asked to point to the word they had seen, they were startled when their left hand (controlled by the right hemisphere) pointed to HE. Given an opportunity to express itself, each hemisphere indicated what it had seen. The right hemisphere (controlling the left hand) intuitively knew what it could not verbally report.

63

image
Figure 2.27: FIGURE 2.27 One skull, two minds When an experimenter flashes the word HEART across the visual field, a woman with a split brain verbally reports seeing the portion of the word transmitted to her left hemisphere. However, if asked to indicate with her left hand what she saw, she points to the portion of the word transmitted to her right hemisphere. (From Gazzaniga, 1983.)

When a picture of a spoon was flashed to their right hemisphere, the patients could not say what they had viewed. But when asked to identify what they had viewed by feeling an assortment of hidden objects with their left hand, they readily selected the spoon. If the experimenter said, “Correct!” the patient might reply, “What? Correct? How could I possibly pick out the correct object when I don’t know what I saw?” It is, of course, the left hemisphere doing the talking here, bewildered by what the nonverbal right hemisphere knows.

A few people who have had split-brain surgery have been for a time bothered by the unruly independence of their left hand, which might unbutton a shirt while the right hand buttoned it, or put grocery store items back on the shelf after the right 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.” With a split brain, both hemispheres can comprehend and follow an instruction to copy—simultaneously—different figures with the left and right hands (Franz et al., 2000; see also FIGURE 2.28). (Reading these reports, one can fantasize a patient enjoying a solitary game of “rock, paper, scissors”—left versus right hand.)

64

image
Figure 2.28: FIGURE 2.28 Try this! People who have had split-brain surgery can simultaneously draw two different shapes.

“Do not let your left hand know what your right hand is doing.”

Matthew 6:3

image IMMERSIVE LEARNING Have you ever been asked if you are “left-brained” or “right-brained”? Consider this popular misconception with LaunchPad’s How Would You Know If People Can Be “Left-Brained” or “Right-Brained”?

When the “two minds” are at odds, the left hemisphere does mental gymnastics to rationalize reactions it does not understand. If a patient follows an order (“Walk”) sent to the right hemisphere, a strange thing happens. The left hemisphere, unaware of the order, doesn’t know why the patient begins walking. If asked why, the patient doesn’t reply, “I don’t know.” Instead, the left hemisphere improvises—“I’m going into the house to get a Coke.” Gazzaniga (1988), who described these patients as “the most fascinating people on earth,” realized that the conscious left hemisphere is an “interpreter” that instantly constructs explanations. The brain, he concluded, often runs on autopilot; it acts first and then explains itself.

RETRIEVE IT

Question

1yY+1Jbp0Mh88fz+3VL5uhQONltujUdCaAH8w05Cj0mH01kw6wid7tlfa/EHv4Yo0D9L/phyjgZXcnVAqA+Hi1xxnHodssBYYrE0HFozSgn+w1gK9rNAgyvf2fwvB0eA8EO5jVQ0svCdtiQL7CzaXNndthLqMlCNO2cpVYLBzJ+uXAaHxIBhjVuo/LS/UDaekSk9d7khTXTEMUpwNjuZZ6khk/JmSQdgMtTEU9faXPOCN4liD8TmTjfm5EoZHyw8rPo+nECk4U01RnKgFhOwN2lAC92qbNdyS5D4f6v7MgtclVIglMtP8Z2/3XitP6aLA/k3Y0ik7k9NeOTTBRCC1pQPmOLw68kY3y57ZeAnZn0O0AJgpPDgOgiDEqSNMCeuq5VSJoh4ayc+BtnbWF6kly4hZMy+OOIe
ANSWERS: 1. yes, 2. no, 3. green

Right-Left Differences in the Intact Brain

So, what about the 99.99+ percent of us with undivided brains? Does each of our hemispheres also perform distinct functions? Several different types of studies indicate they do. When a person performs a perceptual task, for example, brain waves, bloodflow, and glucose consumption reveal increased activity in the right hemisphere. When the person speaks or calculates, activity usually increases in the left hemisphere.

A dramatic demonstration of hemispheric specialization 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 person’s right arm falls limp. If the left hemisphere is controlling language, the patient will be speechless until the drug wears off. If the drug is injected into the artery to the right hemisphere, the left arm will fall limp, but the person will still be able to speak.

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 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 would disrupt a hearing person’s speaking (Corina, 1998).

Although the left hemisphere is skilled at making quick, literal interpretations of language, the right hemisphere excels at making inferences (Beeman & Chiarello, 1998; Bowden & Beeman, 1998; Mason & Just, 2004). It also helps us modulate our speech to make meaning clear—as when we say “Let’s eat, Grandpa” instead of “Let’s eat Grandpa” (Heller, 1990). The right hemisphere also helps orchestrate our self-awareness. People who suffer partial paralysis will sometimes stubbornly deny their impairment—strangely claiming they can move a paralyzed limb—if the damage is to the right hemisphere (Berti et al., 2005).

image For a helpful animated review of split-brain research, see LaunchPad’s PsychSim 6: Hemispheric Specialization.

Simply looking at the two hemispheres, so alike to the naked eye, who would suppose they contribute uniquely to the harmony of the whole? Yet a variety of observations—of people with split brains, of people with normal brains, and even of other species’ brains—converge beautifully, leaving little doubt that we have unified brains with specialized parts (Hopkins & Cantalupo, 2008; MacNeilage et al., 2009).

How does the brain’s intricate networking emerge? How does our heredity—the legacy of our ancestral history—conspire with our experiences to organize and “wire” the brain? To that we turn next.

REVIEW The Cerebral Cortex and Our Divided Brain

Learning Objectives

65

Test Yourself by taking a moment to answer each of these Learning Objective Questions (repeated here from within the chapter). Research suggests that trying to answer these questions on your own will improve your long-term memory of the concepts (McDaniel et al., 2009).

Question

VZikXDyHzQMMPHlQCw5VeEByZMilFxVPyugqI9E2GueT8wrF1IinudJlrAy0v4b2Jg71rNQa4hTqWS7ypQzUrKYOJPG5fmZcFzy0T9P/1tPtNH3OuIk5rIOEjHgi4FCRFC3TtUdQsnvBLuNb9FvtUEjnNx9eahWpuW8RyxdhTDgyBH0VEc0t7VwY8M5YBRpVcwrhUFBXaK67UbwUObTnqijqahxZSMz7TWdGoFtAxIxFrTiq
ANSWER: The cerebral cortex has two hemispheres, and each hemisphere has four lobes: the frontal, parietal, occipital, and temporal. Each lobe performs many functions and interacts with other areas of the cortex. The motor cortex, at the rear of the frontal lobes, controls voluntary movements. The somatosensory cortex, at the front of the parietal lobes, registers and processes body touch and movement sensations. Body parts requiring precise control (in the motor cortex) or those that are especially sensitive (in the somatosensory cortex) occupy the greatest amount of space. Most of the brain's cortex—the major portion of each of the four lobes—is devoted to uncommitted association areas, which integrate information involved in learning, remembering, thinking, and other higher-level functions. Our mental experiences arise from coordinated brain activity.

Question

IqEXAuy1t97lNyicieBkoDR30qDQvDSYFDT1eoD445NZHsr/ka+Qq23KMTOdlVLjiiXdzuvOrAjAOoSICcuYU2FMiJ7L04BpHdG9zuZkh1+JVnexgU6cIjJAkBwwANMdCh8jLxxK0mvLDc50oerpyGapt9OjX3r1XX3rsZ8LpQ9PWjj9n9ZDvNv1oFnO4goJ3xO2O9haZyMywarvNNZ48FwcZfzSkeBcuSGV8OMbiY/gbb7UkWpZ0QCRRRh4+SMzYqnvfg==
ANSWER: If one hemisphere is damaged early in life, the other will pick up many of its functions by reorganizing or building new pathways. This plasticity diminishes later in life. The brain sometimes mends itself by forming new neurons, a process known as neurogenesis.

Question

r+uOZVax3Wsi6QahbMJk0k7Pzwh6/DjkcoM489WMQsChKAE/VczJ6CVR6tzxS76H0kv1WYztZ0CrH7IBLVBDyPT01Q5S0QqWljE2x+XAayZVaMRPSPZvF19DtADOptC3d71izESe5yGjN9tRUGReN3LvinWKFH+rYgIDEWdguGDwt9j6LE7icvMvif5f3a8aVPQ5s9Ui2tyRH9yP+uAl+6/Da0W+JSbiBgtAwKpp4cg7dvtBwkzkCSevofc4Va8P
ANSWER: Split-brain research (experiments on people with a severed corpus callosum) has confirmed that in most people, the left hemisphere is the more verbal, and that the right hemisphere excels in visual perception and the recognition of emotion. Studies of healthy people with intact brains confirm that each hemisphere makes unique contributions to the integrated functioning of the brain.

Terms and Concepts to Remember

Test yourself on these terms.

Question

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

Experience the Testing Effect

Test yourself repeatedly throughout your studies. This will not only help you figure out what you know and don’t know; the testing itself will help you learn and remember the information more effectively thanks to the testing effect.

Question 2.19

b2md3jaavYngp9vaf6gef0doRMtLiX7e1JCD5FUhRQ21nPbUQM1dRfyaW0qBZxRB0gpitX2om3tEiBwxAwo6lrGuUncxbSdid0IFiIhCyEuJV9EkxATtrJVtlSSGX57lB7uYTijE2lJDKnYOnKdimuBT/voJ1gaummD8JF3lUD7ZisLX2LgzrsNNauC62s25t8T0s8COtbx/ZrA/bwLSu990WxH+2hMG2pn+6YGLNwZl8C4++dp50+RtMSOvlSefZ6qjnk3UUczfPDxpkrInaDe7J41WeaQm

Question 2.20

SgzGwqSl6M7skBxsm9I730xUBKYf2yte1L5rAxZ+CkcqVzQ2K9eM/F4UzVXj6mOIKi9jog51Dr9bM/FFV5DfHeOa1Xt57qvboZ5uf+44TnJdFlrSf0mxn4ooOaHaFUDL9Vugt8mtokPOuOhyca4t8TGnmJBifpifmORlr6cDW3RvO6nsZH9zdgfdGL7HghWHhs1w14hYevafRbUefkg4/ZY0+H22vfzZQsGrrL1bgg+B6Q4D8nsfTqgB9BRht5Z/J2wqDGz+a2Lqto8KvTSU676tvps=
ANSWER: The visual cortex is a neural network of sensory neurons connected via interneurons to other neural networks, including auditory networks. This allows you to integrate visual and auditory information to respond when a friend you recognize greets you at a party.

Question 2.21

M+EVkjrR5i9fyELJ7CEYJjJUskmrjyVf9fr0yKw3wTd3alfmR/7v2rcHu7KV46JbE4fWWYAl8jXld8G5qnpDjAJzYB0GdM7gwa0x25gKTf/kmqTzZd0olQxhgguaC5CodzsA7h9Gpo2Rux3GvAbbsool8Mqlldedagl+DjEXsxPZj1097TQ+4QWAVxnHas+tbekT3VbcqNCvBLEemyXyISgIeHF9X2P5YydC9BD62sUZtXDZf3CWVQuqAkai48c2FUcXQtArEw+s6YP88GIzrTRw4F7FtCyM

Question 2.22

4. Judging and planning are enabled by the 94eFyu0V4Eufe7li lobes.

Question 2.23

F2QLPp4CHLiAgeZxeZsUg3SUH1+bQMEujdPuyyW8W09q2ZpO6bSGnnyWuBZ7TGKlwjQiXfhg+Qofbqe1gqmv//ZWV6z9igKTvC+qKMkx0f9iODz9thSH56+BR201Az2FF4bjC2138zffNnH6uv5JhXF1Y2BSDStemzVXKQ9zivaprfUJqGbPT4BqNcE0aoWiw5x01Pk07FhSS504QAa5Z1sI/OKzOobrlmWmKRv9tKSc7Ey7v5xDJwkO90myXwcLXkbADnjaiA7G7M4epHIRBVqI68cuio26vhjR9URyZNiHdTAb9rzwzWros2g=
ANSWER: You would hear sounds, but without the temporal lobe association areas you would be unable to make sense of what you were hearing.

Question 2.24

6. The “uncommitted” areas that make up about three-fourths of the cerebral cortex are called Eo9NHhrSwLoYP6QDjR4lHe5ZKFO6jlOh .

Question 2.25

pqnfo8S2hMGKBfz1GH6+yFCGlZHs8G8/gscJYYuJb2jxGvMnRSo8O7UKz0yb/k0ziyMnOAmEDnCKykzBa44CGN2AC3kRgIrZ+ZELBIfGra9nIJbRUSAwEprET805QWTdv5x1MzPJ+ILRbHT6xa7BUBEDA7PgLsgIWcu87cDbpnBmZVMoQUrjqlSSNB5fhB/+btGbkR9LLIhpxzW0YRAiFD9Z4yrP9dxv6jYuS1eIY7bDXMxW+/hpXogmmhk=

Question 2.26

8. An experimenter flashes the word HERON across the visual field of a man whose corpus callosum has been severed. HER is transmitted to his right hemisphere and ON to his left hemisphere. When asked to indicate what he saw, the man says he saw udFaTX/RGto= but points to 9qBWswHPxKETJYNr .

Question 2.27

OdHhojRgQ5M2ErX5tzGyXYsGCtgtukBRFXRm8jfjBd5GKO1yqxoeNgPckS5riyMARoK0CC6awjzp9IxP68ZKeQZI0K4h4VmPVKQ/JDPjQsnXy279Sz1yNwosUecexezgwi2JrseLFK6agNKMmok1umiJW8NBCHh48gifyDKSlGOWW2IlDhMdqKNy84G2zZPlmwxJxJ4jNxumPyyN3vjRPn4jQonS/fz2p6f1QIBwYNblyLSXE/y0xo1eCbLFfzqdhM29F/ddALRzf08BrUmIgvSKtCZDRk4VYi6gGNEPnKOG6J/UKvHfpxMu1VdIEd6GXRxLbMr/oN7B1RRFlzS5smBu5Sg+4zgi

Question 2.28

7Z6HM0o20nz2fOq2UAuyoUlb1+Kk4slClOEg8fxKZ+H8bM4n7s7+un4XkkHQ4M/dwbNgh1GySqJItbA/YyuhDp6M4cr9gx3PWoy2PSRaG5levS5yZQbulvjvyPENkbl4UX2BsE4z4DB6l7a75eQV0BsB3SzFq0wUop6Syd+eU0rl7+fGYnFY4g7QSyMhesB6p3Q4SeVcl1haLjVISjP+aK7J0C1LuIZKrguONRjB4IouAgPx6AlbWSJoVbmDPEUy6+/yXYutEAzA67N9jl1kYiDsd/de7igVv7mylpvNwIPw5nr+zPGOqIV8PThxj3jXgrG7Uaa8wt8czEny86K+EQ==

Use image to create your personalized study plan, which will direct you to the resources that will help you most in image .