Psych Ch3

Evolutionary Development of the Central Nervous System

The central nervous system evolved from the very simple one found in simple animals to the elaborate nervous system in humans today. Even the simplest animals have sensory neurons and motor neurons for responding to the environment (Shepherd, 1988). For example, single-celled protozoa have molecules in their cell membrane that are sensitive to food in the water. Those molecules trigger the movement of tiny threads called cilia, which help propel the protozoa toward the food source. The first neurons appeared in simple invertebrates, such as jellyfish; the sensory neurons in the jellyfish's tentacles can feel the touch of a potentially dangerous predator, which prompts the jellyfish to swim to safety. If you're a jellyfish, this simple neural system is sufficient to keep you alive. The first central nervous system worthy of the name, though, appeared in flatworms. The flatworm has a collection of neurons in the head—a simple kind of brain— that includes sensory neurons for vision and taste and motor neurons that control feeding behavior. Emerging from the brain is a pair of tracts that form a spinal cord. • During the course of evolution, a major split in the organization of the nervous system occurred between invertebrate animals (those without a spinal column) and vertebrate animals (those with a spinal column). In all vertebrates, the central nervous system is organized into a hierarchy: The lower levels of the brain and spinal cord execute simpler functions, while the higher levels of the nervous system perform more complex functions. As you saw earlier, in humans, reflexes are accomplished in the spinal cord. At the next level, the midbrain executes the more complex task of orienting toward an important stimulus in the environment. Finally, a more complex task, such as imagining what your life will be like 20 years from now, is performed in the forebrain (Addis, Wong, & Schacter, 2007; Schacter, Addis, et al., 2012; Szpunar, Watson, & McDermott, 2007). • The forebrain undergoes further evolutionary advances in vertebrates. In lower vertebrate species such as amphibians (frogs and newts), the forebrain consists only of small clusters of neurons at the end of the neural tube. In higher vertebrates, including reptiles, birds, and mammals, the forebrain is much larger, and it evolves in two different patterns. Reptiles and birds have almost no cerebral cortex. By contrast, mammals have a highly developed cerebral cortex, which develops multiple areas that serve a broad range of higher mental functions. This forebrain development has reached its peak—so far—in humans (FIGURE 3.21). • The human brain, then, is not so much one remarkable thing; rather, it is a succession of extensions from a quite serviceable foundation. Like other species, humans have a hindbrain, and like those species, it performs important tasks to keep us alive. For some species, that's sufficient. All flatworms need to do to ensure their species' survival is eat, reproduce, and stay alive a reasonable length of time. But as the human brain evolved, structures in the midbrain and forebrain developed to handle the increasingly complex demands of the environment. The forebrain of a bullfrog is about as differentiated as it needs to be to survive in a frog's world. The human forebrain, however, shows substantial refinement, which allows for some remarkable, uniquely human abilities: self-awareness, sophisticated language use, abstract reasoning, and imagining, among others. • There is intriguing evidence that the human brain evolved more quickly than the brains of other species (Dorus et al., 2004). Researchers compared the sequences of 200 brain-related genes in mice, rats, monkeys, and humans and discovered a collection of genes that evolved more rapidly among primates. What's more, they found that primate brains not only evolved quickly compared to those of other species, but the brains of the primates who eventually became humans evolved even more rapidly. These results suggest that in addition to the normal adaptations that occur over the process of evolution, the genes for human brains took particular advantage of a variety of mutations (changes in a gene's DNA) along the evolutionary pathway (Vallender, Mekel-Bobrov, & Lahn, 2008). These results also suggest that the human brain is still evolving—becoming bigger and more adapted to the demands of the environment (Evans et al., 2005; Mekel-Bobrov et al., 2005). • Genes may direct the development of the brain on a large, evolutionary scale, but they also guide the development of an individual and, generally, the development of a species. Let's take a brief look at how genes and the environment contribute to the biological bases of behavior.

What Are Genes?

• A gene is the major unit of hereditary transmission. Historically, the term gene has been used to refer to two distinct but related concepts. The initial, relatively abstract concept of a gene referred to units of inheritance that specify traits such as eye color. More recently, genes have been defined as sections on a strand of DNA (deoxyribonucleic acid) that code for the protein molecules that affect traits. Genes are organized into large threads called chromosomes, strands of DNA wound around each other in a double-helix configuration (see FIGURE 3.22). The DNA in our chromosomes produces protein molecules through the action of a molecule known as messenger RNA (ribonucleic acid; mRNA), which communicates a copy of the DNA code to cells that produce proteins. Chromosomes come in pairs, and humans have 23 pairs each. These pairs of chromosomes are similar but not identical: You inherit one of each pair from your father and one from your mother. There's a twist, however: The selection of which of each pair is given to you is random. • Perhaps the most striking example of this random distribution is the determination of sex. In mammals, the chromosomes that determine sex are the X and Y chromosomes; females have two X chromosomes, whereas males have one X and one Y chromosome. You inherited an X chromosome from your mother because she has only X chromosomes to give. Your biological sex, therefore, was determined by whether you received an additional X chromosome or a Y chromosome from your father. • As a species, we share about 99% of the same DNA (and almost as much with other apes), but there is a portion of DNA that varies across individuals. Children share more of this variable portion of DNA with their parents than with more distant relatives or with nonrelatives: They share half their genes with each parent, a quarterof their genes with their grandparents, an eighth of their genes with cousins, and so on. The probability of sharing genes is called degree of relatedness. The most genetically related people are monozygotic twins (also called identical twins), who develop from the splitting of a single fertilized egg and therefore share 100% of their genes. Dizygotic twins (fraternal twins) develop from two separate fertilized eggs and share 50% of their genes, the same as any two siblings born separately. • Many researchers have tried to determine the relative influence of genetics on behavior. One way to do this is to compare a trait shown by monozygotic twins with that same trait among dizygotic twins. This type of research usually enlists twins who were raised in the same household, so that the impact of their environment (their socioeconomic status, access to education, parental child-rearing practices, environmental stressors) remains relatively constant. Finding that monozygotic twins have a higher presence of a specific trait suggests a genetic influence (Boomsma, Busjahn, & Peltonen, 2002). As an example, the likelihood that the dizygotic twin of a person who has schizophrenia (a mental disorder we'll discuss in greater detail in the Psychological Disorders chapter) will also develop schizophrenia is 27%. However, this statistic rises to 50% for monozygotic twins. This observation suggests a substantial genetic influence on the likelihood of developing schizophrenia. Monozygotic twins share 100% of their genes, and if one assumes environmental influences are relatively consistent for both members of the twin pair, the 50% likelihood can be traced to genetic factors. That sounds scarily high . . . until you realize that the remaining 50% probability must be due to environmental influences. In short, genetics can contribute to the development, likelihood, or onset of a variety of traits. But a more complete picture of genetic influences on behavior must always take the environmental context into consideration. Genes express themselves within an environment, not in isolation.

Studying the Brain's Electrical Activity

• A second approach to studying the link between brain structures and behavior involves recording the pattern of electrical activity of neurons. An electroencephalograph (EEG) is a device used to record electrical activity in the brain. Typically, electrodes are placed on the outside of the head, and even though the source of electrical activity in synapses and action potentials is far removed from these wires, the electric signals can be amplified several thousand times by the EEG. This provides a visual record of the underlying electrical activity, as shown in FIGURE 3.27. Using this technique, researchers can determine the amount of brain activity during different states of consciousness. For example, as you'll read in the Consciousness chapter, the brain shows distinctive patterns of electrical activity when awake versus asleep; in fact, there are even different brain-wave patterns associated with different stages of sleep. EEG recordings allow researchers to make these fundamental discoveries about the nature of sleep and wakefulness (Dement, 1978). The EEG can also be used to examine the brain's electrical activity when awake individuals engage in a variety of psychological functions, such as perceiving, learning, and remembering. • A different approach to recording electrical activity resulted in a more refined understanding of the brain's division of responsibilities, even at a cellular level. Nobel laureates David Hubel and Torsten Wiesel used a technique that inserted electrodes into the occipital lobes of anesthetized cats and observed the patterns of action potentials of individual neurons (Hubel, 1988). Hubel and Wiesel amplified the action potential signals through a loudspeaker so that the signals could be heard as clicks as well as seen on an oscilloscope. While flashing lights in front of the animal's eye, Hubel and Wiesel recorded the resulting activity of neurons in the occipital cortex. What they discovered was not much of anything: Most of the neurons did not respond to this kind of general stimulation. This was frustrating to them. Hubel (1988, p. 69) recalled years later that "we tried everything short of standing on our heads to get it to fire," but then they began to notice something interesting. • Nearing the end of what seemed like a failed set of experiments, they projected a glass slide that contained a black dot in front of the cat's eyes and heard a brisk flurry of clicks as the neurons in the cat's occipital lobe fired away! Observing carefully, they realized that the firing did not have anything to do with the black dot, but instead was produced by a faint but sharp shadow cast by the edge of the glass slide. They discovered that neurons in the primary visual cortex are activated whenever a contrast between light and dark occurs in part of the visual field, seen particularly well when the visual stimulus was a thick line of light against a dark background. In this case, the shadow caused by the edge of the slide provided the kind of contrast that prompted particular neurons to respond. They then found that each neuron responded vigorously only when presented with a contrasting edge at a particular orientation. Since then, many studies have shown that neurons in the primary visual cortex represent particular features of visual stimuli, such as contrast, shape, and color (Zeki, 1993). • These neurons in the visual cortex are known as feature detectors because they selectively respond to certain aspects of a visual image. For example, some neurons fire only when detecting a vertical line in the middle of the visual field, other neurons fire when a line at a 45° angle is perceived, and still others in response to wider lines, horizontal lines, lines in the periphery of the visual field, and so on (Livingstone & Hubel, 1988). The discovery of this specialized function for neurons was a huge leap forward in our understanding of how the visual cortex works. Feature detectors identify basic dimensions of a stimulus ("slanted line . . . other slanted line . . . horizontal line"); those dimensions are then combined during a later stage of visual processing to allow recognition and perception of a stimulus ("Oh, it's a letter A"). Other studies have identified a variety of features that are detected by sensory neurons. For example, some visual processing neurons in the temporal lobe are activated only when detecting faces (Kanwisher, 2000; Perrett, Rolls, & Caan, 1982). Neurons in this area are specialized for processing faces; damage to this area results in an inability to perceive faces. These complementary observations (showing that the type of function that is lost or altered when a brain area is damaged corresponds to the kind of information processed by neurons in that cortical area) provide the most compelling evidence linking the brain to behavior

. Components of the Central Nervous System

• Compared to the many divisions of the peripheral nervous system, the central nervous system may seem simple. After all, it has only two elements: the brain and the spinal cord. But those two elements are ultimately responsible for most of what we do as humans. • The spinal cord often seems like the brain's poor relation: The brain gets all the glory and the spinal cord just hangs around, doing relatively simple tasks. Those tasks, however, are pretty important: They keep you breathing, respond to pain, and move your muscles, allowing you to walk. What's more, without the spinal cord, the brain would not be able to put any of its higher processing into action. • Do you need your brain to tell you to pull your hand away from a hot stove? For some very basic behaviors such as this, the spinal cord doesn't need input from the brain at all. Connections between the sensory inputs and motor neurons in the spinal cord mediate spinal reflexes, simple pathways in the nervous system that rapidly generate muscle contractions. If you touch a hot stove, the sensory neurons that register pain send inputs directly into the spinal cord (see FIGURE 3.11). Through just a few synaptic connections within the spinal cord, interneurons relay these sensory inputs to motor neurons that connect to your arm muscles and direct you to quickly retract your hand. • More elaborate tasks require the collaboration of the spinal cord and the brain. The peripheral nervous system sends messages from sensory neurons through the spinal cord into the brain. The brain sends commands for voluntary movement through the spinal cord to motor neurons, whose axons project out to skeletal muscles. Damage to the spinal cord severs the connection from the brain to the sensory and motor neurons that are essential to sensory perception and movement. The location of the spinal injury often determines the extent of the abilities that are lost. As you can see in FIGURE 3.12, different regions of the spinal cord control different systems of the body. Individuals with damage at a particular level of the spinal cord lose sensations of touch and pain in body parts below the level of the injury, as well as a loss of motor control of the muscles in the same areas. A spinal injury higher up the cord usually predicts a much poorer prognosis, such as quadriplegia (loss of sensation and motor control over all limbs), breathing through a respirator, and lifelong immobility. • The late actor Christopher Reeve, who starred as Superman in four Superman movies, damaged his spinal cord in a horseback riding accident in 1995, resulting in loss of sensation and motor control in all of his body parts below the neck. Despite great efforts over several years, Reeve made only modest gains in his motor control and sensation, highlighting the extent to which we depend on communication from the brain through the spinal cord to the body, and showing how difficult it is to compensate for the loss of these connections (Edgerton et al., 2004). Sadly, Christopher Reeve died at age 52 in 2004 from complications due to his paralysis. On a brighter note, researchers are making progress in understanding the nature of spinal cord injuries and how to treat them by focusing on how the brain changes in response to injury (Blesch & Tuszynski, 2009; Dunlop, 2008), a process that is closely related to the concept of brain plasticity that we will examine later in this chapter.

Components of the Neuron

• During the 1800s, scientists began to turn their attention from studying the mechanics of limbs, lungs, and livers to studying the harder-to-observe workings of the brain. Philosophers wrote poetically about an "enchanted loom" that mysteriously wove a tapestry of behavior, and many scientists confirmed the metaphor (Corsi, 1991). To those scientists, the brain looked as though it were composed of a continuously connected lattice of fine threads, leading to the conclusion that it was one big woven web of material. However, in the late 1880s, Spanish physician Santiago Ramón y Cajal (1852-1934) learned about a new technique for staining neurons in the brain (DeFelipe & Jones, 1988). The stain highlighted the appearance of entire cells, revealing that they came in different shapes and sizes (see FIGURE 3.1). • Cajal discovered that neurons are complex structures composed of three basic parts: the cell body, the dendrites, and the axon (see FIGURE 3.2). Like cells in all organs of the body, neurons have a cell body (also called the soma), the largest component of the neuron that coordinates the information-processing tasks and keeps the cell alive. Functions such as protein synthesis, energy production, and metabolism take place here. The cell body contains a nucleus, which houses chromosomes that contain your DNA, or the genetic blueprint of who you are. The cell body is surrounded by a porous cell membrane that allows some molecules to flow into and out of the cell. • Unlike other cells in the body, neurons have two types of specialized extensions of the cell membrane that allow them to communicate: dendrites and axons. Dendrites receive information from other neurons and relay it to the cell body. The term dendrite comes from the Greek word for "tree"; indeed, most neurons have many dendrites that look like tree branches. The axon carries information to other neurons, muscles, or glands. Axons can be very long, even stretching up to a meter from the base of the spinal cord down to the big toe. • In many neurons, the axon is covered by a myelin sheath, an insulating layer of fatty material. The myelin sheath is composed of glial cells (named for the Greek word for "glue"), which are support cells found in the nervous system. Although there are 100 billion neurons busily processing information in your brain, there are 10 to 50 times that many glial cells serving a variety of functions. Some glial cells digest parts of dead neurons, others provide physical and nutritional support for neurons, and others form myelin to help the axon carry information more efficiently. An axon insulated with myelin can more efficiently transmit signals to other neurons, organs, or muscles. In fact, with demyelinating diseases, such as multiple sclerosis, the myelin sheath deteriorates, slowing the transmission of information from one neuron to another (Schwartz & Westbrook, 2000). This leads to a variety of problems, including loss of feeling in the limbs, partial blindness, and difficulties in coordinated movement and cognition (Butler, Corboy, & Filley, 2009). • Cajal also observed that the dendrites and axons of neurons do not actually touch each other. There's a small gap between the axon of one neuron and the dendrites or cell body of another. This gap is part of the synapse, the junction or region between the axon of one neuron and the dendrites or cell body of another (see FIGURE 3.3). Many of the 100 billion neurons in your brain have a few thousand synaptic junctions, so it should come as no shock that most adults have 100 to 500 trillion synapses. As you'll read shortly, the transmission of information across the synapse is fundamental to communication between neurons, a process that allows us to think, feel, and behave.

Functional Brain Imaging

• Functional brain imaging techniques show researchers much more than just the structure of the brain by allowing us to watch the brain in action. These techniques rely on the fact that activated brain areas demand more energy for their neurons to work. This energy is supplied through increased blood flow to the activated areas. Functional imaging techniques can detect such changes in blood flow. In positron emission tomography (PET), a harmless radioactive substance is injected into a person's bloodstream. Then the brain is scanned by radiation detectors as the person performs perceptual or cognitive tasks, such as reading or speaking. Areas of the brain that are activated during these tasks demand more energy and greater blood flow, resulting in a higher amount of the radioactivity in that region. The radiation detectors record the level of radioactivity in each region, producing a computerized image of the activated areas (see FIGURE 3.29). Note that PET scans differ from CT scans and MRIs in that the image produced shows activity in the brain while the person performs certain tasks. So, for example, a PET scan of a person speaking would show activation in Broca's area in the left frontal lobe. • For psychologists, the most widely used functional brain imaging technique nowadays is functional magnetic resonance imaging (fMRI), which detects the difference between oxygenated hemoglobin and deoxygenated hemoglobin when exposed to magnetic pulses. Hemoglobin is the molecule in the blood that carries oxygen to our tissues, including the brain. When active neurons demand more energy and blood flow, oxygenated hemoglobin concentrates in the active areas; fMRI detects the oxygenated hemoglobin and provides a picture of the level of activation in each brain area (see Figure 3.29). Just as MRI was a major advance over CT scans, functional MRI represents a similar leap in our ability to record the brain's activity during behavior. Both fMRI and PET allow researchers to localize changes in the brain very accurately. However, fMRI has a couple of advantages over PET. First, fMRI does not require any exposure to a radioactive substance. Second, fMRI can localize changes in brain activity across briefer periods than PET, which makes it more useful for analyzing psychological processes that occur extremely quickly, such as reading a word or recognizing a face. With PET, researchers often have to use experimental designs different from those they would use in the psychology laboratory in order to adapt to the limitations of PET technology. With fMRI, researchers can design experiments that more closely resemble the ones they carry out in the psychology laboratory. • Functional MRI can also be used to explore the relationship of brain regions with one another, using a recently developed technique referred to as resting state functional connectivity. As implied by the name, this technique does not require participants to perform a task; they simply rest quietly while fMRI measurements are made. Functional connectivity measures the extent to which spontaneous activity in different brain regions is correlated over time; brain regions whose activity is highly correlated are thought to be functionally connected with one another (Lee, Smyser, & Shimony, 2012). Functional connectivity measures have been used extensively in recent years to identify brain networks, that is, sets of brain regions that are closely connected to one another (Yeo et al., 2011). For example, functional connectivity helped to identify the default network (Gusnard & Raichle, 2001), a group of interconnected regions in the frontal, temporal, and parietal lobes that is involved in internally focused cognitive activities, such as remembering past events, imagining future events, daydreaming, and mind wandering (Andrews-Hanna, 2012; Buckner, Andrews-Hanna, & Schacter, 2008; see chapters on Memory and Consciousness). Functional connectivity, along with DTI (which measures structural connectivity), is used in studies conducted by the Human Connectome Project, and will contribute important information to the map of the human connectome

The Role of Environmental Factors

• Genes set the range of possibilities that can be observed in a population, but the characteristics of any individual within that range are determined by environmental factors and experience. The genetic capabilities that another species might enjoy, such as breathing underwater, are outside the range of your possibilities, no matter how much you might desire them. • With these parameters in mind, behavioral geneticists use calculations based on relatedness to compute the heritability of behaviors (Plomin, DeFries, et al., 2001). Heritability is a measure of the variability of behavioral traits among individuals that can be accounted for by genetic factors. Heritability is calculated as a proportion, and its numerical value (index) ranges from 0 to 1.00. A heritability of 0 means that genes do not contribute to individual differences in the behavioral trait; a heritability of 1.00 means that genes are the only reason for the individual differences. As you might guess, scores of 0 or 1.00 occur so infrequently that they serve more as theoretical limits than realistic values; almost nothing in human behavior is completely due to the environment or owed completely to genetic inheritance. Scores between 0 and 1.00, then, indicate that individual differences are caused by varying degrees of genetic and environmental contributions—a little stronger influence of genetics here, a little stronger influence of the environment there, but each always within the context of the other (Moffitt, 2005; Zhang & Meaney, 2010). • For human behavior, almost all estimates of heritability are in the moderate range, between .30 and .60. For example, a heritability index of .50 for intelligence indicates that half of the variability in intelligence test scores is attributable to genetic influences and the remaining half is due to environmental influences. Smart parents often (but not always) produce smart children; genetics certainly plays a role. But smart and not-so-smart children attend good or not-so-good schools, practice their piano lessons with more or less regularity, study or not study as hard as they might, have good and not-so-good teachers and role models, and so on. Genetics is only half the story in intelligence. Environmental influences also play a significant role in predicting the basis of intelligence (see the Intelligence chapter). • Heritability has proven to be a theoretically useful and statistically sound concept in helping scientists understand the relative genetic and environmental influences on behavior. However, there are four important points about heritability to bear in mind. First, remember that heritability is an abstract concept: It tells us nothing about the specific genes that contribute to a trait. Second, heritability is a population concept: It tells us nothing about an individual. Heritability provides guidance for understanding differences across individuals in a population rather than abilities within an individual. • Third, heritability is dependent on the environment. Just as behavior occurs within certain contexts, so do genetic influences. For example, intelligence isn't an unchanging quality: People are intelligent within a particular learning context, a social setting, a family environment, a socioeconomic class, and so on. Heritability, therefore, is meaningful only for the environmental conditions in which it was computed, and heritability estimates may change dramatically under other environmental conditions. Finally, heritability is not fate. It tells us nothing about the degree to which interventions can change a behavioral trait. Heritability is useful for identifying behavioral traits that are influenced by genes, but it is not useful for determining how individuals will respond to particular environmental conditions or treatments.

Types and Functions of Neurotransmitters

• Given that different kinds of neurotransmitters can activate different kinds of receptors, like a lock and key, you might wonder how many types of neurotransmitters are floating across synapses in your brain right now. Today we know that some 60 chemicals play a role in transmitting information throughout the brain and body and differentially affect thought, feeling, and behavior, but a few major classes seem particularly important. We'll summarize those here, and you'll meet some of these neurotransmitters again in later chapters. o Acetylcholine (ACh) is a neurotransmitter involved in a number of functions, including voluntary motor control. Acetylcholine is found in neurons of the brain and in the synapses where axons connect to muscles and body organs, such as the heart. Acetylcholine activates muscles to initiate motor behavior, but it also contributes to the regulation of attention, learning, sleeping, dreaming, and memory (Gais & Born, 2004; Hasselmo, 2006; Wrenn et al., 2006). Alzheimer's disease, a medical condition involving severe memory impairments (Salmon & Bondi, 2009), is associated with the deterioration of ACh-producing neurons. o Dopamine is a neurotransmitter that regulates motor behavior, motivation, pleasure, and emotional arousal. Because of its role in basic motivated behaviors, such as seeking pleasure or associating actions with rewards, dopamine plays a role in drug addiction (Baler & Volkow, 2006). High levels of dopamine have been linked to schizophrenia (Winterer & Weinberger, 2004), whereas low levels have been linked to Parkinson's disease. > o Glutamate is the major excitatory neurotransmitter in the brain, meaning that it enhances the transmission of information between neurons. GABA (gammaaminobutyric acid), in contrast, is the primary inhibitory neurotransmitter in the brain, meaning that it tends to stop the firing of neurons. Too much glutamate, or too little GABA, can cause neurons to become overactive, causing seizures. o Two related neurotransmitters influence mood and arousal: norepinephrine and serotonin. Norepinephrine is particularly involved in states of vigilance, or a heightened awareness of dangers in the environment (Ressler & Nemeroff, 1999). Serotonin is involved in the regulation of sleep and wakefulness, eating, and aggressive behavior (Dayan & Huys, 2009; Kroeze & Roth, 1998). Because both neurotransmitters affect mood and arousal, low levels of each have been implicated in mood disorders (Tamminga et al., 2002). o Endorphins are chemicals that act within the pain pathways and emotion centers of the brain (Keefe et al., 2001). The term endorphin is a contraction of endogenous morphine, and that's a pretty apt description. Morphine is a synthetic drug that has a calming and pleasurable effect; an endorphin is an internally produced substance that has similar properties, such as dulling the experience of pain and elevating moods. The "runner's high" experienced by many athletes as they push their bodies to painful limits of endurance can be explained by the release of endorphins in the brain (Boecker et al., 2008). • Each of these neurotransmitters affects thought, feeling, and behavior in different ways, so normal functioning involves a delicate balance of each. Even a slight imbalance—too much of one neurotransmitter or not enough of another—can dramatically affect behavior. These imbalances sometimes occur naturally: The brain doesn't produce enough serotonin, for example, which contributes to depressed or anxious moods. Other times a person may actively seek to cause imbalances. People who smoke, drink alcohol, or take drugs, legal or not, are altering the balance of neurotransmitters in their brains. The drug LSD, for example, is structurally very similar to serotonin, so it binds very easily with serotonin receptors in the brain, producing similar effects on thoughts, feelings, or behavior. In the next section, we'll look at how some drugs are able to "trick" receptor sites in just this way.

Genes, Epigenetics, and the Environment

• Is it genetics (nature) or the environment (nurture) that reigns supreme in directing a person's behavior? The emerging picture from current research is that both nature and nurture play a role in directing behavior, and the focus has shifted to examining the interaction of the two rather than the absolute contributions of either alone (Gottesman & Hanson, 2005; Rutter & Silberg, 2002; Zhang & Meaney, 2010).

How Drugs Mimic Neurotransmitters

• Many drugs that affect the nervous system operate by increasing, interfering with, or mimicking the manufacture or function of neurotransmitters (Cooper, Bloom, & Roth, 2003; Sarter, 2006). Agonists are drugs that increase the action of a neurotransmitter. Antagonists are drugs that block the function of a neurotransmitter. Some drugs alter a step in the production or release of the neurotransmitter, whereas others have a chemical structure so similar to a neurotransmitter that the drug is able to bind to that neuron's receptor. If, by binding to a receptor, a drug activates the neurotransmitter, it is an agonist; if it blocks the action of the neurotransmitter, it is an antagonist (see FIGURE 3.8). • For example, the drug L-dopa was developed to treat Parkinson's disease, a movement disorder characterized by tremors and difficulty initiating movement, caused by the loss of neurons that use the neurotransmitter dopamine. Dopamine is created in neurons by a modification of a common molecule called L-dopa. Ingesting L-dopa will elevate the amount of L-dopa in the brain and spur the surviving neurons to produce more dopamine. In other words, L-dopa acts as an agonist for dopamine. The use of L-dopa has been reasonably successful in the alleviation of Parkinson's disease symptoms (Muenter & Tyce, 1971; Schapira et al., 2009). However, the effectiveness of L-dopa typically decreases when used over a long period of time, so that many longtime users experience some symptoms of the disease. The actor Michael J. Fox, who was diagnosed with Parkinson's disease in 1991 and takes L-dopa, described the simple act of trying to brush his teeth in his memoir: Grasping the toothpaste is nothing compared to the effort it takes to coordinate the two-handed task of wrangling the toothbrush and strangling out a line of paste onto the bristles. By now, my right hand has started up again, rotating at the wrist in a circular motion, perfect for what I'm about to do. My left hand guides my right hand up to my mouth, and once the back of the Oral-B touches the inside of my upper lip, I let go. It's like releasing the tension on a slingshot and compares favorably to the most powerful state-of-the-art electric toothbrush on the market. With no off switch, stopping means seizing my right wrist with my left hand, forcing it down to the sink basin, and shaking the brush loose as though disarming a knife-wielding attacker. (Fox, 2009, pp. 2-3) • Many other drugs, including some street drugs, alter the actions of neurotransmitters. Let's look at a few more examples. Amphetamine is a popular drug that stimulates the release of norepinephrine and dopamine. In addition, both amphetamine and cocaine prevent the reuptake of norepinephrine and dopamine. The combination of increased release of norepinephrine and dopamine and prevention of their reuptake floods the synapse with those neurotransmitters, resulting in increased activation of their receptors. Both of these drugs therefore are strong agonists, although the psychological effects of the two drugs differ somewhat because of subtle distinctions in where and how they act on the brain. Norepinephrine and dopamine play a critical role in mood control, such that increases in either neurotransmitter result in euphoria, wakefulness, and a burst of energy. However, norepinephrine also increases heart rate. An overdose of amphetamine or cocaine can cause the heart to contract so rapidly that heartbeats do not last long enough to pump blood effectively, leading to fainting and sometimes death. • Methamphetamine, a variant of amphetamine, affects pathways for dopamine, serotonin, and norepinephrine at the neuron's synapses, making it difficult to interpret exactly how it works. But the combination of its agonist and antagonist effects alters the functions of neurotransmitters that help us perceive and interpret visual images, sometimes resulting in strange hallucinations. • Prozac, a drug commonly used to treat depression, is another example of a neurotransmitter agonist. Prozac blocks the reuptake of the neurotransmitter serotonin, making it part of a category of drugs called selective serotonin reuptake inhibitors, or SSRIs (Wong, Bymaster, & Engelman, 1995). People suffering from clinical depression typically have reduced levels of serotonin in their brains. By blocking reuptake, more of the neurotransmitter remains in the synapse longer and produces greater activation of serotonin receptors. Serotonin elevates mood, which can help relieve depression (Mann, 2005). • An antagonist with important medical implications is a drug called propranalol, one of a class of drugs called beta blockers that obstruct a receptor site for norepinephrine in the heart. Because norepinephrine cannot bind to these receptors, heart rate slows down, which is helpful for disorders in which the heart beats too fast or irregularly. Beta blockers are also prescribed to reduce the agitation, racing heart, and nervousness associated with stage fright (Mills & Dimsdale, 1991; for additional discussion of antianxiety and antidepression drug treatments, see the Treatment of Psychological Disorders chapter).

Structural Brain Imaging

• One of the first neuroimaging techniques developed was the computerized axial tomography (CT) scan. In a CT scan, a scanner rotates a device around a person's head and takes a series of X-ray photographs from different angles. Computer programs then combine these images to provide views from any angle. CT scans show different densities of tissue in the brain. For example, the higher-density skull looks white on a CT scan, the cortex shows up as gray, and the least dense fissures and ventricles in the brain look dark (see FIGURE 3.28). CT scans are used to locate lesions or tumors, which typically appear darker because they are less dense than the cortex. • Magnetic resonance imaging (MRI) uses a strong magnetic field to line up the nuclei of specific molecules in the brain tissue. Brief, but powerful, pulses of radio waves cause the nuclei to rotate out of alignment. When a pulse ends, the nuclei snap back in line with the magnetic field and give off a small amount of energy in the process. Different molecules have unique energy signatures when they snap back in line with the magnetic field, so these signatures can be used to reveal brain structures with different molecular compositions. MRI produces pictures of soft tissue at a better resolution than a CT scan, as you can see in Figure 3.28. These techniques give psychologists a clearer picture of the structure of the brain and can help localize brain damage (as when someone suffers a stroke), but they reveal nothing about the functions of the brain. • Diffusion tensor imaging (DTI) is a relatively recently developed type of MRI that is used to visualize white matter pathways, which are fiber bundles that connect both nearby and distant brain regions to one another. DTI measures the rate and direction of diffusion or movement of water molecules along white matter pathways. Because the diffusion of water molecules follows the direction of the pathway, information about the direction in which water molecules diffuse can be used to determine where a white matter pathway goes. Scientists can use measures based on the rate and direction of diffusion to assess the integrity of a white matter pathway, which is very useful in cases of neurological and psychological disorders (Thomason & Thompson, 2011). • Because DTI provides information about pathways that connect brain areas to one another, it is a critical tool in mapping the connectivity of the human brain, and plays a central role in an ambitious undertaking known as the Human Connectome Project. This is a collaborative effort funded by the National Institutes of Health beginning in 2009 that involves a partnership between researchers at Massachusetts General Hospital and UCLA, and another partnership between researchers at Washington University and the University of Minnesota. The main goal of the project is to provide a complete map of the connectivity of neural pathways in the brain: the human connectome (Toga et al., 2012). A unique and exciting feature of the Human Connectome Project is that the researchers have made available some of their results at their Web site (www.humanconnectomeproject.org), which include fascinating colorful images of some of the connection pathways they have discovered.

Investigating the Brain

• So far, you've read a great deal about the nervous system: how it's organized, how it works, what its components are, and what those components do. But one question remains largely unanswered: How do we know all of this? Anatomists can dissect a human brain and identify its structures, but they cannot determine which structures play a role in producing which behaviors by dissecting a nonliving brain. Scientists use a variety of methods to understand how the brain affects behavior. Let's consider three of the main ones: studying people with brain damage; studying the brain's electrical activity; and using brain imaging to study brain structure and watch the brain in action. Let's examine each of these ways of investigating the brain.

Brain Plasticity

• The cerebral cortex may seem like a fixed structure, one big sheet of neurons designed to help us make sense of our external world. Remarkably, though, sensory cortices are not fixed. They can adapt to changes in sensory inputs, a quality researchers call plasticity (i.e., the ability to be molded). As an example, if you lose your middle finger in an accident, the part of the somatosensory area that represents that finger is initially unresponsive (Kaas, 1991). After all, there's no longer any sensory input going from that location to that part of the brain. You might expect the left middle-finger neurons of the somatosensory cortex to wither away. However, over time, that area in the somatosensory cortex becomes responsive to stimulation of the fingers adjacent to the missing finger. The brain is plastic: Functions that were assigned to certain areas of the brain may be capable of being reassigned to other areas of the brain to accommodate changing input from the environment (Feldman, 2009). This suggests that sensory inputs "compete" for representation in each cortical area. (See the Real World box for a striking illustration of phantom limbs.) • Plasticity doesn't only occur to compensate for missing digits or limbs, however. An extraordinary amount of stimulation of one finger can result in that finger "taking over" the representation of the part of the cortex that usually represents other, adjacent fingers (Merzenich et al., 1990). For example, concert pianists have highly developed cortical areas for finger control: The continued input from the fingers commands a larger area of representation in the somatosensory cortices in the brain. Consistent with this observation, recent research indicates greater plasticity within the motor cortex of professional musicians compared with nonmusicians, perhaps reflecting an increase in the number of motor synapses as a result of extended practice (Rosenkranz, Williamon, & Rothwell, 2007). Similar findings have been obtained with quilters (who have highly developed areas for the thumb and forefinger, which are critical to their profession) and taxi drivers (who have overdeveloped brain areas in the hippocampus that are used during spatial navigation; Maguire, Woollett, & Spiers, 2006). • Plasticity is also related to a question you might not expect to find in a psychology text: How much exercise have you been getting lately? While we expect that you are spending countless happy hours reading this text, we also hope that you've been finding enough time for physical exercise. A large of number of studies in rats and other nonhuman animals indicate that physical exercise can increase the number of synapses and even promote the development of new neurons in the hippocampus (Hillman, Erickson, & Kramer, 2008; van Praag, 2009). Recent studies with people have begun to document beneficial effects of cardiovascular exercise on aspects of brain function and cognitive performance (Colcombe et al., 2004, 2006). Although these effects tend to be seen most clearly in older adults (okay, so it's time for your textbook authors to get on a treadmill), benefits have also been documented throughout the life span (Hertig & Nagel, 2012; Hillman et al., 2008; Roig et al., 2012). In fact, some researchers believe that this kind of activity-dependent brain plasticity is relevant to treating spinal cord injuries (which, as we saw, have a devastating impact on people's lives), because understanding how to maximize plasticity through exercise and training may help to guide rehabilitation efforts (Dunlop, 2008). It should be clear by now that the plasticity of the brain is not just an interesting theoretical idea; it has potentially important applications to everyday life (Bryck & Fisher, 2012).

Organization across hemispheres

• The first level of organization divides the cortex into the left and right hemispheres. The two hemispheres are more or less symmetrical in their appearance and, to some extent, in their functions. However, each hemisphere controls the functions of the opposite side of the body. This is called contralateral control, meaning that your right cerebral hemisphere perceives stimuli from and controls movements on the left side of your body, whereas your left cerebral hemisphere perceives stimuli from and controls movement on the right side of your body. • The cerebral hemispheres are connected to each other by commissures, bundles of axons that make possible communication between parallel areas of the cortex in each half. The largest of these commissures is the corpus callosum, which connects large areas of the cerebral cortex on each side of the brain and supports communication of information across the hemispheres (see FIGURE 3.18). This means that information received in the right hemisphere, for example, can pass across the corpus callosum and be registered, virtually instantaneously, in the left hemisphere

The Limbic System

• The hypothalamus also is part of the limbic system, a group of forebrain structures including the hypothalamus, the hippocampus, and the amygdala, which are involved in motivation, emotion, learning, and memory (Maclean, 1970; Papez, 1937). The limbic system is where the subcortical structures meet the cerebral cortex. • The hippocampus (from Latin for "sea horse," due to its shape) is critical for creating new memories and integrating them into a network of knowledge so that they can be stored indefinitely in other parts of the cerebral cortex. Individuals with damage to the hippocampus can acquire new information and keep it in awareness for a few seconds, but as soon as they are distracted, they forget the information and the experience that produced it (Scoville & Milner, 1957; Squire, 2009). This kind of disruption is limited to everyday memory for facts and events that we can bring to consciousness; memory of learned habitual routines or emotional reactions remains intact (Squire, Knowlton, & Musen, 1993). As an example, people with damage to the hippocampus can remember how to drive and talk, but they cannot recall where they have recently driven or a conversation they have just had. You will read more about the hippocampus and its role in creating, storing, and combining memories in the Memory chapter. The amygdala (from Latin for "almond," also due to its shape), located at the tip of each horn of the hippocampus, plays a central role in many emotional processes, particularly the formation of emotional memories (Aggleton, 1992). The amygdala attaches significance to previously neutral events that are associated with fear, punishment, or reward (LeDoux, 1992). As an example, think of the last time something scary or unpleasant happened to you: A car came barreling toward you as you started walking into an intersection or a ferocious dog leaped out of an alley as you passed by. Those stimuli—a car or a dog—are fairly neutral; you don't have a panic attack every time you walk by a used car lot. The emotional significance attached to events involving those stimuli is the work of the amygdala (McGaugh, 2006). When we are in emotionally arousing situations, the amygdala stimulates the hippocampus to remember many details surrounding the situation (Kensinger & Schacter, 2005). For example, people who lived through the terrorist attacks of September 11, 2001 remember vivid details about where they were, what they were doing, and how they felt when they heard the news, even years later (Hirst et al., 2009). In particular, the amygdala seems to be especially involved in encoding events as fearful (Adolphs et al., 1995; Sigurdsson et al., 2007). We'll have more to say about the amygdala in the Emotion and Motivation chapter. For now, keep in mind that a group of neurons the size of a lima bean buried deep in your brain help you to laugh, weep, or shriek in fright when the circumstances call for it.

A Role for Epigenetics

• The idea that genes are expressed within an environment is central to an important and rapidly growing area of research known as epigenetics: environmental influences that determine whether or not genes are expressed, or the degree to which they are expressed, without altering the basic DNA sequences that constitute the genes themselves. To understand how epigenetic influences work, it is useful to think about DNA as analogous to a script for a play or a movie. The biologist Nessa Carey (2012) offers the example of Shakespeare's Romeo and Juliet, which was made into a movie back in 1936 starring classic actors Leslie Howard and Norma Shearer, and in 1996 starring Leonardo DiCaprio and Claire Danes. Shakespeare's script formed the basis of both films, but the directors of the two films used the script in different ways, and the actors in the two films gave different performances. Thus, the final products departed from Shakespeare's script and were different from one another, even though Shakespeare's original script still exists. Something similar happens with epigenetics: depending on the environment, a gene can be expressed or not without altering the underlying DNA code. • The environment can influence gene expression through epigenetic marks, chemical modifications to DNA that can turn genes on or off. You can think of epigenetic marks as analogous to notes that the movie directors made on Shakespeare's script that determined how the script was used in a particular film. There are two widely studied epigenetic marks: o DNA methylation refers to adding a methyl group to DNA. There are special enzymes, referred to as epigenetic writers, whose role is to add methyl groups to DNA. Although adding a methyl group doesn't alter the basic DNA sequence, it switches off the methylated gene (see FIGURE 3.23). This process is roughly analogous to Claire Danes' director making notes that instruct her to ignore a certain portion of the Shakespeare script. That portion of the script—like the switched off gene—is still there, but its contents are not expressed. o Histone modification involves adding chemical modifications to proteins called histones that are involved in packaging DNA. We tend to visualize DNA as the free-floating double helix shown in Figure 3.22, but DNA is actually tightly wrapped around groups of histone proteins, as shown in Figure 3.23. However, whereas DNA methylation serves to switch genes off, histone modification can either switch genes off or turn them on. But just like DNA methylation, histone modifications influence gene expression without altering the underlying DNA sequence (Carey, 2012). • Okay, so now you have learned a lot of strange new terms and may have wondered whether Claire Danes' performance in Romeo and Juliet prepared her to play Carrie in Homeland (come to think of it, Carrie's relationship with Brody bears some resemblance to Shakespeare's doomed lovers). But what is the relevance of epigenetics to the brain and to psychology? It turns out to be more relevant than anyone suspected until the past decade or so. Experiments with rats and mice have shown that epigenetic marks left by DNA methylation and histone modifcation play a role in learning and memory (Bredy et al., 2007; Day & Sweatt, 2011; Levenson, & Sweatt, 2005). Several studies have linked epigenetic changes with responses to stress (Zhang & Meaney, 2010), including recent research with humans. For example, studies of nurses working in high-stress versus low-stress environments found differences between the two groups in DNA methylation (Alasaari et al., 2012). Subjective levels of stress in a sample of 92 Canadian adults, as well as physiological signs of stress, were correlated with levels of DNA methylation (Lam et al., 2012). Another study reported a link between DNA methylation and early life experience, with differences observed between individuals who grew up in relatively affluent households versus those who grew up in poverty, even when controlling for such factors as current socioeconomic status (Lam et al., 2012). Similar findings were reported in a separate study of 40 British adults (Borghol et al., 2012). These observations fit well with an important line of research described in the Hot Science box, showing that DNA methylation and histone modifications play a key role in the long-lasting effects of early experiences for both rats and humans.

Thalamus, Hypothalamus, and Pituitary Gland.

• The thalamus, hypothalamus, and pituitary gland, located in the center of the brain, interact closely with several other brain structures. They relay signals to and from these structures and also help to regulate them. • The thalamus relays and filters information from the senses and transmits the information to the cerebral cortex. The thalamus receives inputs from all the major senses except smell, which has direct connections to the cerebral cortex. The thalamus acts as a kind of computer server in a networked system, taking in multiple inputs and relaying them to a variety of locations (Guillery & Sherman, 2002). However, unlike the mechanical operations of a computer ("send input A to location B"), the thalamus actively filters sensory information, giving more weight to some inputs and less weight to others. The thalamus also closes the pathways of incoming sensations during sleep, providing a valuable function in not allowing information to pass to the rest of the brain. • The hypothalamus, located below the thalamus (hypo- is Greek for "under"), regulates body temperature, hunger, thirst, and sexual behavior. Although the hypothalamus is a tiny area of the brain, clusters of neurons in the hypothalamus oversee a wide range of basic behaviors, keeping body temperature, blood sugar levels, and metabolism within an optimal range for normal human functioning. Lesions to some areas of the hypothalamus result in overeating, whereas lesions to other areas leave an animal with no desire for food at all, highlighting that the hypothalamus plays a key role in regulating food intake (Berthoud & Morrison, 2008). Also, when you think about sex, messages from your cerebral cortex are sent to the hypothalamus to trigger the release of hormones. Finally, electric stimulation of some areas of the hypothalamus in cats can produce hissing and biting, whereas stimulation of other areas in the hypothalamus can produce what appears to be intense pleasure for an animal (Siegel et al., 1999). Researchers James Olds and Peter Milner found that a small electric current delivered to a certain region of a rat's hypothalamus was extremely rewarding for the animal (Olds & Milner, 1954). In fact, when allowed to press a bar attached to the electrode to initiate their own stimulation, rats would do so several thousand times an hour, often to the point of exhaustion! • Located below the hypothalamus is the pituitary gland, the "master gland" of the body's hormone-producing system, which releases hormones that direct the functions of many other glands in the body. The hypothalamus sends hormonal signals to the pituitary gland, which in turn sends hormonal signals to other glands to control stress, digestive activities, and reproductive processes. For example, when a baby suckles its mother's breast, sensory neurons in her breast send signals to her hypothalamus, which then signals her pituitary gland to release a hormone called oxytocin into the bloodstream (McNeilly et al., 1983). Oxytocin, in turn, stimulates the release of milk from reservoirs in the breast. The pituitary gland is also involved in the response to stress. When we sense a threat, sensory neurons send signals to the hypothalamus, which stimulates the release of adrenocorticotropic hormone (ACTH) from the pituitary gland. ACTH, in turn, stimulates the adrenal glands (above the kidneys) to release hormones that activate the sympathetic nervous system (Selye & Fortier, 1950). As you read earlier in this chapter, the sympathetic nervous system prepares the body to either meet the threat head-on or flee from the situation.

Using Brain Imaging to Study Structure and to Watch the Brain in Action

• The third major way that neuroscientists can peer into the workings of the human brain has only become possible within the past several decades. EEG readouts give an overall picture of a person's level of consciousness, and single-cell recordings shed light on the actions of particular clumps of neurons. The ideal of neuroscience, however, has been the ability to see the brain in operation during behavior. This goal has been steadily achieved thanks to a wide range of neuroimaging techniques that use advanced technology to create images of the living, healthy brain (Posner & Raichle, 1994; Raichle & Mintun, 2006). Structural brain imaging provides information about the basic structure of the brain and allows clinicians or researchers to see abnormalities in brain structure. Functional brain imaging, in contrast, provides information about the activity of the brain when people perform various kinds of cognitive or motor tasks.

Divisions of the Nervous System

• There are two major divisions of the nervous system: the central nervous system and the peripheral nervous system (see FIGURE 3.9). The central nervous system (CNS) is composed of the brain and spinal cord. The central nervous system receives sensory information from the external world, processes and coordinates this information, and sends commands to the skeletal and muscular systems for action. At the top of the CNS rests the brain, which contains structures that support the most complex perceptual, motor, emotional, and cognitive functions of the nervous system. The spinal cord branches down from the brain; nerves that process sensory information and relay commands to the body connect to the spinal cord. • The peripheral nervous system (PNS) connects the central nervous system to the body's organs and muscles. The peripheral nervous system is itself composed of two major subdivisions, the somatic nervous system and the autonomic nervous system. The somatic nervous system is a set of nerves that conveys information between voluntary muscles and the central nervous system. Humans have conscious control over this system and use it to perceive, think, and coordinate their behaviors. For example, reaching for your morning cup of coffee involves the elegantly orchestrated activities of the somatic nervous system: Information from the receptors in your eyes travels to your brain, registering that a cup is on the table; signals from your brain travel to the muscles in your arm and hand; feedback from those muscles tells your brain that the cup has been grasped, and so on. • In contrast, the autonomic nervous system (ANS) is a set of nerves that carries involuntary and automatic commands that control blood vessels, body organs, and glands. As suggested by its name, this system works on its own to regulate bodily systems, largely outside of conscious control. The ANS has two major subdivisions, the sympathetic nervous system and the parasympathetic nervous system. Each exerts a different type of control on the body. The sympathetic nervous system is a set of nerves that prepares the body for action in challenging or threatening situations (see FIGURE 3.10). For example, imagine that you are walking alone late at night and frightened by footsteps behind you in a dark alley. Your sympathetic nervous system kicks into action at this point: It dilates your pupils to let in more light, increases your heart rate and respiration to pump more oxygen to muscles, diverts blood flow to your brain and muscles, and activates sweat glands to cool your body. To conserve energy, the sympathetic nervous system inhibits salivation and bowel movements, suppresses the body's immune responses, and suppresses responses to pain and injury. The sum total of these fast, automatic responses is that they increase the likelihood that you can escape. • The parasympathetic nervous system helps the body return to a normal resting state. When you're far away from your would-be attacker, your body doesn't need to remain on red alert. Now the parasympathetic nervous system kicks in to reverse the effects of the sympathetic nervous system and return your body to its normal state. The parasympathetic nervous system generally mirrors the connections of the sympathetic nervous system. For example, the parasympathetic nervous system constricts your pupils, slows your heart rate and respiration, diverts blood flow to your digestive system, and decreases activity in your sweat glands. As you might imagine, the sympathetic and parasympathetic nervous systems coordinate to control many bodily functions. One example is sexual behavior. In men, the parasympathetic nervous system engorges the blood vessels of the penis to produce an erection, but the sympathetic nervous system is responsible for ejaculation. In women, the parasympathetic nervous system produces vaginal lubrication, but the sympathetic nervous system underlies orgasm. In both men and women, a successful sexual experience depends on a delicate balance of these two systems; in fact, anxiety about sexual performance can disrupt this balance. For example, sympathetic nervous system activation caused by anxiety can lead to premature ejaculation in males and lack of lubrication in females

Transcranial Magnetic Stimulation

• We noted earlier that scientists have learned a lot about the brain by studying the behavior of people with brain injuries. But, although brain damage may be related to particular patterns of behavior, that relationship may or may not be causal. Experimentation is the premier method for establishing causal relationships between variables, but scientists cannot ethically cause brain damage in human beings, thus they have not been able to establish causal relationships between particular kinds of brain damage and particular patterns of behavior. Functional neuroimaging techniques such as fMRI don't help on this point because they do not provide information about when a particular pattern of brain activity causes a particular behavior. Happily, scientists have discovered a way to mimic brain damage with a benign technique called transcranial magnetic stimulation (TMS; Barker, Jalinous, & Freeston, 1985; Hallett, 2000). If you've ever held a magnet under a piece of paper and used it to drag a pin across the paper's surface, you know that magnetic fields can pass through insulating material. The human skull is no exception. TMS delivers a magnetic pulse that passes through the skull and deactivates neurons in the cerebral cortex for a short period. Researchers can direct TMS pulses to particular brain regions (essentially turning them off) and then measure temporary changes in the way a person moves, sees, thinks, remembers, speaks, or feels. By manipulating the state of the brain, scientists can perform experiments that establish causal relationships. • For example, in an early study using TMS, scientists discovered that magnetic stimulation of the visual cortex temporarily impairs a person's ability to detect the motion of an object without impairing the person's ability to recognize that object (Beckers & Zeki, 1995). This intriguing discovery suggests that motion perception and object recognition are accomplished by different parts of the brain, but moreover, it establishes that activity in the visual cortex causes motion perception. More recent research has revealed that applying TMS to the specific part of the visual cortex responsible for motion perception also impairs the accuracy with which people reach for moving objects (Schenk et al., 2005) or for stationary objects when there is motion in the background of a visual scene (Whitney et al., 2007). These findings indicate that the visual motion area plays a crucial role in guiding actions when we're responding to motion in the visual environment. • Rather than relying solely on observational studies of people with brain injuries or the snapshots provided by fMRI or PET scans, researchers can also manipulate brain activity and measure its effects. Scientists have also begun to combine TMS with fMRI, allowing them to localize precisely where in the brain TMS is having its effect (Caparelli, 2007). Studies suggest that TMS has no harmful side effects (Anand & Hotson, 2002; Pascual-Leone et al., 1993), and this new tool has changed the study of how our brains create our thoughts, feelings, and actions.

The Distinct Roles of the Left and Right Hemispheres

• You'll recall that the cerebral cortex is divided into two hemispheres, although typically the two hemispheres act as one integrated unit. Sometimes, though, disorders can threaten the ability of the brain to function, and the only way to stop them is with radical methods. This is sometimes the case for people who suffer from severe, intractable epilepsy. Seizures that begin in one hemisphere cross the corpus callosum (the thick band of nerve fibers that allows the two hemispheres to communicate) to the opposite hemisphere and start a feedback loop that results in a kind of firestorm in the brain. To alleviate the severity of the seizures, surgeons can sever the corpus callosum in a procedure called a split-brain procedure. The result is that a seizure that starts in one hemisphere is isolated in that hemisphere because there is no longer a connection to the other side. This procedure helps people with epilepsy, but also produces some unusual, if not unpredictable, behaviors. • Nobel laureate Roger Sperry (1913-1994) was intrigued by the observation that the everyday behavior of people who had their corpus collosum severed did not seem to be affected by the operation. Did this mean that the corpus callosum played no role at all in behavior? Sperry thought that this conclusion was premature, reasoning that casual observations of everyday behaviors could easily fail to detect impairments that might be picked up by sensitive tests. To evaluate this idea experimentally, Sperry and his colleagues first showed that when the corpus callosum was severed in cats, learning was not transferred from one hemisphere to the other (Sperry, 1964). Later, Sperry designed several experiments that investigated the behaviors of people with split brains, and in the process, revealed a great deal about the independent functions of the left and right hemispheres (Sperry, 1964). Normally, any information that initially enters the left hemisphere is also registered in the right hemisphere and vice versa: The information comes in and travels across the corpus callosum, and both hemispheres understand what's going on (see FIGURE 3.25). But in a person with a split brain, information entering one hemisphere stays there. Without an intact corpus callosum, there's no way for that information to reach the other hemisphere. Sperry and his colleagues used this understanding of lateralized perception in a series of experiments. For example, they had a person with a split brain look at a spot in the center of a screen and then projected a stimulus either on the left side of the screen (the left visual field) or the right side of the screen (the right visual field), isolating the stimulus to the opposite hemisphere (see Figure 4.10 in the Sensation and Perception chapter for more details about how information from one visual field enters the opposite hemisphere). • The hemispheres themselves are specialized for different kinds of tasks. You just learned about Broca's and Wernicke's areas, which revealed that language processing is largely a left-hemisphere activity. So imagine that some information came into the left hemisphere of a person with a split brain, and she was asked to describe verbally what it was. No problem: The left hemisphere has the information, it's the "speaking" hemisphere, so she should have no difficulty verbally describing what she saw. But suppose she were asked to reach behind the screen with her left hand and pick up the object she just saw. Remember that the hemispheres exert contralateral control over the body, meaning that the left hand is controlled by the right hemisphere. But this person's right hemisphere has no clue what the object was because that information was received in the left hemisphere and was unable to travel to the right hemisphere! So, even though she saw the object and could verbally describe it, she would be unable to use the right hemisphere to perform other tasks regarding that object, such as correctly selecting it from a group with her left hand (see Figure 3.25). • Of course, information presented to the right hemisphere would produce complementary deficits. In this case, she might be presented with a familiar object in her left hand (e.g., a key), be able to demonstrate that she knew what it was (by twisting and turning the key in midair), yet be unable to verbally describe what she was holding. In this case, the information in the right hemisphere is unable to travel to the left hemisphere, which controls the production of speech. • Furthermore, suppose a person with a split brain was shown the unusual face in FIGURE 3.26. This is called a chimeric face, and it is assembled from half-face components of the full faces also shown in the figure. When asked to indicate which face was presented, she would indicate that she saw both faces because information about the face on the left is recorded in the right hemisphere and information about the face on the right is recorded in the left hemisphere (Levy, Trevarthen, & Sperry, 1972). • These split-brain studies reveal that the two hemispheres perform different functions and can work together seamlessly as long as the corpus callosum is intact. Without a way to transmit information from one hemisphere to the other, information remains in the hemisphere it initially entered and we become acutely aware of the different functions of each hemisphere. Of course, a person with a split brain can adapt to this by simply moving her eyes a little so that the same information independently enters both hemispheres. Split-brain studies have continued over the past few decades and continue to play an important role in shaping our understanding of how the brain works (Gazzaniga, 2006).

Insights from Functional Imaging

• PET and fMRI provide remarkable insights into the types of information processing that take place in specific areas of the brain. For example, when a person performs a simple perceptual task, such as looking at a circular checkerboard, the primary visual areas are activated. As you have read, when the checkerboard is presented to the left visual field, the right visual cortex shows activation, and when the checkerboard is presented to the right visual field, the left visual cortex shows activation (Fox et al., 1986). Similarly, when people look at faces, fMRI reveals strong activity in a region located near the border of the temporal and occipital lobes called the fusiform gyrus (Kanwisher, McDermott, & Chun, 1997). When this structure is damaged, people experience problems with recognizing faces— even faces of friends and family they've known for years—although they don't have problems with their eyes and can recognize visual objects other than faces (Mestry et al., 2012). Finally, when people perform a task that engages emotional processing (e.g., looking at sad pictures), researchers observe significant activation in the amygdala, which you learned earlier is linked with emotional arousal (Phelps, 2006). There is also increased activation in parts of the frontal lobe that are involved in emotional regulation; in fact, in the same areas that were most likely damaged in the case of Phineas Gage (Wang et al., 2005). • You'll recall from the Methods chapter that at the heart of the scientific method is the relationship between ideas and evidence. There is no statute of limitations on scientific investigation. In this case, these modern brain imaging techniques confirm the theories derived from studies of brain damage from over 100 years ago. When Broca and Wernicke reached their conclusions about language production and language comprehension, they had little more to go on than some isolated cases and good hunches. PET scans have since confirmed that different areas of the brain are activated when a person is listening to spoken language, reading words on a screen, saying words out loud, or thinking of related words. This suggests that different parts of the brain are activated during these related but distinct functions. Similarly, it was pretty clear to the physician who examined Phineas Gage that the location of Gage's injuries played a major role in his drastic change in personality and emotionality. fMRI scans have since confirmed that the frontal lobe plays a central role in regulating emotion. It's always nice when independent methods (in these instances, very old case studies and very recent technology) arrive at the same conclusions. As you'll also see at various points in the text, brain imaging techniques such as fMRI are also revealing new and surprising findings, such as the insights described in the Real World box (Brain Death and the Vegetative State). • Although the insights that we are obtaining from fMRI are exciting, it is important that we don't get too carried away with them, as sometimes happens in media depictions of fMRI results (Marcus, 2012). Consider, as an example, the topic of memory accuracy and distortion. Using experimental paradigms that you'll learn about in the chapter on Memory, fMRI studies have shown that activity in some parts of the brain is greater during the retrieval of accurate rather than inaccurate memories (Schacter & Loftus, 2013). Does that mean we are ready to use fMRI in the courtroom to determine whether a witness is recounting an accurate memory or an inaccurate memory? Schacter and Loftus argued that the answer to this question is an emphatic no. For example, we don't yet know whether the results of laboratory fMRI studies of memory, which typically use simple materials like words or pictures, generalize to the kinds of complex everyday events that are relevant in the courtroom. Furthermore, evidence that fMRI can distinguish accurate from inaccurate memories comes from studies in which brain activity is averaged across a group of participants. But in the courtroom we need to determine whether an individual is remembering accurately or not, and there is little evidence yet that fMRI can do so. More generally, it is important to think carefully about how fMRI evidence is obtained before leaping to conclusions about how that evidence can be used in everyday life.

Neurons: The Origin of Behavior

• An estimated 1 billion people watch the final game of World Cup soccer every 4 years. That's a whole lot of people, but to put it in perspective, it's still only a little over 14% of the estimated 7 billion people currently living on Earth. A more impressive number might be the 30 billion viewers who tune in to watch any of the World Cup action over the course of the tournament. But a really, really big number is inside your skull right now, helping you make sense of these big numbers you're reading about. There are approximately 100 billion nerve cells in your brain that perform a variety of tasks to allow you to function as a human being. • Humans have thoughts, feelings, and behaviors that are often accompanied by visible signals. Consider how you might feel on your way to meet a good friend. An observer might see a smile on your face or notice how fast you are walking; internally, you might mentally rehearse what you'll say to your friend and feel a surge of happiness as you approach her. All those visible and experiential signs are coordinated by the activity of your brain cells. The anticipation you have, the happiness you feel, and the speed of your feet are the result of information processing in your brain. In a way, all of your thoughts, feelings, and behaviors spring from cells in the brain that take in information and produce some kind of output trillions of times a day. These cells are neurons, cells in the nervous system that communicate with one another to perform information-processing tasks.

The Emotional Functions of the Frontal Lobes

• As you've already seen, the human frontal lobes are a remarkable evolutionary achievement. However, psychology's first glimpse at some functions of the frontal lobes came from a rather unremarkable fellow; so unremarkable, in fact, that a single event in his life defined his place in the annals of psychology's history (Macmillan, 2000). Phineas Gage was a muscular 25-year-old railroad worker. On September 13, 1848, in Cavendish, Vermont, he was packing an explosive charge into a crevice in a rock when the powder exploded, driving a 3-foot, 13-pound iron rod through his head at high speed (Harlow, 1848). As FIGURE 3.24 shows, the rod entered through his lower left jaw and exited through the middle top of his head. Incredibly, Gage lived to tell the tale. But his personality underwent a significant change. • Before the accident, Gage had been mild mannered, quiet, conscientious, and a hard worker. After the accident, however, he became irritable, irresponsible, indecisive, and given to profanity. The sad decline of Gage's personality and emotional life nonetheless provided an unexpected benefit to psychology. His case study was the first to allow researchers to investigate the hypothesis that the frontal lobe is involved in emotion regulation, planning, and decision making. Furthermore, because the connections between the frontal lobe and the subcortical structures of the limbic system were affected, scientists were able to understand better how the amygdala, hippocampus, and related brain structures interacted with the cerebral cortex (Damasio, 2005).

Neurons Specialized by Location

• Besides specialization for sensory, motor, or connective functions, neurons are also somewhat specialized depending on their location (see FIGURE 3.4). For example, Purkinje cells are a type of interneuron that carries information from the cerebellum to the rest of the brain and spinal cord. These neurons have dense, elaborate dendrites that resemble bushes. Pyramidal cells, found in the cerebral cortex, have a triangular cell body and a single, long dendrite among many smaller dendrites. Bipolar cells, a type of sensory neuron found in the retinas of the eye, have a single axon and a single dendrite. The brain processes different types of information, so a substantial amount of specialization at the cellular level has evolved to handle these tasks.

The Hindbrain

• If you follow the spinal cord from your tailbone to where it enters your skull, you'll find it difficult to determine where your spinal cord ends and your brain begins. That's because the spinal cord is continuous with the hindbrain, an area of the brain that coordinates information coming into and out of the spinal cord. The hindbrain looks like a stalk on which the rest of the brain sits, and it controls the most basic functions of life: respiration, alertness, and motor skills. The structures that make up the hindbrain include: the medulla, the reticular formation, the cerebellum, and the pons (see FIGURE 3.14). • The medulla is an extension of the spinal cord into the skull that coordinates heart rate, circulation, and respiration. Beginning inside the medulla and extending upward is a small cluster of neurons called the reticular formation, which regulates sleep, wakefulness, and levels of arousal. In one early experiment, researchers stimulated the reticular formation of a sleeping cat. This caused the animal to awaken almost instantaneously and remain alert. Conversely, severing the connections between the reticular formation and the rest of the brain caused the animal to lapse into an irreversible coma (Moruzzi & Magoun, 1949). The reticular formation maintains the same delicate balance between alertness and unconsciousness in humans. In fact, many general anesthetics work by reducing activity in the reticular formation, rendering the patient unconscious. • Behind the medulla is the cerebellum, a large structure of the hindbrain that controls fine motor skills. (Cerebellum is Latin for "little brain," and the structure does look like a small replica of the brain.) The cerebellum orchestrates the proper sequence of movements when we ride a bike, play the piano, or maintain balance while walking and running. It contributes to the fine-tuning of behavior: smoothing our actions to allow their graceful execution rather than initiating the actions (Smetacek, 2002). The initiation of behavior involves other areas of the brain; as you'll recall, different brain systems interact and are interdependent with one another. • The last major area of the hindbrain is the pons, a structure that relays information from the cerebellum to the rest of the brain. (Pons means "bridge" in Latin.) Although the detailed functions of the pons remain poorly understood, it essentially acts as a relay station or bridge between the cerebellum and other structures in the brain.

The Electrochemical Actions of Neurons: Information Processing

• Our thoughts, feelings, and actions depend on neural communication, but how does it happen? The communication of information within and between neurons proceeds in two stages: o Conduction is the movement of an electric signal within neurons, from the dendrites to the cell body, then throughout the axon. o Transmission is movement of electric signals from one neuron to another over the synapse. • Together, these stages are what scientists generally refer to as the electrochemical action of neurons.

The Cerebral Cortex

• Our tour of the brain has taken us from the very small (neurons) to the somewhat bigger (major divisions of the brain) to the very large: the cerebral cortex. The cortex is the highest level of the brain, and it is responsible for the most complex aspects of perception, emotion, movement, and thought (Fuster, 2003). It sits over the rest of the brain, like a mushroom cap shielding the underside and stem, and it is the wrinkled surface you see when looking at the brain with the naked eye. • The smooth surfaces of the cortex—the raised part—are called gyri (gyrus if you're talking about just one), and the indentations or fissures are called sulci (sulcus when singular). Sulci and gyri represent a triumph of evolution. The cerebral cortex occupies roughly the area of a newspaper page. Fitting that much cortex into a human skull is a tough task. But if you crumple a sheet of newspaper, you'll see that the same surface area now fits compactly into a much smaller space. The cortex, with its wrinkles and folds, holds a lot of brainpower in a relatively small package that fits comfortably inside the human skull (see FIGURE 3.17).The functions of the cerebral cortex can be understood at three levels: the separation of the cortex into two hemispheres, the functions of each hemisphere, and the role of specific cortical areas.

The Midbrain

• Sitting on top of the hindbrain is the midbrain, which is relatively small in humans. As you can see in FIGURE 3.15, the midbrain contains two main structures: the tectum and the tegmentum. The tectum orients an organism in the environment. The tectum receives stimulus input from the eyes, ears, and skin and moves the organism in a coordinated way toward the stimulus. For example, when you're studying in a quiet room and you hear a click behind and to the right of you, your body will swivel and orient to the direction of the sound; this is your tectum in action. • The tegmentum is involved in movement and arousal; it also helps to orient an organism toward sensory stimuli. The midbrain may be relatively small, but it is a central location of neurotransmitters involved in arousal, mood, and motivation and the brain structures that rely on them (White, 1996). You could survive if you had only a hindbrain and a midbrain. The structures in the hindbrain would take care of all the bodily functions necessary to sustain life, and the structures in the midbrain would orient you toward or away from pleasurable or threatening stimuli in the environment. But this wouldn't be much of a life. To understand where the abilities that make us fully human come from, we need to consider the last division of the brain.

The Development and Evolution of Nervous Systems

• The human brain is surprisingly imperfect. Why? Far from being a single, elegant machine—the enchanted loom the philosophers wrote so poetically about—the human brain is instead a system comprised of many distinct components that have been added at different times during the course of evolution. The human species has retained what worked best in earlier versions of the brain, then added bits and pieces to get us to our present state through evolution. • To understand the central nervous system, it is helpful to consider two aspects of its development. Prenatal development (growth from conception to birth), reveals how the nervous system develops and changes within each member of a species. Evolutionary development reveals how the nervous system in humans evolved and adapted from other species.

Structure of the Brain

• The human brain, weighing in at about three pounds, is really not much to look at. You already know that its neurons and glial cells are busy humming away, giving you potentially brilliant ideas, consciousness, and feelings. But which neurons in which parts of the brain control which functions? To answer that question, neuroscientists had to find a way of describing the brain that allows researchers to communicate with one another. It can be helpful to talk about areas of the brain from "bottom to top," noting how the different regions are specialized for different kinds of tasks. In general, simpler functions are performed at the "lower levels" of the brain, whereas more complex functions are performed at successively "higher" levels (see FIGURE 3.13). Or, as you'll see shortly, the brain can also be approached in a "side-by-side" fashion: Although each side of the brain is roughly analogous, one half of the brain specializes in some tasks that the other half doesn't. Although these divisions make it easier to understand areas of the brain and their functions, keep in mind that none of these structures or areas in the brain can act alone: They are all part of one big, interacting, interdependent whole. Let's look first at the divisions of the brain, and the responsibilities of each part, moving from the bottom to the top. Using this view, we can divide the brain into three parts: the hindbrain, the midbrain, and the forebrain (see Figure 3.13).

Prenatal Development of the Central Nervous System

• The nervous system is the first major bodily system to take form in an embryo (Moore, 1977). It begins to develop within the 3rd week after fertilization, when the embryo is still in the shape of a sphere. Initially, a ridge forms on one side of the sphere and then builds up at its edges to become a deep groove. The ridges fold together and fuse to enclose the groove, forming a structure called the neural tube. The tail end of the neural tube will remain a tube, and as the embryo grows larger, it forms the basis of the spinal cord. The tube expands at the opposite end, so that by the 4th week the three basic levels of the brain are visible. During the 5th week, the forebrain and hindbrain further differentiate into subdivisions. During the 7th week and later, the forebrain expands considerably to form the cerebral hemispheres. • As the embryonic brain continues to grow, each subdivision folds onto the next one and begins to form the structures easily visible in the adult brain (see FIGURE 3.20). The hindbrain forms the cerebellum and medulla, the midbrain forms the tectum and the tegmentum, and the forebrain subdivides further, separating the thalamus and hypothalamus from the cerebral hemispheres. Over time, the cerebral hemispheres undergo the greatest development, ultimately covering almost all the other subdivisions of the brain. • The ontogeny of the brain (how it develops within a given individual) is pretty remarkable. In about half the time it takes you to complete a 15-week semester, the basic structures of the brain are in place and rapidly developing, eventually allowing a newborn to enter the world with a fairly sophisticated set of abilities. In comparison, the phylogeny of the brain (how it developed within a particular species) is a much slower process. However, it, too, has allowed humans to make the most of the available brain structures, enabling us to perform an incredible array of tasks.

The Action Potential: Sending Signals across the Neuron

• The neuron maintains its resting potential most of the time. However, the biologists working with the giant squid axon (see photo on p. 84) noticed that they could produce a signal by stimulating the axon with a brief electric shock, which resulted in the conduction of an electric impulse down the length of the axon (Hausser, 2000; Hodgkin & Huxley, 1939). This electric impulse is called an action potential, an electric signal that is conducted along the length of a neuron's axon to a synapse. • The action potential occurred only when the electric shock reached a certain level, or threshold. When the shock was below this threshold, the researchers recorded only tiny signals, which dissipated rapidly. When the shock reached the threshold, a much larger signal, the action potential, was observed. Interestingly, increases in the electric shock above the threshold did not increase the strength of the action potential. The action potential is all or none: Electric stimulation below the threshold fails to produce an action potential, whereas electric stimulation at or above the threshold always produces the action potential. The action potential always occurs with exactly the same characteristics and at the same magnitude regardless of whether the stimulus is at or above the threshold. The biologists working with the giant squid axon observed another surprising property of the action potential: They measured it at a charge of about +40 millivolts, which is well above zero. This suggests that the mechanism driving the action potential could not simply be the loss of the −70 millivolt resting potential because this would have only brought the charge back to zero. So why does the action potential reach a value above zero? • The action potential occurs when there is a change in the state of the axon's membrane channels. Remember, during the resting potential, the channels that allow K+ to flow out are open. However, when an electric charge is raised to the threshold value, these channels briefly shut down, and channels that allow the flow of positively charged sodium ions (Na + ) are opened (see Figure 3.5b). We've seen already that Na + is typically much more concentrated outside the axon than inside. When the channels open, those positively charged ions (Na + ) flow inside, increasing the positive charge inside the axon relative to that outside. This flow of Na + into the axon pushes the action potential to its maximum value of +40 millivolts. • After the action potential reaches its maximum, the membrane channels return to their original state, and K + flows out until the axon returns to its resting potential. This leaves a lot of extra Na + ions inside the axon and a lot of extra K + ions outside the axon. During this period when the ions are imbalanced, the neuron cannot initiate another action potential, so it is said to be in a refractory period, the time following an action potential during which a new action potential cannot be initiated. The imbalance in ions is eventually reversed by an active chemical "pump" in the cell membrane that moves Na + outside the axon and moves K + inside the axon (the pump does not operate during the action potential; see Figure 3.5c). • So far we've described how the action potential occurs at one point in the neuron. But how does this electric charge move down the axon? It's a domino effect. When an action potential is generated at the beginning of the axon, it spreads a short distance, which generates an action potential at a nearby location on the axon. That action potential also spreads, initiating an action potential at another nearby location, and so on, thus conducting the charge down the length of the axon. This simple mechanism ensures that the action potential travels the full length of the axon and that it achieves its full intensity at each step, regardless of the distance traveled. The myelin sheath facilitates the conduction of the action potential. Myelin doesn't cover the entire axon; rather, it clumps around the axon with little break points between clumps, looking kind of like sausage links. These breakpoints are called the nodes of Ranvier, after French pathologist Louis-Antoine Ranvier, who discovered them (see FIGURE 3.6). When an electric current passes down the length of a myelinated axon, the charge seems to "jump" from node to node rather than having to traverse the entire axon (Poliak & Peles, 2003). This process is called saltatory conduction, and it helps speed the flow of information down the axon.

Electric Signaling: Conducting Information within a Neuron

• The neuron's cell membrane has small pores that act as channels to allow small electrically charged molecules, called ions, to flow in and out of the cell. It is this flow of ions across the neuron's cell membrane that creates the conduction of an electric signal within the neuron. How does it happen?

Organization within hemispheres

• The second level of organization in the cerebral cortex distinguishes the functions of the different regions within each hemisphere of the brain. Each hemisphere of the cerebral cortex is divided into four areas, or lobes: From back to front, these are the occipital lobe, the parietal lobe, the temporal lobe, and the frontal lobe, as shown in Figure 3.17. We'll examine the functions of these lobes in more detail in later chapters, noting how scientists have used a variety of techniques to understand the operations of the brain. For now, here's a brief overview of the main functions of each lobe. • The occipital lobe, located at the back of the cerebral cortex, processes visual information. Sensory receptors in the eyes send information to the thalamus, which in turn sends information to the primary areas of the occipital lobe, where simple features of the stimulus are extracted, such as the location and orientation of an object's edges (see the Sensation and Perception chapter for more details). These features are then processed into a more complex "map" of the stimulus onto the occipital cortex, leading to comprehension of what's being seen. As you might imagine, damage to the primary visual areas of the occipital lobe can leave a person with partial or complete blindness. Information still enters the eyes, which work just fine. But without the ability to process and make sense of the information at the level of the cerebral cortex, the information is as good as lost (Zeki, 2001). The parietal lobe, located in front of the occipital lobe, carries out functions that include processing information about touch. • The parietal lobe contains the somatosensory cortex, a strip of brain tissue running from the top of the brain down to the sides (see FIGURE 3.19). Within each hemisphere, the somatosensory cortex represents the skin areas on the contralateral surface of the body. Each part of the somatosensory cortex maps onto a particular part of the body. If a body area is more sensitive, a larger part of the somatosensory cortex is devoted to it. For example, the part of the somatosensory cortex that corresponds to the lips and tongue is larger than the area corresponding to the feet. The somatosensory cortex can be illustrated as a distorted figure, called a homunculus ("little man"), in which the body parts are rendered according to how much of the somatosensory cortex is devoted to them (Penfield & Rasmussen, 1950). Directly in front of the somatosensory cortex, in the frontal lobe, is a parallel strip of brain tissue called the motor cortex. Like the somatosensory cortex, different parts of the motor cortex correspond to different body parts. The motor cortex initiates voluntary movements and sends messages to the basal ganglia, cerebellum, and spinal cord. The motor and somatosensory cortices, then, are like sending and receiving areas of the cerebral cortex, taking in information and sending out commands as the case might be. • The temporal lobe, located on the lower side of each hemisphere, is responsible for hearing and language. The primary auditory cortex in the temporal lobe is analogous to the somatosensory cortex in the parietal lobe and the primary visual areas of the occipital lobe: It receives sensory information from the ears based on the frequencies of sounds (Recanzone & Sutter, 2008). Secondary areas of the temporal lobe then process the information into meaningful units, such as speech and words. The temporal lobe also houses the visual association areas that interpret the meaning of visual stimuli and help us recognize common objects in the environment (Martin, 2007). • The frontal lobe, which sits behind the forehead, has specialized areas for movement, abstract thinking, planning, memory, and judgment. As you just read, it contains the motor cortex, which coordinates movements of muscle groups throughout the body. Other areas in the frontal lobe coordinate thought processes that help us manipulate information and retrieve memories, which we can use to plan our behaviors and interact socially with others. In short, the frontal cortex allows us to do the kind of thinking, imagining, planning, and anticipating that sets humans apart from most other species (Schoenemann, Sheenan, & Glotzer, 2005; Stuss & Benson, 1986; Suddendorf & Corballis, 2007).

Subcortical Structures

• The subcortical (beneath the cortex) structures are nestled deep inside the brain, where they are quite protected. If you imagine sticking an index finger in each of your ears and pushing inward until they touch, that's about where you'd find the thalamus, hypothalamus, pituitary gland, limbic system, and basal ganglia (see Figure 3.16). Each of these subcortical structures plays an important role in relaying information throughout the brain, as well as performing specific tasks that allow us to think, feel, and behave as humans. Here we'll give you a brief introduction to each, and you'll read more about many of these structures in later chapters.

The Organization of the Nervous System

• We've seen how individual neurons communicate with each other. What's the bigger picture? Neurons work by forming circuits and pathways in the brain, which in turn influence circuits and pathways in other areas of the body. Without this kind of organization and delegation, neurons would be churning away with little purpose. Neurons are the building blocks that form nerves, or bundles of axons and the glial cells that support them. The nervous system is an interacting network of neurons that conveys electrochemical information throughout the body. In this section, we'll look at the major divisions and components of the nervous system.

Preview

• The symptoms of CTE, and the havoc they can wreak in the lives of affected individuals and their families, are stark reminders that our psychological, emotional, and social wellbeing depend critically on the health and integrity of the brain. the consequences of ctE also highlight that understanding neuroscience isn't just an academic exercise confined to scientific laboratories: the more we know about the brain, and the more people who know it, the better our chances of finding solutions to problems such as CTE. IN THIS CHAPTER, WE'LL CONSIDER HOW THE BRAIN WORKS, what happens when it doesn't, and how both states of affairs determine behavior. First, we'll introduce you to the basic unit of information processing in the brain, the neuron. the electrical and chemical activities of neurons are the starting point of all behavior, thought, and emotion. next, we'll consider the anatomy of the central nervous system, focusing especially on the brain, including its overall organization, key structures that perform different functions, and its evolutionary development. Finally, we'll discuss methods that allow us to study the brain and clarify our understanding of how it works. these include methods that examine the damaged brain and methods for scanning the living and healthy brain

Organization within specific lobes

• The third level of organization in the cerebral cortex involves the representation of information within specific lobes in the cortex. There is a hierarchy of processing stages from primary areas that handle fine details of information all the way up to association areas, which are composed of neurons that help provide sense and meaning to information registered in the cortex. For example, neurons in the primary visual cortex are highly specialized: some detect features of the environment that are in a horizontal orientation, others detect movement, and still others process information about human versus nonhuman forms. Secondary areas interpret the information extracted by these primary areas (shape, motion, etc.) to make sense of what's being perceived; in this case, perhaps a large cat leaping toward your face. Similarly, neurons in the primary auditory cortex register sound frequencies, but it's the association areas of the temporal lobe that allow you to turn those noises into the meaning of your friend screaming, "Look out for the cat!" Association areas, then, help stitch together the threads of information in the various parts of the cortex to produce a meaningful understanding of what's being registered in the brain. • A striking example of this property of association areas comes from the discovery of the mirror-neuron system. Mirror neurons are active when an animal performs a behavior, such as reaching for or manipulating an object, and are also activated when another animal observes that animal performing the same behavior. Mirror neurons are found in the frontal lobe (near the motor cortex) and in the parietal lobe (Rizzolatti & Craighero, 2004; Rizzolatti & Sinigaglia, 2010). They have been identified in birds, monkeys, and humans, and their name reflects the function they serve. Neuroimaging studies with humans have shown that mirror neurons are active when people watch someone perform a behavior, such as grasping in midair. But they are more highly activated when that behavior has some purpose or context, such as grasping a cup to take a drink (Iacoboni et al., 2005), and seem to be related to recognizing the goal someone has in carrying out an action and the outcome of the action, rather than to the particular movements a person makes while performing that action (Hamilton & Grafton, 2006, 2008; Iacoboni, 2009; Rizzolatti & Sinigaglia, 2010). In the Learning chapter we'll find out more about the role of mirror neurons in learning. • Finally, neurons in the association areas are usually less specialized and more flexible than neurons in the primary areas. As such, they can be shaped by learning and experience to do their job more effectively. This kind of shaping of neurons by environmental forces allows the brain flexibility our next topic

The Basal Ganglia

• There are several other structures in the subcortical area, but we'll consider just one more. The basal ganglia are a set of subcortical structures that directs intentional movements. The basal ganglia are located near the thalamus and hypothalamus; they receive input from the cerebral cortex and send outputs to the motor centers in the brain stem. One part of the basal ganglia, the striatum, is involved in the control of posture and movement. As we saw in the excerpt from Michael J. Fox's book, people who suffer from Parkinson's disease typically show symptoms of uncontrollable shaking and sudden jerks of the limbs and are unable to initiate a sequence of movements to achieve a specific goal. This happens because the dopamine-producing neurons in the substantia nigra (found in the tegmentum of the midbrain) have become damaged (Dauer & Przedborski, 2003). The undersupply of dopamine then affects the striatum in the basal ganglia, which in turn leads to the visible behavioral symptoms of Parkinson's. • So, what's the problem in Parkinson's: the jerky movements, the ineffectiveness of the striatum in directing behavior, the botched interplay of the substantia nigra and the striatum, or the underproduction of dopamine at the neuronal level? The answer is all of the above. This unfortunate disease provides a nice illustration of two themes regarding the brain and behavior. First, invisible actions at the level of neurons in the brain can produce substantial effects at the level of behavior. Second, the interaction of hindbrain, midbrain, and forebrain structures shows how the various regions are interdependent.

Major Types of Neurons

• There are three major types of neurons, each performing a distinct function: sensory neurons, motor neurons, and interneurons. Sensory neurons receive information from the external world and convey this information to the brain via the spinal cord. They have specialized endings on their dendrites that receive signals for light, sound, touch, taste, and smell. For example, sensory neurons' endings in our eyes are sensitive to light. Motor neurons carry signals from the spinal cord to the muscles to produce movement. These neurons often have long axons that can stretch to muscles at our extremities. However, most of the nervous system is composed of the third type of neuron, interneurons, which connect sensory neurons, motor neurons, or other interneurons. Some interneurons carry information from sensory neurons into the nervous system, others carry information from the nervous system to motor neurons, and still others perform a variety of information-processing functions within the nervous system. Interneurons work together in small circuits to perform simple tasks, such as identifying the location of a sensory signal, and much more complicated ones, such as recognizing a familiar face.

Studying the Damaged Brain

• To understand the normal operation of a process better, it is instructive to understand what happens when that process fails. Much research in neuroscience correlates the loss of specific perceptual, motor, emotional, or cognitive functions with specific areas of brain damage (Andrewes, 2001; Kolb & Whishaw, 2003). By studying these instances, neuroscientists can theorize about the functions those brain areas normally perform. The modern history of neuroscience can be dated to the work of Paul Broca roz (see the Psychology: Evolution of a Science chapter). In 1861, Broca described a patient who had lost the capacity to produce spoken language (but not the ability to understand language) due to damage in a small area in the left frontal lobe. In 1874, Carl Wernicke (1848-1905) described a patient with an impairment in language comprehension (but not the ability to produce speech) associated with damage to an area in the upper-left temporal lobe. These areas were named, respectively, Broca's area and Wernicke's area (see Figure 9.3 in the Language and Thought chapter), and they provided the earliest evidence that the brain locations for speech production and speech comprehension are separate and that for most people, the left hemisphere is critical to producing and understanding language (Young, 1990)

Chemical Signaling: Transmission between Neurons

• When the action potential reaches the end of an axon, you might think that it stops there. After all, the synaptic space between neurons means that the axon of one neuron and the neighboring neuron's dendrites do not actually touch one another. However, the electric charge of the action potential takes a form that can cross the relatively small synaptic gap by relying on a bit of chemistry. Axons usually end in terminal buttons, knoblike structures that branch out from an axon. A terminal button is filled with tiny vesicles, or "bags," that contain neurotransmitters, chemicals that transmit information across the synapse to a receiving neuron's dendrites. The dendrites of the receiving neuron contain receptors, parts of the cell membrane that receive neurotransmitters and either initiate or prevent a new electric signal. • As K + and Na + flow across a cell membrane, they move the sending neuron, or presynaptic neuron, from a resting potential to an action potential. The action potential travels down the length of the axon to the terminal buttons, where it stimulates the release of neurotransmitters from vesicles into the synapse. These neurotransmitters float across the synapse and bind to receptor sites on a nearby dendrite of the receiving neuron, or postsynaptic neuron. A new action potential is initiated in that neuron, and the process continues down that neuron's axon to the next synapse and the next neuron. This electrochemical action, called synaptic transmission (see FIGURE 3.7), allows neurons to communicate with one another and ultimately underlies your thoughts, emotions, and behavior. • Now that you understand the basic process of how information moves from one neuron to another, let's refine things a bit. You'll recall that a given neuron may make a few thousand synaptic connections with other neurons, so what tells the dendrites which of the neurotransmitters flooding into the synapse to receive? One answer is that neurons tend to form pathways in the brain that are characterized by specific types of neurotransmitters; one neurotransmitter might be prevalent in one part of the brain, whereas a different neurotransmitter might be prevalent in a different part of the brain. • A second answer is that neurotransmitters and receptor sites act like a lock-and-key system. Just as a particular key will only fit in a particular lock, so, too, will only some neurotransmitters bind to specific receptor sites on a dendrite. The molecular structure of the neurotransmitter must "fit" the molecular structure of the receptor site. • Another question is what happens to the neurotransmitters left in the synapse after the chemical message is relayed to the postsynaptic neuron? Something must make neurotransmitters stop acting on neurons; otherwise, there'd be no end to the signals that they send. Neurotransmitters leave the synapse through three processes (see Figure 3.7). First, reuptake occurs when neurotransmitters are reabsorbed by the terminal buttons of the presynaptic neuron's axon. Second, neurotransmitters can be destroyed by enzymes in the synapse in a process called enzyme deactivation; specific enzymes break down specific neurotransmitters. Finally, neurotransmitters can bind to the receptor sites called autoreceptors on the presynaptic neurons. Autoreceptors detect how much of a neurotransmitter has been released into a synapse and signal the neuron to stop releasing the neurotransmitter when an excess is present.

The Forebrain

• When you appreciate the beauty of a poem, detect the sarcasm in a friend's remark, plan to go skiing next winter, or notice the faint glimmer of sadness on a loved one's face, you are enlisting the forebrain. The forebrain is the highest level of the brain— literally and figuratively—and controls complex cognitive, emotional, sensory, and motor functions. The forebrain itself is divided into two main sections: the cerebral cortex and the subcortical structures. • The cerebral cortex is the outermost layer of the brain, visible to the naked eye, and divided into two hemispheres. The subcortical structures are areas of the forebrain housed under the cerebral cortex near the center of the brain (see FIGURE 3.16). We'll have much more to say about the two hemispheres of the cerebral cortex and the functions they serve in the next section, fittingly saving the highest level of the brain for last. First, we'll examine the subcortical structures.

The Resting Potential: The Origin of the Neuron's Electrical Properties

•. Neurons have a natural electric charge called the resting potential, the difference in electric charge between the inside and outside of a neuron's cell membrane (Kandel, 2000). When first discovered by biologists in the 1930s, the resting potential was measured at about about −70 millivolts. This is a much smaller charge than a typical battery; for example, a 9-volt battery has 9,000 millivolts (Klein & Thorne, 2007). • The resting potential arises from the difference in concentrations of ions inside and outside the neuron's cell membrane (see FIGURE 3.5a). Ions can carry a positive (+) or a negative (−) charge. In the resting state, there is a high concentration of a positively charged ion, potassium (K + ), as well as negatively charged protein ions (A - ), inside the neuron's cell membrane compared to outside it. By contrast, there is a high concentration of positively charged sodium ions (Na + ) and negatively charged chloride ions (Cl - ) outside the neuron's cell membrane. • The concentration of K + inside and outside a neuron is controlled by channels in the cell membrane that allow K + molecules to flow in and out of the neuron. In the resting state, the channels that allow K + molecules to flow freely across the cell membrane are open, while channels that allow the flow of Na+ and the other ions noted earlier are generally closed. Because of the naturally higher concentration of K+ molecules inside the neuron, some K + molecules move out of the neuron through the open channels, leaving the inside of the neuron with a charge of about −70 millivolts relative to the outside. Like the Hoover Dam that holds back the Colorado River until the floodgates are released, resting potential is potential energy, because it creates the environment for a possible electrical impulse.