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biological foundation of behaviourDocument Transcript
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 76 impos03 414:mhspy7ch03:spy7ch03%0: 3 Biological Foundations of Behavior
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 77 impos03 414:mhspy7ch03:spy7ch03%0: Chapter Outline Learning Goals THE NERVOUS SYSTEM 1 Discuss the nature and basic functions of the nervous system. Characteristics w Pathways in the Nervous System w Divisions of the Nervous System NEURONS 2 Explain what neurons are and how they process information. Specialized Cell Structure w The Neural Impulse w Synapses and Neurotransmitters w Neural Networks STRUCTURES OF THE BRAIN AND THEIR FUNCTIONS 3 Identify the brain’s levels and structures, and summarize How the Brain and Nervous System Are Studied the functions of its structures. w Levels of Organization in the Brain w The Cerebral Cortex w The Cerebral Hemispheres and Split-Brain Research w Integration of Function in the Brain THE ENDOCRINE SYSTEM 4 State what the endocrine system is and how it affects behavior. BRAIN DAMAGE, PLASTICITY, AND REPAIR 5 Describe the brain’s capacity for recovery and repair. The Brain’s Plasticity and Capacity for Repair w Brain Tissue Implants GENETIC AND EVOLUTIONARY BLUEPRINTS OF BEHAVIOR 6 Explain how genetics and evolutionary psychology increase Chromosomes, Genes, and DNA our understanding of behavior. w The Study of Genetics w Genetics and Evolution
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 78 impos03 414:mhspy7ch03:spy7ch03%0: When Brandi Binder was just 6 years old, left side of her brain took over functions surgeons at the University of California at that are based on the right side. Although Los Angeles removed the right side of her her recuperation has not been 100 percent— cerebral cortex (the outermost part and she never regained the use of her left arm, highest level of the brain) in an effort to for example—her recovery is remarkable. subdue frequent seizures caused by very se- Her story shows that if there is a way to vere and uncontrollable epilepsy. compensate for damage, the brain will ﬁnd Epileptic seizures like the ones experi- it (Nash, 1997). enced by Brandi are the result of electrical It is not by coincidence that the human “brainstorms” that ﬂash uncontrollably from brain is so versatile. It has evolved over mil- one side of the brain to the other. Nerve cells lions of years from a small, fairly primitive on one side become overactive and stimulate organ into a very complex network capable overactivity in nerve cells on the other side. of coordinating our body functions, our The excess stimulation produces a seizure in Brandi Binder is evidence of the brain’s thoughts, our emotions, and our behaviors. great power, ﬂexibility, and resilience. which the individual loses consciousness and Despite having had the right side of her Evolutionary psychologists emphasize that goes into convulsions. In severe cases, seizures cortex removed, Brandi engages in many behaviors that increase an organism’s re- activities often portrayed as right-brain can occur numerous times during the day. activities. She loves music, math, and art; productive success and enhance the ability Physicians have discovered that by severing she is shown here working on one of her to pass one’s genes on to the next genera- paintings. the connection between the two sides of the tion eventually prevail in nature over be- brain or by removing the side of the brain in which the overactiv- haviors that do not promote the organism’s survival. From their ity originates, they can eliminate the seizures or at least reduce point of view, the complex human brain has evolved because its their severity. Although not without risks and disadvantages, such increased complexity in some individuals enabled them to be- surgery may greatly improve an individual’s quality of life. have in ways that gave them and their descendants a better After her surgery, Brandi Binder had almost no control over chance of survival—for example, by being able to anticipate ad- muscles on the left side of her body, the side controlled by the right versity and plan for ways to avoid it or cope with it. side of her brain. She needed years of therapy to regain abilities This chapter examines important biological foundations of that she lost with the right side of her brain. At age 13, however, human behavior. The main focus is the nervous system and its Brandi was an A student. She also loved music, math, and art, all command center—the brain. of which are commonly associated with the brain’s right side. It also explores the genetic and evolutionary processes that Brandi’s story illustrates how amazingly adaptive and ﬂexi- have a signiﬁcant inﬂuence on who we are as individuals and ble the brain is, especially at an early age. In Brandi’s case, the how we behave. 1 THE NERVOUS SYSTEM Characteristics Pathways in the Divisions of the Nervous System Nervous System What is the nervous system and what does it do? The nervous system is the body’s electrochemical communication circuitry. The ﬁeld that studies the nervous system is called neuroscience, and the people who study it are nervous system The body’s electro- neuroscientists. chemical communication circuitry, The human nervous system is made up of billions of interconnected cells, and it made up of billions of neurons. is likely the most intricately organized aggregate of matter on planet Earth (Camp- bell, Reece, & Mitchell, 2002). A single cubic centimeter of the human brain consists 78
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 79 impos06 414:mhspy7ch03:spy7ch03%0: The Nervous System 79 of well over 50 million nerve cells, each of which communicates with many other nerve cells in information processing networks that make the most elaborate com- puter seem primitive. Characteristics The brain and nervous system guide our interaction with the world around us, move the body through the world, and direct our adaptation to our environment. Several extraordinary characteristics allow the nervous system to direct our behavior: com- plexity, integration, adaptability, and electrochemical transmission. Complexity The brain and nervous system are enormously complex. The brain itself is composed of billions of nerve cells. The orchestration of all of these cells to allow people to sing, dance, write, talk, and think is an awe-inspiring task. As Brandi Binder paints a piece of art, her brain is carrying out a huge number of tasks— involved in breathing, seeing, thinking, moving—in which extensive assemblies of nerve cells are participating. Integration Neuroscientist Steven Hyman (2001) calls the brain the great integrator. By this, he means that the brain does a wonderful job of pulling information together. Sounds, sights, touch, taste, hearing, genes, environment—the brain integrates all of these as we function in our world. The brain and the nervous system have different levels and many different parts. Brain activity is integrated across these levels through countless interconnections of brain cells and extensive pathways that link different parts of the brain. Each nerve cell communicates, on average, with 10,000 others, making up miles and miles of connections (Bloom, Nelson, & Lazerson, 2001; Johnson, 2003). Consider what hap- pens when a mosquito bites your arm. How does your brain know you were bitten and where? Bundles of interconnected nerve cells relay information about the bite from your arm through the nervous system in a very orderly fashion to the highest level of the brain. Indeed, behaving in just about any way requires a lot of connections in your brain. Brandi Binder’s painting does not occur because of what is going on in a sin- gle brain cell or a single part of her brain but rather because of the coordinated, inte- grated effort of many different nerve cells and parts of her brain. Adaptability The world around us is constantly changing. To survive, we must adapt to new conditions (Bloom, Nelson, & Lazerson, 2001). Our brain and nervous system together serve as our agent in adapting to the world. Although nerve cells reside in certain brain regions, they are not ﬁxed and immutable structures. They have a hereditary, biological foundation, but they are constantly adapting to changes in the body and the environment (Wilson, 2003). The term plasticity denotes the brain’s special capacity for modiﬁcation and change. The experiences that we have contribute to the wiring or rewiring of the brain (Blair, 2002; Greenough, 2000; Nash, 1997; Scharfman, 2002). For example, each time a baby tries to touch an object or gazes intently at a face, electrical impulses and chemical messengers shoot through the baby’s brain, knitting brain cells together into pathways and networks. The brain’s plasticity is nowhere more evident than in Brandi Binder’s case. After she lost much of the right side of her brain, the left side took over many functions that often are thought to reside only in the right side. Electrochemical Transmission The brain and the nervous system function essen- tially as an information processing system, powered by electrical impulses and chem- plasticity The brain’s special capacity ical messengers. When people speak to each other, they use words. When neurons for modiﬁcation and change. communicate with each other, they use chemicals.
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 80 impos06 414:mhspy7ch03:spy7ch03%0: 80 Chapter 3 Biological Foundations of Behavior The electrochemical communication system works effectively in most people to allow us to think and act. However, when the electrochemical system is short- circuited, as in the case of Brandi’s epilepsy, the ﬂow of information is disrupted, the brain is unable to channel information accurately, and the person cannot effectively engage in mental processing and behavior. Epileptic seizures are the result of abnor- mal electrical discharges in the brain. Just as an electrical surge during a lightning storm can disrupt the circuits in a computer, the electrical surge that produces an epileptic seizure disrupts the brain’s information processing circuits. The brains of indi- viduals with epilepsy work effectively to process information between seizures, unless the seizures occur with such regularity that they cause brain damage. In about 75 percent of epilepsy cases, seizures do not cause structural damage to the brain. Pathways in the Nervous System As we interact with and adapt to the world, the brain and the nervous system receive and transmit sensory input, integrate the information received from the environment, and direct the body’s motor activities. Information ﬂows into the brain through sen- sory input, becomes integrated within the brain, and then moves out of the brain to be connected with motor output (Enger & Ross, 2003). This ﬂow of information through the nervous system occurs in specialized path- ways that are adapted for different functions. These pathways are made up of affer- ent nerves, neural networks, and efferent nerves. Afferent nerves, or sensory nerves, carry information to the brain. The word afferent comes from the Latin word meaning “bring to.” These sensory pathways communicate information about exter- nal and bodily environments from sensory receptors into and throughout the brain. Efferent nerves, or motor nerves, carry the brain’s output. The word efferent is derived from the Latin word meaning “bring forth.” These motor pathways commu- nicate information from the brain to the hands, feet, and other areas of the body that allow a person to engage in motor behavior. Most information processing occurs when information moves through neural networks in the central nervous system. The function of these networks of nerve cells is to integrate sensory input and motor output (Peng, Qiao, & Xu, 2002). For example, as you read your class notes, the afferent input from your eye is transmit- afferent nerves Sensory nerves that ted to your brain, then passed through many neural networks, which translate transport information to the brain. (process) your black pen scratches into neural codes for letters, words, associations, and meaning. Some of the information is stored in the neural networks for future efferent nerves Motor nerves that associations, and, if you read aloud, some is passed on as efferent messages to your carry the brain’s output. lips and tongue. Neural networks make up most of the brain. neural networks Clusters of neurons that are interconnected to process Divisions of the Nervous System information. When the nineteenth-century American poet and essayist Ralph Waldo Emerson said, central nervous system (CNS) The “The world was built in order and the atoms march in tune,” he must have had the brain and spinal cord. human nervous system in mind. This truly elegant system is highly ordered and peripheral nervous system (PNS) organized for effective function. The network of nerves that connects the Figure 3.1 shows the two primary divisions of the human nervous system: the brain and spinal cord to other parts of central nervous system and the peripheral nervous system. The central nervous sys- the body. It is divided into the somatic tem (CNS) is made up of the brain and spinal cord. More than 99 percent of all nervous system and the autonomic nerve cells in our body are located in the CNS. The peripheral nervous system nervous system. (PNS) is the network of nerves that connects the brain and spinal cord to other parts of the body. The functions of the peripheral nervous system are to bring information somatic nervous system Division of to and from the brain and spinal cord and to carry out the commands of the CNS to the PNS consisting of sensory nerves, execute various muscular and glandular activities. whose function is to convey informa- The peripheral nervous system itself has two major divisions: the somatic ner- tion to the CNS, and motor nerves, vous system and the autonomic nervous system. The somatic nervous system con- whose function is to transmit informa- sists of sensory nerves, whose function is to convey information from the skin and tion to the muscles. muscles to the CNS about conditions such as pain and temperature, and motor nerves,
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 81 impos03 414:mhspy7ch03:spy7ch03%0: The Nervous System 81 NERVOUS SYSTEM Central Nervous System (CNS) Peripheral Nervous System (PNS) Somatic Nervous Autonomic Nervous Brain Spinal Cord System (Voluntary) System (Involuntary) Sympathetic Nervous Parasympathetic Nervous FIGURE 3.1 Major Divisions of the System (Arousing) System (Calming) Human Nervous System whose function is to tell muscles what to do. The function of the autonomic ner- vous system is to take messages to and from the body’s internal organs, monitoring such processes as breathing, heart rate, and digestion. The autonomic nervous sys- tem also is divided into two parts: the sympathetic nervous system arouses the body and the parasympathetic nervous system calms the body. To better understand the various divisions of the nervous system, let’s see what they do in a particular situation. Imagine that you are preparing to ask a judge to dismiss a parking ticket. As you are about to enter the courtroom, you scan a note card one last time to remember what you plan to say. Your peripheral nervous system carries the written marks from the note card to your central nervous system. Your central nervous system processes the marks, interpreting them as words, while you memorize key points and plan ways to keep the judge friendly. After studying the notes several minutes longer, you jot down an additional joke that you hope will amuse her. Again your peripheral nervous system is at work, conveying to the muscles in your arm and hand the information from your brain that enables you to make the marks on the paper. The information that is being transmitted from your eyes to your brain and to your hand is handled by the somatic nervous system. This is your ﬁrst ticket hearing, so you are a little anxious. Your stomach feels queasy, and your heart begins to thump. This is the sympathetic division of the autonomic nervous system functioning mhhe com/ s a n t rock p7 as you become aroused. You regain your conﬁdence after reminding yourself that you For study tools related to this learning were parked in a legal spot. As you relax, the parasympathetic division of the autonomic goal, see the Study Guide, the CD-ROM, nervous system is working. and the Online Learning Center. Review and Sharpen Your Thinking 1 Identify the parts of the nervous system and explain their role autonomic nervous system Division in behavior. of the PNS that communicates with the • Identify the fundamental characteristics of the brain and nervous system. body’s internal organs. It consists of the • Name and describe the pathways that allow the nervous system to carry out sympathetic and parasympathetic nerv- its three basic functions. ous systems. • Outline the divisions of the nervous system and explain their roles. sympathetic nervous system The di- vision of the autonomic nervous system Try this exercise without looking at Figure 3.1. Suppose you (1) saw a person com- that arouses the body. ing toward you, (2) realized it was someone famous, (3) got excited, (4) waved and shouted, (5) suddenly realized it was not a famous person, and (6) became parasympathetic nervous system suddenly calm again. Which part of your nervous system would have been heavily The division of the autonomic nervous involved at each of these six points? system that calms the body.
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007san9414x_ch03pg76_115 6/25/02 7:08 PM Page 82 impos06 414:mhspy7ch03:spy7ch03%0: For Evaluation Only. 82 Chapter 3 Biological Foundations of Behavior 2 NEURONS Specialized Cell Structure Synapses and Neurotransmitters The Neural Impulse Neural Networks What are neurons and how do they communicate? Within each division of the nervous system, much is happening at the cellular level. Nerve cells, chemicals, and electrical impulses work together to transmit information at speeds of up to 330 miles per hour. As a result, information can travel from your brain to your hands (or vice versa) in a matter of milliseconds (Krogh, 2000; Mar- tini, 2001). There are two types of cells in the nervous system: neurons and glial cells. Neu- rons are the nerve cells that actually handle the information processing function. The human brain contains about 100 billion neurons. The average neuron is as complex as a small computer and has as many as 10,000 physical connections with other cells. To have even the merest thought requires millions of neurons acting simultaneously (Carter, 1998). Glial cells provide support and nutritional beneﬁt functions in the nervous sys- tem (Lemke, 2001; Meller & others, 2002). Glial cells are not specialized to process information in the way that neurons are, although there are many more of them in the nervous system than there are neurons. In one study, neurons placed in a solu- tion containing glial cells grew more rapidly and proliﬁcally than neurons ﬂoating in the same solution without glial cells (Kennedy & Folk-Seang, 1986). This study indi- cates that glial cells function in a supportive or nutritive role for neurons. Specialized Cell Structure Not all neurons are alike. They are specialized to handle different information pro- neuron Nerve cell that is specialized cessing functions. However, all neurons do have some common characteristics. Most for processing information. Neurons are neurons are created very early in life, but their shape, size, and connections can the basic units of the nervous system. change throughout the life span. Thus the way neurons function reﬂects a major glial cells Provide support and nutri- characteristic of the nervous system that we described at the beginning of the chap- tional beneﬁts in the nervous system. ter: plasticity. They are not ﬁxed and immutable but can change. Every neuron has a cell body, dendrites, and axon (see ﬁgure 3.2). cell body Part of the neuron that con- The cell body contains the nucleus, which directs the manufacture of substances tains the nucleus, which directs the that the neuron needs for growth and maintenance. manufacture of substances that the neu- Dendrites receive and orient information toward the cell body. One of the most ron needs for growth and maintenance. distinctive features of neurons is the tree-like branching of their dendrites. Most nerve dendrites Branches of a neuron that cells have numerous dendrites, which increase their surface area, allowing each neu- receive and orient information toward ron to receive input from many other neurons. the cell body; most neurons have nu- The axon is the part of the neuron that carries information away from the cell merous dendrites. body toward other cells. Although very thin (1/10,000th of an inch), axons can be very long, with many branches. In fact, some extend more than three feet—all the axon The part of the neuron that car- way from the top of the brain to the base of the spinal cord. ries information away from the cell Covering all surfaces of neurons, including the dendrites and axons, are very thin body to other cells; each neuron has cellular membranes that are much like the surface of a bubble. The neuronal mem- only one axon. branes are semipermeable, meaning that they contain tiny holes or channels that allow myelin sheath A layer of fat cells that only certain substances to pass into and out of the neurons. encases and insulates most axons. The A myelin sheath, a layer of fat cells, encases and insulates most axons. By insu- myelin sheath speeds up the transmis- lating axons, myelin sheaths speed up transmission of nerve impulses (Mattson, 2002; sion of nerve impulses. Paus & others, 2001). Multiple sclerosis, a degenerative disease of the nervous sys- tem in which a hardening of myelin tissue occurs, disrupts neuronal communication.
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007san9414x_ch03pg76_115 6/25/02 7:08 PM Page 83 impos06 414:mhspy7ch03:spy7ch03%0: For Evaluation Only. Neurons 83 Cell body Dendrites Direction of Nucleus nerve impulse Axon Axon Axon Myelin sheath surrounding the axon Sending Neuron Receiving Neuron FIGURE 3.2 The Neuron The drawing shows the parts of a neuron and the con- The myelin sheath developed as the brain evolved. As brain size increased, it nection between one neuron and an- became necessary for information to travel over longer distances in the nervous sys- other. Note the cell body, branching of tem. Axons without myelin sheaths are not very good conductors of electricity. With dendrites, and the axon with a myelin the insulation of myelin sheaths, they transmit electrical impulses and convey infor- sheath. mation much more rapidly. We can compare the myelin sheath’s development to the evolution of freeways as cities grew. A freeway is a shielded road. It keeps fast- moving, long-distance trafﬁc from getting snarled by slow local trafﬁc. The Neural Impulse A neuron sends information through its axon in the form of brief impulses, or waves, of electricity. In old movies you might have seen telegraph operators tapping out mes- sages one click at a time over a telegraph wire to the next telegraph station. That is what neurons do. To transmit information to other neurons, a neuron sends impulses (“clicks”) through its axon to the next neuron. As you reach to turn this page, hun- dreds of such impulses will stream down the axons in your arm to tell your muscles just when to ﬂex and how vigorously. By changing the rate and timing of the sig- nals or “clicks,” the neuron can vary its message. How does a neuron—a living cell—generate electrical impulses? To answer this question, we need to further examine the nature of a neuron and the ﬂuids in which it ﬂoats. A neuron is like a balloon ﬁlled with one kind of ﬂuid and surrounded by a slightly different kind of ﬂuid. The axon is a piece of the “balloon” that has been stretched to form a long, hollow tube. The axon tube is so thin that a few dozen axons in a bundle would be about as thick as a human hair. Floating in the ﬂuids inside and outside the tube are electrically charged particles called ions. Some of these ions, notably sodium and potassium, carry positive charges. Neg- atively charged ions of chlorine and other elements also are present. The cell mem- brane prevents negative and positive ions from randomly ﬂowing into or out of the cell. The neuron creates electrical signals by moving positive and negative ions back and forth through its outer membrane. How does the movement of ions across the membrane occur? It’s fairly simple. Embedded in the membrane—the wall of our bal- loon—are hundreds of thousands of small gates, called ion channels, that open and close to let the ions pass into and out of the cell. Normally, when the neuron is rest- ing, not transmitting information, the ion channels are closed, and a slight negative charge is present along the inside of the cell membrane. On the outside of the cell membrane, the charge is positive. Because of the difference in charge, the membrane
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 84 impos03 414:mhspy7ch03:spy7ch03%0: 84 Chapter 3 Biological Foundations of Behavior FIGURE 3.3 The Resting Potential An oscilloscope measures the difference in electrical potential between two elec- trodes. When one electrode is placed in- Axon side an axon at rest and one is placed + + + + + + + + + + + + + outside, the electrical potential inside the – – – – – – – – – – – – – cell is Ϫ70 millivolts (mV) relative to the outside. This potential difference is due to the separation of positive (ϩ) and nega- – – – – – – – – – – – – – tive (Ϫ) charges along the membrane. + + + + + + + + + + + + + + + + + + 0 mV + + + ++ –70 mV + ++ Axon + +++ + ++ Polarity ++ + + + + + + FIGURE 3.4 The Action Potential Movement An action potential is a wave of localized of impulse depolarization that travels down the axon Axon Axon Axon + + – + + + + + + + Na+ Na+ Na+ Na+ Na+ as the ion channels in the axon membrane – – + – – – – – – – K+ K+ K+ K+ K+ open and close. (a) The action potential causes a change in electrical potential as it Na+ Na+ moves along the axon. (b) The movements of sodium ions (Na+) and potassium ions – – + – – – – – – – K+ K+ K+ K+ K+ (K+) into and out of the axon cause the + + – + + + + + + + Na+ Na+ Na+ Na+ Na+ electrical changes. +40 mV +40 mV +40 mV 0 mV 0 mV 0 mV –70 mV –70 mV –70 mV Upswing Downswing (a) Action potential generated by an (b) Movement of ions responsible impulse received by the neuron for the action potential of the resting neuron is said to be polarized, like the ends of a ﬂashlight battery. Rest- ing potential is the term given to the stable, negative charge of an inactive neuron (see ﬁgure 3.3). That potential, by the way, is about Ϫ70 millivolts, which is only about 1/14th of a volt, so 14 neurons could make up a 1-volt battery. An electric eel’s 8,400 neurons could generate 600 volts! A neuron becomes activated when an incoming impulse, in reaction to, say, a pinprick or the sight of someone’s face, raises the neuron’s voltage threshold, and the sodium gates at the base of the axon open brieﬂy. This action allows positively charged resting potential The term given to sodium ions to ﬂow into the neuron, creating a more positively charged neuron and the stable, negative charge of an in- depolarizing the membrane by decreasing the charge difference between the ﬂuids active neuron. inside and outside of the neuron. Then potassium channels open, and positively charged potassium ions move out through the neuron’s semipermeable membrane. action potential The term used to de- This returns the neuron to a negative charge. Then the same process occurs as the scribe the brief wave of electrical charge next group of channels ﬂip open brieﬂy. And so it goes all the way down the axon, that sweeps down the axon during the just like a long row of cabinet doors opening and closing in sequence. transmission of a nerve impulse. The term action potential is used to describe the brief wave of positive electri- all-or-none principle Once an cal charge that sweeps down the axon (see ﬁgure 3.4). An action potential lasts only electrical impulse reaches a certain level about 1/1,000th of a second, because the sodium channels can stay open for only a of intensity, it ﬁres and moves all the very brief time. They quickly close again and become reset for the next action poten- way down the axon without losing any tial. When a neuron sends an action potential, it is commonly said to be “ﬁring.” of its intensity. The action potential abides by the all-or-none principle: Once the electrical impulse reaches a certain level of intensity, it ﬁres and moves all the way down the
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 85 impos06 414:mhspy7ch03:spy7ch03%0: Neurons 85 Dendrites A The neural impulse travels down the axon toward dendrites of the next neuron. Direction of nerve impulse Axon Terminal button C At a receptor site on the dendrite of the receiving neuron, the neurotransmitter causes channels to open and creates an action potential. Terminal button Neurotransmitters Axon of sending neuron Synaptic vesicle containing neurotransmitters Synaptic gap Receptor site Dendrite of Channel receiving neuron B In the terminal button, the impulse triggers the release Synaptic vesicle Neurotransmitters on of neurotransmitters into the synaptic gap. releases neurotransmitters. receptor site; channel opens. FIGURE 3.5 How Synapses and Neurotransmitters Work (a) The axon of the presynaptic (sending) neuron meets axon without losing any of its intensity. The impulse traveling down an axon can be dendrites of the postsynaptic (receiving) compared to the burning fuse of a ﬁrecracker. It doesn’t matter whether a match or neuron. (b) This is an enlargement of one blowtorch was used to light the fuse; once the fuse has been lit, the spark travels synapse, showing the synaptic gap be- quickly and with the same intensity down the fuse. tween the two neurons, the terminal but- ton, and the synaptic vesicles containing a neurotransmitter. (c) This is an enlarge- Synapses and Neurotransmitters ment of the receptor site. Note how the neurotransmitter opens the channel on What happens when a neural impulse reaches the end of the axon? Neurons do not the receptor site, triggering the neuron touch each other directly, but they manage to communicate. The story of the con- to ﬁre. nection between one neuron and another is one of the most intriguing and highly researched areas of contemporary neuroscience (Bi & Poo, 2001). Figure 3.5 gives an overview of how this connection between neurons takes place. synapses Tiny junctions between two Synaptic Transmission Synapses are tiny junctions between neurons; the gap neurons, generally where the axon of between neurons is referred to as a synaptic gap. Most synapses lie between the axon one neuron meets the dendrites or cell of one neuron and the dendrites or cell body of another neuron. Before the electri- body of another neuron. cal impulse can cross the synaptic gap, it must be converted into a chemical signal.
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007san9414x_ch03pg76_115 6/14/02 10:24 AM Page 86 impos03 414:mhspy7ch03:spy7ch03%0: For Evaluation Only. 86 Chapter 3 Biological Foundations of Behavior Each axon branches out into numerous ﬁbers that end in structures called ter- minal buttons. Stored in minute synaptic vesicles (sacs) within the terminal buttons are substances called neurotransmitters. As their name suggests, neurotransmitters transmit or carry information across the synaptic gap to the next neuron. When a nerve impulse reaches the terminal button, it triggers the release of neurotransmit- ter molecules from the synaptic vesicles. The neurotransmitter molecules ﬂood the synaptic gap. Their movements are random, but some of them bump into receptor sites in the next neuron. If the shape of the receptor site corresponds to the shape of the neurotransmitter molecule, the neurotransmitter acts like a key to open the receptor site, so that the neuron can receive the electrical signals coming from the previous neuron. After delivering its message, the neurotransmitter is reabsorbed by the axon that released it to await the next neural impulse. Think of the synapse as a river that blocks a road. A grocery truck (the action potential) arrives at one bank of the river, crosses by ferry, and continues its journey to market. Similarly, a message in the brain is “ferried” across the synapse by a neu- rotransmitter, which pours out of the terminal button just as the message approaches the synapse. Neurochemical Messengers There are many different neurotransmitters. Each one plays a speciﬁc role and functions in a speciﬁc pathway. Whereas some neuro- transmitters stimulate or excite neurons to ﬁre, others can inhibit neurons from ﬁr- ing (Heim & Nemeroff, 2002). Some neurotransmitters are both excitatory and inhibitory, depending on what is needed. As the neurotransmitter moves across the synaptic gap to the receiving neuron, its molecules might spread out or be conﬁned to a small space. The molecules might come in rapid sequence or be spaced out. The receiving neuron integrates this information before reacting to it. Most neurons secrete only one type of neurotransmitter, but often many differ- ent neurons are simultaneously secreting different neurotransmitters into the synap- tic gaps of a single neuron. At any given time, a neuron is receiving a mixture of messages from the neurotransmitters. At its receptor sites, the chemical molecules bind to the membrane and either excite the neuron, bringing it closer to the thresh- old at which it will ﬁre, or inhibit the neuron from ﬁring. Usually the binding of an excitatory neurotransmitter from one neuron will not be enough to trigger an action potential in the receiving neuron. Triggering an action potential often takes a num- ber of neurons sending excitatory messages simultaneously or fewer neurons send- ing rapid-ﬁre excitatory messages. So far, researchers have identiﬁed more than 50 neurotransmitters, each with a unique chemical makeup. The rapidly growing list likely will grow to more than 100 (Johnson, 2003). In organisms ranging from snails to whales, neuroscientists have found the same neurotransmitter molecules that our own brains use. Many animal venoms, such as that of the black widow spider, actually are neurotransmitter-like substances that do their harm by disturbing neurotransmission. To get a better sense of what neurotransmitters do, let’s consider just six that have major effects on our behavior. Acetylcholine Acetylcholine (ACh) usually stimulates the ﬁring of neurons and is in- volved in the action of muscles, learning, and memory (Devi & Silver, 2000; McIn- tyre & others, 2002). ACh is found throughout the central and peripheral nervous systems. The venom of the black widow spider causes ACh to gush through the synapses between the spinal cord and skeletal muscles, producing violent spasms. The drug curare, which some South American Indians apply to the tips of poison darts, blocks receptors for ACh, paralyzing muscles. In contrast, nicotine stimulates acetyl- neurotransmitters Chemicals that choline receptors. Individuals with Alzheimer’s disease, a degenerative brain disorder carry information across the synaptic that involves a decline in memory, have an acetylcholine deﬁciency. Some of the gap from one neuron to the next. drugs that alleviate the symptoms of Alzheimer’s disease do so by compensating for the loss of the brain’s supply of acetylcholine.
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007san9414x_ch03pg76_115 6/14/02 10:24 AM Page 87 impos03 414:mhspy7ch03:spy7ch03%0: For Evaluation Only. Neurons 87 FIGURE 3.6 Serotonin Pathways Each of the neurotransmitters in the brain has speciﬁc pathways in which they function. Shown here are the pathways for serotonin. GABA GABA (gamma aminobutyric acid) is found throughout the central nervous system. It is believed to be the neurotransmitter in as many as one-third of the brain’s synapses. GABA is important in the brain because it keeps many neurons from ﬁr- ing (Bou-Flores & Berger, 2001; Ryan, 2001). In this way it helps to control the pre- ciseness of the signal being carried from one neuron to the next. Low levels of GABA are linked with anxiety. Valium and other antianxiety drugs increase the inhibiting effects of GABA. Norepinephrine Norepinephrine usually inhibits the ﬁring of neurons in the central nervous system, but it excites the heart muscle, intestines, and urogenital tract. Stress stimulates the release of norepinephrine (Zaimovic & others, 2000). This neurotrans- mitter also helps to control alertness. Too little norepinephrine is associated with depression, too much with agitated, manic states. For example, amphetamines and cocaine cause hyperactive, manic states of behavior by rapidly increasing brain lev- els of norepinephrine. Recall from the beginning of the chapter that one of the most important char- acteristics of the brain and nervous system is integration. In the case of neurotrans- mitters, they may work in teams of two or more. For example, norepinephrine works with acetylcholine to regulate states of sleep and wakefulness. Dopamine Dopamine mainly inhibits. It helps to control voluntary movement (Jakel & Marangos, 2000). Dopamine also affects sleep, mood, attention, and learning. Stim- ulant drugs, such as cocaine and amphetamines, produce excitement, alertness, ele- vated mood, decreased fatigue, and sometimes increased motor activity mainly by activating dopamine receptors. Low levels of dopamine are associated with Parkinson’s disease, in which physi- cal movements deteriorate (Malapani, Deweer, & Gibbon, 2002). Although the actor Michael J. Fox contracted Parkinson’s disease in his late 20s, the disease is uncom- mon before the age of 30 and becomes more common as people age. High levels of dopamine are associated with schizophrenia, a severe mental disorder that is discussed in chapter 14. Serotonin Serotonin also primarily inhibits. Serotonin is involved in the regulation of sleep, mood, attention, and learning. In regulating states of sleep and wakefulness, it teams with acetylcholine and norepinephrine. Lowered levels of serotonin are asso- ciated with depression (Kanner & Balabanov, 2002; Wagner & Ambrosini, 2001). The antidepressant drug Prozac works by increasing brain levels of serotonin. Figure 3.6 shows the brain pathways for serotonin. Endorphins Endorphins are natural opiates that mainly stimulate the ﬁring of neurons. Endorphins shield the body from pain and elevate feelings of pleasure. A long-distance
Edited by Foxit Reader Copyright(C) by Foxit Software Company,2005-2007san9414x_ch03pg76_115 6/25/02 7:08 PM Page 88 impos06 414:mhspy7ch03:spy7ch03%0: For Evaluation Only. 88 Chapter 3 Biological Foundations of Behavior FIGURE 3.7 An Example of a Neural Network Inputs (informa- tion from the environment and sensory receptors—as when someone looks at a person’s face) become embedded in ex- Inputs Outputs tensive connections between neurons in the brain, which leads to outputs (such as remembering the person’s face). runner, a woman giving birth, and a person in shock after a car wreck all have ele- vated levels of endorphins (Jamurtas & others, 2000). As early as the fourth century B.C., the Greeks used wild poppies to induce euphoria. More than 2,000 years later, the magical formula behind opium’s addictive action was ﬁnally discovered. In the early 1970s, scientists found that opium plugs into a sophisticated system of natural opiates that lie deep within the brain’s path- ways (Pert, 1999; A. B. Pert & Snyder, 1973; Spetea & others, 2002). Morphine (the most important narcotic of opium) mimics the action of endorphins by stimulating receptors in the brain involved with pleasure and pain. Drugs and Neurotransmitters Most drugs that inﬂuence behavior do so mainly by interfering with the work of neurotransmitters (Beatty, 2001; Mader, 2003). Drugs can mimic or increase the effects of a neurotransmitter, or they can block those effects. An agonist is a drug that mimics or increases a neurotransmitter’s effects. For example, the drug morphine mimics the actions of endorphins by stimulating recep- tors in the brain associated with pleasure and pain. An antagonist is a drug that blocks a neurotransmitter’s effects. For example, alcohol blocks serotonin activity (Fils-Aime & others, 1996). Neural Networks So far in the coverage of neurons, I have focused mainly on how a single neuron functions and on how a nerve impulse travels from one neuron to another. Now let’s look at how large numbers of neurons work together to integrate incoming infor- mation and coordinate outgoing information. At the beginning of the chapter, I brieﬂy described neural networks as clusters of neurons that are interconnected to process information. Figure 3.7 shows a sim- pliﬁed drawing of a neural network or pathway (McIntosh, 2000). By looking at this diagram, you can get an idea of how the activity of one neuron is linked with many others. Some neurons have short axons and communicate with other nearby neurons. Other neurons have long axons and communicate with circuits of neurons some dis- tance away. Researchers have found that these neural networks are not static (Carl- son, 2000; Meyer & van Vreeswijk, 2002). They can be altered through changes in the strength of synaptic connections. Any piece of information, such as a name, might be embedded in hundreds or even thousands of connections between neurons (Lee & Farhat, 2001). In this way, such human activities as being attentive, memorizing, and thinking are distributed over a wide range of connected neurons (Bartlett, 2002). The strength of these con- agonist A drug that mimics or in- nected neurons determines how well you remember the information (Golden, 2002; creases a neurotransmitter’s effects. Krause & others, 2000; McClelland & Rumelhart, 1986). antagonist A drug that blocks a neuro- Let’s see how the neural network concept might explain a typical memory, such transmitter’s effects. as the name of a new acquaintance. Initially, the processing of the person’s face might activate a small number of weak neuronal connections that make you remember a
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 89 impos06 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 89 general category (“interesting woman” or “attractive man’’). However, repeated expe- rience with that person will increase the strength and possibly the number of those connections. So you may remember the person’s name as the neurons activated by the name become connected with the neurons that are activated by the face. Chap- ter 8 explores the nature of memory at greater length. Review and Sharpen Your Thinking mhhe com/ s a n t rock p7 2 Explain what neurons are and how they process information. For study tools related to this learning • Differentiate between neurons and glial cells, and describe the functions of goal, see the Study Guide, the CD-ROM, the parts of a neuron. and the Online Learning Center. • Explain what a neural impulse is and how it is generated. • Discuss how a neural impulse is transmitted from one neuron to another. • Describe the function of neural networks. Why is it important to have so many connections and to have integration between neurons? STRUCTURES OF THE BRAIN AND THEIR FUNCTIONS 3 How the Brain and Nervous The Cerebral Cortex Integration of Function System Are Studied in the Brain Levels of Organization The Cerebral Hemispheres in the Brain and Split-Brain Research How is the brain organized? The extensive and intricate networks of neurons that we have just studied are not visible to the naked eye. Fortunately, technology is available to help neuroscientists form pictures of the structure and organization of neurons and the larger structures they make up without harming the organism being studied. This section explores some of the techniques that are used in brain research and discusses what they have shown us about the structures and functions of the brain. Special attention is given to the cerebral cortex, the highest region of the brain. How the Brain and Nervous System Are Studied Much of our early knowledge of the human brain comes from clinical studies of indi- viduals who suffered brain damage from injury or disease or who had brain surgery to relieve another condition (like Brandi Binder). Modern discoveries have relied largely on technology that enables researchers to “look inside” the brain while it is at work. Let’s examine some of these techniques. Brain Lesioning Brain lesioning is an abnormal disruption in the tissue of the brain resulting from injury or disease. The study of naturally occurring brain lesions in humans has provided considerable information about how the brain functions. Neuroscientists also produce lesions in laboratory animals to determine the effects on the animal’s behavior (Krauss & Jankovic, 2002). These lesions may be made by surgically removing brain tissue, destroying tissue with a laser, or elimi- nating tissue by injecting it with a drug. Sometimes transient lesions can be made
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 90 impos03 414:mhspy7ch03:spy7ch03%0: 90 Chapter 3 Biological Foundations of Behavior by administering a drug that temporarily inactivates an area of the brain. The organ- ism’s behavior can be studied while the area is inactivated; after the effects of the drug have worn off, brain activity in the area returns to normal (Gazzaniga, Ivry, & Mangun, 2001). Staining A central interest in neuroscience is to identify the pathways of connec- tivity in the brain and nervous system that allow information to get from one place to another (Sorensen & others, 2002). This is not an easy task because of the com- plexity and extent of the interconnections. Much of the progress in charting these neural networks has come about through the use of stains, or dyes, that are selec- tively absorbed by neurons. One commonly used stain is horseradish peroxidase. A stain will coat only a small portion of neurons so that neuroscientists, using high- powered microscopes, can see which neurons absorb the stains and determine how they are connected. Electrical Recording Also widely used is the electroencephalograph (EEG), which records the electrical activity of the brain. Electrodes placed on the scalp detect brain- FIGURE 3.8 An EEG Recording wave activity, which is recorded on a chart known as an electroencephalogram (see The electroencephalograph (EEG) is widely used in sleep research. It has led ﬁgure 3.8). This device has been used to assess brain damage, epilepsy, and other to some major breakthroughs in under- problems (Meador, 2002; Wallace & others, 2001). standing sleep by showing how the Not every recording of brain activity is made with surface electrodes. In single- brain’s electrical activity changes dur- unit recording, which provides information about a single neuron’s electrical activity, ing sleep. a thin probe is inserted in or near an individual neuron (Seidemann & others, 1996). The probe transmits the neuron’s electrical activity to an ampliﬁer so that researchers can “see” the activity. EEGs are also used to mhhe com/ Brain Imaging For years X rays have been used to reveal damage inside or out- study normal brains. s a n t rock p7 side our bodies, both in the brain and in other locations. But a single X ray of the (Around the Globe) brain is hard to interpret because it shows a two-dimensional image of the three- dimensional interior of the brain. A newer technique called computerized tomography FIGURE 3.9 PET Scan This PET scan of the left half of the brain contrasts the different areas used in aspects of lan- guage activity: generating words, hearing words, seeing words, and speaking words. Generating words Hearing words Seeing words Speaking words
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 91 impos06 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 91 FIGURE 3.10 Embryological Devel- Forebrain opment of the Nervous System The photograph shows the primitive, tu- Midbrain bular appearance of the nervous system Hindbrain at 6 weeks in the human embryo. The drawing shows the major brain regions and spinal cord as they appear early in the development of a human embryo. Spinal cord (CT scan) produces a three-dimensional image obtained from X rays of the head that are assembled into a composite image by a computer. The CT scan provides valuable information about the location and extent of damage involving stroke, language dis- order, or loss of memory. Positron-emission tomography (PET scan) measures the amount of glucose in various areas of the brain, then sends this information to a computer for analysis. Because glucose levels vary with the levels of activity throughout the brain, tracing the amounts of glucose generates a picture of activity levels throughout the brain (Sieb- ner & others, 2002). Figure 3.9 shows PET scans of people’s brain activity while they are hearing, seeing, speaking, and thinking. Another technique, magnetic resonance imaging (MRI), involves creating a magnetic field around a person’s body and using radio waves to construct images of the person’s tissues and biochemical activities. MRI provides very clear pictures of the brain’s inte- rior, does not require injecting the brain with a substance, and, unlike X rays, does not pose a problem of radiation overexposure (Petersen, 2001; Niku & Lu, 2002). In one recent study, Susan Tapert, a neuroscientist who is interested in the brain dysfunction produced by alcoholism, and her colleagues used MRI to determine the effects of alcoholism on the brain (Tapert & others, 2001). They compared MRI brain scans of two groups of young women, one of which had a history of heavy drink- ing, the other no history of alcohol problems. Both groups of women had abstained from alcohol for the previous 24 hours. As they completed a memory task in which they had to remember the location of an object on a screen, they underwent MRI brain scans. The young women who had a history of heavy drinking did more poorly on the memory task, and the MRI scans revealed more sluggish brain activity. Levels of Organization in the Brain As a human embryo develops inside its mother’s womb, the nervous system begins forming as a long, hollow tube on the embryo’s back. At 3 weeks or so after con- ception, cells making up the tube differentiate into a mass of neurons, most of which then develop into three major regions of the brain: the hindbrain, which is adjacent to the top part of the spinal cord; the midbrain, which rises above the hindbrain; and the forebrain, which is the uppermost region of the brain (see ﬁgure 3.10). hindbrain The lowest level of the Hindbrain The hindbrain, located at the skull’s rear, is the lowest portion of the brain, consisting of the medulla, cere- brain. The three main parts of the hindbrain are the medulla, cerebellum, and pons. bellum, and pons. Figure 3.11 shows the location of these brain structures.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 92 impos03 414:mhspy7ch03:spy7ch03%0: 92 Chapter 3 Biological Foundations of Behavior Cerebral Cortex Thalamus Extensive, wrinkled outer layer Relays information between lower of the forebrain governs and higher brain centers higher brain functions, such as thinking, learning, and consciousness Hypothalamus Governs eating, drinking, and sex; plays a role in emotion and stress Reticular Formation Diffuse collection of neurons Eye involved in stereotyped patterns, such as walking Amygdala Involved in discrimination of objects necessary for Medulla organism’s survival Governs breathing and reflexes Pons Pituitary Gland Governs sleep Cerebellum Governs endocrine Hippocampus and arousal system Rounded structure involved Involved in memory Spinal Cord in motor coordination FIGURE 3.11 Structure and Regions in the Human Brain The medulla begins where the spinal cord enters the skull. It helps to control our breathing and regulates reﬂexes that allow us to maintain an upright posture. The cerebellum extends from the rear of the hindbrain, just above the medulla. It consists of two rounded structures thought to play important roles in motor coordi- nation (Middleton & Strick, 2001). Leg and arm movements are coordinated by the cerebellum, for example. When we play golf, practice the piano, or learn a new dance, the cerebellum is hard at work. If a higher portion of the brain commands us to write the number 7, it is the cerebellum that integrates the muscular activities required to do so. Damage to the cerebellum impairs the performance of coordinated movements. When this damage occurs, people’s movements become uncoordinated and jerky. Extensive damage to the cerebellum even makes it impossible to stand up. The pons is a bridge in the hindbrain. It contains several clusters of ﬁbers involved in sleep and arousal. Midbrain The midbrain, located between the hindbrain and forebrain, is an area midbrain Located between the hind- in which many nerve-ﬁber systems ascend and descend to connect the higher and brain and forebrain, a region in which lower portions of the brain. In particular, the midbrain relays information between many nerve-ﬁber systems ascend and de- the brain and the eyes and ears. The ability to attend to an object visually, for exam- scend to connect the higher and lower ple, is linked to one bundle of neurons in the midbrain. Parkinson’s disease, a dete- portions of the brain. rioration of movement that produces rigidity and tremors, damages a section near the bottom of the midbrain. reticular formation A midbrain sys- Two systems in the midbrain are of special interest. One is the reticular for- tem that consists of a diffuse collection mation (see ﬁgure 3.11), a diffuse collection of neurons involved in stereotyped pat- of neurons involved in stereotypical be- terns of behavior such as walking, sleeping, or turning to attend to a sudden noise haviors such as walking, sleeping, or (Soja & others, 2001). The other system consists of small groups of neurons that use turning to attend to a sudden noise. the neurotransmitters serotonin, dopamine, and norepinephrine. Although these brain stem The region of the brain groups contain relatively few cells, they send their axons to a remarkable variety of that includes most of the hindbrain brain regions, perhaps explaining their involvement in high-level, integrative func- (excluding the cerebellum) and the tions (Shier, Butler, & Lewis, 1999). midbrain. A region called the brain stem includes much of the hindbrain (it does not include the cerebellum) and midbrain and is so-called because it looks like a stem.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 93 impos03 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 93 Cerebral cortex Cerebral cortex Cerebellum Cerebellum Brain stem Cerebral Cerebellum cortex Brain stem Cerebral cortex Cerebellum Brain stem Brain stem Embedded deep within the brain, the brain stem connects with the spinal cord at its FIGURE 3.12 The Brain in Different lower end and then extends upward to encase the reticular formation in the mid- Species Note how much larger the cere- brain. The most ancient part of the brain, the brain stem evolved more than 500 mil- bral cortex becomes as we go from the lion years ago (Carter, 1998). It is much like the entire brain of present-day reptiles brain of a rat to the brain of a human. and thus is often referred to as the “reptilian brain.” Clumps of cells in the brain stem determine alertness and regulate basic survival functions such as breathing, heartbeat, and blood pressure. Forebrain You try to understand what all of these terms and parts of the brain mean. You talk with friends and plan a party for this weekend. You remember that it has been 6 months since you went to the dentist. You are conﬁdent you will do well on the next exam in this course. All of these experiences and millions more would not be possible without the forebrain, the highest level of the human brain. Before we explore the structures and function of the forebrain, though, let’s stop for a moment and examine how the brain evolved. The brains of the earliest verte- brates were smaller and simpler than those of later animals. Genetic changes during the evolutionary process were responsible for the development of more complex brains with more parts and more interconnections (Carlson, 2001). Figure 3.12 com- pares the brains of a rat, cat, chimpanzee, and human. In the chimpanzee’s brain, and especially the human’s brain, the hindbrain and midbrain structures are covered by a forebrain structure called the cerebral cortex (Goldsmith & Zimmerman, 2001). The human hindbrain and midbrain are similar to those of other animals, so it is the forebrain structures that mainly differentiate the human brain from the brains of ani- mals such as rats, cats, and monkeys. The human forebrain’s most important struc- tures are the limbic system, thalamus, basal ganglia, hypothalamus, and cerebral cortex. Limbic System The limbic system, a loosely connected network of structures under the cerebral cortex, is important in both memory and emotion. Its two principal struc- tures are the amygdala and hippocampus (see ﬁgure 3.11). The amygdala (from the Latin for “almond” shape) is located within the base of the temporal lobe. It is involved in the discrimination of objects that are necessary for the organism’s survival, such as appropriate food, mates, and social rivals. Neu- forebrain The highest level of the rons in the amygdala often ﬁre selectively at the sight of such stimuli, and lesions in brain. Key structures in the forebrain the amygdala can cause animals to attempt to eat, ﬁght, or mate with inappropriate are the limbic system, thalamus, basal objects such as chairs. The amygdala also is involved in emotional awareness and ganglia, hypothalamus, and cerebral expression through its many connections with higher and lower regions of the brain cortex. (Davidson, 2000). The hippocampus has a special role in the storage of memories (Bannerman & oth- limbic system Loosely connected net- ers, 2002). Individuals who suffer extensive hippocampal damage cannot retain any work of structures—including the new conscious memories after the damage. It is fairly certain, though, that memories amygdala and hippocampus—that are not stored “in” the limbic system. Instead, the limbic system seems to determine play important roles in memory and what parts of the information passing through the cortex should be “printed” into emotion. durable, lasting neural traces in the cortex.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 94 impos03 414:mhspy7ch03:spy7ch03%0: 94 Chapter 3 Biological Foundations of Behavior Thalamus The thalamus is a forebrain structure that sits at the top of the brain stem in the central core of the brain (see ﬁgure 3.11). It serves as a very important relay station, functioning much like a server in a computer network. That is, an impor- tant function of the thalamus is to sort information and send it to the appropriate places in the forebrain for further integration and interpretation (Castro-Alamancos & Calcagnotto, 2001). For example, one area of the thalamus receives information from the cerebellum and projects it to the motor area of the cerebral cortex. Indeed, most neural input to the cerebral cortex goes through the thalamus. While one area of the thalamus works to orient information from the sense receptors (hearing, see- ing, and so on), another region seems to be involved in sleep and wakefulness, hav- ing ties with the reticular formation. Basal Ganglia Above the thalamus and under the cerebral cortex lie large clus- ters, or ganglia, of neurons called basal ganglia. The basal ganglia work with the cerebellum and the cerebral cortex to control and coordinate voluntary movements. Basal ganglia enable people to engage in habitual behaviors such as riding a bicy- cle. Individuals with damage to basal ganglia suffer from either unwanted move- ment, such as constant writhing or jerking of limbs, or too little movement, as in the slow and deliberate movements of those with Parkinson’s disease (Borand & others, 2002). Hypothalamus The hypothalamus, a small forebrain structure located just below the thalamus, monitors three pleasurable activities—eating, drinking, and sex—as well as emotion, stress, and reward (see ﬁgure 3.11 for the location of the hypo- thalamus). As is discussed later, the hypothalamus also helps direct the endocrine sys- tem. Perhaps the best way to describe the function of the hypothalamus is as a regulator of the body’s internal state. It is sensitive to changes in the blood and neu- ral input, and it responds by inﬂuencing the secretion of hormones and neural out- puts. For example, if the temperature of circulating blood near the hypothalamus is increased by just 1 or 2 degrees, certain cells in the hypothalamus start increasing their rate of ﬁring. As a result, a chain of events is set in motion. Increased circula- tion through the skin and sweat glands occurs immediately to release this heat from the body. The cooled blood circulating to the hypothalamus slows down the activity of some of the neurons there, stopping the process when the temperature is just right—37.1º Celsius. These temperature-sensitive neurons function like a ﬁnely tuned thermostat in maintaining the body in a balanced state. The hypothalamus also is involved in emotional states and stress, playing an important role as an integrative location for handling stress. Much of this integration thalamus Forebrain structure that is accomplished through the hypothalamus’s action on the pituitary gland, an impor- functions as a relay station to sort input tant endocrine gland located just below it. and direct it to different areas of the If certain areas of the hypothalamus are electrically stimulated, a feeling of plea- cerebral cortex. It also has ties to the sure results. In a classic experiment, James Olds and Peter Milner (1954) implanted reticular formation. an electrode in the hypothalamus of a rat’s brain. When the rat ran to a corner of an enclosed area, a mild electric current was delivered to its hypothalamus. The basal ganglia Located above the thala- researchers thought the electric current would cause the rat to avoid the corner. Much mus and under the cerebral cortex, to their surprise, the rat kept returning to the corner. Olds and Milner believed they these large clusters of neurons work had discovered a pleasure center in the hypothalamus. Olds (1958) conducted fur- with the cerebellum and the cerebral ther experiments and found that rats would press bars until they dropped over from cortex to control and coordinate volun- exhaustion just to continue to receive a mild electric shock to their hypothalamus. tary movements. One rat pressed a bar more than 2,000 times an hour for a period of 24 hours to hypothalamus Forebrain structure in- receive the stimulation to its hypothalamus (see ﬁgure 3.13). Today researchers agree volved in regulating eating, drinking, that the hypothalamus is involved in pleasurable feelings but that other areas of the and sex; directing the endocrine system brain, such as the limbic system and a bundle of ﬁbers in the forebrain, are also through the pituitary gland; and moni- important in the link between the brain and pleasure. toring emotion, stress, and reward. The Olds studies have implications for drug addiction. In the Olds studies, the rat pressed the bar mainly because it produced a positive, rewarding effect (pleasure),
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 95 impos06 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 95 50,000 FIGURE 3.13 Results of the Experiment by Olds (1958) on the Role of the Hypothalamus in Pleasure The graphed results for one rat show that it pressed the bar more than 2,000 times an hour for a period of 24 hours to receive stimulation to its hypothala- 40,000 mus. One of the rats in Olds and Milner’s experiments is shown pressing the bar. Cumulative bar presses 30,000 20,000 10,000 0 12:00 6:00 P.M. 12:00 6:00 A.M. 12:00 Noon Midnight Noon Time not because it wanted to avoid or escape a negative effect (pain). Cocaine users talk about the drug’s ability to heighten pleasure in food, in sex, and in a variety of activ- ities, highlighting the reward aspects of the drug (Restak, 1988). The Cerebral Cortex The cerebral cortex is the highest region of the forebrain and is the most recently developed part of the brain in the evolution scheme. It is in the cerebral cortex that the highest mental functions, such as thinking and planning, take place. The neural FIGURE 3.14 The Human Brain’s tissue that makes up the cerebral cortex is the largest part of the brain in volume Hemispheres The two halves (hemi- (about 80 percent) and covers the lower portions of the brain like a large cap. In spheres) of the human brain can be seen humans, the cerebral cortex is greatly convoluted with lots of grooves and bulges, clearly in this photograph. which considerably enlarge its surface area (compared to a brain with a smooth sur- face). The cerebral cortex is highly connected with other parts of the brain. Literally millions of axons connect the neurons of the cerebral cortex with those located else- cerebral cortex Highest level of the where in the brain. forebrain, where the highest mental functions, such as thinking and plan- Lobes The wrinkled surface of the cerebral cortex is divided into two halves called ning, take place. hemispheres (see ﬁgure 3.14). Each hemisphere is subdivided into four regions—the frontal lobe, the parietal lobe, the temporal lobe, and the occipital lobe (see ﬁgure occipital lobe The part of the cerebral 3.15). cortex at the back of the head that is in- The occipital lobe, at the back of the head, responds to visual stimuli. Different volved in vision. areas of the occipital lobes are connected to process information about such aspects temporal lobe The portion of the of visual stimuli as their color, shape, and motion. A stroke or wound in the occipi- cerebral cortex just above the ears that tal lobe can cause blindness or, at a minimum, wipe out a portion of the person’s is involved in hearing, language pro- visual ﬁeld. cessing, and memory. The temporal lobe, the portion of the cerebral cortex just above the ears, is involved in hearing, language processing, and memory. The temporal lobes have a frontal lobe The part of the cerebral number of connections to the limbic system. For this reason, people with damage to cortex just behind the forehead that is the temporal lobes cannot ﬁle experiences into long-term memory. involved in the control of voluntary The frontal lobe, the portion of the cerebral cortex behind the forehead, is muscles, intelligence, and personality. involved in the control of voluntary muscles, intelligence, and personality. One fas-
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 96 impos03 414:mhspy7ch03:spy7ch03%0: 96 Chapter 3 Biological Foundations of Behavior Front of head Back of head Sensory cortex Motor cortex Sensory association cortex Frontal lobe Motor Parietal lobe association cortex Prefrontal cortex Occipital lobe Auditory cortex (mostly hidden Visual cortex from view) Temporal lobe Auditory association cortex Visual association cortex FIGURE 3.15 The Cerebral Cortex’s Lobes and Association Areas The cerebral cortex (left) is roughly divided cinating case study illustrates how damage to the frontal lobe can signiﬁcantly alter into four lobes: occipital, temporal, personality. Phineas T. Gage, a 25-year-old foreman who worked for the Rutland and frontal, and parietal. The cerebral cortex Burlington Railroad, met with an accident on September 13, 1848. Phineas and sev- (right) also consists of the motor cortex eral co-workers were using blasting powder to construct a roadbed. The crew drilled and sensory cortex. Further, the cerebral holes in the rock and gravel, poured in the blasting powder, and then tamped down cortex includes association areas, such as the powder with an iron rod. While Phineas was still tamping it down, the powder the visual association cortex, auditory as- blew up, driving the iron rod up through the left side of his face and out through sociation cortex, and sensory association the top of his head. Though the wound in his skull healed in a matter of weeks, cortex. Phineas became a different person. He had been a mild-mannered, hardworking, emotionally calm individual prior to the accident, well liked by all who knew him. Afterward, he became obstinate, moody, irresponsible, selﬁsh, and incapable of par- ticipating in any planned activities. Damage to the frontal lobe of his brain dramati- cally altered Phineas’s personality. Without intact frontal lobes, humans are emotionally shallow, distractible, listless, and so insensitive to social contexts that they may belch with abandon at dinner par- ties (Hooper & Teresi, 1992). Individuals with frontal lobe damage become so dis- tracted by irrelevant stimuli that they often cannot carry out some basic directions. In one such case, an individual, when asked to light a candle, struck a match cor- rectly but instead of lighting the candle, he put the candle in his mouth and acted as if he was smoking it (Luria, 1973). The frontal lobes of humans are especially large when compared with those of other animals. For example, the frontal cortex of rats barely exists; in cats, it occu- pies a paltry 3.5 percent of the cerebral cortex; in chimpanzees, 17 percent; and in humans, approximately 30 percent. Some neuroscientists maintain that the frontal cortex is an important index of evolutionary advancement (Hooper & Teresi, 1992). An important part of the frontal lobes is the prefrontal cortex, which is at the front of the motor cortex (see ﬁgure 3.15). The prefrontal cortex is believed to be involved A computerized reconstruction of Phineas in higher cognitive functions, such as planning and reasoning (Manes & others, 2002). T. Gage’s accident, based on measure- Some neuroscientists refer to the prefrontal cortex as an executive control system ments taken of his skull. because of its role in monitoring and organizing thinking (Owen, 1997). The parietal lobe, located at the top and toward the rear of the head, is involved in registering spatial location, attention, and motor control. Thus the parietal lobes are at work when you are judging how far you have to throw a ball to get it to some- parietal lobe Area of the cerebral cor- one else, when you shift your attention from one activity to another (turn your atten- tex at the top of the head that is in- tion away from the TV to a noise outside), and when you turn the pages of this book. volved in registering spatial location, The brilliant physicist Albert Einstein said that his reasoning often was best when he attention, and motor control. imagined objects in space. It turns out that his parietal lobes were 15 percent larger than average (Witelson, Kigar, & Harvey, 1999).
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 97 impos03 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 97 Pelvis Upper Motor Cortex Lower arm Trunk Neck Sensory Cortex Upper arm Trunk leg Upper Thumb, arm Lower fingers, Lower arm and hand Pelvis leg Hand, fingers, Upper leg and thumb Foot Facial and toes Upper Lower leg face expression Foot and toes Genitals Lips Salivation Teeth Vocalization and gums Mastication Tongue and pharynx Swallowing Motor cortex Sensory cortex Parietal lobe Forebrain Frontal lobe Occipital lobe Temporal lobe FIGURE 3.16 Disproportionate Rep- resentation of Body Parts in the Mo- tor and Sensory Areas of the Cortex In closing this discussion of the cerebral cortex’s lobes, a word of caution is in The amount of cortex allotted to a body part is not proportionate to the body order about going too far in localizing function within a particular lobe. Although I part’s size. Instead, the brain has more have attributed speciﬁc functions to a particular lobe (such as vision in the occipital space for body parts that require precision lobe), there is considerable integration and connection between any two or more and control. Thus the thumb, ﬁngers, and lobes and between lobes and other parts of the brain. hand require more brain tissue than does the arm. Sensory Cortex and Motor Cortex Two other important regions of the cerebral cortex are the sensory cortex and the motor cortex (see ﬁgure 3.15). The sensory cortex processes information about body sensations. It is located at the front of the parietal lobes. The motor cortex, just behind the frontal lobes, processes informa- tion about voluntary movement. The map in ﬁgure 3.16 shows which parts of the sensory and motor cortex are associated with different parts of the body. It is based on research done by Wilder Penﬁeld (1947), a neurosurgeon at the Montreal Neurological Institute. He worked with patients who had severe epilepsy and often performed surgery to remove por- tions of the epileptic patients’ brains. However, he was concerned that removing a portion of the brain might impair some of the individuals’ functions. Penﬁeld’s solu- sensory cortex Area of the cerebral tion was to map the cortex during surgery by stimulating different cortical areas and cortex that processes information about observing the responses of the patients, who were given a local anesthetic so they body sensations. would remain awake during the operation. He found that, when he stimulated cer- tain sensory and motor areas of the brain, different parts of a patient’s body moved. motor cortex Area of the cerebral cor- For both sensory and motor areas, there is a point-to-point relation between a part tex that processes information about of the body and a location on the cerebral cortex. In ﬁgure 3.16, the face and hands voluntary movement. are given proportionately more space than other body parts because the face and
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 98 impos06 414:mhspy7ch03:spy7ch03%0: 98 Chapter 3 Biological Foundations of Behavior hands are capable of ﬁner perceptions and movements than are other body areas and, therefore, need more cerebral cortex representation. The point-to-point mapping of sensory ﬁelds onto the cortex’s surface is the basis of our orderly and accurate perception of the world (Fox, 1996). When something touches your lip, for example, your brain knows what body part has been touched because the nerve pathways from your lip are the only pathways that project to the lip region of the sensory cortex. One familiar example of what happens when these neural pathways get con- nected the wrong way is seen in Siamese cats. Many Siamese cats have a genetic defect that causes the pathways from the eyes to connect to the wrong parts of the visual cortex during development. The result is that these cats spend their lives look- ing at things cross-eyed in an effort to “straighten out” the visual image of their visual cortex. The Association Cortex Embedded in the brain’s lobes, the association cortex makes up 75 percent of the cerebral cortex (see ﬁgure 3.15). Processing information about sensory input and motor output is not all that is taking place in the cerebral cortex. The association cortex (sometimes called association areas) is the region of the cerebral cortex that integrates this information. The highest intellectual functions, such as thinking and problem solving, occur in the association cortex. Interestingly, damage to a speciﬁc part of the association cortex often does not result in a speciﬁc loss of function. With the exception of language areas (which are localized), loss of function seems to depend more on the extent of damage to the association cortex than on the speciﬁc location of the damage. By observing brain- damaged individuals and using a mapping technique, scientists have found that the association cortex is involved in linguistic and perceptual functioning. The largest portion of the association cortex is located in the frontal lobe, directly under the forehead. Damage to this area does not lead to sensory or motor loss. Indeed, it is this area that may be most directly related to thinking and problem solv- ing. Early studies even referred to the frontal lobe as the center of intelligence, but research suggests that frontal lobe damage may not result in a lowering of intelli- gence. Planning and judgment are often associated with the frontal lobe. Personality also may be linked to the frontal lobe. Recall the misfortune of Phineas Gage, whose personality radically changed after he experienced frontal lobe damage. The Cerebral Hemispheres and Split-Brain Research At the beginning of the discussion of the cerebral cortex, I indicated that it is divided into two halves—left and right (see ﬁgure 3.14). Do these hemispheres have differ- ent functions? In 1861, French surgeon Paul Broca saw a patient who had received an injury to the left side of his brain about 30 years earlier. The patient became known as Tan, because Tan was the only word he could speak. Tan suffered from aphasia, a language disorder associated with brain damage. Tan died several days after Broca evaluated him, and an autopsy revealed that the injury was to a precise area of the left hemisphere. Today we refer to this area of the brain as Broca’s area, and we know that it plays an important role in the production of speech. Another area of the brain’s association cortex Region of the cere- left hemisphere that has an important role in language is Wernicke’s area, which, if bral cortex in which the highest intel- damaged, causes problems in comprehending language. Figure 3.17 shows the loca- lectual functions, including thinking tions of Broca’s area and Wernicke’s area. and problem solving, occur (also called Today, there continues to be considerable interest in the degree to which the association areas). brain’s left hemisphere or right hemisphere is involved in various aspects of think- corpus callosum A large bundle of ax- ing, feeling, and behavior (Corballis, Funnell, & Gazzaniga, 2002; Spence & others, ons that connect the brain’s two hemi- 2002). For many years scientists speculated that the corpus callosum, the large bun- spheres. dle of axons that connects the brain’s two hemispheres, had something to do with relaying information between the two sides (see ﬁgure 3.18). Roger Sperry (1974)
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 99 impos03 414:mhspy7ch03:spy7ch03%0: Structures of the Brain and Their Functions 99 conﬁrmed this in an experiment in which he cut the corpus callosum in cats. He also Broca’s area severed certain nerves leading from the eyes to the brain. After the operation, Sperry trained the cats to solve a series of visual problems with one eye blindfolded. After the cat learned the task, say with only its left eye uncovered, its other eye was blind- folded and the animal was tested again. The “split-brain” cat behaved as if it had never learned the task. It seems that the memory was stored only in the left hemi- sphere, which could no longer directly communicate with the right hemisphere. Further evidence of the corpus callosum’s function has come from studies of patients who, like Brandi Binder before surgery, have severe, even life-threatening, forms of epilepsy. Epilepsy is caused by electrical “brainstorms” that ﬂash uncontrol- lably across the corpus callosum. In one famous case, neurosurgeons severed the cor- Wernicke’s area pus callosum of an epileptic patient now known as W. J. in a ﬁnal attempt to reduce his unbearable seizures. Sperry (1968) examined W. J. and found that the corpus cal- FIGURE 3.17 Broca’s Area and losum functions the same in humans as in animals—cutting the corpus callosum Wernicke’s Area Broca’s area is located seemed to leave the patient with “two separate minds” that learned and operated in the brain’s left hemisphere, and it is in- independently. volved in the control of speech. Individu- The right hemisphere, it turns out, receives information only from the left side als with damage to Broca’s area have of the body, and the left hemisphere receives information only from the right side of problems saying words correctly. Also the body. When you hold an object in your left hand, for example, only the right shown is Wernicke’s area, a portion of the hemisphere of your brain detects the object. When you hold an object in your right left hemisphere that is involved in under- hand, only the left hemisphere of the brain detects the object (see ﬁgure 3.19). In a standing language. Individuals with dam- normally functioning corpus callosum, both hemispheres receive this information. age to this area cannot comprehend In people with intact brains, specialization of function occurs in some areas. Fol- words; that is, they hear the words but lowing are the main areas in which the brain tends to divide its functioning into one don’t know what they mean. hemisphere or the other (Gazzaniga, Ivry, & Mangun, 2001; Springer & Deutsch, 1998): • Verbal processing. The most extensive research on the brain’s two hemispheres has focused on language. Speech and grammar are localized to the left hemi- sphere. A common misconception, though, is that all language processing is carried out in the brain’s left hemisphere. However, such aspects of language as appropriate use of language in different contexts, metaphor, and much of our sense of humor reside in the right hemisphere. • Nonverbal processing. The right hemisphere is more dominant in processing non- verbal information, such as spatial perception, visual recognition, and emotion Axons FIGURE 3.18 The Corpus Callosum The corpus callosum is a thick band of about 80 million axons that connect the brain cells in one hemisphere to those in the other. In healthy brains, the two sides engage in a continuous ﬂow of informa- tion via this neural bridge.
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 100 impos06 414:mhspy7ch03:spy7ch03%0: 100 Chapter 3 Biological Foundations of Behavior Fixation point (Corballis, Funnell, & Gazzaniga, 2002). For example, the right hemisphere is mainly at work when we are processing informa- Left Right visual visual tion about people’s faces (O’Toole, 2002). The right hemisphere field field also may be more involved in processing information about emotions, both when we express emotions ourselves and when we recognize others’ emotions (Heller, Etienne, & Miller, 1997). Because differences in the functioning of the brain’s two hemi- spheres are known to exist, people commonly use the phrases left- brained and right-brained as a way of categorizing themselves and others. Such generalizations have little scientiﬁc basis. The most com- mon myth about hemispheric specialization is that the left brain is Optic logical and the right brain is creative. The left-brain, right-brain myth nerve started with the publication of Roger Sperry’s classic split-brain stud- ies. As Sperry’s ﬁndings made their way into the media, they became oversimpliﬁed and people were labeled either right-brained (artistic) or left-brained (logical). Speech Sperry did discover that the left hemisphere is superior in the kind of logic used to prove geometric theorems. But in everyday life, R L our logic problems involve integrating information and drawing con- clusions. In these instances, the right hemisphere is crucial. In most complex activities in which people engage, an interplay occurs Writing between the brain’s two hemispheres (Hoptman & Davidson, 1994). For example, in reading, the left hemisphere comprehends syntax Main language Simple and grammar, which the right does not. However, the right brain is center comprehension better at understanding a story’s intonation and emotion. A similar Corpus callosum interplay is observed in music and art. Pop psychology assigns both Visual half field severed Visual half field music and art to the right brain. The right hemisphere is better at R L some musical skills, such as recognizing chords. But the left hemi- sphere is better at others, such as distinguishing which of two sounds came ﬁrst. FIGURE 3.19 Visual Information in the Split Brain Enjoying or creating music requires the use of both hemispheres. In a split-brain patient, information from the visual ﬁeld’s left One positive result of the left-brain–right-brain myth is a perception side projects only to the right hemisphere. Information from that more right-brain activities and exercises should be incorporated the visual ﬁeld’s right side projects only to the left hemisphere. into school programs (Edwards, 1979). In schools that rely heavily Because of these projections, stimuli can be presented to only on rote learning to instruct students, children probably would ben- one of a split-brain patient’s hemispheres. eﬁt from exercises in intuitive thought and holistic thinking. But a deﬁciency in school curricula has nothing at all to do with left-brain, right-brain spe- cialization. In sum, some specialization of functions exists in both the left hemisphere (pro- cessing of certain verbal information) and the right hemisphere (processing of certain nonverbal information) of the brain. However, in many complex tasks in which humans engage in their everyday lives, integration across the hemispheres is common. To think further about the functioning of the left and right hemispheres, see the Critical Controversy box, which explores similarities and differences in men’s and women’s brains. Integration of Function in the Brain How do all of the regions of the brain cooperate to produce the wondrous complex- ity of thought and behavior that characterizes humans? Neuroscience still doesn’t have answers to such questions as how the brain solves a murder mystery or writes a poem or essay. But we can get a sense of integrative brain function by considering something like the act of escaping from a burning building. Imagine you are sitting at your desk writing letters when ﬁre breaks out behind you. The sound of crackling ﬂames is relayed from your ear, through the thalamus, to the auditory cortex, and on to the auditory association cortex. At each stage, the stim-
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 101 impos03 414:mhspy7ch03:spy7ch03%0: Are There “His” and “Hers” Brains? Critical Controversy Does gender matter when it comes to brain structure and func- cases such differences are small and that the differences do not tion? Human brains are much alike, whether the brain belongs mean that all men are better than all women at such tasks to a man or a woman. However, researchers have found some (Hyde & Mezulis, 2002). Debate about whether there are gen- differences between the male brain and the female brain der differences and about how big or small the differences are (Blum, 1998; Goldstein & others, 2001; Kimura, 2000; Raz & for many human skills continues to ﬂourish. others, 2001). Among differences discovered so far are In one recent neuroimaging study, an area of the parietal lobe that functions in visuospatial skills was larger in men than • One part of the hypothalamus responsible for sexual be- in women (Frederikse & others, 2000). Women, on the other havior is larger in men than in women (Swaab & others, hand, have a better memory for words and objects and are bet- 2001). ter at ﬁne motor skills (Halpern, 1997, 2001). These abilities • Portions of the corpus callosum—the band of tissues may have evolved through making clothes and preparing food. through which the brain’s two hemispheres commu- Are these brain differences truly innate, driven by “nature” nicate—are larger in women than in men (de Lacoste- through evolution, genetic programming, and hormones in the Utamsing & Holloway, 1982; Le Vay, 1994). Might this womb? Or might they be more a consequence of environment, difference mean that men and women process informa- the result of societal inﬂuences that stereotypically deﬁne sex- tion differently? In one study, women were likelier to use speciﬁc roles and characteristics, in effect shaping our brains in both brain hemispheres to process language, whereas men accordance with these roles? Some psychologists argue that the were likelier to use only the left hemisphere (Shaywitz & latter explanation accounts for male/female differences in math others, 1995). Despite this difference, the two sexes per- and verbal achievement (Eagly, 2001). However, many ques- formed equally well on the task, which involved sounding tions regarding men’s and women’s brains are exceedingly out words. The researchers concluded that nature has complex and likely cannot be answered by strictly biological or given the brain different routes to the same ability. environmental arguments. • It has been reported that men lose brain tissue earlier in Also, according to psychologist Diane Halpern (2000, the aging process than women do and that overall they 2001), the fact that there are differences between the brains of lose more of it (Carter, 1998). Further reports suggest women and men does not mean that one sex’s brain is better, that men are especially prone to tissue loss in the frontal any more than one sex’s genitals are better. Different does not (thinking, reasoning) and temporal (hearing) lobes, but mean deﬁcient. People can be different without being unequal women are prone to tissue loss in the parietal lobe (spatial in ability. location) and hippocampus (memory; Nystrand, 1996). Such results, as well as many others in the effort to chart What do you think? sex differences and similarities in human brains, require further research before being fully accepted as reliable and • Could sex differences in the brain be the result rather valid by the scientiﬁc community. than the cause of behavioral differences? Explain. • Have differences in women’s and men’s brains likely Differences in the ways in which men’s and women’s been exaggerated in light of the substantial similarities brains function likely evolved over time. Some of the differ- in their brains? Might the media be involved in any ences appear to be the result of a division of labor dating to exaggerations? Explain. early hunter-gatherer civilizations. For example, men are better • Because of differences in the brains of males and fe- than women at spatial-navigational skills, such as map reading, males, should males and females be educated differ- judging distances, and dart throwing (Kimura, 2000; Majeres, ently? Explain. 1999). However, some psychologists point out that in many ulus is processed to extract information, and, at some stage, probably at the association cortex level, the sounds are ﬁnally matched with something like a neural memory rep- resenting sounds of ﬁres you have heard previously. The association “ﬁre” sets new machinery in motion. Your attention (guided in part by the reticular formation) shifts to the auditory signal being held in your association cortex, and on to your auditory association cortex, and simultaneously (again guided by reticular systems) your head turns toward the noise. Now your visual association cortex reports in: “Objects match- ing ﬂames are present.” In other regions of the association cortex, the visual and audi- tory reports are synthesized (“We have things that look and sound like ﬁre’’), and neural associations representing potential actions (“ﬂee’’) are activated. However, ﬁring the 101
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 102 impos03 414:mhspy7ch03:spy7ch03%0: 102 Chapter 3 Biological Foundations of Behavior neurons that code the plan to ﬂee will not get you out of the chair. The basal ganglia must become engaged, and from there the commands will arise to set the brain stem, motor cortex, and cerebellum to the task of actually transporting you out of the room. Which part of your brain did you use to escape? Virtually all systems had a role; each was quite speciﬁc, and together they generated the behavior. By the way, you would probably remember this event because your limbic circuitry would likely have started the memory formation process when the signiﬁcant association “ﬁre” was ﬁrst triggered. The next time the sounds of crackling ﬂames reach your auditory associa- tion cortex, the associations triggered would include those of this most recent escape. In sum, considerable integration of function takes place in the brain (Gevins, 1999; Miller & Cohen, 2001). Review and Sharpen Your Thinking mhhe com/ s a n t rock p7 3 Identify the brain’s levels and structures, and summarize the functions For study tools related to this learning of its structures. goal, see the Study Guide, the CD-ROM, and the Online Learning Center. • Specify four techniques that are used to study the brain and the nervous system. • Outline the levels of organization in the human brain. • Discuss the areas of the cerebral cortex and their functions. • Explain how split-brain research has increased our understanding of the way the cerebral hemispheres function. • Describe the integration of function in the brain. In your experience, does human behavior differ in important ways from the behavior of other animals? What tasks are human brains able to accomplish that other ani- mals may not be able to? 4 THE ENDOCRINE SYSTEM What is the endocrine system and how does it affect behavior? The endocrine system is a set of glands that regulate the activities of certain organs by releasing their chemical products into the bloodstream. In the past, the endocrine system was considered separate from the nervous system. However, today neurosci- entists know that these two systems are often interconnected. Hormones are the chemical messengers that are manufactured by the endocrine glands. Hormones travel more slowly than nerve impulses. The bloodstream conveys hormones to all parts of the body, and the membrane of every cell has receptors for one or more hormones. The endocrine glands consist of the pituitary gland, the thyroid and parathyroid glands, the adrenal glands, the pancreas, and the ovaries in women and the testes in endocrine system A set of glands that men (see ﬁgure 3.20). In much the same way that the brain’s control of muscular activ- regulate the activities of certain organs ity is constantly monitored and altered to suit the information received by the brain, by releasing hormones into the blood- the action of the endocrine glands is continuously monitored and changed by nervous, stream. hormonal, and chemical signals (Mader, 2003). Recall from earlier in the chapter that the autonomic nervous system regulates processes such as respiration, heart rate, and hormones Chemical messengers man- digestion. The autonomic nervous system acts on the endocrine glands to produce a ufactured by the endocrine glands. number of important physiological reactions to strong emotions such as rage and fear. pituitary gland An important en- The pituitary gland, a pea-sized gland that sits at the base of the skull, controls docrine gland at the base of the skull growth and regulates other glands (see ﬁgure 3.21). The anterior (front) part of the that controls growth and regulates other pituitary is known as the master gland, because almost all of its hormones direct the glands. activity of target glands elsewhere in the body. In turn, the anterior pituitary gland is controlled by the hypothalamus.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 103 impos03 414:mhspy7ch03:spy7ch03%0: The Endocrine System 103 Hypothalamus Pituitary gland Thyroid gland Parathyroid gland Adrenal gland Pancreas Ovary (in female) FIGURE 3.21 The Pituitary Gland The pituitary gland, which Testis (in male) hangs by a short stalk from the hypothalamus, regulates the hormone production of many of the body’s endocrine glands. Here it is enlarged FIGURE 3.20 The Major Endocrine Glands The pituitary gland 30 times. releases hormones that regulate the hormone secretions of the other glands. The pituitary gland is itself regulated by the hypothalamus. The adrenal glands are instrumental in regulating moods, energy level, and the ability to cope with stress. Each adrenal gland secretes epinephrine (also called adren- aline) and norepinephrine (also called noradrenaline). Unlike most hormones, epi- nephrine and norepinephrine act quickly. Epinephrine helps a person get ready for an emergency by acting on smooth muscles, the heart, stomach, intestines, and sweat glands. In addition, epinephrine stimulates the reticular formation, which in turn arouses the sympathetic nervous system, and this system subsequently excites the adrenal glands to produce more epinephrine. Norepinephrine also alerts the individ- ual to emergency situations by interacting with the pituitary and the liver. You may remember that norepinephrine functions as a neurotransmitter when it is released by neurons. In the adrenal glands, norepinephrine is released as a hormone. In both instances, norepinephrine conveys information—in the ﬁrst instance to neurons, in the second to glands (Raven & Johnson, 2002). mhhe com/ s a n t rock p7 For study tools related to this learning Review and Sharpen Your Thinking goal, see the Study Guide, the CD-ROM, and the Online Learning Center. 4 State what the endocrine system is and how it affects behavior. • Describe the endocrine system, its glands, and their functions. Is the behavior of animals such as rats, rabbits, and bulls more likely to be strongly adrenal glands Important endocrine controlled by hormones than that of humans? In answering this question, think glands that are instrumental in regulat- about the differences in the structures of the brains of humans and those animals ing moods, energy level, and the ability that were described earlier in the chapter. to cope with stress.
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 104 impos06 414:mhspy7ch03:spy7ch03%0: 104 Chapter 3 Biological Foundations of Behavior 5 BRAIN DAMAGE, PLASTICITY, AND REPAIR The Brain’s Plasticity and Brain Tissue Implants Capacity for Repair If the brain is damaged through injury or illness, does it have the capacity to repair itself, or can its functions be restored surgically? Recall from the discussion of the brain’s important characteristics earlier in the chap- ter that plasticity is an example of the brain’s remarkable adaptability. Neuroscientists have studied plasticity especially following brain damage, charting the brain’s ability to repair itself. Brain damage can produce horriﬁc effects, including paralysis, sensory loss, memory loss, and personality deterioration. When such damage occurs, can the brain recover some or all of its functions? Recovery from brain damage varies con- siderably from one case to another depending on the age of the individual and the extent of the damage (Garraghty, 1996; Sofroniew & Mobley, 2001). In the case of Brandi Binder, described at the beginning of the chapter, considerable plasticity was present, and the left hemisphere of her cerebral cortex took over many typically right- hemisphere functions after the right hemisphere was surgically removed because of epilepsy. Other people are less fortunate. There is hope that one day surgeons will be able to implant healthy tissue and restore function lost as a result of illness or injury. I discuss this area of research shortly. The Brain’s Plasticity and Capacity for Repair The human brain shows the most plasticity in young children before the functions of the cortical regions become entirely ﬁxed (Kolb, 1989). For example, if the speech areas in an infant’s left hemisphere are damaged, the right hemisphere assumes much of this language function. However, after age 5, damage to the left hemisphere can permanently disrupt language ability. The brain’s plasticity is further discussed in chapter 4 on development throughout the life span. A key factor in recovery is whether some or all of the neurons in an affected area are just damaged or completely destroyed (Black, 1998; Carlson, 2001). If the neurons have not been destroyed, brain function often becomes restored over time. There are three ways in which repair of the damaged brain might take place: • Collateral sprouting, in which the axons of some healthy neurons adjacent to damaged cells grow new branches (Chung & Chung, 2001). • Substitution of function, in which the damaged region’s function is taken over by another area or areas of the brain. This is what happened to Brandi Binder. • Neurogenesis. This is the term given to the generation of new neurons. One of the long-standing beliefs in neuroscience regarding plasticity was that all of the neurons an individual will ever have are present soon after birth. However, neuroscientists have recently found that human adults can generate new neu- rons (Kempermann & Gage, 1999). Researchers also discovered that adult monkeys’ brains can create thousands of new neurons each day (Gould & oth- ers, 1999). Some researchers believe there is good evidence that neurogenesis is much more pervasive than previously thought (Hsu & others, 2001). How- ever, other neuroscientists argue that the evidence is weak (Rakic, 2002). If Actress Patricia Neal suffered a stroke when she was 39 years of age. The stroke researchers can discover how new neurons are generated, possibly the informa- paralyzed one of her legs and left her un- tion can be used to ﬁght degenerative diseases of the brain, such as able to read, write, or speak. However, an Alzheimer’s disease and Parkinson’s disease (Gage, 2000). intensive rehabilitation program and the human brain’s plasticity allowed her to recover her functioning to the point that Brain Tissue Implants she resumed her career as an actress 4 years later. What is the nature of the human The brain naturally recovers some functions lost following damage, but not all. In brain’s plasticity and capacity for repair? recent years, considerable excitement has been generated about brain grafts, implants
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 105 impos03 414:mhspy7ch03:spy7ch03%0: Genetic and Evolutionary Blueprints of Behavior 105 of healthy tissue into damaged brains (Rossi, Saggiorato, & Strata, 2002). The poten- tial success of brain grafts is much better when brain tissue from the fetal stage (an early stage in prenatal development) is used. The neurons of the fetus are still grow- ing and have a much higher probability of making connections with other neurons than do the neurons of adults. In a number of studies, researchers have damaged part of an adult rat’s (or some other animal’s) brain, waited until the animal recovered as much as possible by itself, and assessed its behavioral deﬁcits. Then they took the cor- responding area of a fetal rat’s brain and transplanted it into the damaged brain of the adult rat. In these studies, the rats that received the brain transplants demon- strated considerable behavioral recovery (Dunnett, 1989). Might such brain grafts be successful with humans suffering from brain damage? Research suggests that they might, but ﬁnding donors is a problem (Lindvall, 2001). Aborted fetuses are a possibility, but using them as a source of graft tissue raises eth- ical issues. Another type of treatment has been attempted with individuals who have Parkinson’s disease, a neurological disorder that affects about a million people in the United States. Parkinson’s disease impairs coordinated movement to the point that just walking across a room can be a major ordeal. In one recent study, brain grafts of embryonic dopamine neurons from aborted fetuses into individuals with Parkin- son’s disease resulted in a decrease of negative symptoms in individuals under 60 years of age but not in patients over 60 (Freed & others, 2001). In another study, neuronal cells were transplanted into stroke victims (Kondzi- olka & others, 2000). The motor and cognitive skills of 12 patients who had experi- enced strokes improved markedly after the healthy neuronal cells were implanted in the midbrain. The potential for brain grafts also exists for individuals with Alzheimer’s disease, which is characterized by progressive decline in intellectual functioning resulting from the degeneration of neurons that function in memory. Such degenerative changes can be reversed in rats (Gage & Bjorklund, 1986). As yet, though, no successful brain grafts have been reported for Alzheimer’s patients. Review and Sharpen Your Thinking mhhe com/ s a n t rock p7 5 Discuss the brain’s capacity for recovery and repair. For study tools related to this learning • State the factors that favor recovery of function in damaged brains and list goal, see the Study Guide, the CD-ROM, three ways in which the brain may recover. and the Online Learning Center. • Discuss the possibility of repairing damaged brains with tissue grafts. Suppose someone has suffered a mild form of brain damage. What questions might you ask to determine whether the person’s brain will likely be able to either com- pensate or repair itself? GENETIC AND EVOLUTIONARY BLUEPRINTS OF BEHAVIOR 6 Chromosomes, Genes, The Study of Genetics Genetics and Evolution and DNA How do genetics and evolutionary psychology increase our understanding of behavior? As you saw at the beginning of this chapter, genetic and evolutionary processes favor organisms that have adapted for survival. Successful adaptations can be physical, as in the case of the brain’s increasing complexity, or behavioral, as in the choice of a suitable mate for raising a family.
san9414x_ch03pg76_115 6/25/02 7:08 PM Page 106 impos06 414:mhspy7ch03:spy7ch03%0: 106 Chapter 3 Biological Foundations of Behavior Cell Chromosome DNA Nucleus FIGURE 3.22 Cells, Chromosomes, Genes, and DNA (Left) The body con- tains trillions of cells, which are the basic structural units of life. Each cell contains a central structure, the nucleus. (Middle) Chromosomes, Genes, and DNA Chromosomes and genes are located in the nucleus of the cell. Chromosomes are You began life as a single cell, a fertilized human egg, weighing about one 20-millionth made up of threadlike structures com- of an ounce. From this single cell, you developed into a human being made up of posed mainly of DNA molecules. (Right) A trillions of cells. The nucleus of each human cell contains 46 chromosomes, which gene is a segment of DNA that contains are threadlike structures that come in 23 pairs, one member of each pair coming from the hereditary code. The structure of DNA each parent. Chromosomes contain the remarkable substance deoxyribonucleic resembles a spiral ladder. acid, or DNA, a complex molecule that contains genetic information. Genes, the units of hereditary information, are short segments of chromosomes, composed of DNA. Genes act like blueprints for cells. They enable cells to reproduce and manu- facture the proteins that are necessary for maintaining life. The relationship among chromosomes, genes, and DNA is illustrated in ﬁgure 3.22. When the approximately 30,000 genes from one parent combine at conception with the same number of genes from the other parent, the number of possibilities is staggering. Although scientists are still a long way from unraveling all the mysteries about the way genes work, some aspects of the process are well understood, starting with the fact that every person has two genes for each characteristic governed by chromosomes Threadlike structures principles of heredity (Lewis, 2003; Lewis & others, 2002). that contain genes and DNA. Humans In some gene pairs, one gene is dominant over the other. If one gene of a pair is have 23 chromosome pairs in the nu- dominant and one is recessive, according to the dominant-recessive genes principle, cleus of every cell. Each parent con- the dominant gene overrides the recessive gene. A recessive gene exerts its inﬂuence tributes one chromosome to each pair. only if both genes of a pair are recessive. If you inherit a recessive gene from only one parent, you may never know you carry the gene. In the world of dominant-recessive deoxyribonucleic acid (DNA) A genes, brown eyes, farsightedness, and dimples rule over blue eyes, nearsightedness, and complex molecule that contains genetic freckles. If you inherit a recessive gene for a trait from both of your parents, you will information; makes up chromosomes. show the trait. That’s why two brown-eyed parents can have a blue-eyed child: Each genes The units of hereditary informa- parent would have a dominant gene for brown eyes and a recessive gene for blue eyes. tion. They are short segments of chro- Because dominant genes override recessive genes, the parents have brown eyes. How- mosomes, composed of DNA. ever, the child can inherit a recessive gene for blue eyes from each parent. With no dominant gene to override them, the recessive genes make the child’s eyes blue. dominant-recessive genes principle Unlike eye color, complex human characteristics such as personality and intelli- If one gene of a pair governing a given gence are likely inﬂuenced by many different genes. The term polygenic inheritance is characteristic (such as eye color) is used to describe the inﬂuences of multiple genes on behavior. dominant and one is recessive, the dominant gene overrides the recessive The Study of Genetics gene. A recessive gene exerts its inﬂu- ence only if both genes in a pair are Historically speaking, genetics is a relatively young science. Its origins go back to the recessive. mid-nineteenth century, when an Austrian monk named Gregor Mendel studied heredity in generations of pea plants. By cross-breeding plants with different charac-
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 107 impos03 414:mhspy7ch03:spy7ch03%0: The Human Genome Project and Your Genetic Future Psychology and Life The Human Genome Project, begun in the However, mining DNA variations to 1970s, has made stunning progress in map- discover health risks might increasingly ping the human genome. Goals for the year threaten an individual’s ability to obtain 2003 are to identify all of the genes in hu- and hold jobs, obtain insurance, and keep man DNA and determine the sequence of genetic proﬁles private. For example, 3 billion chemical base pairs that make up should an airline pilot or neurosurgeon human DNA (U.S. Department of Energy, who one day may develop a disorder that 2001). Among the surprise discoveries of makes the hands shake be required to leave the Human Genome Project is that humans that job early? have only about 30,000 genes—it was pre- Answering the following questions viously thought that we had 50,000 to should encourage you to think further 100,000. The project also has revealed that about some of the issues involved in our ge- human DNA is about 98 percent identical to netic future (NOVA, 2001): chimpanzee DNA (U.S. Department of 1. Would you want yourself or a loved Energy, 2001). A positive result from the Human one to be tested for a gene that increases The Human Genome Project has al- Genome Project. Shortly after Andrew your risk for a disease but does not deter- ready linked speciﬁc DNA variations with Gobea was born, his cells were genetically mine whether you will actually develop increased risk of a number of diseases and altered to prevent his immune system the disease? conditions, including Huntington’s disease from failing. 2. Would you want yourself and your (in which the central nervous system mate tested before having offspring to determine your deteriorates), some forms of cancer, asthma, diabetes, hyper- risk for having a child who is likely to contract various tension, and Alzheimer’s disease (Davies, 2001). Other docu- diseases? mented DNA variations affect the way people react to certain 3. Should testing of fetuses be restricted to traits that are drugs. commonly considered to have negative outcomes, such as Every individual carries a number of DNA variations that Huntington’s disease? might predispose that person to a serious physical disease or 4. Should altering a newly conceived embryo’s genes to im- mental disorder. Identifying these ﬂaws could enable doctors to prove qualities such as intelligence, appearance, and estimate an individual’s disease risks, recommend healthy strength be allowed? lifestyle regimens, and prescribe the safest and most effective 5. Should employers be permitted access to your genetic in- drugs. A decade or two from now, parents of a newborn baby formation? may be able to leave the hospital with a full genome analysis of 6. Should life insurance companies have access to your ge- their offspring that reveals various disease risks. netic information? teristics and noting the characteristics of the offspring, Mendel discovered predictable patterns of heredity and laid the foundation for modern genetics. Today researchers continue to apply Mendel’s methods, as well as modern technology, in their quest to expand our knowledge of genetics. This section discusses three ways to study genet- ics: molecular genetics, selective breeding, and behavioral genetics. Molecular Genetics The ﬁeld of molecular genetics involves actual manipulation of genes using technology to determine their effect on behavior. There is currently a great deal of enthusiasm about the use of molecular genetics to discover the speciﬁc locations on genes that determine an individual’s susceptibility to many diseases and other aspects of health and well-being (Dolﬁn, 2002; Mader, 2003). The term genome is used to describe the complete set of instructions for making an organism. It contains the master blueprint for all cellular structures and activities for the life span of the organism. To read about the Human Genome Project and its possible applications, see the Psychology and Life box. 107
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 108 impos03 414:mhspy7ch03:spy7ch03%0: 108 Chapter 3 Biological Foundations of Behavior 20 Selective Breeding Selective breeding is a genetic method in which organisms are chosen for reproduction based on how much Mean errors (normalized scale) 18 of a particular trait they display. Mendel developed this technique Dulls in his studies of pea plants. A more recent example involving 16 behavior is the classic selective breeding study conducted by Robert Tryon (1940). He chose to study maze-running ability in rats. After 14 he trained a large number of rats to run a complex maze, he then mated the rats that were the best at maze running (“maze bright’’) 12 with each other and the ones that were the worst (“maze dull’’) Brights with each other. He continued this process with 21 generations of 10 rats. As can be seen in ﬁgure 3.23, after several generations, the maze-bright rats signiﬁcantly outperformed the maze-dull rats. 8 Selective breeding studies have demonstrated that genes are an important inﬂuence on behavior, but that does not mean that expe- 0 rience is unimportant (Pinel, 2003). For example, in another study, 0 2 4 6 8 10 12 14 16 18 20 22 maze-bright and maze-dull rats were reared in one of two envi- Number of selected generations ronments: (1) an impoverished environment that consisted of a bar- FIGURE 3.23 Results of Tryon’s Selective Breeding ren wire-mesh group cage, or (2) an enriched environment that Experiment with Maze-Bright and Maze-Dull Rats contained tunnels, ramps, visual displays, and other stimulating objects (Cooper & Zubeck, 1958). When they reached maturity, only the maze-dull rats that had been reared in an impoverished environment made more maze-learning errors than the maze-bright rats. Selective breeding is practiced at the Repository for Germinal Choice in Escon- dido, California, which was founded by Dr. Robert Graham as a sperm bank for Nobel Prize winners and other bright individuals with the intent of producing geniuses. The sperm is available to women whose husbands are infertile. What are the odds that the sperm bank will yield that special combination of factors required to produce a creative genius? Twentieth-century Irish-born playwright George Bernard Shaw once told a story about a beautiful woman who wrote to him, saying that, with her body and his mind, they could produce wonderful offspring. Shaw responded by saying that, unfortunately, the offspring might get his body and her mind! What do you think about the Nobel Prize winners’–sperm bank? Is it right to breed for intelligence? Does it raise visions of the German genetics program of the 1930s and 1940s, based on the Nazis’ belief that certain traits were superior? The Nazis tried to produce children with such traits and killed people without them. Or does the sperm bank merely provide a social service for couples who cannot conceive a child, couples who want to maximize the probability that their offspring will have good genes? Behavior Genetics Behavior genetics is the study of the degree and nature of hered- ity’s inﬂuence on behavior. Behavior genetics is less invasive than molecular genet- ics and selective breeding. Using methods such as the twin study, behavior geneticists examine the extent to which individuals are shaped by their heredity and their envi- ronmental experiences (Wahlsten, 2000). In the most common type of twin study, the behavioral similarity of identical twins is compared with the behavioral similarity of fraternal twins. Identical twins develop from a single fertilized egg that splits into two genetically identical embryos, each of which becomes a person. Fraternal twins develop from separate eggs and sep- arate sperm, making them genetically no more similar than nontwin siblings. They may even be of different sexes. By comparing groups of identical and fraternal twins, behavior geneticists capi- talize on the fact that identical twins are more similar genetically than are fraternal twins. In one twin study, 7,000 pairs of Finnish identical and fraternal twins were compared on the personality traits of extraversion (being outgoing) and neuroticism (being psychologically unstable; Rose & others, 1988). The identical twins were much Dr. Graham with the frozen sperm of a more alike than the fraternal twins on both of these personality traits, suggesting that Nobel Prize–winning donor. genes inﬂuence both traits.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 109 impos03 414:mhspy7ch03:spy7ch03%0: Genetic and Evolutionary Blueprints of Behavior 109 What is the nature of the twin-study method? One problem with twin studies is that adults might stress the similarities of iden- tical twin children more than those of fraternal twins, and identical twins might per- ceive themselves as a “set” and play together more than fraternal twins do. If so, observed similarities in identical twins might be more strongly inﬂuenced by envi- ronmental factors than usually thought. In another type of twin study, researchers evaluate identical twins who have been reared in separate environments. If their behavior is similar, the assumption is that heredity has played an important role in shaping their behavior. This strategy is the basis for the Minnesota Study of Twins Reared Apart, directed by Thomas Bouchard and his colleagues (1996). They bring identical twins who have been reared apart to Minneapolis from all over the world to study their behavior. They ask thousands of questions about their family and childhood environment, personal interests, vocational orientation, and values. Detailed medical histories are obtained, including information about their diet, smoking, and exercise habits. One pair of twins in the Minnesota study, Jim Springer and Jim Lewis, were sep- arated at 4 weeks of age and did not see each other again until they were 39 years old. They had an uncanny number of similarities, even though they had lived apart. For example, they both worked as part-time deputy sheriffs, had vacationed in Florida, had owned Chevrolets, had dogs named Toy, and had married and divorced women named Betty. Both liked math but not spelling. Both were good at mechan- ical drawing. Both put on 10 pounds at about the same time in their lives, and both started suffering headaches at 18 years of age. They did have a few differences. For example, one expressed himself better orally, and the other was more proﬁcient at writing. One parted his hair over his forehead, the other wore his hair slicked back with sideburns. Critics argue that some of the separated twins in the Minnesota study had been together several months prior to their adoption, that some had been reunited prior to their testing (in some cases for a number of years), that adoption agencies often put identical twins in similar homes, and that even strangers who spend several hours together are likely to come up with some coincidental similarities (Adler, 1991). Still, even in the face of such criticism, it seems unlikely that all of the similarities in the identical twins reared apart could be due to experience alone. Behavior geneticists also use adoption studies to try to determine whether the The Jim twins: how coincidental? Springer behavior of adopted children is more like that of their biological parents or their adopted (right) and Lewis were unaware of each parents. Another type of adoption study compares biological and adopted siblings. In other for 39 years.
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 111 impos03 414:mhspy7ch03:spy7ch03%0: Genetic and Evolutionary Blueprints of Behavior 111 behavior. For example, Albert Bandura (1998), whose social cognitive theory was described in chapter 1, acknowledges the importance of human adaptation and change. However, he rejects what he calls “one-sided evolutionism,” in which social behavior is the product of evolved biology. Bandura recommends a bidirectional view. In this view, evolutionary pressures created changes in biological structures for the use of tools, which enabled organisms to manipulate, alter, and construct new environmen- tal conditions. Environmental innovations of increasing complexity, in turn, produced new selection pressures for the evolution of specialized biological systems for con- sciousness, thought, and language. Human evolution gave us body structures and biological potentialities, not behav- ioral dictates, according to scientists such as Steven Jay Gould (1981). Having evolved, advanced biological capacities can be instrumental in producing diverse cultures— aggressive or peaceful, for example. And Russian American scientist Theodore Dobzhansky (1977) reminds us that the human species has selected for learnability and plasticity, which allows us to adapt to diverse contexts. Most, if not all, psychol- ogists would agree that the interaction of biology and environment is the basis for our own development as human beings. Chapter 4 further explores the inﬂuence of biology and environment on human development. Review and Sharpen Your Thinking mhhe com/ s a n t rock p7 6 Explain how genetics and evolutionary psychology increase our For study tools related to this learning understanding of behavior. goal, see the Study Guide, the CD-ROM, • Discuss the structures and functions of chromosomes, genes, and DNA. and the Online Learning Center. • Describe three methods for studying genetics. • Explain how evolution might direct human behavior. What ethical issues regarding genetics and behavior might arise in the future?
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 112 impos03 414:mhspy7ch03:spy7ch03%0: Reach Your Learning Goals 1 THE NERVOUS SYSTEM Characteristics Pathways in the Divisions of the Nervous System Nervous System 2 NEURONS Specialized Cell Structure Synapses and Neurotransmitters The Neural Impulse Neural Networks 3 STRUCTURES OF THE BRAIN AND THEIR FUNCTIONS How the Brain and Integration of Function Biological Nervous System Are Studied The Cerebral Cortex in the Brain Foundations Levels of Organization in the Brain The Cerebral Hemispheres and of Behavior Split-Brain Research 4 THE ENDOCRINE SYSTEM 5 BRAIN DAMAGE, PLASTICITY, AND REPAIR The Brain’s Plasticity Brain Tissue Implants and Capacity for Repair 6 GENETIC AND EVOLUTIONARY BLUEPRINTS OF BEHAVIOR Chromosomes, Genes, The Study of Genetics Genetics and DNA and Evolution 112
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 113 impos03 414:mhspy7ch03:spy7ch03%0: q 1 Discuss the nature and basic functions of the nervous system. depending on the nature of the neural impulse. Neuro- transmitters include acetylcholine, GABA, norepineph- rine, dopamine, serotonin, and endorphins. Most drugs • The nervous system is the body’s electrochemical commu- that inﬂuence behavior do so mainly by mimicking neu- nication circuitry. Four important characteristics of the rotransmitters or interfering with their activity. brain and nervous system are complexity, integration, • Neural networks are clusters of neurons that are inter- adaptability, and electrochemical transmission. The brain’s connected to process information. special ability to adapt and change is called plasticity. • The ﬂow of information in the nervous system occurs in specialized pathways of nerve cells. Three of these path- q 3 Identify the brain’s levels and structures and the functions of its structures. ways involve sensory input, motor output, and neural • The main techniques used to study the brain are brain le- networks. sioning, staining, electrical recording, and brain imaging. • The nervous system is divided into two main parts: cen- • The three major levels of the brain are the hindbrain, tral (CNS) and peripheral (PNS). The CNS consists of the midbrain, and forebrain. The hindbrain is the lowest brain and spinal cord. The PNS has two major divisions: portion of the brain. The three main parts of the hind- somatic and autonomic. The autonomic nervous system brain are the medulla (involved in controlling breathing consists of two main divisions: sympathetic and para- and posture), cerebellum (involved in motor coordina- sympathetic. tion), and pons (involved in sleep and arousal). q 2 Explain what neurons are and describe how they process information. • From the midbrain many nerve-ﬁber systems ascend and descend to connect to higher and lower levels of the brain. The midbrain contains the reticular formation, • Neurons are cells that specialize in processing informa- which is involved in stereotypical patterns of behavior tion. They make up the communication network of the (such as walking, sleeping, or turning to a sudden noise), nervous system. Glial cells perform supportive and nutri- and small groups of neurons that communicate with tive functions for neurons. The three main parts of the many areas in the brain. The brain stem consists of much neuron are the cell body, dendrite (receiving part), and of the hindbrain (excluding the cerebellum) and the axon (sending part). A myelin sheath encases and insu- midbrain. lates most axons and speeds up transmission of neural • The forebrain is the highest level of the brain. The key impulses. forebrain structures are the limbic system, thalamus, • A neuron sends information along its axon in the form of basal ganglia, hypothalamus, and cerebral cortex. The brief electric impulses, or waves. Resting potential is the limbic system is involved in memory and emotion term given to the stable, slightly negative charge of an through its two structures, the amygdala (which plays inactive neuron. When the electrical signals exceed a roles in survival and emotion) and the hippocampus certain activation threshold, positively charged sodium (which functions in the storage of memories). The thala- ions rush into the neuron. The brief wave of electrical mus is a forebrain structure that serves as an important charge that sweeps down the axon is called the action relay station for processing information. The basal gan- potential. The neuron returns to a resting potential as glia are forebrain structures that help to control and co- positively charged potassium ions move out of it, return- ordinate voluntary movements. The hypothalamus is a ing the neuron to a negative charge. The action potential forebrain structure that monitors eating, drinking, and abides by the all-or-none principle: Its strength does not sex; directs the endocrine system through the pituitary change during transmission. gland; and is involved in emotion, stress, and reward. • To go from one neuron to another, information must be • The cerebral cortex makes up most of the outer layer of converted from an electrical impulse to a chemical mes- the brain. Higher mental functions, such as thinking and senger called a neurotransmitter. At the synapse where planning, take place in the cerebral cortex. The wrinkled neurons meet, neurotransmitters are released into the surface of the cerebral cortex is divided into hemispheres. narrow gap that separates them. There some neurotrans- Each hemisphere is divided into four lobes: occipital, mitter molecules attach to receptor sites on the receiving temporal, frontal, and parietal. There is considerable in- neuron, where they stimulate another electrical im- tegration and connection between the brain’s lobes. The pulse. Neurotransmitters can be excitatory or inhibitory sensory cortex processes information about body sensa- 113
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 114 impos03 414:mhspy7ch03:spy7ch03%0: tions. The motor cortex processes information about vol- untary movement. Penﬁeld (1947) pinpointed speciﬁc q 5 Describe the brain’s capacity for recovery and repair. areas in the brain that correspond to speciﬁc parts of the body and also mapped sensory ﬁelds onto the cortex’s • The human brain has considerable plasticity, although surface. The association cortex, which makes up 75 per- this plasticity is greater in young children than later in cent of the cerebral cortex, is instrumental in integrating development. Three ways in which a damaged brain information, especially about the highest intellectual might repair itself are collateral sprouting, substitution of functions. function, and neurogenesis. • A controversial topic is the extent to which the left and • Brain grafts are implants of healthy tissue into damaged right hemispheres of the brain are involved in different brains. Brain grafts are more successful when fetal tissue functions. Two areas in the left hemisphere that involve is used. speciﬁc language functions are Broca’s area (speech) and Wernicke’s area (comprehending language). The corpus q 6 Explain how genetics and evolutionary psychol- ogy increase our understanding of behavior. callosum is a large bundle of ﬁbers that connects the two hemispheres. Researchers have studied what happens • Chromosomes are threadlike structures that come in 23 when the corpus callosum has to be severed, as in some pairs, one member of each pair coming from each par- cases of severe epilepsy. Research suggests that the left ent. Chromosomes contain the genetic substance de- brain is more dominant in processing verbal information oxyribonucleic acid (DNA). Genes, the units of (such as language), and the right brain in processing hereditary information, are short segments of chromo- nonverbal information (such as spatial perception, visual somes composed of DNA. The dominant-recessive genes recognition, and emotion). Nonetheless, in a normal in- principle states that if one gene of a pair is dominant and dividual whose corpus callosum is intact, both hemi- one is recessive, the dominant gene overrides the reces- spheres of the cerebral cortex are involved in most sive gene. complex human functioning. • Three methods that are used to study heredity’s inﬂuence • It is extremely important to remember that generally are molecular genetics, selective breeding, and behavior brain function is integrated and involves connections be- genetics. Two methods used by behavior geneticists are tween different parts of the brain. Pathways of neurons twin studies and adoption studies. involved in a particular function, such as memory, are • Several key points in evolutionary psychology center on integrated across different parts and levels of the brain. the idea that nature selects behaviors that increase an or- ganism’s reproductive success, the importance of adaptive q 4 State what the endocrine system is and how it affects behavior. behavior, and specialization of functions. Evolutionary psychologists believe that just as parts of the brain have • The endocrine glands release hormones directly into the become specialized in function through the process of bloodstream for distribution throughout the body. The evolution, so have mental processes and behavior. Critics pituitary gland is the master endocrine gland. The adre- stress that it is important to recognize how evolutionary nal glands play important roles in moods, energy level, advances allow humans to choose and select their envi- and ability to cope with stress. ronments, rather than being completely under the control of their evolutionary past. Key Terms nervous system, p. 78 peripheral nervous system parasympathetic nervous myelin sheath, p. 82 plasticity, p. 79 (PNS), p. 81 system, p. 81 resting potential, p. 84 afferent nerves, p. 80 somatic nervous system, neurons, p. 82 action potential, p. 84 efferent nerves, p. 80 p. 81 glial cells, p. 82 all-or-none principle, p. 84 neural network, p. 80 autonomic nervous cell body, p. 82 synapse, p. 85 central nervous system system, p. 81 dendrite, p. 82 neurotransmitter, p. 86 (CNS), p. 81 sympathetic nervous axon, p. 82 agonist, p. 88 system, p. 81 114
san9414x_ch03pg76_115 6/14/02 10:24 AM Page 115 impos03 414:mhspy7ch03:spy7ch03%0: This is summary text. At the beginning of this chapter, we started six learning goals and antagonist, p. 88 basal ganglia, p. 94 motor cortex, p. 97 deoxyribonucleic acid encouraged 91 to review material related to these goals at three cortex, p. 98 hindbrain, p. you hypothalamus, p. 94 association points in the chapter, (DNA), p.106 page 00, page midbrain, p. 92 00, and page cerebral cortex, p. 95 summary corpus callosum, p. 98 00. The following can be used to guide study genes, p. 106 and understanding of p. 92 chapter. reticular formation, the occipital lobe, p. 95 endocrine system, p. 102 dominant-recessive genes brain stem, p. 92 temporal lobe, p. 95 hormones, p. 102 principle, p. 106 forebrain, p. 93 frontal lobe, p. 95 pituitary gland, p. 102 limbic system, p. 93 parietal lobe, p. 96 adrenal glands, p. 103 thalamus, p. 94 sensory cortex, p. 97 chromosomes, p. 106 q• Explain whatthe scientiﬁc study of behaviordeveloped 1 Psychology is psychology is and how it and mental Apply YourScience uses systematic methods to observe, processes. Knowledge describe, predict, and explain. Behavior includes every- 1. Consider the four characteristics of the nervous system. 4. Ephedra is a drug contained in a number of formulas mar- thing organisms do that can be observed. Mental Suppose you had to do without one of them. Which would keted to enhance athletic performance. Among the actions processes are thoughts, feelings, and motives. you choose, and what would be the consequences for your of ephedra is stimulation of areas that normally respond to q behavior? 2 Discuss contemporary approaches to psychology 2. Do a search on the World Wide Web for “nutrition” and “the epinephrine and norepinephrine. Think about the two dif- ferent kinds of actions (neurotransmitter and hormone) brain.” Examine the claims made by one or more and mental • Psychology is the scientiﬁc study of behavior of the web- these chemicals normally have in the nervous system, and sites. Based onScience uses systematic methods to observe, processes. what you learned in the chapter about how describe the kinds of side effects you might expect from tak- the describe, system works, how could nutrition affect brain nervous predict, and explain. Behavior includes every- ing ephedra. In particular, why might taking ephedra be thing organisms do that can be observed. Mental function? Based on what you know about being a scientist, very dangerous? how believable are the claims on theand motives. processes are thoughts, feelings, website? 5. It’s not unusual to read headlines announcing that genes 3. Imagine that you could make one part of your brain twice as are responsible for a troublesome behavior (for example, big as it is right now. Which part would it be, and how do “Next time you pig out, blame it on the genes,” Los Angeles you think your behavior would change as a result? What if Times, October 19, 2000, or “Men are born ﬁghters,” Times you had to make another part of your brain half its current (London), October 19, 2001). How would you interpret size? Which part would you choose to shrink, and what statements like these in light of the material discussed in the would the effects be? text? mhhe com/ Connections s a n t rock p7 For extra help in mastering the material in this chapter, see the review sections and practice quizzes in the Student Study Guide, the CD-ROM, and the Online Learning Center. 115