chapter 1 Introduction
Miss S. was a 60-year-old woman with a history of high blood pressure, which was not responding well to the medication she was taking. One evening she was sitting in her reclining chair reading the newspaper when the phone rang. She got out of her chair and walked to the phone. As she did, she began to feel giddy and stopped to hold onto the kitchen table. She has no memory of what happened after that.
The next morning, a neighbor, who usually stopped by to have coffee with Miss S., found her lying on the floor, mumbling incoherently. The neighbor called an ambulance, which took Miss S. to a hospital.
Two days after her admission, I visited her in her room, along with a group of neuropsychologists and neurological residents being led by the chief of neurology. We had already been told by the neurological resident in charge of her case that Miss S. had had a stroke in the back part of the right side of the brain. He had attached a CT scan to an illuminated viewer mounted on the wall and had showed us a white spot caused by the accumulation of blood in a particular region of her brain. (You can look at the scan yourself if you like; it is shown in Figure 5.19 .)
About a dozen of us entered Miss S.’s room. She was awake but seemed a little confused. The resident greeted her and asked how she was feeling. “Fine, I guess,” she said. “I still don’t know why I’m here.”
“Can you see the other people in the room?”
“How many are there?”
She turned her head to the right and began counting. She stopped when she had counted the people at the foot of her bed. “Seven,” she reported. “What about us?” asked a voice from the left of her bed. “What?” she said, looking at the people she had already counted. “Here, to your left. No, toward your left!” the voice repeated. Slowly, rather reluctantly, she began turning her head to the left. The voice kept insisting, and finally she saw who was talking. “Oh,” she said, “I guess there are more of you.”
The resident approached the left side of her bed and touched her left arm. “What is this?” he asked. “Where?” she said. “Here,” he answered, holding up her arm and moving it gently in front of her face.
“Oh, that’s an arm.”
“An arm? Whose arm?” “I don’t know…. I guess it must be yours.”
“No, it’s yours. Look, it’s a part of you.” He traced with his fingers from her arm to her shoulder.
“Well, if you say so,” she said, still sounding unconvinced.
When we returned to the residents’ lounge, the chief of neurology said that we had seen a classic example of unilateral neglect, caused by damage to a particular part of the right side of the brain. “I’ve seen many cases like this,” he explained. “People can still perceive sensations from the left side of their body, but they just don’t pay attention to them. A woman will put makeup on only the right side of her face, and a man will shave only half of his beard. When they put on a shirt or a coat, they will use their left hand to slip it over their right arm and shoulder, but then they’ll just forget about their left arm and let the garment hang from one shoulder. They also don’t look at things located toward the left or even the left halves of things. Once I visited a man in his hospital room who had just finished eating breakfast. He was sitting in his bed, with a tray in front of him. There was half of a pancake on his plate. ‘Are you all done?’ I asked. ‘Sure,’ he said. When he wasn’t looking, I turned the plate around so that the uneaten part was on his right. He saw it, looked startled, and said, ‘Where the hell did that come from?’”
The last frontier in this world—and perhaps the greatest one—lies within us. The human nervous system makes possible all that we can do, all that we can know, and all that we can experience. Its complexity is immense, and the task of studying it and understanding it dwarfs all previous explorations our species has undertaken.
One of the most universal of all human characteristics is curiosity. We want to explain what makes things happen. In ancient times, people believed that natural phenomena were caused by animating spirits. All moving objects—animals, the wind and tides, the sun, moon, and stars—were assumed to have spirits that caused them to move. For example, stones fell when they were dropped because their animating spirits wanted to be reunited with Mother Earth. As our ancestors became more sophisticated and learned more about nature, they abandoned this approach (which we call animism) in favor of physical explanations for inanimate moving objects. But they still used spirits to explain human behavior.
From the earliest historical times, people have believed that they possess something intangible that animates them: a mind, or a soul, or a spirit. This belief stems from the fact that each of us is aware of his or her own existence. When we think or act, we feel as though something inside us is thinking or deciding to act. But what is the nature of the human mind? We each have a physical body, with muscles that move it and sensory organs such as eyes and ears that perceive information about the world around us. Within our bodies the nervous system plays a central role, receiving information from the sensory organs and controlling the movements of the muscles. But what role does the mind play? Does it control the nervous system? Is it a part of the nervous system? Is it physical and tangible, like the rest of the body, or is it a spirit that will always remain hidden?
This puzzle has historically been called the mind–body question. Philosophers have been trying to answer it for many centuries, and more recently scientists have taken up the task. Basically, people have followed two different approaches: dualism and monism. Dualism is a belief in the dual nature of reality. Mind and body are separate; the body is made of ordinary matter, but the mind is not. Monism is a belief that everything in the universe consists of matter and energy and that the mind is a phenomenon produced by the workings of the nervous system.
dualism The belief that the body is physical but the mind (or soul) is not.
monism (mahn ism) The belief that the world consists only of matter and energy and that the mind is a phenomenon produced by the workings of the nervous system.
Mere speculation about the nature of the mind can get us only so far. If we could answer the mind–body question simply by thinking about it, philosophers would have done so long ago. Behavioral neuroscientists take an empirical, practical, and monistic approach to the study of human nature. Most of us believe that once we understand the workings of the human body—and, in particular, the workings of the nervous system—the mind–body problem will have been solved. We will be able to explain how we perceive, how we think, how we remember, and how we act. We will even be able to explain the nature of our own self-awareness. Of course, we are far from understanding the workings of the nervous system, so only time will tell whether this belief is justified. In any event there is no way to study nonphysical phenomena in the laboratory. All that we can detect with our sense organs and our laboratory instruments are manifestations of the physical world: matter and energy.
Understanding Human Consciousness: A Physiological Approach
As you will learn from subsequent chapters, scientists have discovered much about the physiology of behavior: of perception, motivation, emotion, memory, and control of specific movements. But before addressing these problems, I want to show you that a scientific approach to perhaps the most complex phenomenon of all—human consciousness—is at least possible.
The term consciousness can be used to refer to a variety of concepts, including simple wakefulness. Thus, a researcher may write about an experiment using “conscious rats,” referring to the fact that the rats were awake and not anesthetized. However, in this context I am using the word consciousness to refer to the fact that we humans are aware of—and can tell others about—our thoughts, perceptions, memories, and feelings.
FIGURE 1.1 Studying the Brain
Will the human brain ever completely understand its own workings? A sixteenth-century woodcut from the first edition of De humani corporis fabrica (On the Workings of the Human Body) by Andreas Vesalius.
(Courtesy of National Library of Medicine.)
We know that consciousness can be altered by changes in the structure or chemistry of the brain; therefore, we may hypothesize that consciousness is a physiological function, just like behavior. We can even speculate about the origins of this self-awareness. Consciousness and the ability to communicate seem to go hand in hand. Our species, with its complex social structure and enormous capacity for learning, is well served by our ability to communicate: to express intentions to one another and to make requests of one another. Verbal communication makes cooperation possible and permits us to establish customs and laws of behavior. Perhaps the evolution of this ability is what has given rise to the phenomenon of consciousness. That is, our ability to send and receive messages with other people enables us to send and receive our own messages inside our own heads—in other words, to think and to be aware of our own existence. (See Figure 1.1 . )
Several phenomena involving the human brain provide insights into the nature of consciousness. One of these phenomena, caused by damage to a particular part of the brain, is known as blindsight (Weiskrantz et al., 1974 ; Cowey, 2010 ). The symptoms of blindsight indicate that the common belief that perceptions must enter consciousness to affect our behavior is incorrect. Our behavior can be guided by sensory information of which we are completely unaware.
blindsight The ability of a person who cannot see objects in his or her blind field to accurately reach for them while remaining unconscious of perceiving them; caused by damage to the “mammalian” visual system of the brain.
Natalie J. had brought her grandfather to see Dr. M., a neuropsychologist. Mr. J.’s stroke had left him almost completely blind; all he could see was a tiny spot in the middle of his visual field. Dr. M. had learned about Mr. J.’s condition from his neurologist and had asked Mr. J. to come to his laboratory so that he could do some tests for his research project.
Dr. M. helped Mr. J. find a chair and sit down.
Mr. J., who walked with the aid of a cane, gave it to his granddaughter to hold for him. “May I borrow that?” asked Dr. M. Natalie nodded and handed the cane to Dr. M. “The phenomenon I’m studying is called blind-sight,” he said. “Let me see if I can show you what it is.
“Mr. J., please look straight ahead. Keep looking that way, and don’t move your eyes or turn your head. I know that you can see a little bit straight ahead of you, and I don’t want you to use that piece of vision for what I’m going to ask you to do. Fine. Now, I’d like you to reach out with your right hand and point to what I’m holding.”
“But I don’t see anything—I’m blind!” said Mr. J., obviously exasperated.
“I know, but please try, anyway.”
Mr. J. shrugged his shoulders and pointed. He looked startled when his finger encountered the end of the cane, which Dr. M. was pointing toward him.
“Gramps, how did you do that?” asked Natalie, amazed. “I thought you were blind.”
“I am!” he said, emphatically. “It was just luck.”
“Let’s try it just a couple more times, Mr. J.,” said Dr. M. “Keep looking straight ahead. Fine.” He reversed the cane, so that the handle was pointing toward Mr. J. “Now I’d like you to grab hold of the cane.”
Mr. J. reached out with an open hand and grabbed hold of the cane.
“Good. Now put your hand down, please.” He rotated the cane 90 degrees, so that the handle was oriented vertically. “Now reach for it again.”
Mr. J. did so. As his arm came up, he turned his wrist so that his hand matched the orientation of the handle, which he grabbed hold of again.
“Good. Thank you, you can put your hand down.” Dr. M. turned to Natalie. “I’d like to test your grandfather now, but I’ll be glad to talk with you later.”
As Dr. M. explained to Natalie afterward, the human brain contains not one but several mechanisms involved in vision. To simplify matters somewhat, let’s consider two systems, which evolved at different times. The more primitive one, which resembles the visual system of animals such as fish and frogs, evolved first. The more complex one, which is possessed by mammals, evolved later. This second, “mammalian” system seems to be the one that is responsible for our ability to perceive the world around us. The first, “primitive,” visual system is devoted mainly to controlling eye movements and bringing our attention to sudden movements that occur off to the side of our field of vision.
Mr. J.’s stroke had damaged the mammalian visual system: the visual cortex of the brain and some of the nerve fibers that bring information to it from the eyes. Cases like his show that after the mammalian visual system is damaged, people can use the primitive visual system of their brains to guide hand movements toward an object even though they cannot see what they are reaching for. In other words, visual information can control behavior without producing a conscious sensation. The phenomenon of blindsight suggests that consciousness is not a general property of all parts of the brain; some parts of the brain, but not others, play a special role in consciousness. Although we are not sure just where these parts are or exactly how they work, they seem to be related to our ability to communicate—with others and with ourselves. The primitive system, which evolved before the development of brain mechanisms that give rise to consciousness, does not have these connections, so we are not conscious of the visual information it detects. It does have connections with the parts of the brain responsible for controlling hand movements. Only the mammalian visual system in the human brain has direct connections with the parts of the brain responsible for consciousness. (See Figure 1.2 . )
Studies of humans who have undergone a particular surgical procedure demonstrate dramatically how disconnecting parts of the brain involved with perceptions from parts that are involved with verbal behavior also disconnects them from consciousness. These results suggest that the parts of the brain involved in verbal behavior may be the ones responsible for consciousness.
FIGURE 1.2 An Explanation of the Blindsight Phenomenon
The surgical procedure is one that has been used for people with very severe epilepsy that cannot be controlled by drugs. In these people, nerve cells in one side of the brain become uncontrollably overactive, and the overactivity is transmitted to the other side of the brain by the corpus callosum. The corpus callosum (“tough body”) is a large bundle of nerve fibers that connect corresponding parts of one side of the brain with those of the other. Both sides of the brain then engage in wild activity and stimulate each other, causing a generalized epileptic seizure. These seizures can occur many times each day, preventing the patient from leading a normal life. Neurosurgeons discovered that cutting the corpus callosum (the split-brain operation ) greatly reduced the frequency of the epileptic seizures.
corpus callosum (core pus ka low sum ) The largest commissure of the brain, interconnecting the areas of neocortex on each side of the brain.
split-brain operation Brain surgery that is occasionally performed to treat a form of epilepsy; the surgeon cuts the corpus callosum, which connects the two hemispheres of the brain.
Figure 1.3 shows a drawing of the split-brain operation. We see the brain being sliced down the middle, from front to back, dividing it into its two symmetrical halves. The artist has created a window in the left side of the brain so that we can see the corpus callosum being cut by the neurosurgeon’s special knife. (See Figure 1.3 . )
FIGURE 1.3 The Split-Brain Operation
A “window” has been opened in the side of the brain so that we can see the corpus callosum being cut at the midline of the brain.
Sperry ( 1966 ) and Gazzaniga and his associates (Gazzaniga and LeDoux, 1978 ; Gazzaniga, 2005 ) have studied these patients extensively. The largest part of the brain consists of two symmetrical parts, called the cerebral hemispheres , which receive sensory information from the opposite sides of the body. They also control movements of the opposite sides. The corpus callosum permits the two hemispheres to share information so that each side knows what the other side is perceiving and doing. After the split-brain operation is performed, the two hemispheres are disconnected and operate independently; their sensory mechanisms, memories, and motor systems can no longer exchange information. You might think that disconnecting the brain hemispheres would be devastating, but the effects of these disconnections are not obvious to the casual observer. The simple reason for this fact is that only one hemisphere—in most people, the left—controls speech. The right hemisphere of an epileptic person with a split brain appears able to understand instructions reasonably well, but it is totally incapable of producing speech.
cerebral hemispheres The two symmetrical halves of the brain; constitute the major part of the brain.
Because only one side of the brain can talk about what it is experiencing, people speaking with a person who has a split brain are conversing with only one hemisphere: the left. The operations of the right hemisphere are more difficult to detect. Even the patient’s left hemisphere has to learn about the independent existence of the right hemisphere. One of the first things that these patients say they notice after the operation is that their left hand seems to have a “mind of its own.” For example, patients may find themselves putting down a book held in the left hand, even if they have been reading it with great interest. This conflict occurs because the right hemisphere, which controls the left hand, cannot read and therefore finds holding the book boring. At other times these patients surprise themselves by making obscene gestures (with the left hand) when they had not intended to. A psychologist once reported that a man with a split brain attempted to hit his wife with one hand and protect her with the other. Did he really want to hurt her? Yes and no, I guess.
The olfactory system is an exception to the general rule that of sensory information crosses from one side of the body to the opposite side of the brain. That is, when a person sniffs a flower through the left nostril, the left brain receives information about the odor. Thus, if the right nostril of a patient with a split brain is closed, leaving only the left nostril open, the patient will be able to tell us what the odors are because the information is received by the side of the brain that controls speech (Gordon and Sperry, 1969 ). However, if the odor enters only the right nostril, the patient will say that he or she smells nothing. But, in fact, the right brain has perceived the odor and can identify it. To show that this is so, we ask the patient to smell an odor with the right nostril and then reach for some objects that are hidden from view by a partition. If asked to use the left hand, which is controlled by the hemisphere that detected the smell, the patient will select the object that corresponds to the odor—a plastic flower for a floral odor, a toy fish for a fishy odor, a model tree for the odor of pine, and so forth. But if asked to use the right hand, the patient fails the test because the right hand is connected to the left hemisphere, which did not smell the odor presented to the right nostril. (See Figure 1.4 . )
FIGURE 1.4 Smelling with a Split Brain
Identification of an object in response to an olfactory stimulus by a person with a split brain.
The effects of cutting the corpus callosum reinforce the conclusion that we become conscious of something only if information about it is able to reach the parts of the brain responsible for verbal communication, which are located in the left hemisphere. If the information does not reach these parts of the brain, then that information does not reach consciousness. We still know very little about the physiology of consciousness, but studies of people with brain damage are beginning to provide us with some useful insights. This issue is discussed in later chapters.
The phenomenon described in the case history at the beginning of this chapter—failure to notice things located to a person’s left—is known as unilateral neglect (Adair and Barrett, 2008 ). Unilateral (“one-sided”) neglect is produced by damage to a particular part of the right side of the brain: the cortex of the parietal lobe. ( Chapter 3 describes the location of this region.) The parietal lobe receives information directly from the skin, the muscles, the joints, the internal organs, and the part of the inner ear that is concerned with balance. Thus, it is concerned with the body and its position. But that is not all; the parietal cortex also receives auditory and visual information. Its most important function seems to be to put together information about the movements and location of the parts of the body with the locations of objects in space around us. This information makes it possible for us to reach for and manipulate objects and to orient ourselves in space.
unilateral neglect A syndrome in which people ignore objects located toward their left and the left sides of objects located anywhere; most often caused by damage to the right parietal lobe.
If unilateral neglect simply consisted of blindness in the left side of the visual field and anesthesia of the left side of the body, it would not be nearly as interesting. But individuals with unilateral neglect are neither half blind nor half numb. Under the proper circumstances, they can see things located to their left, and they can tell when someone touches the left side of their bodies. But normally they ignore such stimuli and act as if the left side of the world and the left side of their bodies do not exist. In other words, their inattention to things to the left means that they normally do not become conscious of them.
Volpe, LeDoux, and Gazzaniga ( 1979 ) presented pairs of visual stimuli to people with unilateral neglect—one stimulus in the left visual field and one stimulus in the right. Invariably, the people reported seeing only the right-hand stimulus. But when the investigators asked the people to say whether the two stimuli were identical, they answered correctly, even though they said that they were unaware of the left-hand stimulus.
If you think about the story that the chief of neurology told about the man who ate only the right half of a pancake, you will realize that people with unilateral neglect must be able to perceive more than the right visual field. Remember that people with unilateral neglect fail to notice not only things to their left but also the left halves of things. But to distinguish between the left and right halves of an object, you first have to perceive the entire object—otherwise, how would you know where the middle was?
FIGURE 1.5 Unilateral Neglect
When people with unilateral neglect attempt to draw simple objects, they demonstrate their unawareness of the left half of things by drawing only the features that appear on the right.
People with unilateral neglect also demonstrate their unawareness of the left half of things when they draw pictures. For example, when asked to draw a clock, they almost always successfully draw a circle; but then when they fill in the numbers, they scrunch them all in on the right side. Sometimes they simply stop after reaching 6 or 7, and sometimes they write the rest of the numbers underneath the circle. When asked to draw a daisy, they begin with a stem and a leaf or two and then draw all the petals to the right. (See Figure 1.5 . )
Bisiach and Luzzatti ( 1978 ) demonstrated a similar phenomenon, which suggests that unilateral neglect extends even to a person’s own visual imagery. The investigators asked two patients with unilateral neglect to describe the Piazza del Duomo, a well-known landmark in Milan, the city in which they and the patients lived. They asked the patients to imagine that they were standing at the north end of the piazza and to describe what they saw. The patients duly named the buildings, but only those on the west, to their right. Then the investigators asked them to imagine themselves at the south end of the piazza. This time, they named the buildings on the east—again, to their right. Obviously, they knew about all of the buildings and their locations, but they visualized them only when the buildings were located in the right side of their (imaginary) visual field.
As you can see, there are two major symptoms of unilateral neglect: neglect of the left halves of things in the environment and neglect of the left half of one’s own body. In fact, although most people with unilateral neglect show both types of symptoms, research indicates that they are produced by damage to slightly different regions of the brain (Hillis et al., 2005 ).
Perception of Self
Although neglect of the left side of one’s own body can be studied only in people with brain abnormalities, an interesting phenomenon seen in people with undamaged brains confirms the importance of the parietal lobe (and another region of the brain) in feelings of body ownership. Ehrsson, Spence, and Passingham ( 2004 ) studied the rubber hand illusion. Normal subjects were positioned with their left hand hidden out of sight. They saw a lifelike rubber left hand in front of them. The experimenters stroked both the subject’s hidden left hand and the visible rubber hand with a small paintbrush. If the two hands were stroked synchronously and in the same direction, the subjects began to experience the rubber hand as their own. In fact, if they were then asked to use their right hand to point to their left hand, they tended to point toward the rubber hand. However, if the real and artificial hands were stroked in different directions or at different times, the subjects did not experience the rubber hand as their own. (See Figure 1.6 . )
While the subjects were participating in the experiment, the experimenters recorded the activity of their brains with a functional MRI scanner. (Brain scanning is described in Chapter 5 .) The scans showed increased activity in the parietal lobe and then, as the subjects began to experience the rubber hand as belonging to their body, in the premotor cortex, a region of the brain involved in planning movements. When the stroking of the real and artificial hands was uncoordinated and the subjects did not experience the rubber hand as their own, the premotor cortex did not become activated. The experimenters concluded that the parietal cortex analyzed the sight and the feeling of brush strokes. When the parietal cortex detected that they were congruent, this information was transmitted to the premotor cortex, which gave rise to the feeling of ownership of the rubber hand.
FIGURE 1.6 The Rubber Hand Illusion
If the subject’s hidden left hand and the visible rubber hand are stroked synchronously in the same direction, the subject will come to experience the artificial hand as his or her own. If the hands are stroked asynchronously or in different directions, this illusion will not occur.
(Adapted from Botwinick, M. Science, 2004, 305, 782–783.)
Another study from the same laboratory provided a particularly convincing demonstration that people experience a genuine feeling of ownership of the rubber hand (Ehrsson et al., 2007 ; Slater et al., 2009 ). The investigators used the procedure previously described to establish a feeling of ownership and then threatened the rubber hand by making a stabbing movement toward it with a needle. (They did not actually touch the hand with the needle.) Brain scans showed increased activity in a region of the brain (the anterior cingulate cortex) that is normally activated when a person anticipates pain and also in a region (the supplementary motor area) that is normally activated when a person feels the urge to move his or her arm (Fried et al., 1991 ; Peyron, Laurent, and Garcia-Larrea, 2000 ). So the impression that the rubber hand was about to receive a painful stab from a needle made people react as they would if their own hand were the target of the threat.
SECTION SUMMARY: Understanding Human Consciousness
The mind–body question has puzzled philosophers for many centuries. Modern science has adopted a monistic position—the belief that the world consists of matter and energy and that the human mind is a manifestation of the human brain. Studies of the functions of the human nervous system tend to support this position, as three specific examples show. These phenomena show that brain damage, by destroying conscious brain functions or disconnecting them from the speech mechanisms in the left hemisphere, can reveal the presence of perceptual mechanisms of which the person is not conscious and that a feeling of ownership of our own body is a function of the human brain.
Blindsight is a phenomenon that is seen after partial damage to the “mammalian” visual system on one side of the brain. Although the person is, in the normal meaning of the word, blind to anything presented to part of the visual field, the person can nevertheless reach out and point to objects whose presence he or she is not conscious of. Similarly, when sensory information about a particular object is presented to the right hemisphere of a person who has had a split-brain operation, the person is not aware of the object but can nevertheless indicate by movements of the left hand that the object has been perceived. Unilateral neglect—failure to become aware of the left half of one’s body, the left half of objects, or items located to a person’s left—reveals the existence of brain mechanisms that control our attention to things and hence our ability to become aware of them. These phenomena suggest that consciousness involves operations of the verbal mechanisms of the left hemisphere. Indeed, consciousness may be, in large part, a matter of our “talking to ourselves.” Thus, once we understand the language functions of the brain, we may have gone a long way toward understanding how the brain can be conscious of its own existence. The rubber hand phenomenon suggests that a feeling of ownership of our own body is a result of brain mechanisms that can be studied with the methods of neuroscience.
■ THOUGHT QUESTIONS
Could a sufficiently large and complex computer ever be programmed to be aware of itself? Suppose that someone someday claims to have done just that. What kind of evidence would you need to prove or disprove this claim?
Is consciousness found in animals other than humans? Is the ability of some animals to communicate with each other and with humans evidence for at least some form of awareness of self and others?
Clearly, the left hemisphere of a person with a split brain is conscious of the information it receives and of its own thoughts. It is not conscious of the mental processes of the right hemisphere. But is it possible that the right hemisphere is conscious too but is just unable to talk to us? How could we possibly find out whether it is? Do you see some similarities between this issue and the one raised in the first question?
The Nature of Behavioral Neuroscience
Behavioral neuroscience was formerly known as physiological psychology, and it is still sometimes referred to by that name. Indeed, the first textbook of psychology, written by Wilhelm Wundt in the late nineteenth century, was titled Principles of Physiological Psychology. In recent years, with the explosion of information in experimental biology, scientists from other disciplines have become prominent contributors to the investigation of the physiology of behavior. The united effort of behavioral neuroscientists, physiologists, and other neuroscientists is due to the realization that the ultimate function of the nervous system is behavior.
When I ask my students what they think the ultimate function of the brain is, they often say “thinking,” or “logical reasoning,” or “perceiving,” or “remembering things.” Certainly, the nervous system performs these functions, but they support the primary one: control of movement. (Note that movement includes talking, an important form of human behavior.) The basic function of perception is to inform us of what is happening in our environment so that our behaviors will be adaptive and useful: Perception without the ability to act would be useless. Of course, once perceptual abilities have evolved, they can be used for purposes other than guiding behavior. For example, we can enjoy a beautiful sunset or a great work of art without the perception causing us to do anything in particular. And thinking can often take place without causing any overt behavior. However, the ability to think evolved because it permits us to perform complex behaviors that accomplish useful goals. And whereas reminiscing about things that happened in our past can be an enjoyable pastime, the ability to learn and remember evolved—again—because it permitted our ancestors to profit from experience and perform behaviors that were useful to them.
The modern history of investigating the physiology of behavior has been written by scientists who have combined the experimental methods of psychology with those of physiology and have applied them to the issues that concern researchers in many different fields. Thus, we have studied perceptual processes, control of movement, sleep and waking, reproductive behaviors, ingestive behaviors, emotional behaviors, learning, and language. In recent years we have begun to study the physiology of human pathological conditions, such as addictions and neurological and mental disorders. All of these topics are discussed in subsequent chapters of this book.
The Goals of Research
The goal of all scientists is to explain the phenomena they study. But what do we mean by explain?Scientific explanation takes two forms: generalization and reduction. All scientists deal with generalization . For example, psychologists explain particular instances of behavior as examples of general laws, which they deduce from their experiments. For instance, most psychologists would explain a pathologically strong fear of dogs as an example of a particular form of learning called classical conditioning. Presumably, the person was frightened earlier in life by a dog. An unpleasant stimulus was paired with the sight of the animal (perhaps the person was knocked down by an exuberant dog or was attacked by a vicious one), and the subsequent sight of dogs evokes the earlier response: fear.
generalization A type of scientific explanation; a general conclusion based on many observations of similar phenomena.
Most physiologists use an additional approach to explanation: reduction . They explain complex phenomena in terms of simpler ones. For example, they may explain the movement of a muscle in terms of the changes in the membranes of muscle cells, the entry of particular chemicals, and the interactions among protein molecules within these cells. By contrast, a molecular biologist would explain these events in terms of forces that bind various molecules together and cause various parts of the molecules to be attracted to one another. In turn, the job of an atomic physicist is to describe matter and energy themselves and to account for the various forces found in nature. Practitioners of each branch of science use reduction to call on sets of more elementary generalizations to explain the phenomena they study.
reduction A type of scientific explanation; a phenomenon is described in terms of the more elementary processes that underlie it.
The task of the behavioral neuroscientist is to explain behavior by studying the physiological processes that control it. But behavioral neuroscientists cannot simply be reductionists. It is not enough to observe behaviors and correlate them with physiological events that occur at the same time. Identical behaviors may occur for different reasons and thus may be initiated by different physiological mechanisms. Therefore, we must understand “psychologically” why a particular behavior occurs—that is, what functions it performs—before we can understand what physiological events made it occur.
Let me provide a specific example: Mice, like many other mammals, often build nests. Behavioral observations show that mice will build nests under two conditions: when the air temperature is low and when the animal is pregnant. A nonpregnant mouse will build a nest only if the weather is cool, whereas a pregnant mouse will build one regardless of the temperature. The same behavior occurs for different reasons. In fact, nest-building behavior is controlled by two different physiological mechanisms. Nest building can be studied as a behavior related to the process of temperature regulation, or it can be studied in the context of parental behavior. Although the same set of brain mechanisms will control the movements that a mouse makes in building a nest in both cases, these mechanisms will be activated by different parts of the brain. One part receives information from the body’s temperature detectors, and the other part is influenced by hormones that are present in the body during pregnancy.
Sometimes, physiological mechanisms can tell us something about psychological processes. This relationship is particularly true of complex phenomena such as language, memory, and mood, which are poorly understood psychologically. For example, damage to a specific part of the brain can cause very specific impairments in a person’s language abilities. The nature of these impairments suggests how these abilities are organized. When the damage involves a brain region that is important in analyzing speech sounds, it also produces deficits in spelling. This finding suggests that the ability to recognize a spoken word and the ability to spell it call on related brain mechanisms. Damage to another region of the brain can produce extreme difficulty in reading unfamiliar words by sounding them out, but it does not impair the person’s ability to read words with which he or she is already familiar. This finding suggests that reading comprehension can take two routes: one that is related to speech sounds and another that is primarily a matter of visual recognition of whole words.
In practice, the research efforts of behavioral neuroscientists involve both forms of explanation: generalization and reduction. Ideas for experiments are stimulated by the investigator’s knowledge both of psychological generalizations about behavior and of physiological mechanisms. A good behavioral neuroscientist must therefore be an expert in the study of behavior and the study of physiology.
Biological Roots of Behavioral Neuroscience
Study of (or speculations about) the physiology of behavior has its roots in antiquity. Because its movement was necessary for life and because emotions caused it to beat more strongly, many ancient cultures, including the Egyptian, Indian, and Chinese cultures, considered the heart to be the seat of thought and emotions. The ancient Greeks did too, but Hippocrates (460–370 B.C.E.) concluded that this role should be assigned to the brain.
Not all ancient Greek scholars agreed with Hippocrates. Aristotle did not; he thought the brain served to cool the passions of the heart. But Galen (130–200 C.E.), who had the greatest respect for Aristotle, concluded that Aristotle’s role for the brain was “utterly absurd, since in that case Nature would not have placed the encephalon [brain] so far from the heart,… and she would not have attached the sources of all the senses [the sensory nerves] to it” (Galen, 1968 translation, p. 387). Galen thought enough of the brain to dissect and study the brains of cattle, sheep, pigs, cats, dogs, weasels, monkeys, and apes (Finger, 1994 ).
René Descartes, a seventeenth-century French philosopher and mathematician, has been called the father of modern philosophy. Although he was not a biologist, his speculations concerning the roles of the mind and brain in the control of behavior provide a good starting point in the modern history of behavioral neuroscience. Descartes assumed that the world was a purely mechanical entity that, once having been set in motion by God, ran its course without divine interference. Thus, to understand the world, one had only to understand how it was constructed. To Descartes, animals were mechanical devices; their behavior was controlled by environmental stimuli. His view of the human body was much the same: It was a machine. As Descartes observed, some movements of the human body were automatic and involuntary. For example, if a person’s finger touched a hot object, the arm would immediately withdraw from the source of stimulation. Reactions like this did not require participation of the mind; they occurred automatically. Descartes called these actions reflexes (from the Latin reflectere, “to bend back upon itself”). Energy coming from the outside source would be reflected back through the nervous system to the muscles, which would contract. The term is still in use today, but, of course, we explain the operation of a reflex differently. (See Figure 1.7 . )
Like most philosophers of his time, Descartes was a dualist; he believed that each person possessed a mind—a uniquely human attribute that was not subject to the laws of the universe. But his thinking differed from that of his predecessors in one important way: He was the first to suggest that a link exists between the human mind and its purely physical housing, the brain. He believed that the mind controlled the movements of the body, while the body, through its sense organs, supplied the mind with information about what was happening in the environment. In particular, he hypothesized that this interaction took place in the pineal body, a small organ situated on top of the brain stem, buried beneath the cerebral hemispheres. He noted that the brain contained hollow chambers (the ventricles) that were filled with fluid, and he hypothesized that this fluid was under pressure. When the mind decided to perform an action, it tilted the pineal body in a particular direction like a little joystick, causing fluid to flow from the brain into the appropriate set of nerves. This flow of fluid caused the same muscles to inflate and move. (See Figure 1.8 . )
reflex An automatic, stereotyped movement that is produced as the direct result of a stimulus.
FIGURE 1.7 Descartes’ Explanation of a Reflex Action to a Painful Stimulus
As a young man, René Descartes was greatly impressed by the moving statues in the grottoes of the Royal Gardens, just west of Paris (Jaynes, 1970 ). He was fascinated by the hidden mechanisms that caused the statues to move when visitors stepped on hidden plates. For example, as a visitor approached a bronze statue of the goddess Diana, bathing in a pool of water, she would flee and hide behind a bronze rose bush. If the visitor pursued her, an imposing statue of Neptune would rise up and bar the way with his trident.
These devices served as models for Descartes in theorizing about how the body worked. The pressurized water of the moving statues was replaced by pressurized fluid in the ventricles; the pipes by nerves; the cylinders by muscles; and, finally, the hidden valves by the pineal body. This story illustrates one of the first times that a technological device was used as a model for explaining how the nervous system works. In science a model is a relatively simple system that works on known principles and is able to do at least some of the things that a more complex system can do. For example, when scientists discovered that elements of the nervous system communicate by means of electrical impulses, researchers developed models of the brain based on telephone switchboards and, more recently, computers. Abstract models, which are completely mathematical in their properties, have also been developed.
model A mathematical or physical analogy for a physiological process; for example, computers have been used as models for various functions of the brain.
FIGURE 1.8 Descartes’ Theory
A woodcut from De homine by René Descartes, published in 1662. Descartes believed that the “soul” (what we would today call the mind) controls the movements of the muscles through its influence on the pineal body. His explanation is modeled on the mechanism that animated statues in the royal gardens. According to his theory, the eyes sent visual information to the brain, where it could be examined by the soul. When the soul decided to act, it would tilt the pineal body (labeled H in the diagram), which would divert pressurized fluid through nerves to the appropriate muscles. His explanation is modeled on the mechanism that animated statues in the Royal Gardens near Paris.
(Courtesy of Historical Pictures Service, Chicago.)
Descartes’ model was useful because, unlike purely philosophical speculations, it could be tested experimentally. In fact, it did not take long for biologists to prove that Descartes was wrong. Luigi Galvani, a seventeenth-century Italian physiologist, found that electrical stimulation of a frog’s nerve caused contraction of the muscle to which it was attached. Contraction occurred even when the nerve and muscle were detached from the rest of the body, so the ability of the muscle to contract and the ability of the nerve to send a message to the muscle were characteristics of these tissues themselves. Thus, the brain did not inflate muscles by directing pressurized fluid through the nerve. Galvani’s experiment prompted others to study the nature of the message transmitted by the nerve and the means by which muscles contracted. The results of these efforts gave rise to an accumulation of knowledge about the physiology of behavior.
FIGURE 1.9 Johannes Müller (1801–1858)
(Courtesy of National Library of Medicine.)
One of the most important figures in the development of experimental physiology was Johannes Müller, a nineteenth-century German physiologist. Müller was a forceful advocate of the application of experimental techniques to physiology. Previously, the activities of most natural scientists had been limited to observation and classification. Although these activities are essential, Müller insisted that major advances in our understanding of the workings of the body would be achieved only by experimentally removing or isolating animals’ organs, testing their responses to various chemicals, and otherwise altering the environment to see how the organs responded. (See Figure 1.9 . ) His most important contribution to the study of the physiology of behavior was his doctrine of specific nerve energies . Müller observed that although all nerves carry the same basic message—an electrical impulse—we perceive the messages of different nerves in different ways. For example, messages carried by the optic nerves produce sensations of visual images, and those carried by the auditory nerves produce sensations of sounds. How can different sensations arise from the same basic message?
doctrine of specific nerve energies Müller’s conclusion that, because all nerve fibers carry the same type of message, sensory information must be specified by the particular nerve fibers that are active.
The answer is that the messages occur in different channels. The portion of the brain that receives messages from the optic nerves interprets the activity as visual stimulation, even if the nerves are actually stimulated mechanically. (For example, when we rub our eyes, we see flashes of light.) Because different parts of the brain receive messages from different nerves, the brain must be functionally divided: Some parts perform some functions, while other parts perform others.
Müller’s advocacy of experimentation and the logical deductions from his doctrine of specific nerve energies set the stage for performing experiments directly on the brain. Indeed, Pierre Flourens, a nineteenth-century French physiologist, did just that. Flourens removed various parts of animals’ brains and observed their behavior. By seeing what the animal could no longer do, he could infer the function of the missing portion of the brain. This method is called experimental ablation (from the Latin ablatus, “carried away”). Flourens claimed to have discovered the regions of the brain that control heart rate and breathing, purposeful movements, and visual and auditory reflexes.
experimental ablation The research method in which the function of a part of the brain is inferred by observing the behaviors an animal can no longer perform after that part is damaged.
Soon after Flourens performed his experiments, Paul Broca, a French surgeon, applied the principle of experimental ablation to the human brain. Of course, he did not intentionally remove parts of human brains to see how they worked but observed the behavior of people whose brains had been damaged by strokes. In 1861 he performed an autopsy on the brain of a man who had had a stroke that resulted in the loss of the ability to speak. Broca’s observations led him to conclude that a portion of the cerebral cortex on the front part of the left side of the brain performs functions that are necessary for speech. (See Figure 1.10 . ) Other physicians soon obtained evidence supporting his conclusions. As you will learn in Chapter 14 , the control of speech is not localized in a particular region of the brain. Indeed, speech requires many different functions, which are organized throughout the brain. Nonetheless, the method of experimental ablation remains important to our understanding of the brains of both humans and laboratory animals.
FIGURE 1.10 Broca’s Area
This region of the brain is named for French surgeon Paul Broca, who discovered that damage to a part of the left side of the brain disrupted a person’s ability to speak.
As I mentioned earlier, Luigi Galvani used electricity to demonstrate that muscles contain the source of the energy that powers their contractions. In 1870, German physiologists Gustav Fritsch and Eduard Hitzig used electrical stimulation as a tool for understanding the physiology of the brain. They applied weak electrical current to the exposed surface of a dog’s brain and observed the effects of the stimulation. They found that stimulation of different portions of a specific region of the brain caused contraction of specific muscles on the opposite side of the body. We now refer to this region as the primary motor cortex, and we know that nerve cells there communicate directly with those that cause muscular contractions. We also know that other regions of the brain communicate with the primary motor cortex and thus control behaviors. For example, the region that Broca found necessary for speech communicates with, and controls, the portion of the primary motor cortex that controls the muscles of the lips, tongue, and throat, which we use to speak.
One of the most brilliant contributors to nineteenth-century science was the German physicist and physiologist Hermann von Helmholtz. Helmholtz devised a mathematical formulation of the law of conservation of energy; invented the ophthalmoscope (used to examine the retina of the eye); devised an important and influential theory of color vision and color blindness; and studied audition, music, and many physiological processes.
Helmholtz was also the first scientist to attempt to measure the speed of conduction through nerves. Scientists had previously believed that such conduction was identical to the conduction that occurs in wires, traveling at approximately the speed of light. But Helmholtz found that neural conduction was much slower—only about 90 feet per second. This measurement proved that neural conduction was more than a simple electrical message, as we will see in Chapter 2 .
Twentieth-century developments in experimental physiology include many important inventions, such as sensitive amplifiers to detect weak electrical signals, neurochemical techniques to analyze chemical changes within and between cells, and histological techniques to see cells and their constituents. Because these developments belong to the modern era, they are discussed in detail in subsequent chapters.
SECTION SUMMARY: The Nature of Behavioral Neuroscience
All scientists hope to explain natural phenomena. In this context the term explanation has two basic meanings: generalization and reduction. Generalization refers to the classification of phenomena according to their essential features so that general laws can be formulated. For example, observing that gravitational attraction is related to the mass of two bodies and to the distance between them helps to explain the movement of planets. Reduction refers to the description of phenomena in terms of more basic physical processes. For example, gravitation can be explained in terms of forces and subatomic particles.
Behavioral neuroscientists use both generalization and reduction to explain behavior. In large part, generalizations use the traditional methods of psychology. Reduction explains behaviors in terms of physiological events within the body—primarily within the nervous system. Thus, behavioral neuroscience builds on the tradition of both experimental psychology and experimental physiology.
A dualist, René Descartes proposed a model of the brain on the basis of hydraulically activated statues.
His model stimulated observations that produced important discoveries. The results of Luigi Galvani’s experiments eventually led to an understanding of the nature of the message transmitted by nerves between the brain and the sensory organs and the muscles. Johannes Müller’s doctrine of specific nerve energies paved the way for study of the functions of specific parts of the brain, through the methods of experimental ablation and electrical stimulation. Hermann von Helmholtz discovered that the conduction through nerves was slower than the conduction of electricity, which meant that it was a physiological phenomenon, not a simple electrical one.
■ THOUGHT QUESTIONS
· 1. What is the value of studying the history of behavioral neuroscience? Is it a waste of time?
· 2. Suppose we studied just the latest research and ignored explanations that we now know to be incorrect. Would we be spending our time more profitably, or might we miss something?
Natural Selection and Evolution
Following the tradition of Müller and von Helmholtz, other biologists continued to observe, classify, and think about what they saw, and some of them arrived at valuable conclusions. The most important of these scientists was Charles Darwin. (See Figure 1.11 . )
Darwin formulated the principles of natural selection and evolution, which revolutionized biology.
Functionalism and the Inheritance of Traits
FIGURE 1.11 Charles Darwin (1809–1882)
Darwin’s theory of evolution revolutionized biology and strongly influenced early psychologists.
(North Wind Picture Archives.)
Darwin’s theory emphasized that all of an organism’s characteristics—its structure, its coloration, its behavior—have functional significance. For example, the strong talons and sharp beaks that eagles possess permit the birds to catch and eat prey. Caterpillars that eat green leaves are themselves green, and their color makes it difficult for birds to see them against their usual background. Mother mice construct nests, which keep their offspring warm and out of harm’s way. Obviously, the behavior itself is not inherited—how could it be? What is inherited is a brain that causes the behavior to occur. Thus, Darwin’s theory gave rise to functionalism , a belief that characteristics of living organisms perform useful functions. So, to understand the physiological basis of various behaviors, we must first understand what these behaviors accomplish. We must therefore understand something about the natural history of the species being studied so that the behaviors can be seen in context.
functionalism The principle that the best way to understand a biological phenomenon (a behavior or a physiological structure) is to try to understand its useful functions for the organism.
FIGURE 1.12 Bones of the Forelimb
The figure shows the bones of (a) human, (b) bat, (c) whale, (d) dog. Through the process of natural selection, these bones have been adapted to suit many different functions.
To understand the workings of a complex piece of machinery, we should know what its functions are. This principle is just as true for a living organism as it is for a mechanical device. However, an important difference exists between machines and organisms: Machines have inventors who had a purpose when they designed them, whereas organisms are the result of a long series of accidents. Thus, strictly speaking, we cannot say that any physiological mechanisms of living organisms have a purpose. But they do have functions, and these we can try to determine. For example, the forelimbs shown in Figure 1.12 are adapted for different uses in different species of mammals. (See Figure 1.12 . )
A good example of the functional analysis of an adaptive trait was demonstrated in an experiment by Blest ( 1957 ). Certain species of moths and butterflies have spots on their wings that resemble eyes—particularly the eyes of predators such as owls. (See Figure 1.13 . ) These insects normally rely on camouflage for protection; the backs of their wings, when folded, are colored like the bark of a tree. However, when a bird approaches, the insect’s wings flip open, and the hidden eyespots are suddenly displayed. The bird then tends to fly away rather than eat the insect. Blest performed an experiment to see whether the eyespots on a moth’s or butterfly’s wings really disturbed birds that saw them. He placed mealworms on different backgrounds and counted how many worms the birds ate. Indeed, when the worms were placed on a background that contained eyespots, the birds tended to avoid them.
FIGURE 1.13 The Owl Butterfly
This butterfly displays its eyespots when approached by a bird. The bird usually will fly away.
Darwin formulated his theory of evolution to explain the means by which species acquired their adaptive characteristics. The cornerstone of this theory is the principle of natural selection . Darwin noted that members of a species were not all identical and that some of the differences they exhibited were inherited by their offspring. If an individual’s characteristics permit it to reproduce more successfully, some of the individual’s offspring will inherit the favorable characteristics and will themselves produce more offspring. As a result, the characteristics will become more prevalent in that species. He observed that animal breeders were able to develop strains that possessed particular traits by mating together only animals that possessed the desired traits. If artificial selection, controlled by animal breeders, could produce so many varieties of dogs, cats, and livestock, perhaps natural selection could be responsible for the development of species. Of course, it was the natural environment, not the hand of the animal breeder, that shaped the process of evolution.
natural selection The process by which inherited traits that confer a selective advantage (increase an animal’s likelihood to live and reproduce) become more prevalent in a population.
Darwin and his fellow scientists knew nothing about the mechanism by which the principle of natural selection works. In fact, the principles of molecular genetics were not discovered until the middle of the twentieth century. Briefly, here is how the process works: Every sexually reproducing multicellular organism consists of a large number of cells, each of which contains chromosomes. Chromosomes are large, complex molecules that contain the recipes for producing the proteins that cells need to grow and to perform their functions. In essence, the chromosomes contain the blueprints for the construction (that is, the embryological development) of a particular member of a particular species. If the plans are altered, a different organism is produced.
The plans do get altered; mutations occur from time to time. Mutations are accidental changes in the chromosomes of sperm or eggs that join together and develop into new organisms. For example, cosmic radiation might strike a chromosome in a cell of an animal’s testis or ovary, thus producing a mutation that affects that animal’s offspring. Most mutations are deleterious; the offspring either fails to survive or survives with some sort of defect. However, a small percentage of mutations are beneficial and confer a selective advantage to the organism that possesses them. That is, the animal is more likely than other members of its species to live long enough to reproduce and hence to pass on its chromosomes to its own offspring. Many different kinds of traits can confer a selective advantage: resistance to a particular disease, the ability to digest new kinds of food, more effective weapons for defense or for procurement of prey, and even a more attractive appearance to members of the other sex (after all, one must reproduce to pass on one’s chromosomes).
mutation A change in the genetic information contained in the chromosomes of sperm or eggs, which can be passed on to an organism’s offspring; provides genetic variability.
selective advantage A characteristic of an organism that permits it to produce more than the average number of offspring of its species.
Naturally, the traits that can be altered by mutations are physical ones; chromosomes make proteins, which affect the structure and chemistry of cells. But the effects of these physical alterations can be seen in an animal’s behavior. Thus, the process of natural selection can act on behavior indirectly. For example, if a particular mutation results in changes in the brain that cause a small animal to stop moving and freeze when it perceives a novel stimulus, that animal is more likely to escape undetected when a predator passes nearby. This tendency makes the animal more likely to survive and produce offspring, thus passing on its genes to future generations.
Other mutations are not immediately favorable, but because they do not put their possessors at a disadvantage, they are inherited by at least some members of the species. As a result of thousands of such mutations, the members of a particular species possess a variety of genes and are all at least somewhat different from one another. Variety is a definite advantage for a species. Different environments provide optimal habitats for different kinds of organisms. When the environment changes, species must adapt or run the risk of becoming extinct. If some members of the species possess assortments of genes that provide characteristics permitting them to adapt to the new environment, their offspring will survive, and the species will continue.
An understanding of the principle of natural selection plays some role in the thinking of every scientist who undertakes research in behavioral neuroscience. Some researchers explicitly consider the genetic mechanisms of various behaviors and the physiological processes on which these behaviors depend. Others are concerned with comparative aspects of behavior and its physiological basis; they compare the nervous systems of animals from a variety of species to make hypotheses about the evolution of brain structure and the behavioral capacities that correspond to this evolutionary development. But even though many researchers are not directly involved with the problem of evolution, the principle of natural selection guides the thinking of behavioral neuroscientists. We ask ourselves what the selective advantage of a particular trait might be. We think about how nature might have used a physiological mechanism that already existed to perform more complex functions in more complex organisms. When we entertain hypotheses, we ask ourselves whether a particular explanation makes sense in an evolutionary perspective.
Evolution of the Human Species
To evolve means to develop gradually (from the Latin evolvere, “to unroll”). The process of evolution is a gradual change in the structure and physiology of plant and animal species as a result of natural selection. New species evolve when organisms develop novel characteristics that can take advantage of unexploited opportunities in the environment.
evolution A gradual change in the structure and physiology of plant and animal species—generally producing more complex organisms—as a result of natural selection.
The first vertebrates to emerge from the sea—some 360 million years ago—were amphibians. In fact, amphibians (for example, frogs and toads) have not entirely left the sea; they still lay their eggs in water, and the larvae that hatch from these eggs have gills and only later transform into adults with air-breathing lungs. Seventy million years later, the first reptiles appeared. Reptiles had a considerable advantage over amphibians: Their eggs, enclosed in a shell just porous enough to permit the developing embryo to breathe, could be laid on land. Thus, reptiles could inhabit regions away from bodies of water, and they could bury their eggs where predators would be less likely to find them. Reptiles soon divided into three lines: the anapsids, the ancestors of today’s turtles; the diapsids, the ancestors of dinosaurs, birds, lizards, crocodiles, and snakes; and the synapsids, the ancestors of today’s mammals. One group of synapsids, the therapsids, became the dominant land animal during the Permian period. Then, about 248 million years ago, the end of the Permian period was marked by a mass extinction. Dust from a catastrophic series of volcanic eruptions in present-day Siberia darkened the sky, cooled the earth, and wiped out approximately 95 percent of all animal species. Among those that survived was a small therapsid known as a cynodont—the direct ancestor of the mammal, which first appeared about 220 million years ago. (See Figure 1.14 . )
FIGURE 1.14 Evolution of Vertebrate Species
(Adapted from Carroll, R. Vertebrate Paleontology and Evolution. New York: W. H. Freeman, 1988.)
The earliest mammals were small nocturnal predators that fed on insects. They (and the other warm-blooded animals: birds) were only a modest success for many millions of years. Dinosaurs ruled, and mammals had to remain small and inconspicuous to avoid the large variety of agile and voracious predators. Then, around 65 million years ago, another mass extinction occurred. An enormous meteorite struck the Yucatan peninsula of present-day Mexico, producing a cloud of dust that destroyed many species, including the dinosaurs. Small, nocturnal mammals survived the cold and dark because they were equipped with insulating fur and a mechanism for maintaining their body temperature. The void left by the extinction of so many large herbivores and carnivores provided the opportunity for mammals to expand into new ecological niches, and expand they did.
The climate of the early Cenozoic period, which followed the mass extinction at the end of the Cretaceous period, was much warmer than is the climate today. Tropical forests covered much of the land areas, and in these forests our most direct ancestors, the primates, evolved. The first primates, like the first mammals, were small and preyed on insects and small cold-blooded vertebrates such as lizards and frogs. They had grasping hands that permitted them to climb about in small branches of the forest. Over time, larger species developed, with larger, forward-facing eyes (and the brains to analyze what the eyes saw), which facilitated arboreal locomotion and the capture of prey.
Plants evolved as well as animals. Dispersal of seeds is a problem inherent in forest life; if a tree’s seeds fall at its base, they will be shaded by the parent and will not grow. Thus, natural selection favored trees that encased their seeds in sweet, nutritious fruit that would be eaten by animals and dropped on the ground some distance away, undigested, in the animals’ feces. (The feces even served to fertilize the young plants.) The evolution of fruit-bearing trees provided an opportunity for fruit-eating primates. In fact, the original advantage of color vision was probably the ability to discriminate ripe fruit from green leaves and eat the fruit before it spoiled—or some other animals got to it first. And because fruit is such a nutritious form of food, its availability provided an opportunity that could be exploited by larger primates, which were able to travel farther in quest of food.
The first hominids (humanlike apes) appeared in Africa. They appeared not in dense tropical forests but in drier woodlands and in the savanna—vast areas of grasslands studded with clumps of trees and populated by large herbivorous animals and the carnivores that preyed on them. Our fruit-eating ancestors continued to eat fruit, of course, but they evolved characteristics that enabled them to gather roots and tubers as well, to hunt and kill game, and to defend themselves against other predators. They made tools that could be used to hunt, produce clothing, and construct dwellings; they discovered the many uses of fire; they domesticated dogs, which greatly increased their ability to hunt and helped warn of attacks by predators; and they developed the ability to communicate symbolically, by means of spoken words.
FIGURE 1.15 Evolution of Primate Species
(Redrawn from Lewin, R. Human Evolution: An Illustrated Introduction, 3rd ed. Boston: Blackwell Scientific Publications, 1993. Reprinted with permission by Blackwell Science Ltd.)
Figure 1.15 shows the primate family tree. Our closest living relatives—the only hominids besides ourselves who have survived—are the chimpanzees, gorillas, and orangutans. DNA analysis shows that genetically, there is very little difference between these four species. (See Figure 1.15 . ) For example, humans and chimpanzees share almost 99 percent of their DNA. (See Figure 1.16 . )
FIGURE 1.16 DNA Among Species of Hominids
The pyramid illustrates the percentage differences in DNA among the four major species of hominids.
(Redrawn from Lewin, R. Human Evolution: An Illustrated Introduction. Boston: Blackwell Scientific Publications, 1993. Reprinted with permission by Blackwell Science Ltd.)
The first hominid to leave Africa did so around 1.7 million years ago. This species, Homo erectus (“upright man”), scattered across Europe and Asia. One branch of Homo erectus appears to have been the ancestor of Homo neanderthalis, which inhabited Western Europe between 120,000 and 30,000 years ago. Neanderthals resembled modern humans. They made tools out of stone and wood and discovered the use of fire. Our own species, Homo sapiens, evolved in East Africa around 100,000 years ago. Some of our ancestors migrated to other parts of Africa and out of Africa to Asia, Polynesia, Australia, Europe, and the Americas. They encountered the Neanderthals in Europe around 40,000 years ago and coexisted with them for approximately 10,000 years. Eventually, the Neanderthals disappeared—perhaps through interbreeding with Homo sapiens, perhaps through competition for resources. Scientists have not found evidence of warlike conflict between the two species. (See Figure 1.17 . )
Evolution of Large Brains
FIGURE 1.17 Migration Routes of Homo Sapiens
The figure shows proposed migration routes of Homo sapiens after evolution of the species in East Africa.
(Redrawn with permission from Cavalli-Sforza, L. L. Genes, peoples and languages. Scientific American, Nov. 1991, p. 75.)
Humans possessed several characteristics that enabled them to compete with other species. Their agile hands enabled them to make and use tools. Their excellent color vision helped them to spot ripe fruit, game animals, and dangerous predators. Their mastery of fire enabled them to cook food, provide warmth, and frighten nocturnal predators. Their upright posture and bipedalism made it possible for them to walk long distances efficiently, with their eyes far enough from the ground to see long distances across the plains. Bipedalism also permitted them to carry tools and food with them, which meant that they could bring fruit, roots, and pieces of meat back to their tribe. Their linguistic abilities enabled them to combine the collective knowledge of all the members of the tribe, to make plans, to pass information on to subsequent generations, and to establish complex civilizations that established their status as the dominant species. All of these characteristics required a larger brain.
A large brain requires a large skull, and an upright posture limits the size of a woman’s birth canal. A newborn baby’s head is about as large as it can safely be. As it is, the birth of a baby is much more arduous than the birth of mammals with proportionally smaller heads, including those of our closest primate relatives. Because a baby’s brain is not large or complex enough to perform the physical and intellectual abilities of an adult, the brain must continue to grow after the baby is born. In fact, all mammals (and all birds, for that matter) require parental care for a period of time while the nervous system develops. The fact that young mammals (particularly young humans) are guaranteed to be exposed to the adults who care for them means that a period of apprenticeship is possible. Consequently, the evolutionary process did not have to produce a brain that consisted solely of specialized circuits of neurons that performed specialized tasks. Instead, it could simply produce a larger brain with an abundance of neural circuits that could be modified by experience. Adults would nourish and protect their offspring and provide them with the skills they would need as adults. Some specialized circuits were necessary, of course (for example, those involved in analyzing the complex sounds we use for speech), but, by and large, the brain is a general-purpose, programmable computer.
How does the human brain compare with the brains of other animals? In absolute size, our brains are dwarfed by those of elephants or whales. However, we might expect such large animals to have large brains to match their large bodies. Indeed, the human brain makes up 2.3 percent of our total body weight, while the elephant brain makes up only 0.2 percent of the animal’s total body weight, which makes our brains seem very large in comparison. However, the shrew, which weighs only 7.5 g, has a brain that weighs 0.25 g, or 3.3 percent of its total body weight. Certainly, the shrew brain is much less complex than the human brain, so something is wrong with this comparison.
The answer is that although bigger bodies require bigger brains, the size of the brain does not have to go up proportionally with that of the body. For example, larger muscles do not require more nerve cells to control them. What counts, as far as intellectual ability goes, is having a brain with plenty of nerve cells that are not committed to moving muscles or analyzing sensory information—nerve cells that are available for learning, remembering, reasoning, and making plans. Figure 1.18 shows a graph of the brain sizes and body weights of several hominid species, including the ancestors of our own species. Note that the brain size of nonhuman hominids increases very little with size: A gorilla weighs almost three times as much as a chimpanzee, but their brains weigh almost the same. In contrast, although the body weight of modern humans is only 29 percent more than that of Australopithecus africanus, our brains are 242 percent larger. Clearly, some important mutations of the genes that control brain development occurred early in the evolution of the primate line. (See Figure 1.18 . )
FIGURE 1.18 Hominid Brain Size
The graph shows average brain size as a function of body weight for several species of hominids.
(Redrawn from Lewin, R. Human Evolution: An Illustrated Introduction, 3rd ed. Boston: Blackwell Scientific Publications, 1993. Reprinted with permission by Blackwell Science Ltd.)
Besides varying is size, brains also vary in the number of neurons found in each gram of tissue. Herculano-Houzel et al. ( 2007 ) compared the weight of the brains of several species of rodents and primates with the number of neurons that each brain contained. They found that primate brains—especially large ones—contain many more neurons per gram than rodent brains do. For example, the brain of a capuchin monkey weighs 52 g and contains 3.7 billion neurons, while the brain of a capybara (a very large South American rodent) weighs 76 g but contains only 1.6 billion neurons. The brain of a capuchin monkey (and a human brain, for that matter) contains 70.7 million neurons per gram, while that of a capybara contains only 21 million neurons per gram.
What types of genetic changes were responsible for the evolution of the human brain? This question will be addressed in more detail in Chapter 3 , but evidence suggests that the most important principle is a slowing of the process of brain development, allowing more time for growth. As we will see, the prenatal period of cell division in the brain is prolonged in humans, which results in a brain that weighs an average of 350 g and contains approximately 100 billion neurons. After birth the brain continues to grow. Production of new neurons almost ceases, but those that are already present grow and establish connections with each other, and other brain cells, which protect and support neurons, begin to proliferate. Not until late adolescence does the human brain reach its adult size of approximately 1400 g—about four times the weight of a newborn’s brain. This prolongation of maturation is known as neoteny (roughly translated as “extended youth”). The mature human head and brain retain some infantile characteristics, including their disproportionate size relative to the rest of the body. Figure 1.19 shows fetal and adult skulls of chimpanzees and humans. As you can see, the fetal skulls are much more similar than are those of the adults. The grid lines show the pattern of growth, indicating much less change in the human skull from birth to adulthood. (See Figure 1.19 . )
FIGURE 1.19 Neoteny in Evolution of the Human Skull
The skulls of fetal humans and chimpanzees are much more similar than are those of the adults. The grid lines show the pattern of growth, indicating much less change in the human skull from birth to adulthood.
(Redrawn from Lewin, R. Human Evolution: An Illustrated Introduction, 3rd ed. Boston: Blackwell Scientific Publications, 1993. Reprinted with permission by Blackwell Science Ltd.)
neoteny A slowing of the process of maturation, allowing more time for growth; an important factor in the development of large brains.
SECTION SUMMARY: Natural Selection and Evolution
Darwin’s theory of evolution, which was based on the concept of natural selection, provided an important contribution to modern behavioral neuroscience. The theory asserts that we must understand the functions that are performed by an organ or body part or by a behavior. Through random mutations, changes in an individual’s genetic material cause different proteins to be produced, which results in the alteration of some physical characteristics. If the changes confer a selective advantage on the individual, the new genes will be transmitted to more and more members of the species. Even behaviors can evolve, through the selective advantage of alterations in the structure of the nervous system.
Amphibians emerged from the sea 360 million years ago. One branch, the therapsids, became the dominant land animal until a catastrophic series of volcanic eruptions wiped out most animal species. A small therapsid, the cynodont, survived the disaster and became the ancestor of the mammals. The earliest mammals were small, nocturnal insectivores who lived in trees. They remained small and inconspicuous until the extinction of the dinosaurs, which occurred around 65 million years ago. Mammals quickly filled the vacant ecological niches. Primates also began as small, nocturnal, tree-dwelling insectivores. Larger fruit-eating primates, with forward-facing eyes and larger brains, eventually evolved.
The first hominids appeared in Africa around 25 million years ago, eventually evolving into four major species: orangutans, gorillas, chimpanzees, and humans. Our ancestors acquired bipedalism around 3.7 million years ago and discovered tool making around 2.5 million years ago. The first hominids to leave Africa, Homo erectus, did so around 1.7 million years ago and scattered across Europe and Asia. Homo neanderthalis evolved in Western Europe, eventually to be replaced by Homo sapiens, which evolved in Africa around 100,000 years ago and spread throughout the world. By 30,000 years ago, Homo sapienshad replaced Homo neanderthalis.
The evolution of large brains made possible the development of tool making, fire building, and language, which in turn permitted the development of complex social structures. Large brains also provided a large memory capacity and the abilities to recognize patterns of events in the past and to plan for the future. Because an upright posture limits the size of a woman’s birth canal and therefore the size of the head that can pass through it, much of the brain’s growth must take place after birth, which means that children require an extended period of parental care. This period of apprenticeship enabled the developing brain to be modified by experience.
Although human DNA differs from that of chimpanzees by only 1.2 percent, our brains are more than three times larger, which means that a small number of genes is responsible for the increase in the size of our brains. As we will see in Chapter 3 , these genes appear to retard the events that stop brain development, resulting in a phenomenon known as neoteny.
■ THOUGHT QUESTIONS
· 1. What useful functions are provided by the fact that a human can be self-aware? How was this trait selected for during the evolution of our species?
· 2. Are you surprised that the difference in the DNA of humans and chimpanzees is only 1.2 percent? How do you feel about this fact?
· 3. If our species continues to evolve (and most geneticists believe that this is the case), what kinds of changes do you think might occur?
Ethical Issues in Research with Animals
Most of the research described in this book involves experimentation on living animals. Any time we use another species of animals for our own purposes, we should be sure that what we are doing is both humane and worthwhile. I believe that a good case can be made that research on the physiology of behavior qualifies on both counts. Humane treatment is a matter of procedure. We know how to maintain laboratory animals in good health in comfortable, sanitary conditions. We know how to administer anesthetics and analgesics so that animals do not suffer during or after surgery, and we know how to prevent infections with proper surgical procedures and the use of antibiotics. Most industrially developed societies have very strict regulations about the care of animals and require approval of the experimental procedures that are used on them. There is no excuse for mistreating animals in our care. In fact, the vast majority of laboratory animals are treated humanely.
Whether an experiment is worthwhile can be difficult to say. We use animals for many purposes. We eat their meat and eggs, and we drink their milk; we turn their hides into leather; we extract insulin and other hormones from their organs to treat people’s diseases; we train them to do useful work on farms or to entertain us. Even having a pet is a form of exploitation; it is we—not they—who decide that they will live in our homes. The fact is, we have been using other animals throughout the history of our species.
Pet owning causes much more suffering among animals than scientific research does. As Miller ( 1983 ) notes, pet owners are not required to receive permission from a board of experts that includes a veterinarian to house their pets, nor are they subject to periodic inspections to be sure that their home is clean and sanitary, that their pets have enough space to exercise properly, or that their pets’ diets are appropriate. Scientific researchers are. Miller also notes that fifty times more dogs and cats are killed by humane societies each year because they have been abandoned by former pet owners than are used in scientific research.
If a person believes that it is wrong to use another animal in any way, regardless of the benefits to humans, there is nothing anyone can say to convince that person of the value of scientific research with animals. For that person the issue is closed from the very beginning. Moral absolutes cannot be settled logically; like religious beliefs they can be accepted or rejected, but they cannot be proved or disproved. My arguments in support of scientific research with animals are based on an evaluation of the benefits the research has to humans. (We should also remember that research with animals often helps other animals; procedures used by veterinarians, as well as those used by physicians, come from such research.)
Before describing the advantages of research with animals, let me point out that the use of animals in research and teaching is a special target of animal rights activists. Nicholl and Russell ( 1990 ) examined twenty-one books written by such activists and counted the number of pages devoted to concern for different uses of animals. Next, they compared the relative concern the authors showed for these uses to the numbers of animals actually involved in each of these categories. The results indicate that the authors showed relatively little concern for animals that are used for food, hunting, or furs or for those killed in animal shelters; but although only 0.3 percent of the animals are used for research and education, 63.3 percent of the pages were devoted to this use. In terms of pages per million animals used, the authors devoted 0.08 to food, 0.23 to hunting, 1.27 to furs, 1.44 to killing in pounds—and 53.2 to research and education. The authors showed 665 times more concern for research and education compared with food and 231 times compared with hunting. Even the use of animals for furs (which consumes two-thirds as many animals as research and education) attracted 41.9 times less attention per animal.
The disproportionate amount of concern that animal rights activists show toward the use of animals in research and education is puzzling, particularly because this is the one indispensable use of animals. We can survive without eating animals, we can live without hunting, we can do without furs; but without using animals for research and for training future researchers, we cannot make progress in understanding and treating diseases. In not too many years our scientists will probably have developed a vaccine that will prevent the further spread of diseases such as malaria or AIDS. Some animal rights activists believe that preventing the deaths of laboratory animals in the pursuit of such a vaccine is a more worthy goal than the prevention of the deaths of millions of humans that will occur as a result of these diseases if vaccines are not developed. Even diseases that we have already conquered would take new victims if drug companies could no longer use animals. If they were deprived of animals, these companies could no longer extract hormones used to treat human diseases, and they could not prepare many of the vaccines we now use to prevent disease.
Our species is beset by medical, mental, and behavioral problems, many of which can be solved only through biological research. Let us consider some of the major neurological disorders. Strokes, caused by bleeding or obstruction of a blood vessel within the brain, often leave people partly paralyzed, unable to read, write, or converse with their friends and family. Basic research on the means by which nerve cells communicate with each other has led to important discoveries about the causes of the death of brain cells. This research was not directed toward a specific practical goal; the potential benefits actually came as a surprise to the investigators.
Experiments based on these results have shown that if a blood vessel leading to the brain is blocked for a few minutes, the part of the brain that is nourished by that vessel will die. However, the brain damage can be prevented by first administering a drug that interferes with a particular kind of neural communication. This research is important, because it may lead to medical treatments that can help to reduce the brain damage caused by strokes. But it involves operating on a laboratory animal, such as a rat, and pinching off a blood vessel. (The animals are anesthetized, of course.) Some of the animals will sustain brain damage, and all will be killed so that their brains can be examined. However, you will probably agree that research like this is just as legitimate as using animals for food.
As you will learn later in this book, research with laboratory animals has produced important discoveries about the possible causes or potential treatments of neurological and mental disorders, including Parkinson’s disease, schizophrenia, manic-depressive illness, anxiety disorders, obsessive-compulsive disorders, anorexia nervosa, obesity, and drug addictions. Although much progress has been made, these problems are still with us, and they cause much human suffering. Unless we continue our research with laboratory animals, they will not be solved. Some people have suggested that instead of using laboratory animals in our research, we could use tissue cultures or computers. Unfortunately, tissue cultures or computers are not substitutes for living organisms. We have no way to study behavioral problems such as addictions in tissue cultures, nor can we program a computer to simulate the workings of an animal’s nervous system. (If we could, that would mean we already had all the answers.)
The easiest way to justify research with animals is to point to actual and potential benefits to human health, as I have just done. However, we can also justify this research with a less practical, but perhaps equally important, argument. One of the things that characterizes our species is a quest for an understanding of our world. For example, astronomers study the universe and try to uncover its mysteries. Even if their discoveries never lead to practical benefits such as better drugs or faster methods of transportation, the fact that they enrich our understanding of the beginning and the fate of our universe justifies their efforts. The pursuit of knowledge is itself a worthwhile endeavor. Surely the attempt to understand the universe within us—our nervous system, which is responsible for all that we are or can be—is also valuable.
Careers in Neuroscience
What is behavioral neuroscience, and what do behavioral neuroscientists do? By the time you finish this book, you will have as complete an answer as I can give to these questions, but perhaps it is worthwhile for me to describe the field—and careers open to those who specialize in it—before we begin our study in earnest.
Behavioral neuroscientists study all behavioral phenomena that can be observed in nonhuman animals. Some study humans as well, using noninvasive physiological research methods. They attempt to understand the physiology of behavior: the role of the nervous system, interacting with the rest of the body (especially the endocrine system, which secretes hormones), in controlling behavior. They study such topics as sensory processes, sleep, emotional behavior, ingestive behavior, aggressive behavior, sexual behavior, parental behavior, and learning and memory. They also study animal models of disorders that afflict humans, such as anxiety, depression, obsessions and compulsions, phobias, psychosomatic illnesses, and schizophrenia.
behavioral neuroscientist A scientist who studies the physiology of behavior, primarily by performing physiological and behavioral experiments with laboratory animals.
Although the original name for the field described in this book was physiological psychology, several other terms are now in general use, such as biological psychology, biopsychology, psychobiology, and—the most common one—behavioral neuroscience. Most professional behavioral neuroscientists have received a Ph.D. from a graduate program in psychology or from an interdisciplinary program. (My own university awards a Ph.D. in neuroscience and behavior. The program includes faculty members from the departments of psychology, biology, biochemistry, and computer science.)
Behavioral neuroscience belongs to a larger field that is simply called neuroscience. Neuroscientists concern themselves with all aspects of the nervous system: its anatomy, chemistry, physiology, development, and functioning. The research of neuroscientists ranges from the study of molecular genetics to the study of social behavior. The field has grown enormously in the last few years; the membership of the Society for Neuroscience is currently over forty thousand.
Most professional behavioral neuroscientists are employed by colleges and universities, where they are engaged in teaching and research. Others are employed by institutions devoted to research—for example, in laboratories owned and operated by national governments or by private philanthropic organizations. A few work in industry, usually for pharmaceutical companies that are interested in assessing the effects of drugs on behavior. To become a professor or independent researcher, one must receive a doctorate—usually a Ph.D., although some people turn to research after receiving an M.D. Nowadays, most behavioral neuroscientists spend two years or more in a temporary postdoctoral position, working in the laboratory of a senior scientist to gain more research experience. During this time they write articles describing their research findings and submit them for publication in scientific journals. These publications are an important factor in obtaining a permanent position.
Two other fields often overlap with that of behavioral neuroscience: neurology and experimental neuropsychology (often called cognitive neuroscience). Neurologists are physicians who are involved in the diagnosis and treatment of diseases of the nervous system. Most neurologists are solely involved in the practice of medicine, but a few engage in research devoted to advancing our understanding of the physiology of behavior. They study the behavior of people whose brains have been damaged by natural causes, using advanced brain-scanning devices to study the activity of various regions of the brain as a subject participates in various behaviors. This research is also carried out by experimental neuropsychologists (or cognitive neuroscientists)—scientists with a Ph.D. (usually in psychology) and specialized training in the principles and procedures of neurology.
Not all people who are engaged in neuroscience research have doctoral degrees. Many research technicians perform essential—and intellectually rewarding—services for the scientists with whom they work. Some of these technicians gain enough experience and education on the job to enable them to collaborate with their employers on their research projects rather than simply working for them.
SECTION SUMMARY: Ethical Issues in Research with Animals and Careers in Neuroscience
Research on the physiology of behavior necessarily involves the use of laboratory animals. It is incumbent on all scientists who use these animals to ensure that they are housed comfortably and treated humanely, and laws have been enacted to ensure that they are. Such research has already produced many benefits to humankind and promises to continue to do so.
Behavioral neuroscience (also called biological psychology, biopsychology, psychobiology, and behavioral neuroscience) is a field devoted to our understanding of the physiology of behavior. Behavioral neuroscientists are allied with other scientists in the broader field of neuroscience. To pursue a career in behavioral neuroscience (or in the sister field of cognitive neuroscience), one must obtain a graduate degree and (usually) serve two years or more as a “postdoc”—a junior scientist working in the laboratory of an established scientist.
■ THOUGHT QUESTION
· Why do you think some people are apparently more upset about using animals for research and teaching than about using them for other purposes?
Strategies for Learning
The brain is a complicated organ. After all, it is responsible for all our abilities and all our complexities. Scientists have been studying this organ for a good many years and (especially in recent years) have been learning a lot about how it works. It is impossible to summarize this progress in a few simple sentences; therefore, this book contains a lot of information. I have tried to organize this information logically, telling you what you need to know in the order in which you need to know it. (After all, to understand some things, you sometimes need to understand other things first.) I have also tried to write as clearly as possible, making my examples as simple and as vivid as I can. Still, you cannot expect to master the information in this book by simply giving it a passive read; you will have to do some work.
Learning about the physiology of behavior involves much more than memorizing facts. Of course, there are facts to be memorized: names of parts of the nervous system, names of chemicals and drugs, scientific terms for particular phenomena and procedures used to investigate them, and so on. But the quest for information is nowhere near completed; we know only a small fraction of what we have to learn. And almost certainly, many of the “facts” that we now accept will some day be shown to be incorrect. If all you do is learn facts, where will you be when these facts are revised?
The antidote to obsolescence is knowledge of the process by which facts are obtained. Scientific facts are the conclusions that scientists reach about their observations. If you learn only the conclusions, obsolescence is almost guaranteed. You will have to remember which conclusions are overturned and what the new conclusions are, and that kind of rote learning is hard to do. But if you learn about the research strategies the scientists use, the observations they make, and the reasoning that leads to the conclusions, you will develop an understanding that is easily revised when new observations (and new “facts”) emerge. If you understand what lies behind the conclusions, then you can incorporate new information into what you already know and revise these conclusions yourself. I can attest to the fact that most of what I know about behavioral neuroscience I learned in the years after receiving my Ph.D.
In recognition of these realities about learning, knowledge, and the scientific method, this book presents not just a collection of facts but a description of the procedures, experiments, and logical reasoning that scientists have used in their attempt to understand the physiology of behavior. If, in the interest of expediency, you focus on the conclusions and ignore the process that leads to them, you run the risk of acquiring information that will quickly become obsolete. On the other hand, if you try to understand the experiments and see how the conclusions follow from the results, you will acquire knowledge that lives and grows.
Enough said. Now let me offer some practical advice about studying. You have been studying throughout your academic career, and you have undoubtedly learned some useful strategies along the way. Even if you have developed efficient and effective study skills, at least consider the possibility that there might be some ways to improve them.
If possible, the first reading of an assignment should be as uninterrupted as you can make it; that is, read the chapter without worrying much about remembering details. Next, after the first class meeting devoted to the topic, read the assignment again in earnest. Use a pen or pencil as you go, making notes. Don’t use a highlighter. Sweeping the felt tip of a highlighter across some words on a page provides some instant gratification; you can even imagine that the highlighted words are somehow being transferred to your knowledge base. You have selected what is important, and when you review the reading assignment, you have only to read the highlighted words. But this is an illusion.
Be active, not passive. Force yourself to write down whole words and phrases. The act of putting the information into your own words will not only give you something to study shortly before the next exam but also put something into your head (which is helpful at exam time). Using a highlighter puts off the learning until a later date; rephrasing the information in your own words starts the learning process right then. Before you begin reading the next chapter, let me say a few things about the design of the book that may help you with your studies. The text and illustrations are integrated as closely as possible. In my experience, one of the most annoying aspects of reading some books is not knowing when to look at an illustration. Therefore, in this book you will find figure references in bold italic letters (like this: Figure 5.6 ), which means “stop reading and look at the figure.” These references appear in locations I think will be optimal. If you look away from the text then, you will be assured that you will not be interrupting a line of reasoning in a crucial place and will not have to reread several sentences to get going again. You will find passages like this: “ Figure 3.1 shows an alligator and two humans. This alligator is certainly laid out in a linear fashion; we can draw a straight line that starts between its eyes and continues down the center of its spinal cord. (See Figure 3.1 . )” This particular example is a trivial one and will give you no problems no matter when you look at the figure. But in other cases the material is more complex, and you will have less trouble if you know what to look for before you stop reading and examine the illustration.
You will notice that some words in the text are italicized, and others are printed in boldface. Italics mean one of two things: Either the word is being stressed for emphasis and is not a new term, or I am pointing out a new term that is not necessary for you to learn. On the other hand, a word in boldface is a new term that you should try to learn. Most of the boldfaced terms in the text are part of the vocabulary of the behavioral neuroscientist. Often, they will be used again in a later chapter. As an aid to your studying, definitions of these terms are printed at the bottom of the page, along with pronunciation guides for terms whose pronunciation is not obvious. In addition, a comprehensive index at the end of the book provides a list of terms and topics, with page references.
At the end of each major section (there are usually three to five of them in a chapter), you will find a Section Summary, which provides a place for you to stop and think again about what you have just read to make sure that you understand the direction the discussion has gone. Taken together, these sections provide a detailed summary of the information introduced in the chapter. My students have told me that they review the interim summaries just before taking a test.
Okay, the preliminaries are over. The next chapter starts with something you can sink your (metaphorical) teeth into: the structure and functions of neurons, the most important elements of the nervous system.
Study and Review on MyPsychLab
Describe blindsight, the behavior of people with split brains, and unilateral neglect and explain the contribution of these phenomena to our understanding of self-awareness.
Describe the nature of physiological psychology and the goals of research.
Describe the biological roots of physiological psychology.
Describe the role of natural selection in the evolution of behavioral traits.
Discuss the evolution of the human species and a large brain.
Discuss the value of research with animals and ethical issues concerning their care.
Describe career opportunities in neuroscience.
Explore the Virtual Brain in MyPsychLab
■ THE VIRTUAL BRAIN
Biopsychology is, in many ways, a visual science. It can be a challenge to picture the location of brain regions and the functional connections among them from written descriptions. Although a cliché, it is true that a picture can be worth a thousand words. The Virtual Brain is a 3-D, interactive resource that will help you visualize the brain regions and circuits described in the text. There are 14 modules in the Virtual Brain, each featuring the neural circuitry underlying a general process.