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The ultimate goal of neural science is to understand how the flow of electrical signals through neural circuits gives rise to mind—to how we perceive, act, think, learn, and remember. Although we are still many decades away from achieving this level of understanding, neuroscientists have made significant progress in gaining insight into the neural mechanisms underlying behavior, the observable output of the nervous systems of humans and other organisms. We are also beginning to understand the disorders of behavior associated with neurological and psychiatric disease. As in the earlier editions of this book, we emphasize in this edition that behavior can be examined in terms of the electrical activity of both individual neurons and systems of nerve cells by seeking answers to five basic questions. How does the brain develop? How do nerve cells in the brain communicate with one another? How do different patterns of interconnections give rise to different perceptions and motor acts? How is the communication between neurons modified by experience? How is that communication altered by disease?

When we published the first edition of this book in 1981, these questions could be addressed only in terms of cellular biology. By the fourth edition in 2000 the same problems were being studied at the molecular level. In the decade intervening between the fourth and the present edition, molecular biology has continued to enlighten the analysis of neurobiological problems. Molecular biology has made it possible to probe the pathogenesis of many neurological diseases, including several devastating genetic disorders: muscular dystrophy, retinoblastoma, neurofibromatosis, Huntington disease, and certain forms of Alzheimer disease. Molecular biology also has greatly expanded our understanding of how the brain develops. Genetically modified worms, flies, and mice have allowed us to relate single genes, including the mutant genes underlying neurological disease, to signaling in nerve cells and to an organism's behavior. At the same time new molecular and optical tools have made it possible to image the activity of individual neurons in the intact brain and to manipulate the electrical activity of neurons and neural circuits to alter behavior. Such experiments have made it possible to examine the molecular dynamics of nerve cells in the circuits responsible for cognitive processes.

Every disease that affects the nervous system has some inherited component. Now that the 20,000 genes of the human genome have been sequenced, it is possible to identify which genes make us susceptible to certain disorders and thus to predict an individual's predisposition to a particular illness. This knowledge of the human genome is beginning to transform the practice of medicine. An individual genome scan can quantify the personal risk for neurological and psychiatric disorders at levels of increasing detail and complexity. We therefore again stress vigorously our view—advocated since the first edition—that the future of clinical neurology and psychiatry depends on the progress of neural science.

Despite the power of molecular biology to elucidate molecular mechanisms of neural function and disease, any detailed understanding of how neurons act to generate complex behaviors requires an analysis of the circuitry in which the neurons participate. Thus, key questions in neuroscience include: How are neuronal assemblies formed during development? What are the computations performed by those neural circuits and how does this activity generate behavior? How are circuits modified during learning and memory? What are the changes in neural circuits that give rise to neurological and psychiatric disease? Although the cellular and molecular biological approaches emphasized in the previous editions will certainly continue to yield important information, knowledge of the function of assemblies of neurons in defined circuits must be attained to arrive at a comprehensive cognitive neuroscience.

To study how we perceive, act, think, learn, and remember, we must develop new approaches and conceptual schemes for understanding the behavior of systems that range from single nerve cells to the substrate of cognition. As a result, in this edition we discuss more fully how the cognitive and behavioral functions of the sensory and motor systems expand our treatment of cognitive processes, and incorporate into our discussion the increasing power and importance of computational neural science. Our ability to record the electrical activity and visualize functional changes in the brain during normal and abnormal mental activity permits even complex cognitive processes to be studied directly. No longer are we constrained simply to infer mental functions from observable behavior. Indeed, a new appreciation of Freud's original insight about the importance of unconscious processes—one of the major new themes to emerge in cognitive neural science—re-emphasizes the great limitation of restricting our analysis of mind to observable behaviors. As a result of its progress in describing unconscious mental processes, neural science may soon develop the tools needed to probe the deepest of biological mysteries—the biological basis of consciousness and free will.

The intuitive insights that guided thinking about the mind at the time of our first edition in 1981 are proving inadequate in the 21st century. To give but one example, we intuitively sense that we perceive an object before interacting with it and we therefore fully expect that the brain acts in this sequential way. But recent studies indicate that at the highest levels the motor and sensory systems act in parallel, not in series, and that the motor system has considerable cognitive capability.

As a result of this progress, it has become easier to write a coherent introduction to the nervous system for students of behavior, biology, and medicine. Indeed, we think such a coherent summary is even more necessary now than it was with the first edition. Today neurobiology is central to the biological sciences— and indeed to science as a whole. Students of biology increasingly want to become familiar with neural science, and most students of psychology consider themselves to be studying the biological basis of behavior. At the same time progress in neural science is providing clearer guidance to clinicians, particularly in the treatment of psychiatric disorders. In fact, perhaps the most significant change in the clinical landscape since the first edition is the realization that psychiatry can be a clinical neural science, that the progress of psychotherapy can be assessed quantitively using brain imaging. We therefore believe it is particularly important to clarify the major principles and mechanisms governing the functions of the nervous system in health and disease without becoming lost in details.

This book provides the detail necessary to meet the interests and requirements of students in particular fields but is organized in such a way that excursions into special topics are not necessary for grasping the major principles of neural science. Toward that end we have continued to refine the illustrations in the book to allow students to understand the fundamental concepts of neural science.

Throughout this book we document the central principle that all behavior is an expression of neural activity and illustrate the insights into behavior that neural science provides. With this fifth edition we again hope to encourage the next generation of undergraduate, graduate, and medical students to approach the study of behavior in a way that unites its biological and social dimensions. From ancient times, understanding human behavior has been central to civilized cultures. Engraved at the entrance to the Temple of Apollo at Delphi was the famous maxim "Know thyself." For us, the study of mind and consciousness defines the frontier of biology.

Eric R. Kandel
Thomas M. Jessell
Steven A. Siegelbaum
A. J. Hudspeth

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