Understanding the Brain: From Cells to Behavior to Cognition
What makes us human and unique among all creatures is our brain. Perception, consciousness, memory, learning, language, and intelligence all originate in and depend on the brain. The brain provides us with wondrous things, from mathematical theories to symphonies, from automobiles and airplanes to trips to the moon. But when it goes awry, we are undone.
Over the past century, our understanding of the brain has raced forward, yet those who study the brain are still scratching the surface, so to speak. What is the mind, after all, and how does it relate to brain function? Most neuroscientists believe the mind originates in brain function, but at the moment no one can define adequately what we mean by mind. “Consciousness” is a particularly elusive subject, though philosophers and others endlessly talk about what it means.
As a neurobiologist, I am forever peppered with questions about the brain and brain function. This is especially true for friends who know about the exciting discoveries in the brain sciences yet witness the consequences of mental illness, aging, or brain injury and want to know more about new drug therapies for treating these problems. No field of medicine is untouched by the advances in the brain sciences, especially as we have come to realize how much the course of a disease and even its outcome, perhaps, can be affected by brain function and mental state. Sound body/sound mind is a two-way street; each is profoundly affected by the other.
This book is intended to answer many of the questions about neuroscience I am often asked. At the same time, I hope to convey to the general reader the essence and vitality of the field—the progress we are making in understanding how brains work—and to describe some of our strategies for studying brain function. Whenever possible, I try to relate topics to something relevant, such as a disease or a consequence of brain function. Much wonderful work in the field is ignored—to keep the book manageable and, I hope, an interesting read.
The first four chapters provide the nuts and bolts necessary for an up-to-date understanding of the brain. The remainder of the book dips into aspects of brain function—vision, perception, language, memory, emotion, and consciousness—seemingly more relevant to cognition and how the brain creates mind. But if an in-depth understanding of these topics is to be gained, the nuts and bolts of brain function must first be sorted out.
An earlier version of this book was published 20 years ago and was called Creating Mind. Where are we today in understanding brain function? Clearly we have learned much in two areas especially—first in neurobiology, the biological mechanisms underlying neuronal function. We can now describe how neurons work in exquisite detail—how they are stimulated, how they generate and propagate electrical signals along their membranes, how they transmit information at synapses, and how they may be modulated or modified on short- or long-term bases. In addition, a wealth of information has been uncovered about the glial cells in the brain—the cells that provide a supporting role to the neurons, in maintaining extracellular space, guiding developing neurons to their proper places in the brain, and pruning and modifying synapses in both the developing and adult brain.
On the other side of the coin, cognitive science has told us much about the human brain and mind and where in the brain behavioral and cognitive phenomena happen. Imaging of the brain, especially functional imaging, has become more and more sophisticated and revealing. Brain imaging has revolutionized psychology: no longer is the brain thought of as a black box.
So where do we need to go? In my view it is to integrate neurobiology and cognitive science. How do groups of neurons interact to underlie complex behaviors? This is the province of systems neuroscience. We are just at the beginning of this quest, which must happen if we are truly going to understand the brain. In my view, it is the major challenge for neuroscience in the twenty-first century. This will be important for understanding not only the normal brain but also the diseased, damaged, or cognitively compromised brain.
Invertebrates have provided some nice examples of understanding simple behaviors neurobiologically (see Chapter 5) but for complete behaviors in vertebrates we have far to go. How will this happen? New electron microscope techniques for imaging and reconstructing brain areas down to the single synapse level, recording the activity of hundreds of neurons simultaneously, and computational modeling are all being worked on intensively and are promising in this regard. In this book, I focus on what we do know presently about how the brain functions. As a neurobiologist, I try to emphasis whenever possible underlying biological mechanisms for the phenomenon under discussion, in an attempt wherever possible to bridge the gap between neurobiology and cognition.
Chapter 1 describes the general organization of the brain. What are the cells like that are found in the brain? How do they differ from cells elsewhere in the body? Chapter 2 discusses how brain cells receive, process, and transmit information. Neural signals travel along cells electrically but between cells chemically. How do cells accomplish this? How do brain cells generate electrical signals, and how do chemical substances pass information to adjacent cells? Chapter 3 discusses in more detail how brain cells talk to one another and the changes that occur in brain cells when they are contacted by other cells. The chemical substances used to communicate signals in the brain are described, and drugs that alter chemical transmission and cause profound alterations in brain function are discussed. Chapter 4 discusses the various sensory receptor systems we have that enable us to experience the world.
Chapter 5 describes how invertebrates—animals without backbones—have been invaluable for elucidating neural and behavioral mechanisms. Animals from the sea, such as squids, horseshoe crabs, and sea slugs, have been particularly useful, and examples of important findings from these animals are presented, as well as the role of genetics in uncovering the mechanisms underlying circadian rhythmicity in fruit flies. Chapter 6 describes the architecture of the human brain—the various parts of the brain and what roles they play. How do the brains of frogs and fishes differ from our brains? Chapters 7 and 8 explore the visual system in depth, from retinal and cortical function to current ideas about visual perception. We know more about the visual system than any other brain system; it provides a wealth of clues about brain function. Chapter 9 deals with development and brain plasticity. How do embryonic brain cells find their way to their targets? How do environment and injury affect the developing and adult brain? The fascinating topics of language, memory, and learning are dealt with in Chapter 10 and 11, along with brain imaging and the question of how we discover new things about the human brain. The neurology clinic has long provided instructive examples of patients with specific brain lesions. Today, brain imaging techniques promise a wealth of new information about the human brain.
Chapter 12 turns to matters we associate more with mind—emotions and rationality. What regions of the brain are involved in emotional behaviors, and what happens when these areas are disrupted? Rationality is an example of an emergent property of the brain, deriving perhaps from emotional behaviors. Finally, Chapter 13 discusses consciousness. What do we mean by consciousness, and what can we say about consciousness from a neuroscience perspective? Throughout the book, I start most chapters with an example where brain function is compromised by injury or disease. Such examples are not presented simply as curiosities. Rather, these alterations in brain function cast light on normalcy. Finally, a glossary is provided to help with unfamiliar terms or concepts, but in almost every case further explanation is provided elsewhere in the book.
The earlier version of this book was written primarily for the non-scientist and it did well, having been reprinted seven times. I have used it for many years as the basic book for my freshman seminar at Harvard. These first-year students, interested in science, but not planning to major in a science, always found it accessible. After 20 years, however, the book clearly needed some updating, so I approached W. W. Norton & Company, its publisher, about doing this. I received an enthusiastic response from Deborah Malmud, a Vice President at Norton, who told me she always enjoyed the book and that it was one of the first books on her shelf when she began working in professional book publishing. She suggested that I not simply update the book, but expand it, essentially making it a new book with a significant amount of new material and with another title. This I have tried to do, describing not only what we presently know about brain function, but where we need to go—the future of brain research.
I owe many thanks to Deborah for her help throughout the writing, from her initial suggestions, to reading partial drafts and eventually the final draft. She has been most supportive. Thanks also to her assistant, Kate Prince, and project editor Mariah Eppes for their patience and enormous help. And finally, many, many thanks to Giselle Grenier, my assistant, who patiently and expertly entered the many drafts of the manuscript into the computer and also carried out all the other things such a project entails.
PART ONE Cellular Neurobiology: The Nuts and Bolts
CHAPTER 1 The Uniqueness of the Brain
Cells of the Brain: Neurons and Glia
How Special Are Neurons?
Development of the Brain
CHAPTER 2 Brain Signals
Electricity and the Brain
Cell Resting Potentials
Transmission Down Axons
Vulnerability of Synapses
A Synaptic Disease: Myasthenia Gravis
CHAPTER 3 Neuromodulation, Drugs, and the Brain
Neurotransmitters and Neuromodulators
Classifying Synaptic Substances
Dopamine, Parkinson’s Disease, and Schizophrenia
Serotonin and Depression
Neuropeptides: The Enkephalins—Endogenous Opiates
CHAPTER 4 Sensing the World
Mechanoreceptors: Touch and Hearing
Hair Cells and Audition
Second-Messenger Receptors: Olfactory and Visual
Discrimination of Odors
PART TWO Systems Neuroscience: Getting at Behaviors
CHAPTER 5 Simpler Nervous Systems: The Invertebrates
Electrical Signaling and the Squid Giant Axon
Mach Bands and the Horseshoe Crab Eye
Learning, Memory, and a Sea Snail
Circadian Rhythms, Behavioral Genetics, and Flies
CHAPTER 6 Vertebrate Brains
The Central Nervous System
Medulla and Pons
CHAPTER 7 Vision: Window to the Brain
Early Processing of Visual Information: The Retina
Next Stages of Visual Processing: The Primary Visual Cortex
Formation of Cortical Receptive Fields
Seeing Depth: Binocular Interactions
Cortical Organization: The Hypercolumn
PART THREE Cognitive Science: Higher Brain Function and Mind
CHAPTER 8 From Brain to Mind: Visual Perception
Area V2 and Higher Visual Areas
Overview of Visual Processing
Modulation of Cortical Responses
CHAPTER 9 The Dynamic Brain: Development and Plasticity
Migration and Differentiation of Neurons
How Do Axons Find Their Way?
The Maturing Brain
Visual System Development and Deprivation
Plasticity of the Adult Cortex
CHAPTER 10 Language and Brain Imaging
Language Areas in Humans
Exploring the Human Brain
CHAPTER 11 Remembering Things: Learning and Memory
Synaptic Mechanisms Underlying LTP
LTP and Memory
Accuracy of Memories
CHAPTER 12 The Emotional Brain: Rationality
The Autonomic Nervous System
CHAPTER 13 Consciousness
Control of Sleep and Arousal
Consciousness and Awareness
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