The Emotional Brain

The Mysterious Underpinnings of Emotional Life

Joseph Ledoux

• Editorial: Simon & Schuster
• Año: 2015
• ISBN:
  9781439126387
• Categoría: Biological Sciences
• Categoría: Science General

What happens in our brains to make us feel fear, love, hate, anger, joy? Do we control our emotions, or do they control us? Do animals have emotions? How can traumatic experiences in early childhood influence adult behavior, even though we have no conscious memory of them? In The Emotional Brain, Joseph LeDoux investigates the origins of human emotions and explains that many exist as part of complex neural systems that evolved to enable us to survive. One of the principal researchers profiled in Daniel Goleman's Emotional Intelligence, LeDoux is a leading authority in the field of neural science. In this provocative book, he explores the brain mechanisms underlying our emotions -- mechanisms that are only now being revealed.

1

WHAT’S LOVE GOT TO DO WITH IT?

“Our civilization is still in a middle stage, scarcely beast, in that it is no longer guided by instinct, scarcely human in that it is not yet wholly guided by reason.”

Theodore Dreiser, Sister Carrie¹

MY FATHER WAS A butcher. I spent much of my childhood surrounded by beef. At an early age, I learned what the inside of a cow looks like. And the part that interested me the most was the slimy, wiggly, wrinkled brain. Now, many years later, I spend my days, and some nights, trying to figure out how brains work. And what I’ve wanted to know most about brains is how they make emotions.

You might think that this would be a crowded field of research. Emotions, after all, are the threads that hold mental life together. They define who we are in our own mind’s eye as well as in the eyes of others. What could be more important to understand about the brain than the way it makes us happy, sad, afraid, disgusted, or delighted?

For quite some time now, though, emotion has not been a very popular topic in brain science.² Emotions, skeptics have said, are just too complex to track down in the brain. But some brain scientists, myself included, would rather learn a little about emotions than more about less interesting things. In this book, I’ll tell you how far we’ve gotten. Skeptics be warned, we’ve gotten pretty far.

Of course, at some level, we know what emotions are and don’t need scientists to tell us about them. We’ve all felt love and hate and fear and anger and joy. But what is it that ties mental states like these together into the bundle that we commonly call “emotions”? What makes this bundle so different from other mental packages, ones that we are less inclined to use the term “emotion” for? How do our emotions influence every other aspect of our mental life, shaping our perceptions, memories, thoughts, and dreams? Why do our emotions often seem impossible to understand? Do we have control over our emotions or do they control us? Are emotions cast in neural stone by our genes or taught to the brain by the environment? Do animals (other than human ones) have emotions, and if so do all species of animals have them? Can we have unconscious emotional reactions and unconscious emotional memories? Can the emotional slate ever be wiped clean, or are emotional memories permanent?

You may have opinions, and even strong ones, about the answers to some of these questions, but whether your opinions constitute scientifically correct answers can’t be determined by intuitions alone. Occasionally, scientists turn everyday beliefs into facts, or explain the workings of intuitively obvious things with their experiments. But facts about the workings of the universe, including the one inside your head, are not necessarily intuitively obvious. Sometimes, intuitions are just wrong—the world seems flat but it is not—and science’s role is to convert these commonsense notions into myths, changing truisms into “old wives’ tales.” Frequently, though, we simply have no prior intuitions about something that scientists discover—there is no reason why we should have deep-seated opinions about the existence of black holes in space, or the importance of sodium, potassium, and calcium in the inner workings of a brain cell. Things that are obvious are not necessarily true, and many things that are true are not at all obvious.

I view emotions as biological functions of the nervous system. I believe that figuring out how emotions are represented in the brain can help us understand them. This approach contrasts sharply with the more typical one in which emotions are studied as psychological states, independent of the underlying brain mechanisms. Psychological research has been extremely valuable, but an approach where emotions are studied as brain functions is far more powerful.

Science works by experimentation, which, by definition, involves the manipulation of some variables and the control of others. The brain is an enormously rich source of variables to manipulate. By studying emotion through the brain, we greatly expand opportunities for making new discoveries beyond what can be achieved with psychological experimentation alone. Additionally, studying the way emotion works in the brain can help us choose between alternative psychological hypotheses—there are many possible solutions to the puzzle of how emotions might work, but the only one we really care about is the one that evolution hit upon and put into the brain.

• • •

I got interested in how emotions come from brains one day in New England. It was the mid-1970s, and I was a graduate student doing my Ph.D. research at the State University of New York at Stony Brook. A decade earlier, my advisor, Mike Gazzaniga, had made a big splash with his thesis research involving the psychological consequences of split-brain surgery in humans, work that he had done at Cal Tech with the late Nobel Laureate Roger Sperry.³

Split-brain surgery is a procedure in which the nerve connections between the two sides or hemispheres of the brain are severed in an attempt to control very severe epilepsy.⁴ A brand-new series of patients was being operated on at Dartmouth and the surgeon had asked Gazzaniga to study them.⁵ We built a laboratory inside a camper-trailer attached to a pumpkin-colored Ford van, and frequently traveled from Long Island to see the patients at their homes in Vermont and New Hampshire.⁶

The earlier studies that Gazzaniga had done showed that when the brain is split, the two sides can no longer communicate with each other. And because language functions of the brain are usually in the left hemisphere, the person is only able to talk about things that the left hemisphere knows about. If stimuli are presented in such a way that only the right hemisphere sees them, the split-brain person is not able to verbally describe what the stimulus is. However, if you give the right hemisphere the opportunity to respond without having to talk, it becomes clear that the stimulus was registered. For example, if the left hand, which sends touch information to the right hemisphere, reaches into a bag of objects, it is able to sort through them and pull out the one that matches the picture seen by the right hemisphere. The right hemisphere can thus match the way the object feels with a memory of the way it looked a few moments earlier and pull out the correct one. The right hand can’t do this because its touch information goes to the left hemisphere, which didn’t see the object. In the split-brain patient, information put into one hemisphere remains trapped on that side of the brain, and is unavailable to the other side. Gazzaniga captured the essence of this remarkable situation in an early article on the topic called “One Brain—Two Minds.”⁷

The split-brain experiment that set my scientific compass in the direction of emotion involved the presentation of stimuli with emotional connotations to the two half-brains of a special patient known as P.S.⁸ He was special because unlike most previous patients of this type, he was able to read words in both hemispheres, although, as with the others, he could only speak through his left hemisphere. So when emotional stimuli were presented to the left hemisphere, P.S. could tell us what the stimulus was and how he felt about it—whether it signified something good or bad. When the same stimuli were presented to the right hemisphere, the speaking left hemisphere was unable to tell us what the stimulus was. However, the left hemisphere could accurately judge whether the stimulus seen by the right was good or bad. For example, when the right hemisphere saw the word “mom,” the left hemisphere rated it as “good,” and when the right side saw the word “devil,” the left rated it as “bad.”

The left hemisphere had no idea what the stimuli were. No matter how hard we pressed, the patient could not name the stimulus that had been presented to the right hemisphere. Nevertheless, the left hemisphere was consistently on the money with the emotional ratings. Somehow the emotional significance of the stimulus had leaked across the brain, even though the identity of the stimulus had not. The patient’s conscious emotions, as experienced by his left hemisphere, were, in effect, being pushed this way and that by stimuli that he claimed to have never seen.

How did this occur? Most likely, the path taken by the stimulus through the right hemisphere forked. One branch brought the stimulus to parts of the right hemisphere that identify what the stimulus is. The split-brain surgery prevented the identification made by the right hemisphere from reaching the left. The other branch took the stimulus to parts of the right hemisphere that determine the emotional implications of the stimulus. The surgery did not prevent the transfer of this information over to the left side.

The left hemisphere, in other words, was making emotional judgments without knowing what was being judged. The left hemisphere knew the emotional outcome, but it did not have access to the processes that led up to that outcome. As far as the left hemisphere was concerned, the emotional processing had taken place outside of its realm of awareness (which is to say, had taken place unconsciously).

Split-brain surgery seemed to be revealing a fundamental psychological dichotomy—between thinking and feeling, between cognition and emotion. The right hemisphere was unable to share its thoughts about what the stimulus was with the left, but was able to transfer the emotional meaning of the stimulus over.

By the way, this work was not at all about the issue of possible hemisphere differences in emotion.⁹ We were simply examining the kinds of information that could and could not flow between the hemispheres when the brain was split.

Freud of course told us long ago that the unconscious is the home of our emotions, which, he said, were often dissociated from normal thought processes. However, decades later, we still had little understanding of how this might take place, and whether it was true at all was often questioned. I set as my goal figuring how the brain processes the emotional meaning of stimuli, a goal that I have since pursued.

After completing my graduate work, I decided that the techniques available for studying the human brain were too limited and that I would never be able to understand the neural basis of emotion by studying humans. I therefore turned to studies of experimental animals, rats, for the purpose of trying to unlock the brain’s emotional secrets. As important as the human split-brain observations were in getting me going on this topic, it has been the animal studies that have really shaped my view of the emotional brain.

• • •

This book will tell you what I’ve learned from my researching and thinking about brain mechanisms of emotions. It gives a scientific account of what emotions are, how they operate in the brain, and why they have such important influences on our lives.

Several themes about the nature of emotions will emerge and recur. Some of these will be consistent with your commonsense intuitions about emotions, whereas others will seem unlikely if not strange. But all of them, I believe, are well-grounded in facts about the brain, or at least in hypotheses that have been inspired by such facts, and I hope that you will hear them out.

• The first is that the proper level of analysis of a psychological function is the level at which that function is represented in the brain. This leads to a conclusion that clearly falls into the realm of the bizarre at first—that the word “emotion” does not refer to something that the mind or brain really has or does.¹⁰ “Emotion” is only a label, a convenient way of talking about aspects of the brain and its mind. Psychology textbooks often carve the mind up into functional pieces, such as perception, memory, and emotion. These are useful for organizing information into general areas of research but do not refer to real functions. The brain, for example, does not have a system dedicated to perception. The word “perception” describes in a general way what goes on in a number of specific neural systems—we see, hear, and smell the world with our visual, auditory, and olfactory systems. Each system evolved to solve different problems that animals face. In a similar vein, the various classes of emotions are mediated by separate neural systems that have evolved for different reasons. The system we use to defend against danger is different from the one we use in procreation, and the feelings that result from activating these systems—fear and sexual pleasure—do not have a common origin. There is no such thing as the “emotion” faculty and there is no single brain system dedicated to this phantom function. If we are interested in understanding the various phenomena that we use the term “emotion” to refer to, we have to focus on specific classes of emotions. We shouldn’t mix findings about different emotions all together independent of the emotion that they are findings about. Unfortunately, most work in psychology and brain science has done this.

• A second theme is that the brain systems that generate emotional behaviors are highly conserved through many levels of evolutionary history. All animals, including people, have to satisfy certain conditions to survive in the world and fulfill their biological imperative to pass their genes on to their offspring. At a minimum, they need to obtain food and shelter, protect themselves from bodily harm, and procreate. This is as true of insects and worms as it is of fish, frogs, rats, and people. Each of these diverse groups of animals has neural systems that accomplish these behavioral goals. And within the animal groups that have a backbone and a brain (fish, amphibians, reptiles, birds, and mammals, including humans), it seems that the neural organization of particular emotional behavioral systems—like the systems underlying fearful, sexual, or feeding behaviors—is pretty similar across species. This does not imply that all brains are the same. It instead means that our understanding of what it means to be human involves an appreciation of the ways in which we are like other animals as well as the ways in which we are different.

• A third theme is that when these systems function in an animal that also has the capacity for conscious awareness, then conscious emotional feelings occur. This clearly happens in humans, but no one knows for sure whether other animals have this capacity. I make no claims about which animals are conscious and which are not. I simply claim that when one of these evolutionarily old systems (like the system that produces defensive behaviors in the presence of danger) goes about its business in a conscious brain, emotional feelings (like being afraid) are the result. Otherwise, the brain accomplishes its behavioral goals in the absence of robust awareness. And absence of awareness is the rule of mental life, rather than the exception, throughout the animal kingdom. If we do not need conscious feelings to explain what we would call emotional behavior in some animals, then we do not need them to explain the same behavior in humans. Emotional responses are, for the most part, generated unconsciously. Freud was right on the mark when he described consciousness as the tip of the mental iceberg.

• The fourth theme follows from the third. The conscious feelings that we know and love (or hate) our emotions by are red herrings, detours, in the scientific study of emotions. This will surely be hard to swallow at first. After all, what is an emotion but a conscious feeling? Take away the subjective register of fear and there’s not much left to a dangerous experience. But I will try to convince you that this idea is wrong—that there is much more than meets the mind’s eye in an emotional experience. Feelings of fear, for example, occur as part of the overall reaction to danger and are no more or less central to the reaction than the behavioral and physiological responses that also occur, such as trembling, running away, sweating, and heart palpitations. What we need to elucidate is not so much the conscious state of fear or the accompanying responses, but the system that detects the danger in the first place. Fear feelings and pounding hearts are both effects caused by the activity of this system, which does its job unconsciously—literally, before we actually know we are in danger. The system that detects danger is the fundamental mechanism of fear, and the behavioral, physiological, and conscious manifestations are the surface responses it orchestrates. This is not meant to imply that feelings are unimportant. It means that if we want to understand feelings we have to dig deeper.

• Fifth, if, indeed, emotional feelings and emotional responses are effects caused by the activity of a common underlying system, we can then use the objectively measurable emotional responses to investigate the underlying mechanism, and, at the same time, illuminate the system that is primarily responsible for the generation of the conscious feelings. And since the brain system that generates emotional responses is similar in animals and people, studies of how the brain controls these responses in animals are a pivotal step toward understanding the mechanisms that generate emotional feelings in people. Studies of the neural basis of emotion in humans vary from difficult to impossible for both ethical and practical reasons. The study of experimental animals is, as a result, both a useful and a necessary enterprise if we are to understand emotions in the human brain. Understanding emotions in the human brain is clearly an important quest, as most mental disorders are emotional disorders.

• Sixth, conscious feelings, like the feeling of being afraid or angry or happy or in love or disgusted, are in one sense no different from other states of consciousness, such as the awareness that the roundish, reddish object before you is an apple, that a sentence just heard was spoken in a particular foreign language, or that you’ve just solved a previously insoluble problem in mathematics. States of consciousness occur when the system responsible for awareness becomes privy to the activity occurring in unconscious processing systems. What differs between the state of being afraid and the state of perceiving red is not the system that represents the conscious content (fear or redness...