SCIENCE 15 MAY 1998 p. 1005


Probing the Biology of Emotion

Neuroscientists using an array of new methods to explore the anatomy and chemistry of emotion are finding that intense emotions can leave a long-lasting physical imprint in the brain


After he cracked the origin of species, Charles Darwin turned to another of life's mysteries: the nature of emotions. In his 1872 book The Expression of the Emotions in Man and Animals, Darwin took an outside-in approach, scrutinizing sulky monkeys, snarling dogs, faces of the insane, even his own wailing infant. He found that different species have common ways of expressing certain emotions, reinforcing his belief in the shared ancestry of animals.

A century later, researchers have I picked up on Darwin's cross-species approach, but they've turned the study of emotions inside out. In rats, monkeys, and I people, neuroscientists are looking beyond the surface expression of emotion into the pockets of brain from which they arise. By I recording the activity of single neurons and I analyzing brain chemistry in rats and other animals, and scanning brain activity in humans, they are beginning to map the neural circuits that send emotional messages. And I although their field is in its infancy, they I are poised to integrate these studies into an understanding of the biological basis of emotion. "Emotion is now tractable at a mechanistic level," says neuroscientist Richard J. Davidson of the University of Wisconsin, Madison. "It's been a huge advance, just enormous."

Increasingly, researchers are finding that intense emotions, particularly at key times in early life, can trigger not only behavioral changes but long-lasting physical changes in the brain. These persist long after the emotions themselves have passed and shape emotional responses later in life. This inside-out approach is also lending new insight into another favored Victorian notion—that of emotional temperaments. Individuals who are fearful or resilient not only have characteristic behaviors, they have distinct patterns of brain activity, too.

The new findings have sparked excitement—and more research. In the past month, scientists have gathered at three major meetings on the science of emotions. * And many young clinicians are eager to enter the field, hoping for better ways to diagnose and treat emotional disorders, which afflict an estimated 15 million adults in the United States. Meanwhile, spending by the National Institute of Mental Health (NIMH) on the neurobiological basis of emotion has been relatively modest, but it's growing, reaching $6.3 million in fiscal year 1997. "We think the funding future is very bright," says psychiatrist Ned H. Kalin, who directs the 2year-old Health Emotions Research Institute at the University of Wisconsin, Madison. Scientifically, "emotion is way behind in many respects," admits Antonio R. Damasio of the University of Iowa College of Medicine in Iowa City. "But it's catching up."

The anatomy of emotion

Emotions were long the province of behavioral scientists, while neuroscientists typically focused on cognitive or sensory functions such as vision. Emotions were considered "too vague and difficult to quantify," says Damasio. "It's not as bad as being a sex researcher," adds Stanford University psychiatrist David Speigel, who pioneered studies of stress and cancer survival, "but it comes close."

When researchers did begin to probe the neurobiology of feelings, they first focused on anatomy. They have traced emotional messages to such areas as the prefrontal cortex, just behind the forehead, and the ventral striatum, deep in the brain. But one of the most important emotional sites, as shown over the last 15 years by New York University neuroscientist Joseph LeDoux and others, is the amygdala—an almond-shaped structure in the center of the brain that is a key station in the processing of fear. Over the last several years, work in rats has identified finer and finer areas within the amygdala that are part of the neural fear circuits, says LeDoux. Researchers have shown, for example, that a cluster of neurons called the lateral nucleus brings the fear message in from the senses, and another cluster, the central nucleus, sends it out to other brain structures. Researchers are now piripointing the neural fear connections at even finer scales.

Meanwhile, less detailed imaging I 0 studies show that when humans feel fear, the amygdala becomes active. "It's I 0 the hot area," says Yale University psychologist Elizabeth Phelps. She, LeDoux, and others report in a study in press in Neuron that when human subjects who have been conditioned to associate a visual cue with a shock see the I telltale cue, blood flow to the amygdala increases. In another new study from Damasio's lab, patients with damaged amygdalas rated faces with negative expression to be as trustworthy as faces with positive expression. Without the amygdala to issue a waming, these patients apparently don't feel the usual wariness sparked by a stranger, Damasio said at the NIMH meeting.

Now that this wave of research has marked off some of the brain territory crucial to emotions, other researchers are studying the biochemical events that take place there. They are detailing how intense or even mundane emotional experiences leave their marks on the chemistry of the developing mammalian brain. For example, Trevor W. Robbins's group at the University of Cambridge has been comparing adult rats subjected to two different kinds of early life stress. One set of rats is stressed by being raised in solitary cages after weaning. The other group is stressed early by repeated separations from their mothers but then grow up among other rats.

The two different stressors produced almost opposite behavioral syndromes, Robbins said at the Madison meeting. Isolation-reared rats appear frenzied, becoming overexcited in response to food cues and a new environment, and they are unusually sensitive to amphetamines—similar in some ways to the attention dysfunction seen in schizophrenia and other diseases. In contrast, maternally de

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'Wisconsin Symposium on Emotion, Madison,

17-18 April; Society for Neuroscience 1998 Brain Awareness Symposium, Chicago, 24 April; Na

tional Institute of Mental Health and Library of

Congress, "Discovering Our Selves: The Science of Emotion," Washington, D.C., 5 6 May.