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Of course, what you would really like to know is whether any of the work done in rats applies to humans. Clearly researchers can't go around performing brain surgery on the amygdalas of living patients to see if it affects their anxiety levels. But the fascinating case of a woman known only by her research number, SM046, suggests that when it comes to fear, rodents and hominids really aren't so different.
Owing to an unusual brain disorder, SM046 has a defective amygdala. As a result, her behavior is abnormal in a very particular way. When scientists at the University of Iowa show SM046 pictures of a series of faces, she has no trouble picking out those that are happy, sad or angry. But if the face is displaying fear, she cannot recognize the feeling. She identifies it as a face expressing some intense emotion, but that is all. Her unusual condition strongly suggests that even in Homo sapiens, fear takes hold in the amygdala.
But studying brain-damaged patients can teach scientists only so much. They would also like to know how anxiety works in normal, intact brains. For this, brain scans have proved invaluable.
For years, doctors have used CAT scans and MRIs to help them diagnose strokes, brain tumors and other neurological conditions. But as the technology has become more sophisticated, researchers have started to employ it to tease out some of the subtle changes associated with mental illness. "We're not yet able to use these scans in a diagnostic way," says Dr. David Silbersweig of the Weill Cornell Medical College in New York City. "But we're getting pretty specific about the areas of the brain that are implicated in a number of psychiatric disorders."
One type of brain scan helps identify structures that are the wrong size or shape. Two years ago, researchers at the University of Pittsburgh showed that the amygdalas of a group of overanxious young children were, on average, much larger than those of their unaffected peers. Perhaps they just had more fear circuits to contend with? Neuroscientists are tempted to say yes, but they admit the conclusion is pretty speculative. Another group of researchers found that patients with post-traumatic stress disorder had a smaller hippocampus than normal. Perhaps their stressful experiences had somehow interfered with the hippocampus' ability to make new memories and, just as important, forget the old ones? Again, no one knows for sure.
Another type of brain scan tells scientists which brain cells are using the most oxygen or soaking up the most nutrients. The idea, explains Dr. Scott Rauch of Massachusetts General Hospital, is that any area that seems more active than usual while someone is anxious may play an important role in making the person that way. Rauch's team has spent the past eight years scanning groups of combat veterans, some with post-traumatic stress disorder and some without, to see which areas of the brain light up when they hear tapes recounting their most troubling memories. So far, the signals in the amygdala appear to be more active in those with PTSD than in those without. In addition, signals to the prefrontal cortex of PTSD subjects seem to be weaker than in those without the disorder. Perhaps this explains why the patients still feel threatened even when they are perfectly safe.