The Science of Anxiety All animals, especially the small kind, appear to feel anxiety. Humans have felt it since the days they shared the planet with saber-toothed tigers. But we live in a particularly anxious age. A recent study found that eight months after the September 11 event, nearly two-thirds of Americans think about the terror attacks at least several times a week. And it doesn't take much for all the old fears to come rushing back. What was surprising about the recent drumbeat of terror warnings was how quickly it triggered the anxiety so many of us thought we had put behind us. This is one of the mysteries of anxiety. While it is a normal response to physical danger and can be a useful tool for focusing the mind when there's a deadline looming-anxiety becomes a problem when it persists too long beyond the immediate threat. Sometimes there's an obvious cause. Other times, we don't know why we can't stop worrying. Anxiety disorder—which is what health experts call any anxiety that persists to the point that it interferes with one's life—is the most common mental illness in the US which in its various forms, afflicts 19 million Americans. In recent years, researchers have made significant progress in nailing down the underlying science of anxiety. In just the past decade, they have come to appreciate that whatever the factors that trigger anxiety, it grows out of a response that is rooted in our brains. They have learned, among other things: -There is a genetic component to anxiety some people seem to be born worriers. -Brain scans can reveal differences in the way patients who suffer from anxiety disorders respond to danger signals. -Due to a shortcut in our brain's information-processing system, we can respond to threats before we become aware of them. -The root of an anxiety disorder may not be the threat that triggers it but a breakdown in the mechanism that keeps the anxiety response from careering out of control. Before we dig into the latest research, let's define a few terms. Though we all have our own intuitive sense of what the words stress and fear mean, scientists use these words in very specific ways. For them, stress is an external stimulus that signals danger, often by causing pain. Fear is the short-term response such stresses produce in men, women or lab rats. Anxiety has a lot of the same symptoms as fear, but it's a feeling that lingers long after the stress has lifted and the threat has passed. In general, science has a hard time pinning down emotions because they are by nature so slippery and subjective. Most people are as clueless about why they have certain feelings. But fear is the one aspect of anxiety that's easy to recognize. Humans break out in a cold sweat. Heartbeats race, and blood pressure rises. That gives scientists something they can control and measure. Indeed, a lot of what researchers have learned about the biology of anxiety comes from scaring rats and then cutting them open. The researchers destroy small portions of the rats' brains to see what effect that has on their reactions. By painstakingly matching the damaged areas with changes in behavior, scientists have, bit by bit, created a road map of fear as it travels through the rat's brain. The journey begins when a rat feels the stress, in this case an electric shock. The rat's senses immediately send a message to the central portion of its brain, where the stimulus activates two neural pathways. One of these pathways is a relatively long, circuitous route(迂回径路) through the cortex(脑皮层), where the brain does its most elaborate and accurate processing of information. The other route is a kind of emergency shortcut that quickly reaches a cluster of cells called the amygdale(扁桃体). What's special about the amygdala is that it can quic