In a purely biological sense, fear begins with the body's system for reacting to things that can harm us-- the so-called fight-or-flight response. 'An animal that can't detect danger can't stay alive. 'says Joseph LeDoux. Like animals, humans evolved with an elaborate mechanism for processing information about potential threats. At its core is a cluster of neurons (神经元)deep in the brain known as the amygdala(扁桃棱). LeDoux studies the way animals and humans respond to threats to understand how we form. memories of significant events in our lives. The amygdala receives input from many parts of the brain, including regions responsible for retrieving memories. Using this information, the amygdala appraises a situation I think this charging dog wants to bite me--and triggers a response by radiating nerve signals throughout the body. These signals produce the familiar signs of distress: trembling, perspiration and fast--moving feet, just to name three. This fear mechanism is critical to the survival of all animals, but no one can say for sure whether beasts other than humans know they're afraid. That is all LeDoux says,' if you put that system into a brain that has consciousness, then you get the feeling of fear.' Humans, says Edward M. Hallowell, have the ability to call up images of bad things that happened in the past and to anticipate future events. Combine these higher thought processes with our hardwired danger-detection systems, and you get a near-universal human phenomenon: worry. That's not necessarily a bad thing, says Hallowell. 'When used properly, worry is an incredible device. 'he says. After all, a little healthy worrying is okay if it leads to constructive action--like having a doctor look at that weird spot on your back. Hallowell insists, though, that there's a right way to worry. 'Never do it alone, get the facts and then make a plan.' he says. Most of us have survived a recession, so we're familiar with the belt-tightening strategies needed to survive a slump. Unfortunately, few of us have much experience dealing with the threat of terrorism, so it's been difficult to get facts about how we should respond. That's why Hallowell believes it was okay for people to indulge some extreme worries last fall by asking doctors for Cipro(抗炭疽茵的药物)and buying gas masks. The 'so-called fight-or-flight response' (Line 2,Para. 1) refers to ______