The main fear center in the brain is the amygdala. If the amygdala is damaged, a wild animal will sometimes become tame. Rats will lose their fear of cats, and monkeys will approach people and novel objects without hesitation. When the amygdala and the brain structures around it are removed, fear is abolished. Current research is now showing that the amygdala has other circuits that are not related to fear, but overall its function is biased toward fear.
