Further research revealed that the amygdala—an almond-shaped cluster of neurons deep within the brain—plays a pivotal role in the fear-association response in rats, and also in humans. The sight of a loaded gun, for example, triggers activity in this part of the brain. People with an injured amygdala have dampened emotional responses and so do not learn to fear new things through association.
In the 1980s, Caroline and Robert Blanchard, working together at the University of Hawaii, carried out a pioneering study on the natural history of fear. They put wild rats in cages and then brought cats gradually closer to them. At each stage, they carefully observed how the rats reacted. They found that the rats responded to each kind of threat with three distinct sets of behaviors.
The first kind of behavior is a reaction to a potential threat, in which a predator, such as a cat, is not visible but there is good reason to worry that it might be nearby, such as the scent of fresh cat urine. In such a case, a rat will proceed cautiously, assessing the risk. The second behavior occurs when the rat see the cat. The rat will freeze and then make a choice about what to do next, either remain immobile or run away. In the third behavior, the cat notices something and walks toward the rat to investigate. At this point, the rat will flee if it has an escape route. If the cat gets close, the rat will choose either to fight or to run for its life.