USC Trojan Family Magazine -Spring 2000: What's New: The Anatomy of Fear

IF CONQUERING fear is the beginning of wisdom – as Bertrand Russell wrote – then we’re on the threshold of becoming a wiser species.
Scientists from USC and the Université de Bordeaux recently identified the brain circuit where memories of fear are evaluated and expressed. Their discovery points the way toward possible treatment of panic attacks, phobias and other anxiety disorders.
The demon’s lair, it turns out, lies within the amygdala, a large structure deep inside each cerebral hemisphere. “In the presence of threatening stimuli, the amygdala signals to the prefrontal cortex, triggering the expression of fearful behavior,” says USC neuroscientist Richard F. Thompson, co-author of the study published in the British journal Nature.
Thompson and his colleagues tested their theory by observing laboratory mice into whose prefrontal cortex tiny electrodes had been implanted. Whenever the mice heard a chime, the sound was followed by an electric jolt. The study animals soon learned to associate the tone with the impending shock and would freeze in fearful an-ticipation. At the same time, the researchers detected changes in the electrical im-pulses measured by the electrode. After the study animal’s amygdala was surgically removed, however, both the freezing behavior and the altered neuronal activity disappeared.
The experiment has direct bearing on the study of human fear. While a mouse’s brain is far smaller than a human’s, Thompson notes, it has essentially the same structures and operates similarly.
“The prefrontal cortex acts as a kind of ‘executive office,’ controlling other parts of the brain,” he says. It decides how you will react. The amygdala stores memories of fear, codes them into signals and transmits those signals to the frontal cortex for action.

ANXIETY DISORDERS are expressions of one’s memories of fear. “Nearly all our fears are learned fears,” says Thompson, who is director of the USC Program in Neural, Informational and Behavioral Sciences. “Why are you afraid when you’re alone in the dark and hear footsteps behind you? You have learned to be afraid.”
And how do we unlearn fear?
“If we could find a drug or genetic treatment that would stop the amygdala from signaling to the frontal cortex, then we could effectively treat anxiety disorders,” Thompson says.