Scientists have identified the brain circuit where memories of fear are evaluated and expressed. Their discovery, published Nov. 18, 1999, in the British journal Nature, points the way toward possible treatments for anxiety disorders.
"The amygdala, a large structure deep within each cerebral hemisphere, is the place where the brain stores memories of fear," said University of Southern California neuroscientist Richard F. Thompson, co-author of the Nature article. "In the presence of threatening stimuli, the amygdala signals to the prefrontal cortex, triggering the expression of fearful behavior."
Researchers at USC and the Université de Bordeaux (France) trained laboratory mice by sounding a tone and then administering a small electric shock. The mice soon learned to associate the tone with the impending shock and froze in fear as soon as they heard it. Simultaneously, the researchers detected changes in the electrical impulses measured by electrodes implanted in the subjects' prefrontal cortex. When the amygdala was then surgically removed, both the freezing behavior and the altered neuronal activity disappeared.
Lead author René Garcia, of Bordeaux's Laboratoire de Neurosciences Cognitives, designed and completed the experiment in Thompson's USC laboratory during the summer of 1999."While a mouse's brain is far smaller than a human's, it has essentially the same structures and operates in analogous ways," Thompson explained.
"The prefrontal cortex acts as a kind of 'executive office,' controlling other parts of the brain. It makes decisions that determine how you will react. Memories of fear are stored in the amygdala, which codes them into signals and transmits those signals to the frontal cortex for action.
"Why are you afraid when you're walking alone in the dark and hear footsteps behind you? You have learned to be afraid. Nearly all of our fears are learned fears."
And anxiety disorders, such as panic attacks and phobias, are expressions of your memories of fear, said Thompson.
"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," he suggested.
Thompson, director of the USC Program in Neural, Informational and Behavioral Sciences, holds the William M. Keck Chair in Biological Sciences and Psychology in the USC College of Letters, Arts and Sciences.
In addition to Garcia and Thompson, USC neurobiologist Michel Baudry and Université de Bordeaux neuroscientist Rose Marie Vouimba were co-authors of the Nature article.
The study was funded by grants from the National Science Foundation, NATO and Fondation Fyssen.
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