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By Suzanne Rostler
NEW YORK, Mar 26 (Reuters Health) - Just hearing that a dog that once bit you is roaming the neighborhood leashless is enough to cause shivers of fright. Now researchers reveal just how these physical responses to the idea of a threat are triggered.
The investigators used functional magnetic resonance imaging (fMRI) to examine the brains of people who had been told they might receive an electric shock when a square of one color appeared, but not when a square of a different color was presented.
Previous studies have shown that the amygdala, a pair of symmetrical brain structures, is involved in how humans and animals learn to respond to threats with fear.
The imaging revealed that the left amygdala of all 12 study participants was activated when they saw the square associated with the shock. There was less activation when a different-colored square was presented. Study participants never actually received the electric shock, the authors note in the April issue of Nature Neuroscience.
The researchers also used skin electrodes to measure the physical expression of the fear response. The strength of the fear response was correlated with the activity in the left amygdala.
"This study tells us that the amygdala is involved in the processing of fears that are imagined and anticipated but never actually happen," Dr. Elizabeth A. Phelps, a psychologist at New York University and the study's lead author, told Reuters Health.
This type of fear, known as instructed fear, is unique to humans and is how we learn about potential dangers in our environment.
"For example, you know that a particular dog is dangerous because it once bit you," Phelps said.
An actual threat--an attack dog actually running toward you, for instance--has also been shown to activate the amygdala, but the response is seen mainly in the right side.
The current findings demonstrate that "fears that exist only in our minds activate some of the same neural systems as fears that are learned through experience," Phelps explained.
SOURCE: Nature Neuroscience 2001;4:437-441.
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