By Suzanne Rostler
NEW YORK (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.
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