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WASHINGTON
(Reuters), Thu
August 21, 2003
By Maggie Fox, Health and Science Correspondent
Scientists
who trained volunteers to react like Pavlov's dogs to peanut
butter and
ice cream said on Thursday their brain scans help explain why
we fill up on dinner yet have room for dessert.
The volunteers were conditioned to become hungry when they
saw certain abstract pictures, just as Pavlov's dogs salivated
at the sound of a bell, the researchers said.
"Instead of using a bell and meat powder, which is
what Pavlov originally used, we used visual pictures of little
intrinsic
significance and coupled those to food smells," said Dr.
Jay Gottfried of the Wellcome Department of Imaging Neuroscience
at University College London.
Gottfried was trying to explain what he calls the "restaurant
phenomenon."
"You sit down to your eight-course meal for your birthday
and you have gone though all the appetizers and entrees and
just
as you feel you can't fit one more thing in your tummy, then
they bring the dessert menu or the dessert cart rolls by and
suddenly you discover you have room for the chocolate fondant," Gottfried
said in a telephone interview.
"This is specific satiation -- you are full of one
thing but not another."
The phenomenon may help explain why diets fail, but it also
sheds light on how the brain works. Gottfried, who reports
his findings in Friday's issue of the journal Science, said
he wanted to find out how the brain learns.
"We wanted to look for brain regions that showed decreased
activity going from pre- to post-feeding," he said.
LIVE BRAIN SCANS
The 13 volunteers underwent functional magnetic resonance imaging
-- a way of looking at brain activity "live" -- while
doing what they thought were simple computer tasks.
Gottfried and colleagues showed them abstract, computer-generated
images while at the same time wafting their way the odors of
either vanilla ice cream or peanut butter.
"At various points before, during and after scanning
we asked them to give us pleasantness ratings for the smells," Gottfried
said. Unconsciously, the volunteers began to associate the
images with the smells.
Then they fed them either peanut butter or ice cream.
They imaged the brains again and found strong emotional responses
to the smells got weaker after the volunteers ate the corresponding
food.
A person's response to the peanut butter odor changed after
eating some peanut butter, but a vanilla smell made the brain
light up again. Eventually, the abstract picture associated
with vanilla evoked the responses, but again they weakened
after the volunteers ate.
Gottfried said specific brain circuits are involved in this
process. The researchers found heavy involvement of the amygdala
-- the area of the brain best known for processing emotions
-- and the orbitofrontal cortex.
"If every time you drove past a McDonald's and saw
the golden arches, you felt compelled to go inside and get
a Big Mac,
this would be destructive after time," he said.
Something must tell the brain when to respond and when not
to, and this does not necessarily stop at food.
"Whether we are talking about food or sex or even
things on the aversive scale such as dangers and threats and
predators,
the brain also needs to know how to update ... and modulate
these associations so you don't get stuck in a rut."
© Copyright
Reuters 2002
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