Washington, Aug 22: Scientists who trained volunteers to react like Pavlov's dogs to peanut butter and ice cream said 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.


''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 today'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.


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.

Bureau Report