Brain scans show why we always have room for dessert

"Instead of using a bell and meat powder, which is what Pavlov originally used, researchers used visual pictures of little intrinsic significance and coupled those to food smells," said Dr. Jay Gottfried 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. 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, said he wanted to find out how the brain learns and wanted to look for brain regions that showed decreased activity going from pre- to post-feeding .

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.

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.