Not only do we mentally connect the enjoyment of certain foods with unrelated stimuli, our brains can also relax these connections once we're full of that food, the researchers discovered. They report their findings in the journal Science, published by AAAS, the science society.
If that mental relaxing process doesn't happen properly, it might leave us with the urge to eat even once we're full. An inability to disconnect the anticipation of food from various sights, sounds, or other stimuli may play a role in compulsive eating, according to author Jay Gottfried of the Institute of Neurology in London.
The brain's connection-making tendencies, or "conditioning," may extend to other substances that also trigger the brain's reward circuitry, Gottfried said. The ease with which we seem to associate pleasure-giving substances with other items supports the idea that one reason it's so hard for addicts to stay clean is that the smallest signs can bring back the cravings for drugs or alcohol.
The ability to make connections between eating and various stimuli in one's environment is a fundamental part of learning that is probably common, to some degree, in all animals.
"Say a rabbit is hopping around its little grassy patch and he's learned how to find clover," Gottfried explained. "So, if he sees a particular tree stump, or rocky slope, that predicts clover."
As if nature knew we wouldn't just stop eating when we were full, it also evolved a "brake system" for the conditioning process. Gottfried and his colleagues discovered that the brain tones down certain associations, once a person is full of the desired substance.
Such a system spares the rabbit, for example, from spending all its waking hours nosing around a particular tree stump. Humans need a brake system as well, according to Gottfried.
"If every time we smelled a grilled steak we dashed for the door, it would be a little bit impractical," he said.
A faulty brake system seems to be at least part of the problem for patients with Kluver-Bucy syndrome, who often consume huge amounts of food, even putting non-food items in their mouth. These patients have damage to brain regions including those involved in the conditioning system Gottfried's team studied, the amygdala and the orbitofrontal cortex.
"You could conjecture that a similar thing may be going on in certain eating disorders, where the routine brakes on the whole system are tweaked somehow, so they're no longer responding to normal cues," said Gottfried.
For their Science study, the scientists used brain imaging on a group of 13 hungry human volunteers. The experiments involved an initial training period, in which the volunteers were shown abstract images in association with the smell of vanilla or peanut butter. Meanwhile, a functional magnetic resonance imaging (fMRI) machine monitored their brain activity.
The volunteers began to automatically associate certain images with either smell, according to the researchers.
Then, the volunteers ate their fill of either vanilla ice cream or peanut butter sandwiches, being asked to eat until they didn't want any more, but weren't uncomfortably full.
Back in the fMRI machine, the volunteers again experienced the various combinations of images and the two food smells. The researchers observed a change in brain activity for the responses related to the food that they had just eaten, but not for the other food.
The change was primarily in the brain's amygdala and orbitofrontal cortex, where the activity decreased significantly. Previous studies have also implicated these regions in conditioning. The researchers also observed some activity differences in other areas, including the ventral striatum, which is associated with the reward pathways in drug addiction.
About the role the amygdala and orbitofrontal cortex might play in the conditioning process, Gottfried stressed the importance of the fact that this activity decreased only when the volunteers were shown images corresponding to the particular food they had eaten.
Thus, this sort of brain activity is likely involved with anticipating the enjoyment of a given food – which also decreased after the volunteers had eaten until they didn't want any more. The volunteers' amygdala and orbitofrontal cortex responses remained the same for the smell (or corresponding picture) of the second food, which they did not eat.
Ultimately, this brain system may be far more versatile and wide-reaching than just a possible explanation for why food cravings can strike out of nowhere. It may be offer an adaptable system for learning, Gottfried said, that allows us to recognize cues that predict important events, and to discard cues that are no longer useful.
Jay Gottfried's co-authors are John O'Doherty and Raymond J. Dolan of the Institute of Neurology in London, U.K. The study was funded by the Howard Hughes Medical Institute and the Wellcome Trust.
The American Association for the Advancement of Science (AAAS) is the world's largest general scientific society, and publisher of the journal, Science (www.sciencemag.org). AAAS was founded in 1848, and serves some 265 affiliated societies and academies of science, serving 10 million individuals. Science has the largest paid circulation of any peer-reviewed general science journal in the world, with an estimated total readership of one million. The non-profit AAAS (www.aaas.org) is open to all and fulfills its mission to "advance science and serve society" through initiatives in science policy; international programs; science education; and more. For the latest research news, log onto EurekAlert!, www.eurekalert.org, the premier science-news Web site, a service of AAAS.
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