Russian scientist Ivan Pavlov conditioned his dogs to associate the sound of a bell with food. Eventually, the animals would drool in response to a ring, even when no reward was available. Jay A. Gottfried and his colleagues at the Wellcome Department of Imaging Neuroscience trained people undergoing brain scans to link abstract images on a computer scene with either the smell of vanilla or peanuts. After an eight-minute training period, the subjects showed heightened levels of activity in areas known to be part of the brain's reward circuitry, the amygdala and the orbitofrontal cortex (OFC), associated with the pictures alone. The researchers then instructed the patients to eat as much vanilla ice cream or peanut butter sandwiches as they desired, without becoming uncomfortably full. When the participants were retested using the MRI machine, the scientists found that the image associated with the food they had just eaten evoked a lower response than it did before the snack. The images linked to the other food, in contrast, continued to trigger a hunger response.
The results suggest that our brains can put the brakes on our desires for certain foods once our cravings have been satisfied. The authors hypothesize that malfunctions in this mechanism could be a driving force behind compulsive eating or addictions. For example, sufferers of Kluver-Bucy syndrome--who often gorge themselves or resort to eating nonfood items--have damage to brain regions including the amygdala and the OFC. "You could conjecture that a similar thing may be going on in certain eating disorders," Gottfried notes, "where the routine breaks on the whole system are tweaked somehow, so they're no longer responding to normal cues."