By forcing people to eat chocolate until the point of disgust, researchers have demonstrated what happens to the brain when a good stimulus turns bad. The results shed light on the brain regions involved in addictions and eating disorders, the researchers say, which seem to tap into the same brain circuitry.
"Chocolate is the number one craved food, so this is sort of an in-house model of addiction," says cognitive neuroscientist Dana Small at Northwestern University School of Medicine in Chicago. To look at what brain regions become active when people eat chocolate, Small and colleagues fed chocolate bars to nine self-proclaimed chocoholics and measured blood flow in the brain using positron emission tomography (PET). After each of seven snacks, the subjects were asked whether the sweets tasted good and whether they wanted more.
The brain's limbic system--which processes emotion--came into play in different ways, depending on whether the subjects found the chocolate rewarding or revolting. When the subjects still wanted more, Small's team saw activity in brain regions implicated in craving, such as the midbrain and insula. That's known to also happen when people take cocaine. But when the chocolate lovers ate more even though they didn't really want to, blood flowed to areas such as the prefrontal region. These parts of the brain normally help make the decision to stop eating, the team says. Additionally, different regions of the orbitofrontal cortex--which is involved in weighing motivations--were activated depending on whether the subjects enjoyed the chocolate or not, the team reports in the September issue of Brain.
The study makes a "major contribution" to ongoing efforts to map the brain circuitry involved in motivation because it used a constant stimulus, even as the sensation switched from pleasure to disgust, says neurobiologist Peter Shizgal of Concordia University in Montreal, Canada.