Jay A. Gottfried, John O'Doherty and Raymond J. Dolan of the Institute of Neurology in London, U.K. have found that human brains, not too surprisingly, form connections between environmental stimuli and desired foods rather like Pavlov's dog.
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 speculates that people who suffer from obesity may not respond as well to eating some kind of food by having a decrease in the stimulus effect of images that are correlated with availability of that type of food. How quickly we get disgusted wtih a food may in part determine how well we can regulate our eating.
Importantly, the team also showed that the human brain can put a "brake" on the powerful desire for certain foods once the appetite has been sated. This system to turn the "delectable into the distasteful" may be crucial in regulating behaviour, they say. Detecting faults in this system might in future help shed light on compulsive eating disorders and substance addictions, speculates Gottfried, a neurologist.
Since satiation tends to be at least partially specific for particular foods people are more likely to overeat if presented with a succession of different kinds of foods.
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 in a telephone interview.
"This is specific satiation -- you are full of one thing but not another."
"These processes operate in a very food-specific fashion, and this is important," Gottfried explains. "If, ultimately, what you need is a good balance of nutrients, vitamins, minerals, and so forth, it's important for you to be sampling different foods." A braking effect is in place as long as it comes to the same food item.
This result suggests a couple of potential strategies for limiting calorie consumption. One is to avoid any environmental stimulation separate from food itself that reminds you of some kind of food. Another is to avoid food pictures, food smells, or real food. Another is to eat meals with fewer courses.
|Share |||Randall Parker, 2003 August 25 04:07 PM Biological Mind|