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THURSDAY, Aug. 21, 2003 (HealthDayNews) -- Pavlov's dogs were trained to salivate at the sound of a bell. Humans, it seems, can be trained to crave vanilla ice cream and peanut butter at the sight of an abstract computer image.
And trained scientists are now speculating that the brain mechanisms involved in these processes are much more malleable and adaptable than once thought.
A central part of learning is the ability to connect unrelated environmental stimuli with behavior. "What these emotional stimuli do is orient your attention to something important, have you target your behavior toward something beneficial, whether it is escaping from danger or going toward a potential food source," says Dr. Jay Gottfried, a neurologist and neuroscientist at the Wellcome Department of Imaging Neuroscience in London and lead author of a study appearing in the Aug. 22 issue of Science.
But there apparently is a flip side to this process. Pavlov's dogs associated the bell with the arrival of dog food. If the food were to stop accompanying the bell, the dogs' brains would eventually eliminate the association.
"The brain needs a way to put a brake on the system," Gottfried says.
David Zald, assistant professor of psychology at Vanderbilt University in Nashville, Tenn., puts it another way. "If we are to appropriately adjust our behavior based on our current need states, we have to be able to modify how we respond relative to our current needs," Zald says. "If we didn't, we would respond the same way to food-related stimuli whether we were hungry or full, and probably all end up obese. The present study helps us to start unraveling how our brains allow us to modify our responses based on our current need states."
This system appears to be extremely finely tuned when it comes to food. Consider the last time you gorged on T-bone steak, convinced you had no more room in your stomach until someone suggested chocolate cake for dessert.
"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.
The latest experiment replaced dogs with 13 humans and dog food with vanilla ice cream and peanut butter sandwiches.
Each participant was shown two abstract visual images in association with the smell of either vanilla or peanut butter. As soon as the volunteers began to associate certain images with a certain smell, they sat down to eat vanilla ice cream or peanut butter sandwiches. They were asked to eat until they were satisfied, but not uncomfortably full. (The study was done in England, where peanut butter and jelly is considered a culinary travesty. As it was, the researchers had to turn away about 70 percent of the volunteers because they didn't like peanut butter.)
Participants who had eaten their fill of vanilla ice cream no longer responded to the visual cue associated with that flavor, but they did respond to the picture associated with peanut butter. The opposite was true of volunteers who had just finished eating peanut butter sandwiches.
Functional magnetic resonance imaging revealed that the amygdala and orbitofrontal cortex (OFC) regions of the brain were key players in the process. Activity in those areas increased when participants were learning to associate smells with images. They decreased when the participants had finished eating.
"While we know that the brain is capable of making these associations, it also needs to be able to update these associations and modulate them," Gottfried says. "Critical regions in this regard would be the amygdala and the OFC."
What happens when the brake or updating mechanism doesn't work? "One could imagine a case where someone might have difficulty disengaging from this pattern of behavior, and this could lead to things like compulsive eating or other eating disorders," Gottfried says.
People with Kluver-Bucy syndrome, for example, consume huge quantities of food and even put nonfood items in their mouth. These individuals also have damage in various brain regions, including the amygdala and the OFC.
"It's possible that part of the explanation is that the amygdala and the OFC are no longer able to provide an appropriate update or commentary on what's happening environmentally," Gottfried explains.
"This is a really important paper in that it links together several different important areas of psychology research," Zald says. "Specifically, it brings together the classical conditioning literature with the emerging literature on the coding of motivational value. In doing so, it suggests that responses to conditioned stimuli are sensitive to the current motivational value of the stimuli that they are associated with. Moreover, it points to the specific brain regions that appear to mediate this process."
More information
Harvard Medical School has information on different brain structures, while How Stuff Works has a user-friendly explanation of how the brain works.
