Food cravings and brain activity

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http://www.sciam.com/article.cfm?chanID=sa011&articleID=3CEE4262-E7F2-99DF-3053861E739B32E2

September Issue, 2007 "Scientific American"

OBESITY AND ADDICTION
This is Your Brain on Food (extended version)

Neuroimaging reveals a shared basis for chocoholia and drug addiction
By Kristin Leutwyler Ozelli


Nora D. Volkow is director of the National Institute of Drug Abuse. Before her appointment in 2003, she held various positions at Brookhaven National Laboratory and also served as professor of psychiatry and associate dean for the medical school at Stony Brook University. In her research, she was first to use imaging technology to investigate neurochemical changes associated with addiction

Mounting evidence shows that compulsive eating and drug abuse engage some of the same brain circuits in similar ways, offering a new angle for understanding and treating obesity. In an interview with Scientific American, Nora D. Volkow, director of the National Institute on Drug Abuse and a pioneer in the study of addiction, explains.

How do foods and drugs affect the brain in the same way?
The system in the brain that both drugs and food activate is basically the circuitry that evolved to reward behaviors that are essential for our survival. One of the reasons why humans are attracted to food is because of its rewarding, pleasurable properties. When we experience pleasure, our brains learn to associate the pleasurable experience with the cues and conditions that predict it. In other words, the brain remembers not just what the food tasted like but also the sensation of pleasure itself, and the cues or behaviors that preceded it. That memory becomes stronger and stronger as the cycle of predicting, seeking and obtaining pleasure becomes more reliable. When you remember that food, you also automatically expect the pleasure that comes from it. So when you like something very much, the mere fact of being re-exposed to it, even if it is out of reach, will trigger the desire to get it. In scientific terms, we call this process conditioning.

Conditioned cues or memories are very powerful and can profoundly affect our behavior. And when conditioning occurs to a positive stimulus, such as food, you are much more likely to repeat a particular action to obtain it. Drugs are particularly effective as conditioning stimuli, primarily by virtue of their chemical properties. They can directly stimulate areas of the brain involved with pleasure in a way that is more efficient than natural reinforcers, such as food or sex. You get an exaggerated response (supraphysiological) partly because the drug can get to the brain very fast, in a matter of seconds. With natural reinforcers the process of activating the reward pathway is more prolonged. Importantly, the conditioning that takes place links the behavior not just to the stimulus itself but to the environment and other cues that might have been only peripherally associated with it.

That’s exactly what nature intended: if the behavior necessary to seek a pleasurable experience was triggered exclusively by the object, the conditioned response would be very ineffective indeed; think about the need to find food to survive, for example: say we are primitive creatures in the jungle and you by pure chance taste a banana. The banana tastes good, but if you were just conditioned to remember that it tasted good—and not to the smell, the shape, the color, or the location of the banana—your ability to find it again would be impaired. Once you create this conditioned memory, though, it’s just like Pavlov’s dogs; the response becomes a reflex. This conditioned response underlies both the drive in drug addiction and the drive in compulsive eating.

What’s going on in the brain during cravings?

[see link for full text]
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