NEW ORLEANS — Scientists are on the cusp of understanding the role of sensory factors related to food intake control, and the orbitofrontal cortex appears to be hub of activity.
“In food intake control, sensory factors such as taste, smell, sight, and texture are first decoded in the brain as being taste and smell, independently of reward,” Dr. Edmund T. Rolls said at the annual scientific sessions of the American Diabetes Association. “Then they project into structures such as the orbitofrontal cortex, which decodes them in terms of their reward value. It is there that neurons determine if the food tastes pleasant, or smells pleasant, or looks pleasant. That is a crucial part of the brain for sending signals to make us want to eat: because of food reward.”
Dr. Rolls said that functional magnetic resonance imaging (fMRI) is helping scientists track the complex circuitry involved in food cravings and food reward. When someone finds a food pleasant, for example, fMRI reveals blood oxygen level-dependent (BOLD) signals in the orbitofrontal cortex, where scores of neurons are tuned into the sensory properties of food “like a wonderfully sensitive antenna,” he said. “The reason, I think, that you have neurons like that is so that during a meal you could have a response to one food as a reward. When you have eaten it, you no longer like that food, but then you will still like other foods. You compute that property very simply by having these neurons that respond to different combinations of foods.”
The concept is known as sensory-specific satiety: the notion that when you eat, your appetite can go down for one food, but remain high for other foods.
The reward decoding in parts of the brain such as the orbitofrontal cortex “is crucial to understand and may be different in obese people,” added Dr. Rolls, of the Oxford (England) Centre for Computational Neuroscience Research.
“Also, when we see, taste, or smell food, we're going to get autonomic responses, changes in insulin and changes in glucose. So understanding this circuitry and how it could be different in some individuals is also important to understanding obesity.”
In one study conducted by Dr. Rolls and his associates, participants rated the pleasantness of the flavor of chocolate milk and tomato juice, then half of the participants consumed chocolate milk to satiety and the other half consumed tomato juice to satiety, after which all of them underwent brain fMRI. The researchers found that the pleasantness of the flavor of the food eaten to satiety decreased, and that the decrease in pleasantness was reflected in decreased neuronal activation in the orbitofrontal cortex (Cereb. Cortex 2003;13:1064-71).
“The initial BOLD signal response indicates that the food is pleasant,” Dr. Rolls said. “As you feed to satiety, one of those foods becomes less pleasant, and the orbitofrontal cortex is no longer activated.”
Other fMRI studies have demonstrated that when fat is placed into the mouth, some neurons increase their firing rate to about 15 times per second. “There seems to be a sensing of texture reward in the orbitofrontal cortex, as well as olfactory reward, taste reward, visual reward, and temperature,” he said.
Dr. Rolls concluded his remarks by expressing doubt that endocrine or genetic factors directly contributed to the spike in obesity that has occurred over the past 3 decades.
“We think it is these sensory input signals and the rewards they produce that drive people to eat too much,” he said. “Palatability is another factor. Food companies produce highly palatable foods; that will tend to produce an imbalance with respect to our evolutionary old satiety signals. Food variety, portion size, and stress-induced eating are also factors.”
Dr. Rolls had no conflicts of interest to disclose.