Delicious! Disgusting! So Say Our Brains


by Scott P. Edwards

February, 2008

We all remember occasions when we ate something that just did not taste right: the overripe apple, the rotten meat, the sour milk. Our ability to reject these foods as unpleasant, even dangerous, resides in part in the gustatory cortex, a small region of the insula, deep inside the brain. Recent findings indicate that reduced activity in the gustatory cortex may be connected to anorexia nervosa.

The gustatory system is anything relating to the sense of taste, including the smell, texture, look and feel of foods. “The gustatory process is inherently multisensory,” says Duke University neurobiologist Sidney A. Simon.

When you put something in your mouth and begin to chew, all of these senses act at once to give you a “oneshot taste” of something, Simon says. “You drink lemonade, not citric acid, sugar, pH this, pH that. You’re getting lemonade.”

The second thing that happens— in milliseconds— is that you decide either that you like what you put in your mouth, in which case you swallow it, or you decide that you do not like it and spit it out. We react much differently to a fresh potato chip than to a soggy one, for example.

A recent study found that women suffering from anorexia nervosa have less activity in the insula, including the gustatory cortex—the primary cortical taste region—than those who do not have the eating disorder. This is the first evidence that individuals with anorexia process taste stimuli differently than others and  that some alteration in the function of the insula may have predisposed them to the disorder.

Researchers at the University of Pittsburgh and University of California at San Diego (UCSD) medical schools used functional magnetic resonance imaging to measure the brain activity of32 women, half of whom had recovered from anorexia nervosa. The imaging results showed that the women who had recovered from anorexia had significantly reduced responses in the insula and related brain regions to pleasant and neutral tastes compared to the control group. Further, the control group showed a strong relationship between how they judged the pleasantness of a taste and activity of the insula, a relationship not seen in the scans of those who had recovered from anorexia.

“It is possible that people with anorexia have difficulty recognizing taste or responding to the pleasure that is associated with food,” says Walter H. Kaye, a psychiatrist who directs the Eating Disorders Program at UCSD and co-author of the paper, which appeared in the journal Neuropsychopharmacology in May 2007.

CONTEXT AND EMOTIONAL RESPONSES TO TASTE

The gustatory cortex is the primary pathway for taste, connecting the ventromedial prefrontal cortex (an area of the brain that is involved with emotion) to the insular cortex. Similar to how the brain processes sounds via the auditory cortex, the brain identifies different tastes by way of signals that travel along the gustatory pathway to the insular cortex.

But that is not all that happens. Simon says that responses in the insular cortex are context-dependent. If, for example, you gagged on lima beans when you were young and vomited after eating them, you are not likely to like them when you are older. This is called conditioned taste aversion, a survival mechanism that trains the body to avoid certain substances before they cause harm.

“This shows that there’s not just a wire from the tongue to the brain [telling us what a taste is],” he says. “It’s modulated all over the place based on expectations and experiences. If it was simply what goes on on the tongue, then expectations [of taste] wouldn’t matter.”

In addition, the insular cortex is connected to the brain’s limbic system, which includes the hypothalamus, hippocampus and amygdala and is important for emotions and memory. When taste messages arrive in this part of the brain, we experience pleasant—or unpleasant—feelings and memories. Thus, we eat foods that bring us pleasure and stay away from those that taste bad or make us sick.

Kaye and his colleagues think decreased activity in the insula may be the reason that people with anorexia often avoid food. “It may be that food is aversive [to them], rather than rewarding,” Kaye says.