When your toddler gets angry at being pulled away from the television to have his diaper changed--and when your teenaged daughter is disgusted by having to change that toddler's diaper--you read those emotions instantaneously in their faces. How does the human brain do that?---American Neurological Association
An article published February 14 in the on-line edition of the Annals of Neurology, the scientific journal of the American Neurological Association, provides clues about how the brain recognizes disgust in the faces of other people. The results or this and similar studies may ultimately have important implications for understanding devastating brain diseases like schizophrenia or dementia.
Neuropsychologists--specialists in the no-man's-land between the study of the brain (neurobiology) and the mind (psychology)--have recently begun to find evidence supporting the theory that the brain is "hard-wired" for the perception of emotion. Magnetic resonance imaging (MRI) scans of subjects looking at pictures of faces showing fear, happiness, disgust and other emotions indicate that different cell groupings in the brain become active in response to different expressions.
The MRI research supports older evidence from patients with damage to isolated areas of the brain, whether from stroke or trauma. In the most well-studied example, people with damage to an area called the amygdala have trouble understanding the facial expression of fear in other people, though they are able to identify happiness, sadness, and other emotions. Interestingly, the amygdala is also implicated in the sensation of fear, suggesting that it coordinates both the experience of fear and the recognition of fear in others.
Recently, researchers have identified an area deep in the brain called the insula as being important for the recognition of disgust in other people's faces. This is partly a result of studying patients with Huntington's disease, which damages nerve cells in the insula and related areas. Huntington's patients have particular trouble recognizing facial expressions of disgust.
Researchers at the INSERM Institute in Lyon, France and at the University of Lyon recently had an unprecedented opportunity to apply more precise mapping tools than MRI to the question of how the brain processes disgust. They studied epilepsy patients who had been implanted with electrodes in preparation for possible surgery to remove sections of the brain that generate recurrent and debilitating seizures.
When the subjects viewed pictures of faces showing disgust, nerve cells in very specific subregions of the insula became active. Nerve cells in other parts of the insula or surrounding brain areas did not respond in this way. On the other hand, the areas that responded to disgust did not respond to happiness, fear, or neutral expressions.
The researchers also noted that the insula did not respond as quickly to the pictures as do other areas that respond to facial expressions. This supports the idea that the insula plays a more complicated role in integrating disgust recognition.
"This is the first time that data specify where and when the insula participates in the recognition of disgust. And we know that this part of the insula is connected to areas of the brain involved in taste, smell, and control of the visceral organs," said lead author Pierre Krolak-Salmon, M.D. of the INSERM Institute.
The authors stress, however, that the insula is probably not the single center for disgust processing, but is likely an integral part of a large network that processes disgust and perhaps other emotions, and may be involved in both the experience of disgust and the recognition of disgust in others.
"Facial expression recognition is impaired in schizophrenia, some types of dementia, Huntington's disease and others. This deficit may interfere with social contact and communication in these patients, which is why it's very important to define which neural networks are implicated in the processing of facial expressions," said Krolak-Salmon.