by Linda Ruggiero, PhD
The mirror system in humans has gained a great deal of attention for its putative role in enabling individuals to engage in social interactions. Because of its implications in imitation, empathy and language, this system is of great interest in the study of the causes underlying autism spectrum disorders (ASDs).
In the early 1990s, a team of scientists working in Parma, Italy, placed electrodes in the ventral premotor cortex of the macaque monkey in to order study the function of single cells involved in the control of movements, such as reaching for and picking up objects. In their experiments, they recorded the activity of neurons in the brain while a monkey performed these tasks and found that some of the cells they recorded from became active both when the monkey picked up the object as well as when the monkey saw a person pick up the object. These cells were later termed “mirror neurons” because they seemed to “mirror” the actions of another animal.
Though the invasive studies that looked at single cell function in monkeys cannot easily be applied to humans, evidence suggests that a mirror system is present in humans as well. Functional Magnetic Resonance Imaging (fMRI) studies, which allow for imaging of activity within specific areas of the human brain, have been used to study the mirror system in humans. In one study, fMRIs recorded brain activity while an individual was asked to perform a task or behavior. Similar to what was observed in monkeys by single-cell electrode measures, the fMRIs showed that the human inferior frontal cortex and superior parietal lobe were active when a person performed an action and also when the person observed another individual performing the action. It was suggested, therefore, that these brain areas contain mirror neurons and appear to be involved in imitation.
Studies have suggested that in humans the mirror system may allow for empathy and language acquisition as well. A large number of experiments have shown that certain brain areas, including the anterior insula, anterior cingulate cortex, and inferior frontal cortex, respond when a person experiences an emotion such as happiness. These areas are also active when he or she watches another person experiencing a similar emotion, suggesting that mirror neurons are located in these areas. This may explain why when we see another person in pain or upset, we often “feel” their grief. In addition, fMRI studies reported that mirror neurons appear in human brain areas involved in language, which suggested that the mirror system could be involved in language acquisition. Further research supported this when a group of scientists, using fMRI studies, showed that similar brain activity was observed when individuals watched the movements of another individual and when they read phrases relating to movements.
These results suggest that the mirror system may play a role in understanding behaviors through the descriptive use of language.
Because individuals with ASDs are often characterized as having difficulty with language and social skills, such as imitation and empathy, it has been theorized that dysfunction in the mirror system may be a contributing cause. Research has examined the possibility that the mirror system in children with ASDs may not be functioning properly. Electroencephalography (EEG) records brain activity in individuals by placing small, painless electrodes on a person’s scalp.
The activity is observed as graphical waves of neural activity at a certain frequency in response to an action or a cue. It has been demonstrated that mu waves are blocked when a person makes a voluntary movement. These waves are also suppressed when a person observes another person making a movement. To test the function of the mirror system in autistic children, mu waves were measured in 10 children with ASDs and in 10 controls while each of the children performed specific movements as well as watching the same movements. Both children with and without ASD showed suppression of mu waves during voluntary movements. However, the control, but not the ASD children, showed suppression of mu waves while they watched the movements done by another person. These findings suggest dysfunction within the mirror neuron systems of individuals with ASD. Similar findings were shown with fMRI studies done in 12 children with autism as they viewed and imitated faces depicting several emotions, including anger, fear, happiness or sadness. These children were tested on an Interpersonal Reactivity Index (IRI), a test that can determine different levels of empathy. The researchers found that when children appeared to imitate social behaviors or to empathize with others, enhanced brain activity was seen in the mirror system. However, the greater the deficits in imitation and empathy, the less activity was seen in the mirror areas of their brains. This suggests that changes in the mirror-neuron system may affect the ability to imitate and empathize in these children.
The findings so far provide evidence that the mirror system may be an affected area of the brain in ASD children. This allows scientists to have an anatomical substrate to study, in addition to the behavioral symptoms of the disorder. Also, EEGs are beginning to provide a promising way to diagnose ASDs early in childhood development, which would allow for early treatment in children. Because dysfunction in the mirror system does not explain all of the symptoms of ASDs, continued research is critical in understanding how this system may interact with other neural networks.
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