CHICAGO — The impaired social interaction and communication characteristic of autistic children is the result of abnormally functioning mirror neurons in the brain, judging from the findings of a novel imaging study.
A controlled study of 25 children revealed those with autism have increased gray matter in several areas of the parietal lobes, Manzar Ashtari, Ph.D., said at the annual meeting of the Radiological Society of North America.
“What we found was that the larger the brain matter, the more restrictive the child's interest and the more stereotypical his or her behavior, indicating the increased gray matter in autistic children is abnormal,” said Dr. Ashtari, senior neuroscientist at Children's Hospital of Philadelphia. “This suggests that the inability of autistic children to relate to people and life situations in an ordinary way may result from an abnormally functioning mirror neuron system,” she said.
Mirror neurons are brain cells that are active both when an individual is performing an action and experiencing an emotion or sensation, and when that individual witnesses the same actions, emotions, and sensations in others, Dr. Ashtari explained. “Mirror neurons were first discovered in the macaque monkey, and there is a similar system in the human brain,” she said, adding that the mirror neuron system is part of the motor system and plays an essential role in controlling our own actions. The “broken mirror” theory of autism, which was first proposed about a decade ago, argues that dysfunction of the mirror neuron system is a root cause of social disability in autism.
The study led by Dr. Ashtari was conducted at the Fay J. Lindner Center for Autism, North Shore-Long Island Jewish Health System, Bethpage, N.Y., and involved 13 boys diagnosed with high-functioning autism or Asper-ger syndrome who had IQs greater than 70, and 12 healthy controls. The subjects, average age 11 years, underwent diffusion tensor imaging (DTI), a technique that tracks the movement of water molecules in the brain.
Although DTI traditionally is used to study the brain's white matter and fiber content, Dr. Ashtari's team applied it to the assessment of gray matter by employing apparent diffusion coefficient based morphometry, which highlights brain regions with changes in gray matter volume.
In addition to the gray matter abnormalities linked to the mirror neuron system, the investigators reported that the amount of gray matter in the left parietal area correlated with higher IQs in the control group but not in the autistic children. While this finding was interesting, said Dr. Ashtari, the difference did not reach statistical significance. “However, this does suggest that the gray matter in children with autism is dysfunctional.”
Dr. Antonia Hamilton doubts the “broken mirror” theory. “I am skeptical of the mirror neuron-autism link, and the Ashtari study does nothing to change my mind,” she said in an interview. In her own study, Dr. Hamilton reported that children with autism do not suffer general imitation impairment or a global mirror neuron system deficit (Neuropsychologia 2007;45:1859–68).
“Mirror neurons are active any time you perform an action with your own hand. When you pick up a cup of coffee, or see another person picking up a cup of coffee, the same neurons are involved,” said Dr. Hamilton, a lecturer at the School of Psychology, University of Nottingham (England). “My experiment found that autistic children do fine when it comes to these practical, goal-oriented actions; however, they do not do well with social actions that involve imitation, such as smiling or waving at another person,” she explained.
Dr. Hamilton studied 25 children with an independent clinical diagnosis of autism or autism spectrum disorder (ASD). The group had a mean age of 8 years and a mean verbal mental age of just over 4 years and were compared with 29 controls. Children were tested in their ability to copy the experimenter's hand movement to a target location on a table top, using mirror imitation. The investigators found no evidence for differences in performance between the ASD group and the matched controls. Both showed the typical pattern of hand errors on contralateral trials. “We can conclude that typical and autistic children have the same tendency to imitate the goal of another person's action,” the scientists said, noting the concurrency of their results with previous studies.
In a second experiment, 23 children with ASD and 31 controls completed a grasp imitation and motor planning task. “Motor planning is known to rely on the frontoparietal circuit which makes up the mirror neuron system, so the [autistic mirror neuron dysfunction, or] AMND predicts poor performance in autism spectrum disorder, which was not found,” they wrote.