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A Crack in the Mirror Neuron Hypothesis of Autism
12 May 2010 12:32 pm
Brain cells thought to underlie our ability to understand one another work just fine in people with autism spectrum disorders (ASD), according to the authors of a controversial new study. Other researchers had proposed that these cells, called mirror neurons, malfunction in people with ASD, disrupting their ability to understand what someone else is experiencing. If the results hold up, researchers will need another way to explain the social deficits that characterize the disorder.
First identified in monkeys, mirror neurons fire when an animal performs particular movements but also when it sees another monkey or a person perform the same movement. Such neurons allow monkeys—and presumably humans—to learn actions by imitating others, and, some researchers believe, to understand other people and empathize with them. People on the autism spectrum struggle to understand what’s happening in other people’s minds, which makes it hard for them to connect socially. Some neuroscientists had proposed that mirror neuron deficits were at the root of their social problems.
Several groups had found evidence supporting the mirror neuron hypothesis; for example, neuroscientist Marco Iacoboni of the University of California, Los Angeles, and colleagues reported in 2005 that children with ASD show reduced mirror neuron activity compared with healthy controls when they watch and imitate others making faces. But neuroscientists Ilan Dinstein and David Heeger of New York University and their colleagues considered the previous results in humans inconsistent and inconclusive and designed what they considered “a more in-depth test,” Dinstein says.
That test consisted of two types of experiments. In the first, the researchers scanned neural activity in the brains of healthy adults and high-functioning autistic adults in a functional MRI scanner while showing them one photograph after another with one of six simple hand gestures: rock, paper, scissors, thumbs up, gun, or the hang-loose hand signal favored by Hawaiian surfers. In the second experiment, they scanned subjects’ brains after instructing them through headphones to make a series of the same six hand gestures.
Two brain areas thought to contain mirror neurons, the anterior intraparietal sulcus and the ventral premotor cortex, responded similarly in autistic and control groups, whether subjects observed a hand signal or made one themselves. This suggested that the autistic group’s mirror neurons were functioning normally.
The researchers sought to confirm this with additional experiments that relied on a clever trick. When activated repeatedly, many neurons fatigue and their signals fade. What’s more, individual mirror neurons are believed to respond to specific movements—for example, the “paper” neurons are distinct from the “hang loose” neurons.
Dinstein and his colleagues assumed that if the activity of a brain region faded more when a subject repeatedly saw or reenacted a single hand signal than when he or she saw or reenacted a mixed series of hand signals, then it contained mirror neurons specific for that hand signal. In both autistic and nonautistic individuals, signals from the two mirror regions of the brain faded to a similar degree when they saw or reenacted the same hand signal over and over, the team will report tomorrow in Neuron. That suggests to Dinstein that autistic subjects’ mirror neurons are behaving normally.
Instead, the team noted that brain regions that process visual images and execute movements in autistic people fired more variably than did those in healthy people. This could be caused by malfunctioning synapses, which would make neural networks less reliable and would make signals from the world more difficult to interpret, Dinstein speculates. This faulty signaling, rather than mirror neuron problems, could underlie some of their social difficulties, he says.
Psychiatrist Justin Williams of the University of Aberdeen in the United Kingdom says that although the study demonstrates “that people with autism are as good at recognizing actions as anyone else, mirror neurons play a more complex role” and may not function normally during the childhood development of real-world social skills.
But Iacoboni says that the results don’t prove what the researchers say they do, because their trick of focusing on fatigued neurons will miss some mirror neurons. Furthermore, he says, simply testing brain activity involved in identifying and mimicking movements does not accurately model the brain activity underlying complex social tasks.