Mirror neurons seen behaving normally in autism

People with autism seem to have normal "mirror" neurons after all. A popular theory has it that these neurons – brain cells that fire both when you perform an action and when you watch someone else doing the same thing – don't work properly in people with autism. Now it looks as though that isn't so.

The idea was that malfunctioning mirror neurons underlie the difficulties that people with autism have in interpreting the intentions of others. It seemed to be backed up by studies in which the brain activity of normal and autistic people was measured while they watched and performed simple actions, such as hand movements. In many, but not all of these studies, brain areas rich with mirror neurons proved less active in the people with autism.

But Ilan Dinstein, a neuroscientist at the Weizmann Institute in Rehovot, Israel, says other differences besides faulty mirror neurons could explain these results. For instance, if it turns out that people with autism imitate or detect hand movements more slowly than others, brain scans could mistake this delay for dysfunction in mirror neurons, he says.

No mirror dysfunction

So Dinstein and his colleagues at New York University turned to a more sensitive test for mirror neuron activity that he and others discovered recently in healthy people. Like brain cells that respond to sounds and smells, mirror neurons fire a little more weakly in response to repeated activation by an exactly repeated movement.

Dinstein's team asked 13 autistic adults and 10 controls to watch or perform a series of hand signals – thumbs up and miming holding a gun, for instance – while in a functional MRI scanner. In some trials they performed or watched the same hand movement over and over again, while in others they performed or watched successions of different signals.

Brain areas linked to the mirror neuron system – parts of the premotor and the parietal cortices – lit up in both groups, whether they watched or performed a hand-movement. What's more, the mirror neuron activity quieted when both groups observed or performed the same signal over and over, but not when they performed a succession of different movements, suggesting that the system was working normally in people with autism. "That argues against a mirror system dysfunction in autism," says Dinstein.

However, Marco Iacoboni, a neuroscientist at the University of California, Los Angeles, one of the first scientists to suggest that mirror neurons were dysfunctional in autism, says that 13 autistic and 10 normal subjects are too few to draw any conclusion about brain differences between these groups.

Noisy brains

Iacoboni's UCLA colleague Mirella Dapretto adds that even if the findings do hold in larger groups of people, they still don't rule out the idea that mirror neurons could be behaving abnormally in autism. She has previously argued that the more extreme a person's autism symptoms, the more problems they have in their mirror neuron system.

Dinstein stands by his team's conclusions. The number of participants he examined is typical for brain imaging studies, he says, and their autistic participants, though high-functioning, possessed the most extreme form of autism spectrum disorder, not milder forms such as Asperger's syndrome.

He supports a different theory for autism: that it is the product of "noisy brain networks" that don't communicate as predictably as those in normal people. He says his latest study offers support for this, as his team noticed more variability in the brain activity of people with autism, compared with controls.

He plans to probe this theory by searching for noise in other brain areas in people with autism.

Journal reference: Neuron, DOI: 10.1016/j.neuron.2010.03.034

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