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But while human mirror systems are similar, they are not identical. Individuals vary widely, for example, in their capacity to resonate with the emotional state of others--something that can be measured by psychological tests. In a sequel to their rancid-butter experiment, Keysers' team found that subjects with higher empathy scores on such tests also exhibited stronger mirror reactions to facial expressions of both disgust and pleasure.
An even more provocative result comes from a study undertaken in 2005 by UCLA developmental psychologist Mirella Dapretto and her colleagues. They found that autistic children, compared with other children, showed depressed activity in their premotor cortex while imitating or observing facial expressions--and the more severe the autism, the more depressed the activity was. The results did not surprise Dapretto. A central problem in autism, after all, is an impaired ability to understand the feelings of others, and it seems plausible, if far from proven, that a deficiency in the mirror-neuron system could be involved.
Whether insights into the mind's hall of mirrors will actually lead to a better understanding of autism is another question, of course, and it's only one of many that remain unanswered. But as a number of researchers see it, this is a strength rather than a weakness. For the mirror-neuron system has provided neuroscientists with a powerful new probe into the biological roots of the human psyche and prompted them to take a fresh look at old questions. Indeed, says Parma's Gallese, that's what makes the research so exciting--it's still in an early phase, and the fun has just begun.