There is something obviously wrong in the brains of people with autism, and one of the chief symptoms of that is the difficulty they have in understanding other people's emotions and intentions.
But exactly what causes that social awkwardness is still being debated among neuroscientists -- and the debate only got sharper on Wednesday.
That's when scientists from New York and Pittsburgh published a study in the journal Neuron that concludes that one of the most prominent theories for what causes social problems in autism is flatly wrong.
The theory involves brain cells known as mirror neurons, which were first discovered in monkeys' brains about 15 years ago, and have since been identified in the human brain.
Mirror neurons are active not only when someone performs an action, like grabbing a cup, but when he sees someone else do the same thing.
That led to the idea that mirror neurons might be the basis of empathy -- understanding someone else's motives and goals -- and that notion was strengthened by some studies that seemed to show less activity in mirror neurons in people with autism.
The Neuron study, however, found just the opposite.
Lead author Ilan Dinstein, now a postdoctoral researcher at the Weizmann Institute of Science in Israel, along with three researchers from Carnegie Mellon University, one from the University of Pittsburgh and one from New York University, tested 13 adults with autism and 10 without the condition.
They put all of them in a magnetic resonance imaging scanner and asked them either to watch certain hand signals like the thumbs-up or rock, paper and scissors signs, or to imitate them.
The mirror neurons in both the autistic and non-autistic people's brains lit up just as strongly in the scanner while performing both tasks.
To prove the brain cells were reacting to specific hand motions, the researchers also gave the participants an "adaptation test" in which they were shown repeated images of the same hand signal or were asked to repeat the same motion. As with other brain cells exposed to repeated stimuli, the mirror neurons in both groups decreased in activity. But when the hand signals were varied each time, the cells leaped back to life, proving they were responding to individual signals.
The experiment did not dissuade advocates of the mirror neuron theory of empathy.
One of the leading scientists in that group, Marco Iacoboni of UCLA, told the magazine New Scientist that the small number of people in the new study made the results questionable, and said he stood by earlier research that showed autistic people had lower activity in mirror neurons.
If the mirror neurons aren't responsible for the social problems of autistic people, what is?
One possibility, said CMU psychologist and study author Marlene Behrmann, is "noisy circuits" in the brain.
Work she and others have done shows that the connections between different parts of the brain are disorganized in people with autism.
One common symptom in autistic people is hypersensitivity to noise, light or touch, she said, and "that idea can be well captured by a brain system that has noisy circuitry."
The idea that autism is caused by a global brain problem also makes sense because it has such varied effects, including repetitive motions, lack of eye contact, language delay and obsessive, narrow interests, added Dr. Dinstein.
"We have to ask," he said in a video on the Neuron website, "could such a heterogeneity of symptoms be caused by a single malfunctioning set of neurons, or are they more likely caused by widespread abnormalities throughout the brain?"
Mark Roth: firstname.lastname@example.org or 412-263-1130.