Recently researchers from Carnegie Mellon University, University of Pittsburgh, and New York University published an article in the journal Neuron regarding the responses of those with Autism using their "mirror neurons", or neurons that fire when a person performs an action, and also views an action being performed. It has been a theory that diminished action within the mirror neurons would cause autism, supported by the idea that autistic children cannot learn from mimicking others.
This study placed 13 autistic adults and 10 neurotypical adults within an MRI machine and had them perform actions and observe actions being performed. The results showed that in both the study group and the control group, the performance was almost identical. Their conclusion is that the study argues against a failure in the mirror neurons as a cause for autism.
When the researchers were asked by the Pittsburgh Post-Gazette as to what could be causing autism, they said, "noisy circuits". Noisy circuits? What's that? Basically it is recircuiting the brain with neurons, a large abundance of neurons, and often times an overabundance of neurons that fire at the same time to the same stimuli. This would account for the varied symptoms of Autism, along with varying levels of severity within autism. It would also account for the increased size and/or density of the brain in autistic individuals.
So, who is right? I have always sided with the noisy circuits theory, and was not even aware of the mirror neuron theory until the article has come out. Both, of course, are theories, and require more research to be specific to their cause. Still, it's good research that is being done, and that alone gives me some hope.