Recently, neuroimaging studies have helped reveal a specific and significant set of abnormalities present in the brain with autism. A system of "mirror neurons" located in various parts of human and animal brains have been identified that become activated equally well when people do something such as wave their hand or smile, and when they simply observe those things being done. Because mirror neurons do not discriminate between performed and viewed actions, they are very likely the neurological means through which animals (including people) are able to recognize the intentions of other animals. For example, a set of mirror neurons will activate when you smile, and also when you observe someone else smiling. The fact that the mirror neurons fire when you observe someone else smiling helps you to relate what you are seeing to your own experience of smiling, and thus you are able to infer what the other person is likely to be feeling when they smile at you. Mirror neurons are likely to be the basis for people's instinctual ability to recognize emotions from facial expressions, and perhaps even more significantly, to imitate and match those expressions. They are perhaps in some fundamental way, the neurological foundation for communication.
People with autism's mirror neuron systems appear to be less active than people without autism's mirror neuron systems. This relative lack of mirror neuron activity appears to impair people with autism's ability to automatically and instinctually intuit what other people are feeling, and to imitate them. For example, people with autism's mirror neurons were found to be less active than were people without autism's when both watched an image of a hand waving on a video screen. In another study focused on mirror neuron reactions to facial emotion, people without autism's mirror neuron systems and also their limbic systems (brain parts most responsible for emotion) became activated when they viewed pictures of people's emotional facial expressions. In contrast, people with autism's mirror neuron systems did not activate as strongly, and they did not show limbic activation either, suggesting that they did not pick up the emotional content of what they were seeing. Research on mirror neuron systems' impact on autistic social functioning is only just beginning. It is not possible to say that mirror neuron system dysfunction is the 'cause' of people with autism's problems at this time, but it certainly appears that mirror neurons play an important role in creating autistic problems. As is usually the case, more research is necessary, and time will tell.