One of the most prominent and potentially important extrapolations of the mirror neuron theory of action understanding concern autism spectrum disorder (ASD). The "broken mirrors" hypothesis of ASD, exemplified by a Scientific American article by V.S. Ramachandran, is built on the following logic.
1. The mirror system allows us to understand the action of others.
2. The mirror system, by extrapolation, allow us to understand the emotions, intentions, and perspectives of others.
3. ASD involves a lack of sensitivity to the emotions, intentions, and perspectives of others (in particular a lack of empathy).
4. Therefore ASD results from functional disruption to the mirror system, that is, from "broken mirrors".
Since I've argued that assumption #1 is false, the logic simply falls apart. However, let me attack another of these assumptions, namely #3, that ASD involves a lack of sensitivity to emotion, etc. I'm going to argue instead that ASD involves, in fact, hyper-sensitivity to emotional states, both their own and others. This hypothesis is not new. In fact, Henry Markram, Tania Rinaldi, and Kamila Markram proposed exactly this in their 2007 paper titled, The Intense World Syndrome -- An Alternative Hypothesis for Autism. I would just like to underline their perspective here.
An analogy is useful for seeing why hyper- rather than hypo-sensitivity makes more sense. Imagine a person who is hypo-sensitive to sound. Is such a person more or less likely to walk into very loud environments? More likely! If you are less sensitive to sound you might actually prefer loud environments because that's how you get your acoustic sensation to a normal level. Conversely, a person who is hyper-sensitive to sound is going to avoid loud environments because it hurts.
Now consider the same scenario translated to the social/emotional domain. For starters let's agree that a large part of one's emotional stimulation comes from social situations. Not all emotional stimulation comes from the social domain, to be sure: I can get pretty emotional when I can't figure out how to fix my leaky faucet. But we get a lot more emotional and more often when a family member is sick or injured, or when a colleague rolls his or her eyes when we try to make a point, and so on. Now, imagine a person who is hyper-sensitive emotionally. Is such a person more or less likely to engage in social situations? Less! For someone who is hyper-sensitive to emotion, engaging in a normal social situation would be like walking into an excessively loud environment: it's uncomfortable and causes an avoidance response. ASD individuals may avoid social interaction not because they lack empathy but rather because social interaction is simply too stressful.
Some data: A characteristic of autism is decreased gaze fixation, particularly on faces. Correspondingly, the fusiform face area (FFA) appears to be less active in autistic individuals during face processing tasks. This might be interpreted as social indifference, a lack of interest in faces. However, another interpretation is that looking at faces, which are a major source of emotional information, is stressful for individuals who are hyper-sensitive to emotion. An imaging study of face perception published by Dalton et al. in 2005 supports this view. This study found that the autistic group did indeed spend less time fixating the eyes and that there was less activity overall in the FFA for the autistic than control group. However, this is not all that surprising because if you spend less time looking at a stimulus, your brain activation in regions sensitive to that stimulus will naturally be lower. And in fact, when Dalton et al. looked the correlation between FFA activity and gaze duration they found a strong and positive correlation in autistic subjects. In other words, the FFA of autistic folks is working fine, it's just that they spend less time looking at the stimulus. More importantly, they report that amygdala activity was strongly correlated with gaze duration in the autistic group but not in the control group. Interpretation: looking at faces is more emotionally arousing in autistic than control individuals.
Hyper-sensitivity to emotion is perfectly consistent with the well-known sensory sensitivity noted in ASD. In other words, there are other reasons to believe that hyper-sensitivity is a key feature of the syndrome across the board.
The mirror neuron folks got it perfectly backwards again. You don't have to take my word for it though. Here are some links to essays written by individuals with ASD or by a parent. The links were provided by Morton Gernsbacher, an author on the study referred to above. I found them particularly illuminating.
Dalton, K., Nacewicz, B., Johnstone, T., Schaefer, H., Gernsbacher, M., Goldsmith, H., Alexander, A., & Davidson, R. (2005). Gaze fixation and the neural circuitry of face processing in autism Nature Neuroscience DOI: 10.1038/nn1421
Hickok, G., Eight problems for the mirror neuron theory of action understanding in monkeys and humans. J Cogn Neurosci, 2009. 21(7): p. 1229-43.
Hickok, G. and M. Hauser, (Mis)understanding mirror neurons. Curr Biol, 2010. 20(14): p. R593-4.
Hickok, G., J. Houde, and F. Rong, Sensorimotor integration in speech processing: computational basis and neural organization. Neuron, 2011. 69(3): p. 407-22.
Lotto, A.J., G.S. Hickok, and L.L. Holt, Reflections on Mirror Neurons and Speech Perception. Trends Cogn Sci, 2009. 13: p. 110-114.