The current issue of Nature Reviews Neuroscience has a Research Highlights piece on a new "mirror neuron" paper by Catmur, Walsh, & Hayes (2007, Curr. Biol., 17, 1527-1531). Although I've only read the highlight piece, the paper looks pretty interesting. The authors used TMS to induce motor evoked potentials in the abductor muscles of the hand. When subjects watched a video of a hand with the index finger moving the MEPs were greater in the subjects own index finger, whereas when the video showed movement of the little finger, MEPs were greater in the little finger of the observer. Standard "mirror" effect. Note that an action-based theory of perception would hold that this motor activity in the observer reflects the subject's "understanding" of the observed action by mapping the action onto his or her own motor system.
But there's more: the study authors then trained subjects to move in a manner incongruent with the hand in the video: move little finger when index finger movement is shown and vise versa. After training MEPs were greater in the little finger when index finger movement was observed, and vise versa. So "mirror" effects are easily trained simply by association. Nice result.
Question: does the subject now fail to correctly understand the movement of the hand in the video? If asked, would subjects report that index finger movement had taken place when in fact the pinky moved? Of course not. So this is another demonstration of a dissociation between "mirror neuron" activity and action comprehension. Conclusion: the "mirror system" reflects sensory-motor associations, NOT the neural foundation of action understanding.
Although I don't know what the study authors actually concluded about their experiment, the Nature Reviews Neuroscience piece concluded that, "These findings imply that insufficient social interactions and consequent inadequate sensory experience might affect the development of the mirror neuron system, for example in children with autism" (p. 737). Seriously? That's the logical equivalent of trying to leap across the Grand Canyon from a crumbly ledge. Hmm... I wonder if skull measurements might be able to detect this "mirror neuron" dysfunction in autism.