I recently had the pleasure of giving a lecture on mirror neurons at UC San Diego which is a very active locale for folks working on the human mirror system. I expected a lot of push-back on my critical views of mirror neurons, and I wasn't disappointed.
One of my major points of emphasis is and has been that if the mirror neuron system is really important for action understanding, then damage to action execution should result in action understanding deficits. I have pointed out that this prediction doesn't hold, either in apraxia or with more force in aphasia.
A typical response to this argument is that "the mirror system" involves lots of areas working together (George Lakoff, who was at the UCSD lecture even seemed willing to include the STS in this network) and so it is no surprise that damage to fronto-parietal areas doesn't result in the expected action understanding deficits. Along these lines, some folks pointed out that mirror responses have now been found in lots of brain areas, not just the frontal and parietal areas where they have been documented in monkeys. In other words, the mirror system is expanding.
At this point in the talk, Pat Churchland, who was my host, jumped in and said (and I paraphrase here), "Now wait a minute. If mirror neurons are all over the brain then don't they lose their explanatory power? Aren't we now just back to our old friend, the How Does the Brain Work Problem?" Very true, I think.
I ran into this same issue in my debate with Fadiga at the Neurobiology of Language Conference in Chicago. I said, here are several examples of preserved speech perception in the face of an absence of speech production ability, and the mirror neuron proponents said basically, it's a large system that's too complicated to succumb to the loss of the motor system.
A new review by Rizzolatti and Sinigaglia (2010) also pushes back against some of the critiques that have been raised lately. One concerns the observation that mirror responses in a TMS paradigm can be re-trained such that they no longer mirror and dissociated from understanding. RS: "The reason is that, in the task, there was nothing to understand: the investigated movements were meaningless." Another is the "surprising" claim (mine) that damage to the fronto-parietal motor system should be associated with deficits in action understanding. RS:" As clearly shown by electrophysiological mapping, there are motor sectors in the monkey inferior parietal lobule (and even in area PFG) with and without mirror neurons. Thus, dissociations between motor deficits and action understanding deficits can and do occur." Wait, I thought the mirror system got is magical powers from being part of the motor system. If the motor system is functionally disrupted, shouldn't the action understanding system also be disrupted? Or is there a parallel mirror motor system that can't control movement but can understand action (kind of sounds like a sensory system to me).
I think the mirror neuron folks have a serious problem on their hands: there is apparently no empirical result that can falsify the theory. If a mirror neuron shows up in an unexpected place, it is a new part of the mirror system. If a mirror neuron's activity dissociates from action understanding, it was not coding understanding at that moment. If damage to the motor system doesn't disrupt understanding, it is because that part of the motor system isn't mirroring.
Can someone from the mirror neuron camp come forward and provide us with an example of what kind of empirical result would falsify the theory? Because if you can't falsify it, it's no longer a scientific theory, it's religion.
Rizzolatti G, & Sinigaglia C (2010). The functional role of the parieto-frontal mirror circuit: interpretations and misinterpretations. Nature reviews. Neuroscience PMID: 20216547