In discussion summary #2, we commented on Rizzolatti & Craighero's (RC) admission that the mirror neuron (MN) system isn't the only mechanism for action understanding, and why this is a serious problem for the MN theory. Here we discuss the two functions that have been ascribed to the MN system, (1) action understanding, and (2) imitation.
"Two main hypotheses have been advanced on what might be the functional role of mirror neurons. The first is that mirror-neuron activity mediates imitation (see Jeannerod 1994); the second is that mirror neurons are at the basis of action understanding (see Rizzolatti et al. 2001). Both these hypotheses are most likely correct." (p. 172).
This is a funny position for reasons that RC correctly raise: "...imitation, the capacity to learn to do an action from seeing it done ... is present among primates, only in humans, and (probably) in apes..." (p. 172). So real mirror neurons, those documented in macaque, cannot be the neural basis for imitation because monkeys don't imitate. RC therefore claim, quite reasonably, that "... the mirror neuron system is the system at the basis of imitation in humans." (p. 172, italics mine).
Why is this a funny position? There are two reasons.
First, since real mirror neurons -- i.e., cells that respond both during the execution of an action and during the perception of a similar action, and that have sensory-related activity that cannot be explained by movement preparation, etc. -- have not been documented in humans, we are left to assume without empirical confirmation that regions involved in action imitation in human functional imaging studies (there are many) in fact involve cells with mirror neuron properties. If instead imitation activations in humans involve a circuit that is related to, say movement preparation instead of action perception (has this ever been tested?), then we are talking about entirely different animals.
The second reason it is funny to ascribe action understanding and imitation to the same system stems from RC's point about the complexity of imitation. "Although laymen are often convinced that imitation is a very primitive cognitive function, they are wrong." (p. 172). In other words, imitation is cognitively complicated, explaining why apes can do it and monkeys can't. Now I don't know the literature on imitation, but this seems reasonable. The complexity of imitation is a problem for the joint understanding/imitation theory of MNs, because it seems to me that one would need to understand an action in order to know whether it is worth imitating, yet without having performed the task yourself, you can't understand it. So this either means that imitation is a blind reflex where apes and humans go around imitating EVERYTHING and then when they blindly hit on a useful action they see the consequences of their own action, and then understand it, (in which case imitation is NOT a complicated function at all), OR, the usefulness of actions one has never performed and therefore are not part of the MN "vocabulary" can be understood "directly" without the MN system, and then selected for imitation because of its usefulness.
Put simply, if the MN system is required for action understanding and imitation, how does the animal know what to imitate? Unless you are imitating reflexively, you need action understanding to drive imitation, but without having previously imitated or performed an action, you can't understand it. So there's a bit of circularity in this position. Of course, if you admit of some other mechanism that allows for action understanding without the MN system, then you can crack the circuit. But this raises serious problems of its own: see discussion summary #2.
It seems to me that what people are calling the mirror system in humans really does support imitation (in the form of a network doing some form of sensory-motor integration). But the association of this system with the macaque mirror system is tenuous given that they don't imitate.
Caveat: I don't know much about imitation! Is imitation reflexive? Or is it selective? I would love to hear from someone who knows about this stuff.