The section titled, "Evidence in Favor of the Mirror Mechanism in Action Understanding" (p. 173) gets to the heart of the matter. So what's the evidence?
The first point made by RC is that the "simplest, and most direct, way to prove that the mirror-neuron system underlies action understanding..." -- namely to lesion the relevant area(s) and show that monkeys can no longer understand actions -- is not a feasible research strategy. RC provide three reasons why this is the case:
"First, the mirror-neuron system is bilateral and includes ... large portions of the parietal and premotor cortex." (p. 173). Hmmm. It's interesting that the Parma group report that "... inactivation of a large part of F5 [on one side] produced a bilateral deficit in preshaping and grasping." (Gallese, Fadiga, Fogassi, Luppino, & Murata. (1997). A parietal-frontal circuit for hand grasping movement in the monkey: evidence from reversible inactivation experiments. In "parietal lobe contribution to orientation in 3D Space." P. Thier and H.-O, Karnath (eds). Springer-Verlag, Heidelberg. p. 264, italics theirs). If you can disrupt grasping, why wouldn't a similar deficit be evident in the understanding of grasping actions?
"Second, there are other mechanisms that may mediate action recognition..." (p. 173). Why do we need the mirror system then? See previous post.
"Third, vast lesions as those required to destroy the mirror neuron system may produce more general cognitive deficits that would render difficult the interpretation of the results." (p. 173). See my comment on their first reason.
Sounds to me like somebody needs to do the critical experiment.
So, instead of using the "simplest, most direct" method to test the MN theory, RC have to resort to correlative methods: "If mirror neurons mediate action understanding, their activity should reflect the meaning of the observed action, not its visual features." (p. 173). The problem here is the usual problem with correlation studies: there's no way to assess causality. Do MNs cause action understanding? Or is their activity just correlated with action understanding that is achieved by "other mechanisms that may mediate action recognition." So none of these studies actually test the hypothesis.
RC describe two such studies. In one it is shown that F5 neurons respond to action-associated sounds (ripping paper). This shows that sounds can be associated with actions. Cool, but it doesn't prove a thing regarding understanding. (We're reading this empirical paper for next week.)
The other is potentially interesting. Here's the logic: "If mirror neurons are involved in action understanding, they should discharge also in conditions in which [the] monkey does not see the occurring action but has sufficient clues to create a mental representation of what the experimenter does." (p. 173). The study by Umilta et al. (2001, Neuron, 32: 91-101), recorded cells in a full vision condition (monkey sees an action toward a visible object), and a hidden condition. The hidden condition was this: while the monkey is watching, the experimenter places a piece of food behind a screen. The action is then directed toward an object that the monkey can't actually see. (Recall that pantomiming actions doesn't activate MNs.) The result was that "more than half of the tested neurons" (p. 174) responded during the hidden condition. RC conclude, "It was ... the understanding of the meaning of the observed actions that determined the discharge in the hidden condition."
What can we conclude from this result? First, following the logic of the authors, the study shows that a little less than half of mirror neurons are NOT involved in action understanding. Second, we can conclude that monkeys can mentally represent objects in working memory (no surprise) and that this representation can interact with response properties of (some) MNs. There is no evidence that the MNs supported the understanding of these actions, as the actual understanding of the hidden action could have been achieved by the OTHER action understanding system.
RC conclude the section by stating that "... the activity of mirror neurons correlates with action understanding" (p. 174). I don't think action understanding was ever measured, and only about half of MNs seem to correlate, but even ignoring these limitations, correlation doesn't test the hypothesis and so proves nothing.
So, in fact, the main conclusion from the "Evidence in Favor of the Mirror Mechanism in Action Understanding" section is that there is no evidence.