What is driving the perceptual response to non-action stimuli in premotor cortex? (see last post for the context of this question.) Iacoboni et al. write, "During all scans the participants knew that the task was either to move a finger or to refrain from moving it. Thus mental imagery of their finger (or of the finger movement) should have been present even during simple observation." p. 2526-2527. In other words, finger movements, which activate prefrontal cortex, are associated not only with actions, but in these subjects also with non-action stimuli and so simply presenting the stimuli becomes sufficient to elicit motor imagery and therefore activate premotor cortex. A very reasonable explanation; I fully agree. But this raises a question: if any old stimulus can elicit activity in the mirror system simply as a result of association, why can't we explain action-perception responses the same way? On this view, the only reason why there is more activation for action via imitation compared with action cued by rectangles with symbols on them (the paper's main result) is because the association is stronger in the former condition.
I actually like this paper. I even agree with the main conclusion that the circuit highlighted by the study supports imitation (it's mostly the action understanding part of MNs that I think is seriously flawed). I just think that it might be useful to think of imitation as part of a broader system involved in sensory-motor integration that, as a network, isn't restricted to actions.