A nice motor-learning experiment was reported in J. Neuroscience recently (Lahav et al., 2007, 27, 308-314). The authors trained non-musicians to play a piece of music on a keyboard. They then scanned the subjects using fMRI while they passively listened to the piece they learned or to a piece the hadn't learned to play. Both trained and untrained pieces activated auditory regions in the superior temporal lobe. However, the trained piece in addition activated posterior frontal areas, whereas the untrained piece did not. This is a very nice demonstration that motor associations of the trained musical piece were indeed acquired, and that these associations involve posterior frontal areas.
Here's the puzzling thing: the title of the paper is "Action Representation of Sound: Audiomotor Recognition Network While Listening to Newly Acquired Actions" and the frontal activations are attributed to, The Mirror Neuron System.
First of all, what evidence is there that the frontal activations are "action representations of sound?" Why can't they be motor representations of movement? Second, how is this an experiment on "listening to actions"? And what does listening to actions mean? Third, what is being "recognized" by this mirror system? The melody? Can't be, unless one holds that subjects are incapable of recognizing melodies that they don't know how to play. In fact, the data make a very nice case for the idea that one can recognized action-generated sensory events without mapping that sensory event onto the motor system. Maybe the mirror system is "recognizing" the unseen actions that produce the melody. Ok fine, but how is this different from, or more to the point, what is the evidence that it is different from the simpler idea that frontal activation during perception of trained melodies reflects (pun intended) the motor side of a learned sensory-motor association? You pair a tone with a puff of air to the eye and pretty soon, the tone elicits an eye blink response. Is the eye blink recognizing the tone? Or the air puff? Or anything? You associate a melody with a sequence of finger movements and pretty soon the melody activates motor areas involved in coding those finger movements. What is the evidence that activation of the "mirror system" is more than simple sensory-motor association?