I'm only about halfway through so far but already I find the book both useful in terms of its summary of the functional anatomy of the macaque motor system and frustratingly sloppy in terms of its theoretical logic.
Let me provide one example of the latter. At the outset of the book the authors describe the functional properties motor neurons (not necessarily mirror neurons) in macaque area F5. They argue that F5 motor cells
code motor acts (i.e., goal-directed movement) and not individual movements (p. 23).
As evidence they note that
...many F5 neurons discharge when the monkey performs a motor act, for example when it grasps a piece of food, irrespective of whether it uses its right or left hand or even its mouth ... [and] a particular movement that activates a neuron during a specific motor act does not do so during other seemingly related acts; for example, bending the index finger triggers a neuron when grasping, but not when scratching. (p. 23)
Therefore the activity of these neurons cannot be adequately described in terms of pure movement, but taking the efficacy of the motor act as the fundamental criterion of classification they can be subdivided into specific categories, of which the most common are 'grasping-with-the-hand-and-the-mouth', 'grasping-with-the-hand', 'holding', 'tearing', 'manipulating', and so on. (p. 23)
So the claim is that F5 cells are coding something higher-level that is defined by the goal, the "efficacy", of movement.
Clearly, F5 cells are coding something that is at least one-step removed from specific movements (e.g., finger flexion), but the leap from this observation to the idea that it is coding categories or goals such as 'tearing' is suspect. Perhaps these complex movements are being coded -- separately for the mouth and hand, for tearing in one manner versus another, etc. -- by the population of cells in F5 rather than in individual cells. In other words, the fact that a single cell responds to grasping with the hand and grasping with the mouth doesn't necessarily mean that it is coding an abstract concept of grasping.
But we don't need to argue with Rizzolatti and Sinigaglia on this theoretical point because they argue against their own view rather convincingly (although unwittingly) on empirical grounds. Specifically, in contrast to the claim that F5 cells code goal-directed actions, they give examples of how these cells code specific, albeit complex, movements.
Most F5 neurons ... also code the shape the hand has to adopt to execute the act in question... (p. 25)
This strikes me as a rather specific individual movement that, for example, would not apply to the same "act" executed by the mouth. More pointedly though, in their discussion of mirror neurons in F5, Rizzolatti and Sinigaglia make a big deal of cells that show a strict relation between the observed and executed act. They provide a striking example:
...the monkey observes the experimenter twisting a raisin in his hands, anti-clockwise and clockwise, as if to break it in two: the neuron discharges for one direction only. (p. 82)
So here is a case where two movements have the same goal (breaking the raisin in two), but the F5 cell only fires in response to one of the movements. Apparently this cell is coding movements not goals.
Has anyone else read Rizzolatti and Sinigaglia's book? Any thoughts?