Some interesting commentary on motor somatotopy (or lack thereof) from Greig de Zubicaray's website which I've copied and pasted here (with permission of course):
We're very interested in claims of a so-called "semantic somatotopy" in primary and premotor cortex (i.e., overlapping activity for executing/observing actions performed with a specific effector and retrieving word meanings related to that effector). See Tash Postle's paper in NeuroImage last year in which the peak maxima from neuroimaging studies cited as evidence for this claim were plotted against cytoarchitectonic maps of BA4 and BA6 in the language dominant left hemisphere. Friedemann Pulvermüller and colleagues have a new fMRI paper using cluster analysis in press in Human Brain Mapping in which they conclude that "The motor-premotor region was dominated by semantic somatotopy... with face, arm, and leg word activation foci at inferior lateral (-49/11/16), dorsolateral (-32/-5/52), and dorsal sites (-19/-21/61)." So, here are 10 mm spheres centred on those peak maxima plotted in relation to the cytoarchitectonic maps of BA4 (red) and BA6 (blue) from Eickhoff et al. (2006). Again, not much congruency with respect to the actual cortical motor areas, and as motor execution and/or observation conditions weren't included in the study, we have no evidence that these clusters are representative of the actual neuronal ensembles used by the participants to perform effector movements. Interestingly, though, the leg peak is in BA6 this time.
Pulvermüller, F., Kherif, F., Hauk, O., Mohr, B., & Nimmo-Smith, I. (in press). Distributed cell assemblies for general lexical and category-specific semantic processing as revealed by fMRI cluster analysis, Human Brain Mapping.
Eickhoff, S.B., Heim, S., Zilles, K., & Amunts, K., (2006). Testing anatomically specified hypotheses in functional imaging using cytoarchitectonic maps. NeuroImage, 32, 570–582.