TMS to motor cortex affects lexical decision to body-part related words - What does this tell us?

One of the most impressive demonstrations of the functional relevance of motor cortex to action-word processing comes from a TMS study by Friedemann Pulvermuller and colleagues (2005, European Journal of Neuroscience, 21:793-97). These researchers stimulated motor cortex for hand or leg areas while subjects performed a lexical decision task. TMS to hand areas led to faster reaction times to hand-related words (e.g., pick) than leg-related words (e.g., kick), whereas the reverse held for TMS to leg areas. This seems to be a clear case of motor cortex involvement in motor-related language processing.

Does this mean that the semantics of the word kick is encoded in the motor representation of kicking? Not at all. All it means is that there is an association between the semantics of the word kick and motor programs associated with kicking. It tells us nothing about the nature of the association. Suppose the lexical/conceptual representation of the word kick is stored in neural ensemble A which is distinct from, but connected to, the motor programs for the action kicking which is stored in neural ensemble M in motor cortex. Stimulation of M results in partial activation of A to which it is connected, thus priming the lexical/conceptual representation, which in turn results in faster RTs.

The simplicity and intuitive appeal of embodied accounts of complex processes make it all too easy to accept empirical results as indisputable evidence in support of such accounts when in fact there is much to dispute.

Friedemann Pulvermuller, Olaf Hauk, Vadim V. Nikulin, Risto J. Ilmoniemi (2005). Functional links between motor and language systems European Journal of Neuroscience, 21 (3), 793-797 DOI: 10.1111/j.1460-9568.2005.03900.x