The earliest use of TMS involved the motor system (Barker et al., 1985). TMS can be used to clarify the relationship between cognition and action in the human brain (Oxford Handbook).

Ongoing research

The best way to describe a complex act to someone else is to demonstrate it directly and several transcranial magnetic stimulation (TMS) studies have reported facilitation of the primary motor cortex (M1) during the mere observation of actions (Alaerts et al., 2010).

Using rTMS in healthy individuals, previous work has shown that interference with the extrastriate body area impairs the discrimination of bodily forms and interference with the ventral premotor cortex impairs the discrimination of bodily actions. This double dissociation suggests that the premotor cortex is crucial for visual discriminations of actions, whereas the extrastriate body area mainly processes an actor’s body identity (Urgesi et al. 2006).

In an interesting study by Alaerts et al. (2010) excitability of M1 was shown to modulate in a muscle-specific way, such that only the cortical representation areas in M1 that control the specific muscles used in the observed lifting action became increasingly facilitated. Moreover, muscle-specific M1 facilitation was shown to modulate to the force requirements of the observed actions, such that M1 excitability was considerably higher when observing heavy object lifting compared with light object lifting. Overall, these results indicate that different levels of observed grip force are mirrored onto the observer’s motor system in a highly muscle-specific manner. The measured force-dependent modulations of corticospinal excitability in M1 are hypothesized to be functionally relevant for scaling the observed grip force in the observer’s own motor system. In turn, this mechanism may contribute, at least partly, to the observer’s ability to infer the weight of the lifted object.

References

• Barker et al., 1985
• The Oxford Handbook of Transcranial Magnetic Stimulation
• Urgesi et al., J Neurosci, 2006
• Alaerts et al., Eur J Neurosci, 2010