T029 Where do we stimulate M1? A combined neurophysiological and modelling approach

Publication date: March 2017
Source:Clinical Neurophysiology, Volume 128, Issue 3
Author(s): A. Thielscher
Much of our knowledge on the physiological mechanisms of transcranial magnetic stimulation (TMS) stems from studies which targeted the human motor cortex. Very surprisingly, however, it is still unclear which part of M1 is stimulated by TMS when a muscle twitch is elicited. Considering that the motor cortex consists of functionally and histologically distinct subareas, this also renders the hypotheses on the physiological TMS effects uncertain.I will report on two recent studies which combined electrophysiological measurements of muscle responses to TMS with realistic estimates of the induced electric field, based on the finite-element method (FEM) and MRI-derived individual head models. In the first study, the orientation of a standard figure 8 coil was systematically varied and the field changes in different subparts of the motor cortex were compared with the electrophysiological threshold changes. In the second study, three figure 8 coils having different field decays were used and the differences in the electrophysiological thresholds were correlated with the differences in the calculated field distributions. Both studies consistently demonstrate that TMS stimulates the region around the gyral crown and that the maximal electric field strength in this region is significantly related to the electrophysiological response.Our studies are one of the most extensive comparisons between FEM-based field calculations and physiological TMS effects so far, being based on data for two hand muscles in 9 subjects. The results help to improve our understanding of the basic mechanisms of TMS. They also pave the way for a systematic exploration of realistic field estimates for dosage control in TMS.



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