Where does transcranial magnetic stimulation (TMS) stimulate? Modelling of induced field maps for some common cortical and cerebellar targets
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Computational models have been be used to estimate the electric and magnetic fields induced by transcranial magnetic stimulation (TMS) and can provide valuable insights into the location and spatial distribution of TMS stimulation. However, there has been little translation of these findings into practical TMS research. This study uses the International 10-20 EEG electrode placement system to position a standard figure-of-eight TMS coil over 13 commonly adopted targets. Using a finite element method and an anatomically detailed and realistic head model, this study provides the first pictorial and numerical atlas of TMS-induced electric fields for a range of coil positions. The results highlight the importance of subject-specific gyral folding patterns and of local thickness of subarachnoid cerebrospinal fluid (CSF). Our modelling shows that high electric fields occur primarily on the peaks of those gyri which have only a thin layer of CSF above them. These findings have important implications for inter-individual generalizability of the TMS-induced electric field. We propose that, in order to determine with accuracy the site of stimulation for an individual subject, it is necessary to solve the electric field distribution using subject-specific anatomy obtained from a high-resolution imaging modality such as MRI.
KeywordsTranscranial magnetic stimulation Electric field Cerebrospinal fluid Finite element method Site of stimulation
J. Bijsterbosch is supported by a Medical Research Council Ph.D. studentship. We thank Dr. Pedro Crespo Valero from SEMCAD for his assistance in optimizing the SEMCAD Talairach tool for use in this work. A.T.B. and J.D.B. contributed equally to this work and jointly discussed the results and implications. A.T.B. conceived the study, designed and ran the models and commented extensively on the manuscript at all stages. J.D.B. contributed to the design of the models, designed and performed the post-processing of the modelling data and wrote the manuscript. K-H.L. and P.W.R.W. supervised J.D.B. and commented on the manuscript.
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