Effect of Fiber Tracts and Depolarized Brain Volume on Resting Motor Thresholds During Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a treatment procedure for some neuropsychiatric disorders, and it has been used for brain mapping, as well as diagnosis and treatment of neuromuscular dysfunctions. There is a disconnect between TMS modeling and clinical data: several groups have reported the simulated induced electric field and measured resting motor threshold (RMT) with inconsistent results in the relationship between RMT and brain scalp distance. This necessitates the use of simulation parameters that further account for individual differences in neuroanatomy. We recruited ten healthy subjects and obtained empirical RMT, magnetic resonance imaging (MRI), and diffusion tensor imaging (DTI). We developed anatomically accurate brain models from MRI and simulated TMS to determine the percent depolarized volume of gray matter (DVG) from TMS-induced electric fields. Corticospinal fiber tracts were extracted from the primary motor cortex from DTI to obtain fiber tract surface areas (FTSAs) for each participant. Linear mixed-effects models were used to evaluate the effect of DVG and FTSA on RMT. We report that DVG correlates with RMT when accounting for corticospinal FTSA.