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Previous research has demonstrated that anatomical complexities of cortical regions determine transcranial magnetic stimulation (TMS) induced electric fields in the brain, which impact the response to TMS-based therapies. The current objective was to investigate the effect of individual neuroanatomy on first dorsal interosseous (FDI) and biceps brachii resting motor thresholds (RMT) in response to TMS.
Cross-sectional study using a convenience sample.
Ten healthy individuals (7 females, 23.5 ± 5 years) participated in this study. Each participant completed two TMS sessions (one each targeting the FDI and biceps cortical hotspots) and an MRI of the head on separate days.
Main Outcome Measures
RMTs were determined using a Magstim Super BiStim stimulator via a 70 mm figure-of-eight coil to the left primary motor cortex and electromyography signals were measured from right FDI and biceps. Head models were generated based on T1 & T2 weighted MRI, while diffusion tensor imaging was used to determine fiber tract geometry for FDI and biceps corticospinal tracts. Via the models, we established neuroanatomical parameters including: fiber tract surface area (FTSA), tract fiber count (TFC), and brain scalp distance (BSD). Cortical electric field strength (EFS) was calculated using simulated stimulation of head models and finite element analysis.
For the FDI, RMT was dependent on the interaction between individually modeled parameters: 1) EFS and FTSA (p = 0.036), and 2) EFS and TFC (p = 0.004). For the biceps, RMT was dependent on the interaction between 1) EFS and FTSA (p = 0.022) and 2) EFS and BSD (p = 0.010).
Our study results show that MRI-based measures of neuroanatomy, specifically cortical architecture and tract anatomy, differentially impact how the motor system responds to TMS. MRI-based modeling of individual neuroanatomy may be a useful approach to select appropriate motor targets when designing TMS based therapies.
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