Effect of Neuroanatomy on Motor Evoked Potentials after Intermittent Theta Burst Stimulation

Abstract

Introduction: Transcranial magnetic stimulation (TMS) can be affected by anatomical features. Therefore, neuroanatomy could influence corticomotor excitability after intermittent theta burst stimulation (iTBS). The relationship between iTBS aftereffects and neuroanatomy was investigated via motor evoked potentials (MEPs) and finite element analysis modeling (FEM) of TMS of MRI-derived head models. Targets were the biceps brachii and first dorsal interosseus (FDI) due to their relevance in activities of daily living. Experimental Details: Ten nonimpaired individuals participated, completing sham-controlled iTBS sessions (one each targeting the biceps and FDI cortical hotspots). MEPs were acquired at an intensity of 120% of resting motor threshold (RMT) while iTBS was delivered at 80% of active motor threshold, both via 70 mm figure-of-eight coils targeting the primary motor cortex. MEPs were recorded with surface electromyography. Each participant underwent an MRI of the head, and models were generated from T1 & T2 weighted images. Fiber tract geometry was extracted from diffusion tensor images for the biceps and FDI corticospinal tracts. Neuroanatomical parameters established were fiber tract surface area (FTSA), tract fiber count (TFC), and brain scalp distance (BSD) at the point of stimulation. Cortical electric field strength (EFS) was obtained by FEM of TMS of the head models. A linear mixed effects model was used to assess effects of these parameters and RMT on MEPs recorded after iTBS. RMT was included as a metric of the muscle's measured responsiveness to TMS. Results: iTBS targeting the biceps was faciliatory (p = 0.013), and dependent on FTSA (p < 0.001) and TFC (p < 0.001). There was no effect of iTBS on the FDI (p = 0.223) but individual changes in corticomotor excitability due to stimulation type scaled with RMT (p < 0.001), EFS (p = 0.001), BSD (p = 0.004), and FTSA (p = 0.011). Conclusion: There were unique relationships between neuroanatomical parameters and empirical iTBS response. MRI-based measures of target-specific neuroanatomy impact how the motor system responds to iTBS, making MRI-based modeling potentially useful in selection of motor targets when designing iTBS protocols.