Investigation of Resting Motor Threshold Variabiliy in Schizophrenia Patients during Transcranial Magnetic Stimulation

Transcranial magnetic stimulation (TMS) is a noninvasive neuromodulation technique utilized to treat several neuropsychiatric disorders. Schizophrenia patients typically experience positive and negative symptoms which are commonly treated with antipsychotic medications. Repetitive (r)TMS is being tested as a treatment to alleviate these symptoms by increasing the cortical stimulation. TMS stimulation strength is determined by the resting motor threshold (RMT) which is collected from single pulse stimulation to the M1 brain region. However, the effective stimulation intensity within the cortex is dependent on many individual factors. In this study, 23 Schizophrenia participants' magnetic resonance imaging (MRI) were used to create anatomically accurate head models. These head models were then used to calculate the coil to cortex distance (CCD), and finite element analysis (FEA) was performed to calculate the induced electric field on the gray matter. Functional MRI was also performed to evaluate functional connectivity. These MRI derived parameters were then evaluated to establish their relationship with empirically collected RMT. CCD did not correlate with RMT suggesting CCD alone is not indicative of patient specific cortical architecture. Simulated electric field strength (EFS) negatively correlated with CCD suggesting the induced electric field in the brain is influenced by the CCD. We also observed a negative linear relationship between RMT and functional connectivity suggesting functional connectivity influences TMS treatment outcomes. Future investigations into RMT variability should incorporate quantitative fMRI Independent Component data with a larger sample size to further establish the relationship with RMT.