Where and what TMS activates: Experiments and modeling.

Background: Despite recent developments in navigation and modeling techniques, the type and location of the structures that are activated by transcranial magnetic stimulation (TMS) remain unknown.
Objective: We studied the relationships between electrophysiological measurements and electric fields induced in the brain to locate the TMS activation site.
Methods: The active and resting motor thresholds of the first dorsal interosseous muscle were recorded in 19 subjects (7 female, 12 male, age 22 ± 4 years) using anteromedially oriented monophasic TMS at multiple locations over the left primary motor cortex (M1). Structural MR images were used to construct electric field models of each subject's head and brain. The cortical activation site was estimated by finding where the calculated electric fields best explained the coil-location dependency of the measured MTs.
Results: The experiments and modeling showed individual variations both in the measured motor thresholds (MTs) and in the computed electric fields. When the TMS coil was moved on the scalp, the calculated electric fields in the hand knob region were shown to vary consistently with the measured MTs. Group-level analysis indicated that the electric fields were significantly correlated with the measured MTs. The strongest correlations (R ²  = 0.69), which indicated the most likely activation site, were found in the ventral and lateral part of the hand knob. The site was independent of voluntary contractions of the target muscle.
Conclusion: The study showed that TMS combined with personalized electric field modeling can be used for high-resolution mapping of the motor cortex.
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