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Abstract:
Knowledge of the type and position of the neural population stimulated by transcranial magnetic stimulation (TMS) is pivotal for a better understanding of the underlying physiological mechanisms and for advancing a systematic targeting and dosage approach. We will provide evidence that realistic field modeling informed by individual structural MRI, combined with measurements of the elicited motor evoked potentials (MEPs) can help to pinpoint the stimulated brain region and reveal the stimulation depth. We will further show that field modeling can contribute to the understanding of the neural origin of inter-individual differences in MEP latencies.
In 9 healthy participants, we systematically varied the orientation of a standard figure-8 coil and compared the MEP threshold changes for monophasic TMS with the electric field changes in the motor cortex that were calculated using the Finite-Element Method (FEM). In addition, in another 9 participants, we used three figure-8 coils having different field decays to correlate the differences in the electrophysiological thresholds for current direction posterior-to-anterior (PA) with the differences in the calculated field distributions. These two experiments consistently showed that TMS stimulates the region of the crown and posterior lip of the precentral gyrus and that the maximal electric field strength in this region is significantly related to the MEP threshold. Finally, in 13 participants, we tested the correlation between the field in the motor cortex and the MEP onset delays for anterior-to-posterior (AP) current orientation. We demonstrated that the part of the motor cortex found in the two prior experiments also exhibited a significant negative correlation between the onset delays and the field strength.
The results of our experiments validate the FEM-based field calculations by demonstrating a significant correlation between the electric field estimates and the physiological response to TMS. They further help to resolve uncertainties on the stimulation depth of TMS. They suggest that TMS at the optimal current orientation might mainly stimulate subarea BA 4a of the motor cortex at the transition from the posterior wall to the crown of the precentral gyrus. In addition, in those subjects in which lower field strengths are sufficient to induce a motor response for current orientation AP, later I-waves are recruited. This suggests that local inter-individual differences in cortical organization in the upper part of M1 might underlie the observed latency differences.
