Multi-locus TMS transducer for probing orientation dependency of mechanisms in the primary motor cortex

Mechanisms underlying short-interval intracortical inhibition (SICI) and facilitation (SICF) in the motor cortex seem to be sensitive to conditioning-pulse orientation. Available devices require manual coil rotation to adjust the electric field orientation, making it impractical to apply two consecutive pulses with different orientations in millisecond scale. We developed a multi-locus transcranial magnetic stimulation (mTMS) transducer to control the pulse orientation electronically and investigated the orientation dependency of the mechanisms associated with the motor evoked potential (MEP) generation, SICI, and SICF. The mTMS transducer was manufactured by stacking two perpendicular figure-of-eight coils. The pulse direction is changed electronically by adjusting the amplitudes of simultaneously-triggered pulses in these coils, driven by a custom stimulator. Thirteen healthy subjects participated in the study. The hotspot, optimal orientation, and resting motor threshold (rMT) of the right abductor pollicis brevis muscle were measured. MEP amplitude stimulus-orientation curve was built by applying five 110%-rMT pulses every 3°; 0° as the current flowing in posterior-anterior (PA) direction perpendicular to the central sulcus. To assess SICI and SICF, paired-pulses were applied with 0.5, 2.5, 6, and 8 ms interstimulus intervals. Conditioning- and test-pulse intensities were 80- and 110%-rMT, respectively. Twenty paired-pulses were administered in each condition, with the conditioning-pulse orientation set to anterior-posterior (AP), PA, lateral-medial, or medial-lateral. Test pulses were always PA-oriented. The MEP amplitudes had consistent maxima around the PA and AP orientations. SICI was evident for conditioning pulses in all directions. SICF was observed only for PA- and AP-oriented conditioning pulses. Neural elements responsible for SICF seem to be aligned with the cortical columns, whereas a distinct mechanism of SICI activation makes it independent of pulse direction. Possibly, SICF and SICI mechanisms are initiated in the sulcus wall and the gyrus, respectively. The demonstrated control of stimulus orientation opens a possibility for new TMS protocols.