Shape of the Pulse: Pulse Width and Current Direction Effects on Motor Evoked Potentials Using a Cobot-assisted...

Transcranial magnetic stimulation (TMS) is a noninvasive technique for stimulating the cerebral cortex. The purpose of the study is to investigate the effect of current direction and pulse width on motor evoked potentials (MEPs), using a controllable TMS (cTMS) stimulator in combination with cobot-assisted neuronavigation. The setup allows a modulation of pulse width and current direction within one TMS system while precise and reliable coil placement motor cortex is ensured.

In 30 healthy participants, MEPs were recorded from the first dorsal interosseous muscle. Biphasic pulses with different initial phase durations (280 and 160 μs) and current directions (posterior-anterior [PA] and anterior-posterior [AP] in second phase) were used both for determining resting motor threshold (RMT) and MEPs. To assess MEPs, 100 pulses were applied with an intensity of 110% RMT in three conditions (PA_280μs, AP_280μs, PA_160μs).

Stimulation with the cTMS device was well tolerated, with no serious adverse events or uncommon side effects. The RMT differed significantly among pulse widths, indicating an inversely proportional relationship of pulse width and stimulation intensity. Differences in RMT and MEP readout show sensitivity of the motor cortex and indicate AP-PA currents as more effective than PA-AP currents when adjusted to threshold. For pulse width, no differences were found for electromyography readout, indicating shorter pulses as more efficient given they produce the same level of electrophysiologic response but with less energy loss.

This study emphasizes that TMS outcomes depend on the current direction and pulse width of stimulation parameters. The finding that shorter pulses achieve equivalent responses with lower energy use suggests more efficient stimulation protocols, whereas the sensitivity to current direction underlines the need for parameter optimization to improve reliability and comparability across studies and devices. These insights support the improvement of TMS protocols in both research and clinical applications.