Impact of the number of cycles per pulse on the efficiency of single-pulse transcranial magnetic stimulation

Introduction: Waveform and current direction of transcranial magnetic stimulation (TMS) at the primary motor cortex essentially influence both amplitudes of motor evoked potentials (MEP) and resting motor threshold (RMT). We investigated the effect of concatenating multiple biphasic waves on these parameters.

Methods: We measured RMT and input-output (IO) curves in 8 healthy volunteers using a burst stimulator prototype which allows the application of polyphasic waves consisting of 1 to 6 full biphasic sinus waves. For IO curves, we recorded motor evoked potentials (MEP) in steps of 10% RMT (ranging from 90% to 140% RMT) with 10 MEPs per step. In 4 subjects we also reversed the current direction of TMS.

Results: When using polyphasic waves, RMT was significantly decreased and MEP amplitudes significantly increased compared to a single biphasic wave (RMT: 79.6% at 1 wave, 68.0% at 6 waves, p<0.001; MEP: 1.29 mV at 1 waves, 2.84 mV at 6 waves, p<0.001). The slope of IO curves was smallest with a single biphasic wave. Reversal of current direction modulated amplitude and latency of MEPs for single but not for polyphasic waves consisting of 6 waves.

Conclusion: Concatenating multiple biphasic waves led to a decreased RMT and increased MEP amplitudes. This effect is already present with two waves and can be only minimally enhanced by adding more waves. In addition, differences between current directions are less pronounced for polyphasic waves.