Enhancement of Neuromodulation with Novel Pulse Shapes Generated by Controllable Pulse Parameter Transcranial Magnetic Stimulation

- To enhance neuromodulation, rTMS pulse shape can be altered via a controllable pulse parameter TMS (cTMS) device.
- Among four different pulse configurations in 1 Hz rTMS, the conventional bidirectional cosine pulse showed the weakest effect on motor cortex excitability in the measurements.
- The on average greatest reduction in excitability was observed for a unidirectional rectangular pulse.
- Differences in MEP onset latency across the various rTMS pulse configurations suggest that they may be activating distinct subsets of the cortical microcircuitry.
- The direction and amplitude of the pulses probing the MEP amplitude were observed to influence the detected excitability changes.

BackgroundStandard repetitive transcranial magnetic stimulation (rTMS) devices generate bidirectional biphasic sinusoidal pulses that are energy efficient, but may be less effective than monophasic pulses that induce a more unidirectional electric field. To enable pulse shape optimization, we developed a controllable pulse parameter TMS (cTMS) device.ObjectiveWe quantified changes in cortical excitability produced by conventional sinusoidal bidirectional pulses and by three rectangular-shaped cTMS pulses, one bidirectional and two unidirectional (in opposite directions), and compared their efficacy in modulating motor evoked potentials (MEPs) produced by stimulation of motor cortex.MethodsThirteen healthy subjects completed four sessions of 1 Hz rTMS of the left motor cortex. In each session, the rTMS electric field pulse had one of the four shapes. Excitability changes due to rTMS were measured by applying probe TMS pulses before and after rTMS, and comparing resultant MEP amplitudes. Separately, we measured the latency of the MEPs evoked by each of the four pulses.ResultsWhile the three cTMS pulses generated significant mean inhibitory effects in the subject group, the conventional biphasic cosine pulses did not. The strongest inhibition resulted from a rectangular unidirectional pulse with dominant induced current in the posterior-anterior direction. The MEP latency depended significantly on the pulse shape.ConclusionsThe pulse shape is an important factor in rTMS-induced neuromodulation. The standard cosine biphasic pulse showed the smallest effect on cortical excitability, while the greatest inhibition was observed for an asymmetric, unidirectional, rectangular pulse. Differences in MEP latency across the various rTMS pulse shapes suggest activation of distinct subsets of cortical microcircuitry.