Increasing pulse widths and intensity increase the efficacy of high frequency rTMS in inducing excitatory aftereffects

Introduction: High frequency repetitive transcranial magnetic stimulation induces excitation (i.e. post-stimulation increase of motor evoked potential (MEP) amplitudes) when applied to the motor cortex. The effects differ according to pulse width, probably by selectively stimulating distinct segments of neurons due to their different membrane properties. Here we focus on the aftereffects generated with high frequency controllable pulse TMS (cTMS). Objectives: To investigate the influence of different stimulation intensities, pulse widths and direction on the excitatory plastic aftereffects of high frequency (HF) rTMS reflected by MEP amplitudes. Methods: Using a controllable pulse stimulator TMS (cTMS), we stimulated the hand motor cortex of 20 healthy subjects with 5 Hz rTMS applying 1200 bidirectional pulses with the main component widths of 80 and 120 microseconds. 80% resting motor threshold (RMT) intensity was investigated for both anterior-posterior (AP) and posterior-anterior (PA), and in a second experiment at 90% RMT in the AP direction. Results: 80% HF rTMS did not produce any significant excitation in either AP or PA direction. 90% RMT AP stimulation with 120 microsecond-wide pulses induced more significant excitatory aftereffect, when compared to the 80 microsecond-wide pulses. Conclusions: HF rTMS with wider pulses is more effective in producing excitatory aftereffects, which may be due to the different membrane properties of the various neuronal segments such as dendrites whose activation plays a major role in long term potentiation. Significance: The findings here may contribute to better results in future clinical studies performed with longer cTMS pulses.