A physiological characterization of biphasic transcranial magnetic stimulation

Question: In transcranial magnetic stimulation (TMS), two pulse waveforms are mainly used to study cortical function. Although monophasic stimuli are optimal to measure cortical excitability, the biphasic pulse waveform is increasingly used to induce cortical plasticity and to treat neurological as well as psychiatric illnesses.

Methods: We used a novel stimulation device (flexTMS) to study variable pulse configurations in young healthy volunteers. Primary outcome measure was cortical resting motor threshold (RMT). We determined the strength-duration relationship for single-pulse biphasic TMS using pulse durations between 120 and 400µs. To further characterize the biphasic stimulus, we systematically varied amplitude and current direction of the first and second part of the stimulus waveform.

Results: The strength-duration relation has a parabolic shape for biphasic stimulation with a minimum at a pulse length of approximately 240µs. Varying the amplitude of the part inducing a posterior-anterior oriented current considerably raised RMT, whereas changes of the part which induces an anterior-posterior current had little effect