Differences between 3 Hz monophasic and biphasic repetitive transcranial magnetic stimulation

摘要

Objective: To compare motor-evoked potentials (MEPs) between monophasic and biphasic repetitive transcranial magnetic stimulation (rTMS). Methods: Thirteen volunteers were studied. Surface EMG responses were recorded from the right FDI in three stimulation conditions. We gave both monophasic and biphasic 3-Hz rTMS over the motor cortex and 3-Hz rTMS at the foramen magnum level. We measured the size of each MEP recorded in three conditions. Results: Stimulation (3 Hz) with either monophasic or biphasic pulses evoked MEPs that gradually increased in amplitude, with the later MEPs being significantly larger than the earlier ones. Monophasic rTMS showed much more facilitation than biphasic stimulation. Stimulation at the foramen magnum level elicited fairly constant MEPs. Conclusions: These findings suggest that the facilitation occurred at the motor cortex. We hypothesize that monophasic TMS has a stronger short-term effect during repetitive stimulation than biphasic TMS because monophasic pulses preferentially activate one population of neurons oriented in the same direction so that their effects readily summate. Biphasic pulses in contrast may activate several different populations of neurons so that summation of the effects is not so clear as with monophasic pulses.