Triad TMS of the human motor cortex.

Usually, cortical rhythmic activities are studied with local field potentials. To overcome small amplitude of EEGs easily disturbed by several factors, we developed a new method to study motor cortical rhythm using Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS). We, here, review triad-conditioning TMS technique for investigating the intrinsic rhythm of the human primary motor cortex (M1). MEP was recorded from the first dorsal interosseous muscle (FDI). TMS was applied over the M1 to study its frequency dependency. In the intervention condition, the subthreshold, same intensity three conditioning stimuli separated by a certain interval were given prior to the supra-threshold test stimulus. In the control condition, the test stimulus was given alone. MEPs were compared between the two conditions. In healthy volunteers, triad-conditioning stimulus (TCS) at an interval of 25 ms induced MEP facilitation, whereas the other intervals TCS induced no facilitation. This frequency dependent facilitation may reflect some intrinsic rhythm of M1 (25 ms, i.e. 40 Hz). In cortical myoclonus, the 40 ms TCS induced facilitation whereas 25 ms TCS induced no facilitation, which is consistent with abnormal rhythm of M1 at 25 Hz (40 ms interval) reported previously. In Parkinson's disease (PD), 25 ms TCS evoked no facilitation. Triad-conditioning TMS may enable us to investigate the intrinsic rhythmic activity of M1 and its abnormality.