Effects of subthreshold transcranial magnetic stimulation on choice reaction time and correlation with motor cortical activation.

To study the effect of subthreshold transcranial magnetic stimulation (TMS), we measured choice reaction time (RT), with or without TMS, in 7 healthy participants. TMS over the hand motor area was randomly delivered at variable delays after the imperative signal, while participants performed right or left abduction of the thumb. Lateralized movement-related potentials (MRPs) were recorded in a separate session to link the TMS effect with the motor cortical circuitry. For the right hand, the coexistence of a motor specific and non-specific effects of TMS was clearly evidenced, by the RT shortening at delays of 0 and 150 ms. The response dependency of the specific TMS effect was also demonstrated through the response-locked analysis, showing a maximal shortening at 120-ms bin before EMG onset. Furthermore, the lateralized MRP commenced at about 80 ms before EMG onset, indicating that TMS influences the cortical motor circuitry around 30-40 ms before the activation of the primary motor cortex. In contrast, for the left hand, we were faced with some uncertain concepts as human handedness and hemisphere asymmetries in both measures of RT and MRP, and thus it was not possible to substantiate about the motor specific effect of subthreshold TMS.