Alterations in Motor Cortical Excitability Induced by Peripheral Stimulation With Magnetic Stimulation

The conditioning of cortical excitability by transcranial magnetic stimulation (TMS) has become a valuable technique to promote recovery of motor function after stroke. As TMS is not used in all patients, we investigated the hypothesis that peripheral stimulation may have an adjustment effect on motor cortical excitability. Our experimental paradigm was divided into three phases. In the first phase, TMS was delivered to the left or right primary motor cortex to induce a motor evoked potential (MEP) from the contralateral first dorsal interosseous muscle. The measured MEPs in this phase were used to evaluate the effect of peripheral stimulation. In the second phase, 1 Hz magnetic stimulation was applied over the contralateral or ipsilateral forearm for motor cortex as peripheral stimulation. In the third phase, the MEP was evoked by TMS and recorded using the same setting as the first phase. We found that a decrease in MEP amplitude was observed in the left motor cortex following peripheral stimulation over the right forearm. By contrast, the MEP amplitude was not altered in the right motor cortex by peripheral stimulation over the left forearm. An increase in MEP amplitude was observed in the ipsilateral motor cortex induced by peripheral stimulation over the left or right forearm. We also found that by changing the MEP amplitude, the motor cortex excitability varied according to magnetic stimulation of the forearm. These data suggest that peripheral stimulation may have an adjustment effect on motor cortical excitability, via changes in the stimulus site.