Differential modulation of motor evoked potential and silent period by activation of intracortical inhibitory circuits.

Objectives: To investigate the effect of activation of intracortical inhibitory circuits, as tested by short interval (3 ms) paired-pulse transcranial magnetic stimulation (TMS) with a conditioning-test paradigm, on the electromyographic (EMG) pause (silent period, SP) following the motor evoked potential (MEP) in normal subjects. Methods: SPs and MEPs were recorded from the right first dorsal interosseous (FDI) muscle during a tonic voluntary contraction (from 70 to 90% of the maximum). Using a focal coil, we compared the SP duration after single-pulse TMS, paired-pulse TMS and single-pulse TMS of reduced intensity such as to evoke MEPs matched in size to the conditioned ones after paired-pulse TMS. In addition, we compared in a control experiment the duration of the SP following matched size MEPs evoked, respectively, by focal TMS with preferential activation of indirect I1- or I3-waves. Results: SP duration after paired-pulse TMS was significantly longer than after single-pulse TMS evoking MEPs of a similar size. In no case the SP duration was longer when focal TMS preferentially activated I1-waves. Conclusions: The conditioning sub-threshold stimulus is more powerful in reducing the MEP size than in cutting down the subsequent EMG silence, suggesting that the neural circuits underlying MEP and SP are, at least in part, different.