Conditioning the Cortical Silent Period with Paired Transcranial Magnetic Stimulation

BackgroundA single supra-threshold pulse of transcranial magnetic stimulation (TMS) over human motor cortex elicits multiple descending volleys (I-waves) that generate a motor evoked potential (MEP) followed by a period of electromyographic silence in the tonically contracted target muscle (silent period; SP). A sub-threshold conditioning stimulus (CS) delivered at inter-pulse intervals (IPIs) of 1-5 ms after a supra-threshold test stimulus (TS) conditions I-waves elicited by TS and can increase MEP amplitude (short-interval intracortical facilitation; SICF), however its effect on the SP remains unknown.ObjectiveWe investigated whether it is possible to modulate the SP resulting from a TS by delivering a sub-threshold CS 1–5 ms later.MethodsPaired-pulse TMS was delivered while subjects performed slight contraction of the first dorsal interosseous muscle. SICF and SP duration were measured at each IPI and compared to amplitude-matched MEPs evoked by single-pulse TMS.ResultsPaired stimulation at IPI 2–5 ms prolonged the SP by 21 ± 3% (P < 0.001) but had no effect on MEP amplitude. At shorter IPIs the CS increased MEP amplitude (by 170 ± 31%), but the SP was not prolonged when compared to an amplitude-matched single-pulse stimulus.ConclusionThe SP can be modified by a CS applied during the early phase of its genesis. We suggest that this is in keeping with an early GABA_A contribution to the SP, and it is possible that this new conditioning paradigm may offer another means for probing the excitability of cortical inhibitory networks in human motor cortex.