Conditioning the cortical silent period with paired transcranial magnetic stimulation.

Background: A 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.

Objective: We investigated whether it is possible to modulate the SP resulting from a TS by delivering a sub-threshold CS 1-5 ms later.

Methods: Paired-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.

Results: Paired 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.

Conclusion: The 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 GABAA 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.