Conventional or threshold-hunting TMS? A tale of two SICIs

Background: In human primary motor cortex (M1), the paired-pulse transcranial magnetic stimulation (TMS) paradigm of short-interval intracortical inhibition (SICI) can be expressed conventionally as a percent change in the relative amplitude of a conditioned motor evoked potential to non-conditioned; or adaptive threshold-hunting a target motor evoked potential amplitude in the absence or presence of a conditioning stimulus, and noting the relative change in stimulation intensity. The suitability of each approach may depend on the induced current direction, which probe separate M1 interneuronal populations. Objective: To examine the influence of conditioning stimulus intensity, interstimulus interval (ISI) and current direction for adaptive threshold-hunting and conventional SICI using equivalent TMS intensities. Methods: In 16 participants (21-32 years), SICI was examined using adaptive threshold-hunting and conventional paired-pulse TMS with posterior-anterior and anterior-posterior stimulation, ISIs of 2 and 3 ms, and a range of conditioning intensities. Results: Inhibition with adaptive threshold-hunting was greater for anterior-posterior stimulation with an ISI of 3 ms (23.6 +/- 9.0%) compared with 2 ms (7.5 +/- 7.8%, P < 0.001) and posterior-anterior stimulation at both ISIs (2 ms 8.6 +/- 8.7%, 3 ms 5.9 +/- 4.8%; P < 0.001). There was an association between inhibition obtained with conventional and adaptive threshold-hunting for posterior-anterior but not anterior-posterior stimulation (2 ms only, r = 0.68, P = 0.03). Conclusions: More inhibition was evident with anterior-posterior than posterior-anterior current for both adaptive threshold-hunting and conventional paired-pulse TMS. Assessment of SICI with anteriorposterior stimulation was not directly comparable between the two approaches. However, the amount of inhibition was dependent on conditioning stimulus intensity and ISI for both SICI techniques. (C) 2018 Elsevier Inc. All rights reserved.