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

John Cirillo; John G Semmler; Ronan A Mooney; Winston D Byblow

doi:10.1016/j.brs.2018.07.047

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

Brain Stimul. 2018 Nov-Dec;11(6):1296-1305. doi: 10.1016/j.brs.2018.07.047. Epub 2018 Jul 18.

Authors

John Cirillo¹, John G Semmler², Ronan A Mooney¹, Winston D Byblow³

Affiliations

¹ Department of Exercise Sciences, University of Auckland, Auckland, New Zealand; Centre for Brain Research, University of Auckland, Auckland, New Zealand.
² Discipline of Physiology, Adelaide Medical School, University of Adelaide, Adelaide, Australia.
³ Department of Exercise Sciences, University of Auckland, Auckland, New Zealand; Centre for Brain Research, University of Auckland, Auckland, New Zealand. Electronic address: w.byblow@auckland.ac.nz.

PMID: 30033042
DOI: 10.1016/j.brs.2018.07.047

Abstract

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 anterior-posterior 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.

Keywords: Adaptive threshold-hunting; Current direction; Interstimulus interval; Short-interval intracortical inhibition; Transcranial magnetic stimulation.

Publication types

Comparative Study
Research Support, Non-U.S. Gov't

MeSH terms

Adult
Conditioning, Classical / physiology
Conditioning, Operant / physiology
Electromyography / methods
Evoked Potentials, Motor / physiology*
Female
Humans
Interneurons / physiology
Male
Motor Cortex / physiology*
Neural Inhibition / physiology*
Transcranial Magnetic Stimulation / methods*