Fast acquisition of resting motor threshold with a stimulus-response curve - Possibility or hazard for transcranial magnetic stimulation applications?

Elisa Kallioniemi; Friedemann Awiszus; Minna Pitkänen; Petro Julkunen

doi:10.1016/j.cnp.2021.10.005

Fast acquisition of resting motor threshold with a stimulus-response curve - Possibility or hazard for transcranial magnetic stimulation applications?

Clin Neurophysiol Pract. 2021 Dec 17:7:7-15. doi: 10.1016/j.cnp.2021.10.005. eCollection 2022.

Authors

Elisa Kallioniemi¹, Friedemann Awiszus², Minna Pitkänen³, Petro Julkunen^{3

4}

Affiliations

¹ Department of Psychiatry, UT Southwestern Medical Center, Dallas, TX, United States.
² Neuromuscular Research Group at the Department of Orthopaedics, Otto-von-Guericke University, Magdeburg, Germany.
³ Department of Applied Physics, University of Eastern Finland, Kuopio, Finland.
⁴ Department of Clinical Neurophysiology, Kuopio University Hospital, Kuopio, Finland.

Abstract

Objective: Previous research has suggested that transcranial magnetic stimulation (TMS) related cortical excitability measures could be estimated quickly using stimulus-response curves with short interstimulus intervals (ISIs). Here we evaluated the resting motor threshold (rMT) estimated with these curves.

Methods: Stimulus-response curves were measured with three ISIs: 1.2-2 s, 2-3 s, and 3-4 s. Each curve was formed with 108 stimuli using stimulation intensities ranging from 0.75 to 1.25 times the rMT_guess, which was estimated based on motor evoked potential (MEP) amplitudes of three scout responses.

Results: The ISI did not affect the rMT estimated from the curves (F = 0.235, p = 0.683) or single-trial MEP amplitudes at the group level (F = 0.90, p = 0.405), but a significant subject by ISI interaction (F = 3.64; p < 0.001) was detected in MEP amplitudes. No trend was observed which ISI was most excitable, as it varied between subjects.

Conclusions: At the group level, the stimulus-response curves are unaffected by the short ISI. At the individual level, these curves are highly affected by the ISI.

Significance: Estimating rMT using stimulus-response curves with short ISIs impacts the rMT estimate and should be avoided in clinical and research TMS applications.

Keywords: APB, abductor pollicis brevis; EMG, electromyography; ISI, interstimulus interval; Interstimulus interval; MEP, motor evoked potential; MRI, magnetic resonance imaging; MSO, maximum stimulator output; Motor evoked potential; Motor threshold; SI, stimulation intensity; Stimulus-response curve; TMS, transcranial magnetic stimulation; rMT, resting motor threshold; rMTRR, resting motor threshold estimated with the Rossini-Rothwell method; rMTestimate, resting motor threshold estimated with stimulus–response curves; rMTguess, resting motor threshold estimated with prior information and three scout pulses; rMTthreshold, resting motor threshold estimated with the threshold-hunting method; rMTtrue, true resting motor threshold in simulations.