RMT is a baseline characteristic of corticospinal excitability commonly used to adjust stimulation parameters for other TMS protocols. Conventionally, RMT is defined as stimulus intensity required to obtain a response in 50% of consecutive trials, and currently two probabilistic approaches – relative frequency and adaptive maximum-likelihood – are recommended by the International Federation of Clinical Neurophysiology [1].
Alternatively, motor threshold can be defined as stimulation intensity required to maintain a response of a specific size and obtained by threshold-tracking procedures [2]. Here, we compare threshold-tracking and its reliability to well-established RMT estimation methods.
RMT with conventional cut-off value of 0.05 mV was measured in 24 healthy volunteers (11 men; median age 22 years, range 18-55 years; all self-reported right-handed) from first dorsal interosseous muscle of the dominant hand using three methods: i) relative frequency (RF, 10/20 positive trial rule); ii) best PEST (parameter estimation by sequential testing) – an adaptive maximum-likelihood procedure; and iii) threshold-tracking (TT). Measurements were repeated twice by a single operator on the same day. Data is presented as mean ± standard deviation.
No difference in mean group RMT estimates was observed between the methods (RF 50.6±8.3, PEST 50.3±8.2, TT 50.9±9.1 % maximum stimulator output, p>0.05), but the duration of procedure differed considerably (number of stimuli required: RF 60±26, PEST 19±2, TT 12±4, p<0.05). All methods had excellent reproducibility (intraclass correlation coefficients: RF 0.89, PEST 0.90, TT 0.88) and similar test-retest repeatability (smallest detectable change: RF 7.8%, PEST 7.3%, TT 8.9% maximum stimulator output).
In conclusion, all methods provide the same value of RMT, but threshold-tracking offers an improved speed without fundamentally compromising the reliability of repeated measurements. While probabilistic methods provide point estimates only, threshold-tracking allows uninterrupted monitoring of RMT and therefore opens new avenues in TMS research and its clinical application.