Research articleThe minimal number of TMS trials required for the reliable assessment of corticospinal excitability, short interval intracortical inhibition, and intracortical facilitation
Introduction
Transcranial magnetic stimulation (TMS) of primary motor cortex (M1) is a non-invasive technique frequently used to study the human motor system [31]. A supra-threshold single-pulse TMS (spTMS) over M1 elicits a motor-evoked potential (MEP) in the corresponding peripheral muscle providing information about the excitability of the motor cortex and its descending projections, referred to as corticospinal excitability (CSE) [1]. In paired-pulse TMS (ppTMS) the supra-threshold stimulus (test) is preceded by a subthreshold pulse (conditioning), which inhibits MEPs amplitude at inter-stimulus intervals (ISIs) of 1–6 ms and facilitates them at ISIs of 8–30 ms [22,36]. These paradigms, known as short interval intracortical inhibition (SICI) and intra-cortical facilitation (ICF), give information about inhibitory and facilitatory intracortical processes [22,36].
Today, findings from TMS assessments are expanding our understanding of the pathophysiology of neurological disorders and the effects of therapeutic interventions. However, questions persist about the variability of the measures, and definitive strategies to address this confounding variance are yet to be established. Recent studies suggest that MEP-related parameters are highly variable within and between individuals; and different stimulation paradigms (i.e. spTMS and ppTMS) have shown different levels of reproducibility [4,10,17,30]. The inconsistency observed in TMS outcomes have been attributed to numerous technical and physiological factors such as coil positioning methods [10,18], stimulus intensity [30], inter pulse intervals [25], and the background neural activity [28]. To minimize variability, a generally accepted method is to elicit a number of MEPs and use their average for analysis purposes [9,15,37]. However, due to various technical and physiological limitations, such as inducing plastic changes with higher numbers of stimuli [7], eliciting numerous MEPs is not always feasible. In addition, recent studies have shown that estimates of MEP values tend to plateau with averaging, suggesting that redundant stimuli could be avoided by determining the optimal number for stable averages [9,15].
The only study that has systematically investigated the effect of the number of trials on MEP measures suggested 30 as the optimal number of stimuli needed to achieve a reliable and stable level of MEPs [15]. This finding has significant implications for the specific spTMS protocol employed in that study, but cannot be generalized to other frequently used single pulse or to paired pulse protocols. In addition, the high variability of MEP measures between individuals [24] can affect the results of reliability tests and lead to misinterpretation of findings [2]. Hence, defining the level of inter-individual variability permits more reasonable interpretation of the results [9,15]. In addition, SICI and ICF measures have been found to be even more variable and less reliable than spTMS outcomes, limiting their ability to detect neurophysiological changes in repeated measure designs [30]. Various studies have been conducted to optimize the reliability of ppTMS protocols by altering stimulation parameters including intensity and frequency [4,10,13,17], but the effect of the number of pulses remains unclear. This study was conducted to define the minimum number of trials to obtain reliable and stable measures of CSE using spTMS, and SICI and ICF using ppTMS, within and between sessions. Also, the inter-individual variability was investigated, in order to facilitate interpretation of reliability results.
Section snippets
Participants
Fifteen healthy volunteers (8 females, 7 males), between 18 and 30 years (mean ± standard deviation (SD) = 21.5 ± 3.1) were recruited for this study. All participants were right-handed based on the 10-item version of the Edinburgh Handedness Inventory [29] (mean ± SD laterality index = 81% ± 18%) and were safe to receive TMS according to their responses to the Adult Safety Screening Questionnaire [21]. Prior to the experiments, all participants provided informed consent to participate in the
RMT
The mean ± SD for RMTs were 31.8 ± 4.7%, 31.5 ± 5.6 and 32.1 ± 6.3% of maximal stimulator output for three sessions, respectively. Both intra- and intersession reliability analyses disclosed significant correlations for these measures (r = 0.93, and r = 0.89, respectively). The intra- and inter-session CV were 2.6% and 3.0%, respectively.
Single-pulse paradigm
The mean ± SD of CSE estimates for each session and each number of trials are summarized in Table 1. Fig. 1(a) shows the intra- and intersession reliability of
Discussion
The current study aimed to optimize sp- and ppTMS protocols to examine cortical changes by determining the minimum number of trials required to provide reliable intra- and inter-session outcomes. According to the findings, it can be suggested that the minimum number of trials is 20 for spTMS and 26 for SICI to ensure high intra and inter-session reliability. Although 30 consecutive trials produced highly reliable ICF measures within a session, inter-session reliability was not significant
Conclusion
The present study revealed increasing the number of stimuli improves the reliability of TMS measures, but to a certain level. The optimal ranges of 20–25, 25–30 and 30–35 trials were defined for the assessments of CSE, SICI and ICF, respectively. Excitability measures showed high variability and instability below these ranges, and further elicitation of MEPs did not lead to significant improvements in the reliability of these measures. These findings can have significant implications for
Acknowledgements
This manuscript is based on research conducted by Mana Biabani, PhD candidate at Monash University, Melbourne, Australia. This project received no external financial support, and has no conflict of interest to report. The authors would like to highly appreciate Dr Nigel Rogasch for his valuable comments and suggestions, which greatly improved the manuscript.
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