Optimization of the transcranial magnetic stimulation protocol by defining a reliable estimate for corticospinal excitability

Koen Cuypers, Herbert Thijs, Raf L J Meesen, Koen Cuypers, Herbert Thijs, Raf L J Meesen

Abstract

The goal of this study was to optimize the transcranial magnetic stimulation (TMS) protocol for acquiring a reliable estimate of corticospinal excitability (CSE) using single-pulse TMS. Moreover, the minimal number of stimuli required to obtain a reliable estimate of CSE was investigated. In addition, the effect of two frequently used stimulation intensities [110% relative to the resting motor threshold (rMT) and 120% rMT] and gender was evaluated. Thirty-six healthy young subjects (18 males and 18 females) participated in a double-blind crossover procedure. They received 2 blocks of 40 consecutive TMS stimuli at either 110% rMT or 120% rMT in a randomized order. Based upon our data, we advise that at least 30 consecutive stimuli are required to obtain the most reliable estimate for CSE. Stimulation intensity and gender had no significant influence on CSE estimation. In addition, our results revealed that for subjects with a higher rMT, fewer consecutive stimuli were required to reach a stable estimate of CSE. The current findings can be used to optimize the design of similar TMS experiments.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1. Data for subject 36 (a)…
Figure 1. Data for subject 36 (a) and 34 (b) is illustrated.
The Y-axis shows the MEP amplitude (mV), while the number of TMS stimuli (n) is shown on the X-axis. White dots represent the individual (raw) MEPs, whereas the black dots represent the average of consecutive MEPs (). Dashed lines represent the 95% confidence interval (CI), which is based upon all 40 stimuli. The upper panel (a) illustrates data that was included in the statistical analysis (slope estimate: 0.007; p = 0.355). For this particular subject, 8 consecutive stimuli were sufficient to enter the CI. The lower panel (b) shows data that has been excluded due to a significant change in slope over time (slope estimate: 0.062; p<0.001).
Figure 2. Results for gender (a) and…
Figure 2. Results for gender (a) and intensity (b) based upon the raw data are illustrated.
The Y-axis shows probability of inclusion in the 95% CI, while the number of TMS stimuli (n) is shown on the X-axis. The upper panel (a) illustrates the probability of inclusion in the 95% CI for females (white dots) and males rMT (black dots). The lower panel (b) illustrates the probability of inclusion in the 95% CI for the stimulation intensity of 110% rMT (white dots) and 120% rMT (black dots).

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