The variability of intracortical inhibition and facilitation

Objective: To assess the variability of transcranial magnetic stimulation paired pulse measurements of cortical excitability between subjects, between sessions and within subjects within sessions. Methods: In experiment 1, intracortical inhibition and facilitation were assessed with a fixed conditioning stimulus intensity (CSI) of 80% of active motor threshold (AMT) whereas in experiment 2, the effect of different CSIs (60–110% of AMT) was investigated. Results: Experiment 1 revealed that subjects differed significantly in the degree of inhibition and facilitation. Between sessions the variability was substantial as predicted by high within session variability. Experiment 2 allowed determination of individual thresholds for inhibition and facilitation. These thresholds were the best predictor of the amount of inhibition or facilitation at a given CSI. Across subjects we observed a high correlation of the threshold for inhibition (expressed in terms of maximum stimulator output) with AMT (r=0.93). Results for facilitation were more variable. Conclusions: The variability was high if a single CSI was used to compare the percent intracortical inhibition or facilitation between subjects, or between sessions. Much less variable was the threshold for intracortical inhibition/facilitation, which was highly correlated to AMT. We suggest that the ratio of CSI:AMT is a robust and useful additional measure of the integrity of neuronal circuits underlying intracortical inhibition/facilitation.