Inter- and intra-individual variability of paired-pulse curves with transcranial magnetic stimulation (TMS)

Objectives: Previous studies have evaluated the variability of motor thresholds (MTs) and amplitude of motor-evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) within and across individuals. Here we evaluate the reproducibility and inter-hemispheric variability of measures of cortical excitability using the ‘conventional’ paired-pulse (PP) TMS technique. Methods: We studied PP curves of the left and right hemisphere in 10 healthy subjects on two separate days 2 weeks apart. The inter-stimulus intervals studied were 1, 3, 6, 8, 10 and 12 ms with the conditioning stimulus being 80% of the resting MT, and a single test stimulus producing MEPs of approximately 0.8 mV peak-to-peak amplitude. Results: As a group, the PP curves of the left and right hemispheres, and of Day1 and Day2 were not significantly different. The intracortical inhibition (ICI), but not the intracortical facilitation, showed a good correlation across days within the individuals. Conclusions: Cortical excitability, particularly ICI, measured by PP TMS shows no inter-hemispheric asymmetry and is reproducible within individuals.