Stimulus parameters influencing cortical responses by transcranial magnetic stimulation targeting I-wave dynamics : duration, intensity and interval between interventions

Transcranial magnetic stimulation (TMS) modulates corticomotor excitability and plasticity; however, the magnitude of the effects appears to depend on a range of stimulus parameters. Three possible critical factors of repetitive TMS (rTMS) interventions are the duration of application, the intensity of the stimulation and the interval between interventions. However, no previous studies have systematically investigated the effect of these factors. The purpose of this research was to investigate the effects of duration (Study 1), intensity (Study 2) and interval between interventions (Study 3) comprising of equi-intensity paired pulses (1.5 ms between pulses) corresponding to indirect (I)-wave periodicity (iTMS) on cortical excitability changes. To assess cortical excitability changes, single-pulse motor evoked potential (MEP) amplitude before and following the intervention, and paired-pulse motor evoked potential (iMEP) amplitude during the intervention from the first dorsal interosseous (FDI) muscle were recorded in the studies. In Study 1, the effect of intervention duration was investigated by comparing 15 min and 30 min to test the hypothesis that doubling the duration of the intervention would further increase the excitability. Ten (7 women and 3 men) healthy individuals (21 - 27 years) performed the 15- and 30-min interventions in a counterbalanced, crossover fashion, and changes in iMEP amplitudes during the intervention and MEP amplitudes before and for 30 min following the intervention were compared between the interventions by a two-way repeated measures ANOVA. Paired-pulse iMEP amplitude increased (