Modulation of I-wave generating pathways with repetitive paired-pulse transcranial magnetic stimulation: A TMS-EEG study

ObjectivesRepetitive paired-pulse transcranial magnetic stimulation (iTMS) at indirect (I) wave intervals increases motor-evoked potentials (MEPs) produced by TMS to primary motor cortex (M1). However, the effects of iTMS at early and late intervals on the plasticity of specific I-wave circuits remains unclear. The current study therefore aimed to assess how the timing of iTMS influences intracortical excitability within early and late I-wave circuits. To investigate the cortical effects of iTMS more directly, changes due to the intervention were also assessed using combined TMS-electroencephalography (EEG). Material and MethodsEighteen young adults (24.6 {+/-} 4.2 years) participated in four sessions in which iTMS targeting early (1.5 ms interval; iTMS1.5) or late (4.0 ms interval; iTMS4.0) I-waves was applied over M1. Neuroplasticity was assessed using both posterior-to-anterior (PA) and anterior-to-posterior (AP) stimulus directions to record MEPs and TEPs before and after iTMS. SICF at inter-stimulus intervals of 1.5 and 4.0 ms was also used to index I-wave activity. ResultsMEP amplitude was increased after iTMS (P < 0.01) and this was greater for PA responses (P < 0.01), but not different between iTMS intervals (P = 0.9). Irrespective of iTMS interval and coil current, SICF was facilitated after the intervention (P < 0.01). While the N45 produced by AP stimulation was reduced by iTMS1.5 (P = 0.04), no other changes in TEP amplitude were observed. ConclusionThe timing of iTMS failed to influence which I-wave circuits were potentiated by the intervention. In contrast, reductions in the N45 suggest that the neuroplastic effects of iTMS may include disinhibition of intracortical inhibitory processes.