Repetitive paired-pulse TMS at I-wave periodicity markedly increases corticospinal excitability: A new technique for modulating synaptic plasticity

Objective: We hypothesised that facilitatory I-wave interaction set up by paired-pulse transcranial magnetic stimulation delivered with I-wave periodicity (iTMS) may reinforce trans-synaptic events and provide a means for modulating synaptic plasticity and cortical excitability. Our objective was to determine whether prolonged iTMS can increase corticospinal excitability, and whether this form of stimulation can have lasting aftereffects.Methods: Paired stimuli of equal strength with a 1.5 ins inter-stimulus interval were delivered for 30 min at a rate of 0.2 Hz. Motor threshold and motor evoked potential (MEP) amplitude to single-pulse TMS was compared before and after intervention.Results: Paired-pulse MEP amplitude increased linearly throughout the period of iTMS, and had increased five-fold by the end of the stimulation period. Single-pulse MEP amplitude was increased a mean of four-fold for 10 min after stimulation. Motor threshold was unaffected.Conclusions: iTMS is an effective method for increasing excitability of the human motor cortex, and probably acts by increasing synaptic efficacy.Significance: Reinforcement of trans-synaptic events by iTMS may provide a means to investigate and modulate synaptic plasticity in the brain. (c) 2005 International Federation of Clinical Neurophysiology. Published by Elsevier Ireland Ltd. All rights reserved.