Background: Transcranial magnetic stimulation (TMS) induced I-wave behavior can be demonstrated at neuronal population level using paired-pulses and by observing short-interval cortical facilitation (SICF). Advancements in stimulator technology have made it possible to apply biphasic paired-pulses to induce SICF.
Objective: Our aim was to characterize the SICF I-wave interaction by biphasic paired-pulses with the ultimate objective to enhance TMS effects via SICF in various TMS-applications.
Methods: We used biphasic paired-pulses in 15 volunteers to characterize corticospinal SICF using various 1.2-8.0ms inter-stimulus intervals, and measuring SICF input-output response.
Results: SICF interaction with the first I-wave (I1) was observed in the output responses (motor evoked potentials; MEPs) in all subjects. Most subjects (≥80%) also exhibited later SICF I-wave interaction. SICF at I1 was present at all applied intensities below 140% of resting motor threshold. At I2, we observed SICF only with intensities just above motor threshold.
Conclusions: Biphasic paired-pulses can reliably induce SICF shown by the facilitatory I-wave interaction, and could therefore be applied with repetitive bursts to enhance responsiveness to TMS.
Keywords: Biomedical engineering; Indirect wave; Motor cortex; Motor evoked potential; Short-interval intracortical facilitation; Transcranial magnetic stimulation.
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