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Impact of Current Direction on Short Latency Intracortical Facilitation (SICF)

Jaron Kee Photo

University of New Mexico (New Mexico)

Jaron Kee, Nivethida Thirugnanasambandam, Mark Hallett


The human motor cortex discharges high frequency I (indirect)-waves (~670 Hz) following transcranial magnetic stimulation (TMS). These waves can be recorded as descending volleys in the spinal cord using epidural electrodes. Short latency intracortical facilitation (SICF), a paired pulse TMS paradigm, has been used to investigate I-wave activity non-invasively. SICF is elicited by combining a supra-threshold conditioning pulse (S1) with a near-threshold test pulse (S2) at distinct interstimulus intervals (ISIs). A facilitatory effect on motor evoked potentials (MEPs) is observed around 1.5, 3, and 4.5ms ISIs when I-waves generated by the two stimuli fall in-phase with each other. Past TMS protocols have investigated SICF using a posterior-anteriorly (PA) directed current that generates I1 waves preferentially. An anterior-posterior (AP) current stimulates different neuronal systems and generates later I-waves preferentially. In this study we aim to investigate the effects of current direction on SICF by inducing both PA and AP currents and identify the I-wave contributing to each peak. Our results show 3 peaks at expected ISIs with the PA stimulation. We did not see any peaks with AP stimulation when S2 was just sub-threshold. On slightly increasing the S2 intensity, we observed 3 peaks. Based on MEP latency we can conclude that I1 contributes primarily to the MEPs from PA stimulation whereas I2 contributes significantly to the MEPs generated by the AP current. Thus, the SICF peaks in the PA direction are generated by I1, I2 and I3 waves of S2 aligning with the I2, I3 and I4 waves of S1, whereas those in the AP direction are generated by I2, I3 and I4 waves of S2 falling in phase with the I3, I4 and I5 of S1. Thus for future studies, we can non-invasively determine the impact of physiological/pathological factors on specific I-waves.

Last Modified November 27, 2013