P51 Effect of stimulus orientation and intensity on short-interval intracortical inhibition (SICI) and facilitation (SICF)
Paired-pulse transcranial magnetic stimulation (TMS) allows investigating inhibitory and excitatory interactions in the human motor cortex noninvasively. Short-interval intracortical inhibition (SICI) and facilitation (SICF) are used to measure cortico–cortical excitability in patients with, e.g., stroke, dystonia, and Parkinson’s disease. However, the role of the induced electric field (E-field) orientation remains partly unclear. Posterior–anterior (PA)-oriented E-field elicits motor evoked potentials (MEPs) with the lowest stimulus intensities due to the recruitment of corticospinal neurons, indirectly via excitatory synaptic inputs to corticospinal axons (indirect (I-) waves). Stimulation in the lateral–medial (LM) orientation directly activates corticospinal axons, which leads to the generation of both direct (D-) and I-waves. Conditioning stimulus (CS) with an intensity between 50% and 90% of resting motor threshold (RMT) induces activation of GABAA inhibitory mechanisms observed as the SICI (inhibitory) effect on MEP amplitude. In contrast, if the CS intensity is above RMT, the SICF (excitatory) phenomenon can be present due to the superposition of D- and I-waves. Our aim was to investigate the dependence of inhibitory and facilitatory mechanisms on the orientation of the induced E-field of CS and TS. We developed a multi-locus TMS (mTMS) transducer, which allowed us to control the E-field orientation independently for CS and TS at a millisecond inter-pair interval (IPI). Eight healthy subjects (five males; mean age 29, range 21–35 years) participated in the study. mTMS was applied to the hotspot of the abductor pollicis brevis (APB) muscle in the left primary motor cortex. The stimulus intensities were based on the individual RMT of APB for PA and LM orientations. TS and single pulses were administered at 110% RMT. Twenty single pulses were applied for each TS orientation and for each of the 32 paired-pulse conditions. CS and TS stimuli were applied in every combination of the PA and LM orientations with four CS intensities (50, 70, 90, and 110% RMT) and two IPIs (1.5 and 2.7 ms) in a random order. Interaction between CS orientation, IPI, and CS intensity significantly affected TS MEP amplitudes. We observed no statistically significant difference between the responses induced by PA- and LM-oriented TS. CS at 70% RMT for SICI and at 110% RMT for SICF induced similar effects regardless of the TS orientation. We established that LM-oriented CS at 90% RMT produced a greater inhibition than stimuli at the same intensity in the PA orientation. Our results emphasize the minimal influence of the CS E-field dorientation on the test pulse. Additionally, we demonstrate the pivotal role of the stimulus intensity for any CS orientation. SICI and SICF evoked using perpendicular CS and TS directions indicate that we stimulated overlapping neuronal populations with both pulses.