P26-T Coil orientation matters: TMS-EEG to optimise the activation of pre-Supplementary motor area

Abstract

Background Studies using Transcranial Magnetic Stimulation (TMS) have demonstrated the possibility to target different neuronal populations within the primary motor cortex (M1) by changing coil orientation and stimulation intensity. The absence of a direct electrophysiological response such as motor evoked potentials has limited this investigation in areas outside M1. This problem can be solved with the use of combined TMS and electroencephalography, which allows direct recording of cortical responses to TMS pulses. Materials and methods In this work on healthy human subjects we compared early (10–70 ms) and late (70–250 ms) transcranial evoked potentials (TEP) and TMS-related spectral perturbation (TRSP) obtained by delivering magnetic pulses over the pre-SMA with variable coil orientation (0°; 90°; 180° and 270°) and stimulation intensity (100%, 120% and 140% of resting motor threshold). Results both early and late TEP were generally larger when the coil handle was pointing backwards (0[o). This result was true also for early TRSP but, surprisingly, late TRSP was greater when the coil was pointing at 180°. Additionally, stimulating the pre-SMA with a 90° orientation activated the right M1, thus limiting the spatial specificity of the stimulation. Conclusions these results suggest that parameters such as coil orientation and stimulation intensity are critical when stimulating the pre-SMA and they should be taken into account in physiological and clinical studies.