Effects of coil orientation on Motor Evoked Potentials from Orbicularis Oris and First Dorsal Interosseous

Objective This study aimed to characterise effects of coil orientation on the size of Motor Evoked Potentials (MEPs) from both sides of Orbicularis Oris (OO) and compare these effects with those reported for First Dorsal Interosseous (FDI), following stimulation to left lip and left hand Primary Motor Cortex. Methods Using a 70 mm figure-of-eight coil, we collected MEPs from eight different orientations while recording from contralateral and ipsilateral OO and FDI using a monophasic pulse. Results MEPs from OO were evoked consistently for six out of eight orientations for contralateral and ipsilateral sites. When latency and silent periods were taken into account, contralateral orientations 0°, 45°, 90°, and 315° were found to best elicit OO MEPs with a likely cortical origin. As expected, the largest FDI MEPs were recorded with an orientation of 45°, invoking a posterior-anterior (PA) current flow, from the contralateral location. Conclusion Orientations traditionally used for FDI were also found suitable for eliciting OO MEPs. Individuals vary more in their optimal coil orientation for eliciting MEPs from OO than for FDI. It is recommended that researchers iteratively probe several orientations when eliciting MEPs from OO. Care must be taken however because several orientations likely induced direct activation of facial muscles.