Effects of Coil Orientation on Motor Evoked Potentials From Orbicularis Oris

Abstract

This study aimed to characterize effects of coil orientation on the size of Motor Evoked Potentials (MEPs) from both sides of Orbicularis Oris (OO) and both First Dorsal Interosseous (FDI) muscles, following stimulation to left lip and left hand Primary Motor Cortex. 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 delivered at 120% active motor threshold. MEPs from OO were evoked consistently for six orientations for contralateral and ipsilateral sites. Contralateral orientations 0°, 45°, 90°, and 315° were found to best elicit OO MEPs with a likely cortical origin. The largest FDI MEPs were recorded for contralateral 45°, invoking a posterior-anterior (PA) current flow. Orientations traditionally used for FDI were also found to be suitable for eliciting OO MEPs. Individuals vary more in their optimal orientation for OO than for FDI. It is recommended that researchers iteratively probe several orientations when eliciting MEPs from OO. Several orientations likely induced direct activation of facial muscles.

Keywords: coil orientation; facial muscle; hand muscle; motor cortex; motor evoked potentials; transcranial magnetic stimulation.

FIGURE 1

Eight coil orientations used in the lip and hand conditions. The intersection of the lines was placed on the subject’s hot spot for lip or hand M1 and the coil handle was aimed toward the angle tested (here: 45°).

FIGURE 2

Polar plot of average Area Under the Curve (AUC) elicited from contralateral (A) Orbicularis Oris (OO) in mV⋅ms for Middle Latency MEPs only. Only average values with >5 contributing subjects are included. Values normalized relative to the largest value, set to 1.

FIGURE 3

MEP EMG traces for a single MEP for subject 9 for four orientations: contralateral 45 (A), contralateral 135 (B), ipsilateral 0 (C), and 180 (D) in mV. Arrows indicates approximate start of MEP, measured from TMS pulse at 40 ms into the trial.

FIGURE 4

Polar plot of average Area Under the Curve (AUC) elicited from contralateral (A) First Dorsal Interosseous (FDI) in mV⋅ms for Middle Latency MEPs only. Only average values with >5 contributing subjects are included. Values normalized relative to the largest value, set to 1.