Computer-based model of epidural motor cortex stimulation: effects of electrode position and geometry on activation of cortical neurons.

OBJECTIVE The aim of this study was to determine the effects of electrode placement, geometry, and polarity during epidural cortical stimulation (ECS) on thresholds for direct activation of cortical neurons. METHODS We used a computational model of epidural electrical stimulation of the motor cortex coupled to compartmental models of cortical neurons. RESULTS Thresholds varied with stimulation polarity and neuron position, and neurons deep within the sulci had much larger thresholds than those on the crowns or lips of the gyri. Axons were more excitable than cell bodies or dendrites. Delivering stimulation with the lead placed above or perpendicular to the sulci resulted in substantial stimulation of the gyri adjacent to the target gyrus. Electrode diameter and inter-electrode spacing influenced thresholds and affected the spread of activation in the cortex. CONCLUSIONS Electrode placement, geometry, and polarity during ECS influence excitation properties of cortical neurons substantially. SIGNIFICANCE Epidural leads have varying geometries, and in clinical studies of ECS the placement of the lead has been inconsistent. These results provide an improved understanding of the effects of electrode placement, geometry, and polarity on the outcome of ECS and can facilitate the rational implantation and programming of ECS systems.