FIGURE 1.

Effect of stimulation on a single neuron and on a population of neurons. (A) Stimulation along a nerve fiber results in depolarization beneath the cathode, and hyperpolarization beneath an anode. (B) Single neurons can be stimulated by both anodal and cathodal stimulation depending on their orientation. In this example, anodal current enters the dendrites of the neuron and leaves through the axon, which results in depolarization of the axon and an action potential. (C) In the case of stimulation through ECoG electrodes, a large population of neurons can be activated by stimulation. Shown are approximate scales of an ECoG electrode relative to the precentral and postcentral gyrus, along with a representative mixed population of pyramidal neurons potentially depolarized by stimulation. In the zoomed-in region, we highlight the multiple orientations of neurons that could be activated. (D) An axial slice in a co-registered CT and MRI image following implantation with an ECoG array shows the potential current paths that different stimulation configurations would have to pass through, illustrating the large populations of neurons present within the potential current path. (A,B) Inspired by Ranck (1975).