This document describes systems, methods, and devices for sensing electrical activity in a brain of a mammal Some aspects include a cortical electrode assembly configured, in use, to sense electrical activity at a surface of a brain, the assembly comprising: a ring-shaped substrate; and a plurality of electrical sensors affixed to the ring-shaped substrate, wherein the plurality of electrical sensors are spaced along the ring-shaped substrate so as to form a ring of electrical sensors that substantially encircle an aperture formed by the ring-shaped substrate.