The cortical node was a Borg implant that sat in a socket above the right eyebrow as a part of a Borg drone's cortical array. (VOY: "Human Error") It was the single most important component a Borg drone possessed, as it served in controlling all of the drone's other cybernetic implants. If the cortical node malfunctioned, the implants would fail and be rejected, killing the drone. Its technology was too advanced to attempt any form of repair; in the Borg Collective, if a drone's cortical node malfunctioned, the node was replaced entirely. If a drone had been dead for more than an unspecified period, its cortical node became inactive and therefore could not be used to replace a malfunctioning node. (VOY: "Imperfection")

Built into every cortical node was a fail-safe mechanism designed to shut down the higher brain functions when the drone achieved a certain level of emotional stimulation, in order to deactivate drones who started to regain their emotions. This was to prevent the drone's mind from regaining a sense of individuality, breaking the Collective's control over it. (VOY: "Human Error")