The cortical processing subunit was a device aboard Borg vessels that downloaded newly assimilated data into the cortical implants of individual drones. There was a subunit located inside each unimatrix.
In 2376, Seven of Nine modified her alcove to act as a cortical processing subunit, so that she could assimilate information more quickly. However, her brain was unable to deal with the amount of information and she became paranoid as a result. (VOY: "The Voyager Conspiracy")