"Cybernetic matrix" was a term used to describe the complex and interrelated systems responsible for the maintenance Borg drone's conscious thoughts and personality.
In 2377, when faced with the possible decompiliation of The Doctor's matrix by a Lokirrim photonic disruptor, Seven of Nine was forced to download The Doctor's program into her own cybernetic matrix, leaving The Doctor in full control of Seven's body. This was accomplished by interfacing her assimilation tubules with The Doctor's mobile emitter, which allowed The Doctor to survive the Lokirrim attack. Later, Seven was able to regain control of her own body by repeating the procedure, reactivating The Doctor's program with the mobile emitter. (VOY: "Body and Soul")