Greg Egan's short story Closer has people implanted with a device, a "jewel", that learns to mimic the behaviour of their brain - to the point where people actually choose to switch from their biological brain controlling their body to their jewel....
Egan's story 'Learning to Be Me', available in the same collection as 'Closer' (Axiomatic), is also an exploration of this idea with relevance to sodiumphosphate's fear.
http://eidolon.net/?story=Closer&pagetitle=Closer&se...