Gut bacteria and virus have both been implicated in "mental" illness. Look up "Toxoplasmosis".
There's a reductionist orthodoxy, which views man as a brain on a stick - or a monkey driving a robot. Transplant the monkey in a new robot, and you have the same being. Only it's just not true.
Your entire nervous system is an extended "brain", in some regards. The entire "Me" that we have is a hive, and a colony of interdependence. Without getting all speculative or "holistic" examine mitochondria, for Pete's sake!
For the geek set: Luke is as much a manifestation of his midichloreans as he is a history of brain impulses. Put his brain in C3PO and you don't get Luke+Life Extension. You get a limited Luke simulacrum, able to replay Luke memory without new Luke experience or interaction. Plop his brain into Han Solo's body, and he will not be Luke anymore - He may be surprisingly like Han, with amnesia.