We know that people 3,000 years ago when beliefs about reincarnation where first formed - didn't know about the body and brain what we know. THey were probably under the belief that they were a "whole" entity inside a body! Science now knows that we are not "whole" but are a lot of parts working together and if one part stops working then that impairs the function of the rest of our bodies or mind! However the brain is a kind of elastical ability and in some cases parts of the brain used for other functions takes over functions of a part of the brain that is damaged beyond repair.
Anyone been watching Tony Robinsons "Gods and Monsters." in which he shows what irrational beliefs we used to have even a few 100 years ago!
Check out this video about Reincarnation!