Joni Mitchell's story and Pat Martino's story are remarkably similar (brain aneurysm). The question is, did they regain access to the part of their brain that allowed them to play guitar, or did they really re-learn?
In Martino's case, after his full amnesia, he was able to retrieve his unique playing style, but was that because he learned by listening to his recordings (just like Joni did), or because the style never left him but was somehow locked?