There was actually a story on NPR recently that mentioned such a brain operation where the patient needed brain surgery to alleviate some type of recurring seizure. And he was also a music lover, and so there was an inherent risk the surgeons said that he might lose those abilities.

So rather than take that risk blindly, they had him play a guitar while they were doing surgery on his brain! And if at any point, while they were operating, if his playing ability was affected they would stop operating in that region of the brain.

The kicker was that the side effect of the surgery ( his second ), was that they inadvertently affected a part of the brain that is responsible for 'self-control, judgement' and he ended up becoming a child porn addict, which got him arrested and convicted.

There's about 5 different unbelievable parts to this story, worth a listen if you can find it.

The Human Brain.. continually amazed at how much we still don't know about this part of our anatomy.