YouTube has a number of these videos. Doctors have to go into some poor guy’s skull to do some repairs. But because they can’t just poke around blindly, the patient has to be awake so the surgeons can assess the effects of touching this spot vs. that spot has. A millimeter the wrong way and the patient could lose some sort of vital function.
Take this guy. The only way his doctors could know they weren’t hitting the wrong parts of his brain was to have him play and sing Beatles songs while his brain was being probed. (Via Garnet)