Offbeat

This is pretty common practice for neurostimulation surgery - we can't be certain of what exact specific bits of the brain are responsible for "play the clarinet", its not common across every person and there's no imaging system that could tell you in detail, so experimental stimulation is the best way to determine placement. While playing a full instrument is admittedly uncommon, people are regularly encouraged to talk at length, do basic coordination tests, sing, describe colors etc. during the procedure. Basically do whatever they have problems with, so their reaction can be used diagnostically in realtime.