The brain activity appears to differ depending on the sort of playing I am engaged in. With technical practice, on my silent Virgil Practice Clavier, I think very hard about my physical movements, make mental notes about what is working and what isn't, and constantly experiment with new haptic means. It took me many years to realise that how I perceive movements is very important, and does not necessarily correspond to how they look. So during technical practice I think about nothing save technique, which process is aided by the instrument being silent.
The situation with improvisation, at least for me, is tremendously complicated. I regard it as nothing less than an abstract mapping of my entire consciousness over seventy years including memories, dream content, arbitrary associations, visions, music itself and countless other things. It also contains abstract beauty and form perceived within the present moment. “Thinking” falls a long way short of what goes on, which is more akin to what Aldous Huxley termed “opening the doors of perception”, escaping the quotidian reducing valves of the mind and letting in “mind at large”. I do not think about physical technique at all during this activity.
With pieces I am least qualified to pass an opinion, as my repertoire is small and I do not perform. However, I do not just repeat the same movements and thoughts day in and day out. I am regularly thinking about possible new ways of playing a piece, even one I have played for many years. Sometimes it seems to me that at its best, performance of a piece should seek to produce the mental states of improvisation, but again, that is just my private fancy.