Psychology-Medical question.?

What do you call on that act or maybe thing in which the individual is in a very very deep concentration. And that it looks like he or she's possessed by that thing. He does not mind on any other things. As if he was hypnotized to that kind of thing. For instance, when one plays an instrument, he plays it with pion and feelings, and to the extent that he plays and plays when one notices it looks like he is mad or something. He cannot stop playing and it looks like he is apathetic on the outside, making him being possessed but in the inside there is the boiling emotions?