From TFA:

> "To play an instrument means that you can understand music, which is a high cognitive function. It means you can interact with the instrument, you can coordinate both hands, you can exercise memory, you can count — because music is mathematics — you can test vision because the patient has to see the instrument, and you can test the way the patient interacts with the rest of the team," he said.

I take that to mean that playing the instrument uses many brain functions and so they're able to monitor many things at once.