#1
This short video is something to be pointed out to people that reckon musician's are dumb!  This one is how playing an instrument affects the brain.  Pretty motivating.  Want to get smarter in general ... play an instrument!  

This other short video is about effective practice and the brain's role ...
#2
It's amazing what goes on if your just playing and singing a song.. Thinking about the rhythm, lyrics, and chords your playing while trying to sing in key all while simultaneously thinking about what the next chords and lyrics are going to be. Hearing what is going on and making adjustments accordingly. It definitely keeps the mind sharp
#3
babysmasher Yup.  Software for processing sound to extract musical components is still way off being able to successfully separate out all the instruments, especially in real-time.  And handling bass in real-time requires extra shenanigans (e.g. guitar to MIDI conversion).   And we do all this.  And remember enough of what we've heard to make sense of what's coming up. Let alone speech on top.  And process vision.  While minutely controlling all the joints and muscles involved playing.  And still catch the eye of a mate at the bar, while gigging, to get another pint.