I can tell you first hand it went a long way in my mental state after getting divorced in '99. I was working on a roof in a neighborhood that was having a garage sale. Needing many household items as it was, one thing caught my eye was a black Strat Squire and a funky little amp (originally sold as a starter set together). I bargained with the dude and for $35, I had him throw in an electric juicer. Having last played in the late 60s, it was a long shot but I didn't want to start stupid drinking and I could see were this might guide my mind. Someone told me to check out Youtube(at that time I never heard of this!). I couldn't believe the stuff I learned on line for free! Anyway, fast forward, I've been playing steady for these past twenty plus years and great sites like GuitarTricks have catapulted my skills! Along with my spiritual faith, my guitar settled down my mind and gave me accomplishments and feeling of success! 
i like what Faith 83 said about Alzheimer's therapy; I don't doubt guitar is a positive connection!! Anyway, sorry for rambling but I hope guitar helps someone going through tough stretches in life (along with faith in God).