This study shows brief mediation gives improvement, however I'm not sure how they controlled for those who didn't complete the experiment. Could those people be self-selecting out of it due to difficulties - ADD perhaps?
Overall - I'm love to try a simple form of meditation to see if it'd work for my ADD self.
http://www.sciencedaily.com/releases/2010/04/100414184220.ht...
I prefer just vipassana, watching the breath and watching thoughts as they arise and fade. No chanting or 'OMMM'.
An interesting book is "Zen and the Brain" by an emeritus professor of neurology, published by MIT Press. The author started doing zen while in Japan in the late 60s. Over about 1000 pages, he tries to come up with testable hypotheses explaining the phenomena and results of long-term meditation practice.
It's very detailed and dense. There are a few chapters that describe his subjective experiences, but the rest is pretty scientific. There are few illustrations, it's a massive block of text.