The voice you hear in your head spouting seemingly randomness.
Your internal dialogue.
For example, if you read my post in your head.. that's your mind chatter/internal dialogue.
Actually it might be more readily understood when phrased as a more personal thought exercise, e.g. -- if you are hearing your (own) last post in your head, rephrasing it, hearing imagined responses, re-rephrasing again, thinking about dinner, re-rephrasing again etc... *that* is mind chatter.
This breath/mind activity connection was something I noticed maybe 20 years ago after I had been meditating for a few years and at times my breathing would naturally slow way down. I began to notice the interim after exhaling and before starting the next inhale... a definite effect on mental activity.
This is noted clearly in Pranayama practice - the prana & manas (breath & mind) connection;http://www.yogamag.net/archives/1997/ajan97/pranbran.shtmlThe RAS is the trigger for other parts of the brain. Man is able to affect the RAS through the breath only. No other function of the autonomic nervous system can be controlled by conscious human activity. Control of the brain through the RAS by means of conscious breathing is a method by which other functions of the body may be controlled, for example, heart rate, blood pressure, digestion, excretion and absorption. Therefore, control of the subconscious is achieved through conscious activity of prana nigraha and then pranayama (7).