This is a really fascinating study.  Do people who are asleep or comatose hear AND understand what is said to them?  While by no means definitive or conclusive, this study does look at one variable in the brain.  By using special imaging, they evaluated brain activity in healthy volunteers falling asleep as someone talked to them.  Apparently, the brains seemed to "hear" the speech but areas of the brain that would "interpret" the speech stopped functioning when someone was asleep.  Also It appears that the brain did not make any "memories" of this speech either.  This suggest that asleep and comatose people hear but, do not understand or remember what they hear.   This seems to make sense however it is a little disappointing as we often wish to "speak" to our loved ones when they are sick with comas in the hospital.  Would I let this study stop me?  Absolutely not... Communication with our loved ones, even if it's one sided, is ALWAYS good.

Watching Speech Comprehension Fade As The Brain Falls Asleep