It's somewhat different but along the same line...my father was at home for hospice until mid-February and the nurses and social worker told us the last week or so that we couldn't know what he was aware of, and to assume he could hear us just fine (hearing is thought to be the last sense that goes.) He was on morphine and anti-anxiety medications, in and out of a coma, yet 2 days before he passed he knew I took my grandson in to say good morning, as I did every day, and stirred ever so slightly without opening his eyes and said, "Baby, oh, baby..." That same day my mother went in to sit with Dad while the hospice chaplain was here, and she spoke a few words softly to Dad. He didn't open his eyes or turn his head or try to speak, but his hand came out ever-so-slowly from under the covers to touch hers. It was the most profound act of love I have ever seen. The chaplain was astounded as well, and said, "Isn't that amazing, he can't respond and is in a coma, yet he knew his wife was there." And that night when one of my brothers played his guitar softly and sang worship songs for him Dad reached his arms out and was saying something so I bent down to hear him, he was saying, "Thank you, Father." This was the man who had been comatose moments earlier and went back to that state moments later.
I myself have been heavily sedated after surgeries and heard visitors, even though I couldn't rouse myself or speak. Knowing they were there meant so much to me, and I let them know when I was more alert. So as a rule I always go to see people regardless of their apparent state of awareness, because I know that they CAN know I'm there and feel comforted and loved.