"I have had oxygen deprivation and I never had any visions. Is there any study that shows at what levels of oxygen deprivation one might "see" things? Any data on those who have been strangled, put in a sleeper hold or drowned?"
Actually, quite a bit. I only googled for one, but I'm sure a smart guy like you can find more. That's assuming you are more interested in a scientific answer than trolling people who don't have your religious beleifs.
When mountain climbing, hypoxia causes hallucinations, for example.
http://www.pbs.org/wgbh/nova/everest/exposure/hackett.html "I could share some of my own experiences with this kind of hypoxia on Everest. One time, when I was sleeping in my tent and ran out of oxygen during the night at about 25,000 feet, I had this most vivid hallucination that John West, our expedition leader, had come up to the camp with a full bottle of oxygen, with no regulator on it. He had put it inside the tent and just opened it and filled the whole tent with oxygen. I was very grateful for him. I felt better right away. And the next morning, I was trying to figure out where the bottle was and how he had gotten up there in the middle of the night."
"It dawned on me that obviously this was a hallucination and I knew I wasn't dreaming because it was so incredibly vivid."
Conclusion: John West is really god! duh.
When brain cells die they release glutamate which can stimulate other cells. When large numbers of cells die (as in cases where blood or oxygen can't reach the brain the volume of glutamate can cause other cells to die from over stimulation. In order to prevent this the brain will begin releaseing glutamate blockers which prevents more cells from dieing but also interferes with the normal functioning of the brain.
Ketamine, which is used as a vetranary anesthetic, is also a glutamate blocker and has been shown to produce near death experiences when administered to humans.
Here are a few links:
Using Ketamine to Induce the Near-Death Experience
Near-Death Experiences The Skeptic's Dictionary: Near Death Experiences