I find these results surprising. For what it's worth, I've been in a sensory deprivation tank around five times (for an hour the first time, and for an hour and a half all subsequent times), known a handful of other people who've tried it too, talked to the two women who ran the place I went to, and leafed through The Deep Self by John Lilly, which includes many descriptions of people's time in the tank. All of these things led me to believe that hallucinations after just 15 minutes were quite rare. Granted, half the participants were picked because they tested as prone to hallucinations, and maybe there's something about the room they used that makes it different from the tank, but I'm still quite surprised.
I'm not sure, but I think an anechoic chamber is a different, more deprived, experience. I've been in the one at NPS in Monterey, and it's a very strange experience. No echos, no external sounds, no vibrations, still body temp air, and no light. In a water tank, you still hear the water, feel the edges of the tank, feel the water move when you move your body, etc, right?
I was also in an anechoic chamber a few times years ago. I remember once, I had been working on a robot for the FIRST competition for about 10 hours straight. I was exhausted, so I thought, 'I'll take a nap in the anechoic chamber'. It was probably the only quiet, dark place around.
I couldn't sleep, and lasted only about 20 minutes in the chamber. I didn't have hallucinations, but I did start feeling paranoid and the lack of sensation was (for lack of a better word) intense. I couldn't stand it. Perhaps I would have started hallucinating if I had stayed longer. Sensory deprivation is a truly bizarre experience.
Interesting. That makes me want to try an anechoic chamber :-). However, the flotation tank also has body temp air and water, no light, good soundproofing (I've also always gone in wearing earplugs). It's true that if I were splash inside, I'd probably hear it some, and that if I floated to the side, I'd feel the edge. But when I went floated, I always lay as still as I could for the whole time, which was pretty still. So I don't really see what would make it all that different, other than maybe the fact that you're lying down instead of sitting.
I've been in an isolation tank, too. The most noteworthy aspect was how, after a time* , my sense of balance / spatial orientation had gotten enough repetitive input that it checked out. At the same time, my body had gotten used to being supported fully by the water, and I could feel muscles in my back, shoulders, etc. relaxing that I didn't know I had. (Sometimes the same thing happens when I do zazen, though that usually focuses more on letting my mind give up on its chattering.)
A flotation tank is a bit different than what's described in the article - if you move, you feel waves in the water, hear splashes echo, etc.
All of this focusing on hallucinations seems kind of odd. People hallucinate all the time, they just rationalize it as misreading signs and whatnot. Big deal.
* I'm guessing half an hour, but the isolation unsurprisingly made it seem far longer. Isolation without control would be terrifying.
Maybe the standards are different. I wonder how many of the tank people had used hallucinogens, and were only reporting really noticeable hallucinations?
> Among the nine participants who scored high on the first survey, five reported having hallucinations of faces during the sensory deprivation, and six reported seeing other objects or shapes that weren’t there. Four also noted an unusually heightened sense of smell, and two sensed an “evil presence” in the room. Almost all reported that they had “experienced something very special or important” during the experiment.
I remember reading an account in Omni magazine from Dr. John Lily of one of his experiences with LSD and a deprivation tank. He became a point of light, racing at ever higher speed through the cosmos. He reaches the edge of the universe, and two other points of light accost him and announce that they are the "Intergalactic Thought Police" and that he's not allowed to proceed further.
Dr. Lily inspired the movies "Altered States" and "The Day of the Dolphin."
If anyone's interested in reading about my experience the first time I went, which was less dramatic, I described what I could remember here: http://divia.posterous.com/my-first-hour-in-the-flotation-ta....