What happens if you are left in the dark in solitary confinement for days on end? The result is called sensory deprivation and the mind struggles to cope with it.
Adam Bloom is a stand-up comic, an extreme extrovert who admits he thrives on stimulation.
He is one of six volunteers who have agreed to be shut inside a cell in a nuclear bunker, alone and in the dark. And for Bloom it will be particularly hard.
Within half an hour of being locked up at the start of the experiment, all of the subjects lie down and go to sleep. But the real ordeal will begin when they wake up and find they have no idea what time it is.
Throughout the 48 hours they are being monitored by a psychologist and filmed for a BBC Horizon documentary.
As the hours pass, seeing and hearing nothing, they become increasingly disoriented.
After 24 hours Bloom is suffering. The subjects have been encouraged to describe how they are feeling but know that while their words may be heard, no one will respond.
"Its really hard to stimulate your brain with no light. It's blanking me. I can feel my brain just not wanting to do anything," he says.
Sensory deprivation is a controversial subject, with allegations the technique has been used at Guantanamo Bay as an interrogation strategy. And thousands of prisoners around the world are kept in solitary confinement, often with a significant degree of sensory deprivation.
The tests are exploring the theory that sensory deprivation makes subjects much more suggestible.
Some of the first research on this subject was carried out after the Korean War in the 1950s. The Canadian military wanted to investigate what had happened to POWs who appeared at international press conferences confessing that they were war criminals. It was thought they had been brainwashed following solitary confinement.
North American scientists paid students to stay in conditions of sensory deprivation for varying lengths of time. Most dropped out after 72 hours, and very few were able to stay more than four or five days. The boredom and oppression of the experiments' conditions became overpowering.
After just 30 hours of the same treatment, Adam is one of several inmates who are pacing up and down their cells, again a common reaction.
"This behaviour of pacing up and down is something we see in animals as well as people when they are kept in confinement," says Prof Ian Robbins, a clinical psychologist at St George's Hospital who is supervising the experiment.
"It could be just seen as something you can do without thinking about it, it may be in part attempting to exercise, but I think it is reaction to the lack of input and you provide the input physically."
Trauma of captivity
Brian Keenan is all too familiar with some of what they are experiencing. He spent four years as a hostage in Lebanon. "I reckon I was in the dark about seven or eight months. I can't be sure; it's very hard to tell the time.
"The nothingness, that was extremely hard. Because the question in your head is how am I going to get through the next 10 minutes? Or months later, how am I going to get through the next day? Is there enough left in my head?"
"I remember one occasion waking up and having to squeeze my face and my chest and thinking to myself 'Am I still alive?'"
After just 30 hours, Adam is in trouble.
Brian Keenan was held in the dark for over seven months
"I'm hallucinating! I thought I could see a pile of oyster shells, five thousand oyster shells, empty, to represent all the nice food I could have eaten while I was inside here."
Keenan also experienced periods when his brain conjured up images. One particularly unpleasant time involved musical instruments getting louder and louder.
"I got really afraid and that's when I kind of started banging my head against the wall, just to make this go away.
"You would try and engage your mind forcefully in something else .... it was not comforting. And it went on for a very, very long time."
Back in the nuclear bunker, some of the other guinea pigs have also been hallucinating.
Mickey, a postman is seeing mosquitoes and fighter planes buzzing around his head and it's frightening him.
Claire a psychology student doesn't mind the little cars, snakes and zebras. But she gets scared when she suddenly feels somebody is in the room.
"In the dark room there is nothing to focus on," says Prof Robbins as he monitors their behaviour. "In the absence of information the human brain carries on working and processing information even if there is no information to process and after a while it starts to create that information itself."
Comedian Adam Bloom is used to stimulation and interaction
Finally Adam and the others are released and given the psychological tests. Suggestibility is measured as the subjects are asked to read out what colour is printed on a card although the letters appear in a different colour, for instance the word black printed in red.
Several of the subjects including Adam show very high levels of suggestibility.
The results give Prof Robbins an insight into "what can happen to people kept in solitary confinement over possibly many months and even years".
"Evidence that has accumulated in those places must be considered very unreliable because people will after a while start to take on board the views of their interrogators," he says.
"Our volunteers were in a sensory deprivation environment for 48 hours and being treated humanely."
After just 48 hours, Adam wanted to kiss the man who opened the door to let him out.
"I was let outside and saw the sun and the sky, for the first time in 48 hours. My senses were overwhelmed totally and utterly by the sights, sounds and smells."
He is glad he did it and proud he didn't give up early. But he would not do it again.
"It was an amazing experience that was very much worth going through once. It taught me to appreciate my senses and all forms of interaction."
Horizon: Total Isolation is on BBC Two at 2100GMT on Tuesday 22 January.
Below is a selection of your comments.
I spent five hours in asesory deprevation experiment whilst at university. Thirty years later I still remeber the disorientation and discomfort. I had to keep count of buzzers and bleeps. In that short time I overcounted by almost double and halucinated about parents and conversations.
Quite terrible. I guess that it is used as part of an interrogation because it does make you feel disorientated and thereore reliant on your interrogator. Part of "being broken".
Having said that, I would really prefer this as an alternative to permanent physical damage.
Jon Shamah, London, UK
Wonderful concept. For my thoughts, it opens the door to how we understand information, and the thought of, if it is our thoughts that make us or do we make our thoughts.
Howard Palmer, Atlanta, Ga
Our mind is an amazing "computer" that manages all the information given to it. It then starts making it's own information when other the input is lacking. We must not abuse our system
Ghanashyam Master, London, UK
Brian Keenan is a remarkable man: he went to hell - and survived. A modern-day Dante, if you like. His book, An Evil Cradling, will make you value the worth of human life. A masterpiece.
S. Martin, Highbridge, Somerset
As a claustrophobic, subway/elevator avoiding new yorker, I can't think of a more horrible experiment to be part of.
Deborah, New York
I can't agree with this part: "Suggestibility is measured as the subjects are asked to read out what colour is printed on a card although the letters appear in a different colour, for instance the word black printed in red.
Several of the subjects including Adam show very high levels of suggestibility. "
The task (aka Stroop Task) measures attention - it requires a good deal of forced attention to overcome the automatic process of reading the word. If you've been hallucinating and (presumably?) food deprived for days on end, then your attention is going to be low, and hard to focus. I don't agree with the methodology cited here.
It would be interesting to see how this correlates with people living in the arctic where it is dark for months.
Bob Johnson, near Kuopio, Finland