pitch black

You are in a small underground chamber, no bigger than a prison cell. The door is locked and the lights are switched out.
It is not just dark; it's pitch black.

In these conditions, it would be impossible to see even the faintest hint of your hand were it inches from your eyes. And the silence is all-encompassing.

The only noise you can hear is the sound of your own breathing . . . in and out, in and out.

The sense of isolation is not just eerie, it's as if you had been cast into the furthest reaches of space, or into the deepest subterranean cavern.

Now imagine staying in these conditions for hours on end. With no way to tell whether it was night or day, would you pass the time asleep?

Or, if awake, how would you stay alert in the absence of a single sight, sound or stimulus?

Could any human, in fact, endure such total sensory deprivation without losing their sanity?

That was what British scientists sought to discover in one of the most extreme and controversial experiments to be conducted on the human mind.

They put a group of six volunteers into a total isolation chamber, constructed in a former nuclear bunker in Hertfordshire, to monitor the effects it would have on their mind and physical health over 48 hours.

The results, to be shown on a BBC2 documentary tonight, are intriguing. But this was not just a reality show stunt.

The researchers involved hoped to shed new light on the validity of statements given by terror suspects who have been held in extreme conditions in camps such as Guantanamo Bay.

"It is important to look at the impact of sensory deprivation because of the number of places around the world where it is used as a weapon or to aid interrogation," says Professor Steven Robbins, who oversaw the experiments and is one of the Britain's leading experts on the effects of psychological torture.

"We know that stimulating the brain helps increase connections in the brain that speeds up information processing, but we wanted to find out if the reverse occurs."

The risks were considerable. Similar experiments carried out in the Fifties by Canadian psychologist Professor Donald Hebb had to be abandoned after volunteers were unable to endure the conditions for more than 48 hours.