Two of the British prisoners who have returned to the UK from Guantanamo Bay reportedly spent 18 months in solitary confinement. What impact does being locked up all day, away from any other human beings, have on an individual's mind?
The four Britons who were still being held by US forces at Guantanamo Bay returned to the UK on Tuesday.
Feroz Abbasi, Martin Mubanga and Richard Belmar from London and Moazzam Begg from Birmingham were detained as part of the US-led War on Terror and have been held for nearly three years.
At least two of them were apparently held in solitary confinement, in small cells, apparently with no natural light, nothing to do or read, and nobody but the occasional guard to talk to.
Abbasi, a former computer studies student from Croydon in south London who was arrested in Afghanistan as an "unlawful combatant", and Begg, a father-of-four from Sparkhill in Birmingham, arrested by the CIA in the Pakistani capital Islamabad in February 2002, were reportedly held in isolation cells for 18 months after the Pentagon decided they should stand trial as al-Qaeda members.
These cells are in Camp Echo, a high-security facility within Camp Delta. Inmates there do not have rights to association, recreation or joint prayers, which are held by other detainees.
Camp authorities reportedly withdrew a guard from outside Begg's cell and put a CCTV camera in his place, after it was discovered that the guard had been talking to Begg.
Abbasi and Begg were moved out of solitary confinement and transferred from Echo to Delta in December, following protests from the Foreign Office about their health. The men's lawyers claimed both prisoners were suffering from post-traumatic stress and had attempted suicide.
Now that Abbasi, Begg and the two other prisoners are returning to the UK, should they expect to suffer from any long-term problems as a result of their experiences at Guantanamo? What can solitary confinement, the denial of contact with others and even of basic sensory stimulation, do to people?
Craig Haney, professor of psychology at the University of California at Santa Cruz, has written extensively about prisoners kept in solitary confinement.
Feroz Abassi, Richard Belmar and Moazzam Begg
He warns that they sometimes experience great difficulty adjusting to life in the real world.
"The most common problems are depression and feelings of profound alienation," he says. "There is a sense that no one in the free world quite understands what you have gone through or, frankly, understands what you are feeling.
"The routines of normal social life seem foreign and unfamiliar and, in many cases, become anxiety-arousing.
"A number of people who have been in long-term solitary-type confinement also experience some of the symptoms of post-traumatic stress disorder, including nightmares, anxiety or panic attacks, and instances in which they re-live aspects of the trauma to which they were exposed."
Professor Haney has noted that some prisoners released from solitary confinement even attempt to re-create the conditions they were held in, in an attempt to cope with how they feel.
"Some attempt to manage their feelings by literally re-creating the kind of environment from which they came - they live alone, literally or figuratively, arrange their rooms much as they were during their incarceration, and adopt many of the same behavioural patterns that they used in confinement to create a sense of internal control."
But the intensity of such feelings, says Professor Haney, varies from one inmate to another - depending on the prisoner's own levels of resilience and the kind of support he or she receives after being released.
"Prisoners vary in their response to this kind of confinement. We are not exactly sure why. But the most accepted explanations focus, first, on the nature of the social support and other resources that one has upon release and, second, on a kind of 'resiliency' variable - the notion that some people are better able to bounce back from trauma more quickly and effectively than others as a result of their personal or psychological resources."
British psychologists claim to have found similar symptoms in individuals held under anti-terror laws at Belmarsh high-security jail in south London. These people are not held in solitary confinement, but like the Guantanamo detainees have not had a trial. Ministers say the Belmarsh detainees receive proper care and attention.
Ian Robbins, professor of mental health practice at the European Institute of Health and Medical Sciences, along with other top psychiatrists, wrote a report on the Belmarsh prisoners' health.
They concluded that: "Indefinite detention is linked to deterioration in mental health and fluctuations in mental state are related to the prisoner regime and to the vagaries of the appeal system."
Prof Haney says that health problems can be more acute when prisoners are unsure of why they are being held and when they will be released - such as those at Guantanamo, which has been described by some as a kind of "legal limbo".
"Domestic prisoners who are held in solitary-like confinement under similar circumstances [as those in Guantanamo] often complain about the uncertainty of their confinement - not knowing why they are being held there or, and this is most important, when they are getting out or what have to do in order to be released."