Segregation units can be the quietest part of a prison
As part of our week-long series on prisons we speak to those who have experienced segregation and solitary confinement.
It is difficult for most people to imagine what it must be like to live in a world without other people.
To some it is "a prison within a prison" to others simply "seg".
Those who are held there have broken the prison rules, are dangerous, or are in danger.
Prisoners describe segregation units as having a very different atmosphere to the hustle and bustle of a normal prison wing.
At best those who are held there have contact with a handful of prisoners as opposed to the dozens on their normal wing.
Robert Maudsley, who killed a fellow patient while at Broadmoor secure hospital, and then a further two after being transferred to a prison, and Charles Bronson, who has a lengthy history of assaults and hostage-taking, have both spent many years in solitary.
Frank Cook knows what life is like for those who serve their time alone.
Cook was in prison for 27 years for crimes including armed robbery, gangland warfare, and firearms.
"I was born into it. It came from my parents, a family of criminals.
"Other kids got praised for getting a certificate at school, I got praised for ripping people off."
He estimates 11 of his years inside were spent in some form of segregation or solitary confinement.
"My record was appalling. My record inside was worse than my record outside - hostage-taking, slashing and smashing other inmates."
It is 12 years since he last spent time in prison, and he has since written a book, Hard Cell, about his experiences.
His voice full of nervous energy, Cook confides: "I found comfort in the end. It was like going back to the womb.
"It hurts you in comparison to what you are used to, but once you get used to it, it becomes a cocoon. You become safe there because nobody cares about you.
"I became conditioned to it in the end. I had won. They had nothing else to give me. I was actually in charge of my predicament."
Cook eventually benefited from a rehabilitation scheme aimed at disruptive prisoners.
"In the end they won through with their kindness and the ability to understand me. They talked to me properly, called me Frank."
Charles Bronson's violent record kept him in solitary
John Kamara spent nearly two decades in prison for a murder - that of a Liverpool bookmaker in 1981 - he did not commit. His conviction was quashed in March 2000.
Knowing he was not guilty made obeying the myriad rules and regulations of prison life hard to stomach, and he spent virtually all of his sentence in segregation.
"I wasn't accepting the prison and really there was nowhere for them to send me. They couldn't send me any lower.
"I wouldn't work and they used to fine me and put me in the [segregation] block. I eventually thought I'll just stay in the block. Sometimes I refused to leave.
"You were allowed an hour of exercise, library once a week and the rest was behind your door for 23 hours. You might get exercise with one other person but that was it."
There were was one unexpected benefit of his "punishment" for refusing to accept the prison regime.
"I found that I could get on with fighting my case because I wasn't getting distracted, with all my documents, writing letters to people."
But clearly spending so much time alone has had its effect.
He adds: "Even now, six years on, I still like being on my own. Sometimes I still go out all hours of the night driving. I still get a bit panicky in crowds."
Professor David Wilson knows the effects segregation and solitary can have on mental health.
The academic, formerly Britain's youngest prison governor before he became disillusioned with the system worked at Grendon, Wormwood Scrubs and Woodhill.
He is now one of the most outspoken critics of the way the prison system is administered.
"Long-term solitary confinement has a very detrimental effect.
"It cuts them off. They don't interact, they don't eat in company, not able to socialise, they lose interpersonal skills, become withdrawn and depressed and in some cases, can become suicidal."
Prof Wilson said it is nearly impossible to truly isolate a prisoner.
"The ability of prisoners to overcome their particular circumstances that they are in is legendary. They will shout, try and have notes passed, tap messages on pipes."
He admitted: "There are some prisoners who are such a threat to other prisoners that they have to be kept separate.
"Charles Bronson would routinely take hostages, routinely assault staff and was therefore somebody you couldn't even in segregation allow to have access [to other prisoners].
"There are some but those prisoners are a very small minority."
When a prisoner has had a form indicating they are a suicide or self-harm risk, the Prison Service says segregation must only be used in exceptional circumstances.
Prof Wilson adds: "Segregation units are usually prisons within prisons, not very visible. A lot of the maltreatment of prisoners has historically gone on in segregation units.
"They are far more spartan, there is a sense of us and them, a sense of they only get what they are entitled to, and that has to be guarded against."
Indeed the most serious allegations of abuse within prisons in recent years centred on the segregation unit at Wormwood Scrubs.
Dr Kimmett Edgar, of the Prison Reform Trust, said a disproportionate number of prison suicides happened in segregation.
But he emphasises: "It isn't meant to be psychologically cruel."
The Prison Service says that no figures are kept on the use of segregation and solitary across the prison system.
While each prison does keep records, these figures are not collated centrally.
The Prison Service emphasises it is only sparingly used as a disciplinary tool.
But the issue will be examined in detail by Prisons Inspector Ann Owers, who will publish a report on segregation units and close supervision centres in high security prisons in July.