I first entered Louisiana State Penitentiary in the early 60s, at the age of 18. I was in and out of that place for the rest of the decade. Back then, if you were young, black and had a record, police in New Orleans would come looking for you when they had a backlog of unsolved cases: it was called cleaning the books.

In 1969, I was locked up for a robbery I didn't do and, while inside, I joined the Black Panthers. Three years later, an inmate was stabbed to death on my prison block and, because of my politics, the authorities saw a chance to pin it on me. In 2001, I was cleared of this killing but, by then, I had spent 29 years alone in a cell.

It was a dimly lit box, 9ft by 6ft, with bars at the front facing on to the bare cement walls of a long corridor. Inside was a narrow bed, a toilet, a fixed table and chair, and an air vent set into the back wall.

Some days I would pace up and down and from left to right for hours, counting to myself. I learned to know every inch of the cell. Maybe I looked crazy walking back and forth like some trapped animal, but I had no choice – I needed to feel in control of my space.

At times I felt an anguish that is hard to put into words. To live 24/7 in a box, year after year, without the possibility of parole, probation or the suspension of sentence is a terrible thing to endure.

I was kept in the closed cell restricted (CCR) wing of the penitentiary, which is also known as Angola, after the slave plantation that was on the site prior to the prison. Three times a week I was let out for an hour to go to the exercise yard, where I was kept separate from other prisoners by razor wire.

The wardens tried to discourage us from talking, but we defied them. We were beaten up and prisoners were found hanging in their cells. Whenever I was disciplined, it was for talking. I didn't care, I refused to let them dehumanise me.

The worst punishment was the "cold box", our name for the cell within Camp J. It was down a long hallway through three sets of secure doors, and when they pushed me inside, the isolation was total. They would keep me there for a month, in blocks of 10 days, shoving food through a slot in the door. I went for days without speaking to anyone. That kind of sensory deprivation was torture for me – to survive I knew I had to keep my mind active.

One pastime I had was smuggling out praline candies that I made on my cell floor. I traded tobacco to get the ingredients of sugar, peanuts and powdered milk. I made them using a cold drink can for a pot and burning toilet paper to melt sugar.

Another thing I did was to fold up toilet paper into squares and stick them to the floor with toothpaste to make a chessboard. I would call out moves to other inmates. When we were in nearby cells I played with Herman Wallace or Albert Woodfox. Like me, they were Black Panthers kept in solitary because they were seen as a threat. They had started a chapter of the Panthers, which had helped mobilise inmates to curb some of the abuse going on inside Angola at the time.

They are still in solitary after nearly 38 years – more than any other inmate in the American prison system. They were convicted of killing a prison guard in 1972, but there's a lot of evidence that they're innocent.

Since my conviction was overturned in 2001, I have travelled constantly, educating people about the widespread use of solitary confinement in America. The words of the US Constitution prohibit what is called "cruel and unusual punishment", and yet that phrase could have been written to describe solitary confinement.

When I walked out of Angola, I didn't realise how permanently the experience of solitary would mark me. Even now my sight is impaired. I find it very difficult to judge long distances – a result of living in such a small space. Emotionally, too, I've found it hard to move on. I talk about my 29 years in solitary as if it was the past, but the truth is it never leaves you. In some ways I am still there. I made a statement when I was released that although I was free of Angola, it would never be free of me. Until Herman and Albert can join me on the outside, I have to make good on that promise.

As told to Paul Willis

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