CHICAGO (AFP) - Robert King sat, slept and ate every meal alone in a cramped cell at Louisiana's notorious Angola prison for 29 long and lonely years.
His eyesight failed from lack of stimulation, but Mr King insists his resolve to fight the injustice of solitary confinement never did.
The Black Panther movement member said he managed to stay sane through the strength of his political convictions.
Many of the more than 80,000 prisoners currently held in solitary confinement in US prisons - and countless more around the world - are not as fortunate.
The human mind is not built for the sensory and social isolation of solitary confinement, researchers said on Friday at the annual meeting of the American Association for the Advancement of Science in Chicago.
Depriving someone of visual stimulation, human interaction, sunlight or physical activity can change their brain structure in a matter of days - let alone decades - said University of Michigan neuroscientist Huda Akil.
"Positive experience like touch actually activate positive things in our brains, including molecules we can measure that are like the fertilizers that make things growth," Dr Akil told reporters.
Depriving people of these important stimulants can be toxic, she said, adding that "a lot of structures in the brain actually shrink."
About a third of US prisoners in solitary confinement suffer from mental illnesses, said Dr Craig Haney, a psychology professor at the University of California, Santa Cruz.
Plenty of prisoners who entered solitary without mental ailments end up developing them as a result of the severe conditions.
Anxiety, depression and difficulties with impulse control are common. As are social phobias. The loneliness spurs some people to start hearing voices.
"So much of who we are depends on social contact," Dr Haney said. "When you remove people from that social contact, they lose their sense of self."
A major psychological experiment was carried out in the 19th century when hundreds of thousands of prisoners were subjected to solitary confinement.
The hope was that the isolation would rehabilitate prisoners by getting them to turn their thoughts inward and meet God. It soon became apparent that it actually drove many of them mad.
The reason it is still so widely used is because it is seen as the easiest way to protect guards from violent prisoners.
"You can either go for dynamic security where you do as much as you can to achieve low levels of violence by having a reasonably good culture," said Peter Scharff Smith of the Danish Institute for Human Rights.
Or, you can simply "give up" and stick prisoners in solitary.
Corrections officials in the United States were fully aware of the harm that solitary confinement causes to individual prisoners when it became embraced on a wider scale beginning in the late 1970s, Dr Haney said.
"We did it anyway because prisons were becoming overcrowded, there was an influx of the mentally ill into the prisons and we had abandoned rehabilitation," he explained.
"So they embraced solitary as a form of control and they have forgotten how do it any other way - it is inertia."
Mr King, who managed to win his freedom in 2001 after his conviction for a prison murder was overturned, continues to fight to free two other men known as the "Angola Three" who are approaching their fourth decade in solitary.
"The psychological impact, it extends to this day," Mr King told reporters.
But while he was crippled by the experience to the point where he still gets lost within a block or two of his home, Mr King said he never cracked.
"I said in my mind that even though I was in prison, I was not going to let prison get to me," Mr King said.
"We spent our time fighting with the system in order to affect change."