Scientists call solitary confinement 'damaging and unnecessary'
"To this day, I find it difficult to associate myself with my surroundings. My terrain, my geography is way off," says Robert King.
"It is off to the point where I get lost even when I am walking just around the corner from where I live."
King spent 29 years in solitary confinement, living in a box just 6ft by 9ft by 12ft.
He says it conditioned him. "But I managed to weather it. I like to say that although I was in prison, I didn't allow prison to get in me," he told BBC News.
King is famous as one of the "Angola Three", who were convicted of murdering a prison guard in 1972.
The trio were all put in solitary at the Louisiana State Penitentiary, also known as Angola Prison.
King won his freedom 12 years ago, his conviction quashed, and Herman Wallace likewise last year, although he died from cancer a few days after his release.
Albert Woodfox, however, continues to fight his incarceration. He has been in solitary now for 41 years.
King took to the platform here at the annual meeting of the American Association for the Advancement of Science (AAAS) to decry the practice of solitary confinement.
He was joined by experts in the fields of law, neuroscience, and psychology. Their complaint was that solitary was both damaging and unnecessary.
Prof Craig Haney said it was remarkable how well Robert King had managed to adjust to freedom after so many years alone in a cell "not much bigger than a king-sized bed".
The University of California, Santa Cruz, researcher has been studying conditions in US prisons since the late 1970s, and in particular the impacts that long periods of isolation can have on inmates' state of mind.
Although definitions vary, some 80,000 individuals across America are thought currently to be held in solitary, spending sometimes 23 hours a day in their cells; and even when being taken for exercise, doing that activity on their own.
Prof Haney has interviewed more than 500 prisoners in such situations, and is in no doubt that the added deprivation these people face over and above their incarceration is extremely destructive.
"People who are in these units are in pain," he said.
"This is not an experience that is easily tolerated psychologically, and people suffer in response to it. There are variations in the response, of course.
"Not everybody is in agony. Some people appear to be somewhat more resilient than others. Some people are able to implement a strategy that enables them to survive the severe sense of loss and functioning. But these people are in many ways unusual.
"The more common response is for people to become increasingly depressed, and to be enveloped by a sense of hopelessness."
Self-harming is not an uncommon practice, he says, and there are documented cases of individuals with no pre-existing mental illness or psychiatric symptoms going on to develop them in the course of their isolation.
Prof Huda Akil from the University of Michigan studies the brain biology of stress, emotions and depression. She said there was ample data in the literature to demonstrate the deleterious effects of solitary confinement.
"We have a vast amount of knowledge about the brain and how it responds to each one of the elements [of solitary confinement]. The lack of physical activity, the lack of interaction with the natural world like sunlight, the lack of interactions with other human beings, the lack of visual stimulation, the lack of touch - each one of those has been studied not just in humans but in animal models such as rodents. And each one by itself is sufficient to change the brain and change it dramatically."
The use of solitary confinement in the US grew rapidly during the 1980s with the spread of so-called "supermax", or "super-maximum security", prisons.
Legal challenges are now being brought against these jails.
In California, 1,000 inmates at the Pelican Bay facility are part of a class action being taken to the federal court. The prisoners are arguing that their isolation violates the American Constitution and international law because it is cruel, inhumane and degrading.
Prof Jules Lobel from Pittsburgh Law School is representing them. He rejected the notion that these were all violent people, a danger to others and themselves.
"In Pelican Bay, many of the prisoners are there, not because they have committed any violent act within prison, but simply because they're alleged to be associated with some gang. Often, that's just because they have artwork or tattoos that are seen as gang-related.
"And even those few people that might warrant segregation, there's no justification for treating them inhumanely. There is no reason penologically why you can't have windows in a cell, why you can't give people some out-of-cell time to recreate in small groups with very high security, or why you can't give people phone calls."
Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos