"Extreme solitary confinement and reduced environmental stimulation" is how Thomas Silverstein, a convicted multiple murder, has described conditions at the Supermax prison.

In a 2007 lawsuit challenging the conditions of his imprisonment, Silverstein said he was subject to 24-hour surveillance, locked in an 8.5ft by 10ft cell for as many as 23 hours a day, and took meals and exercised alone.

The cells in "Z-Unit" - a special segregation unit - contained a bed with metal restraint rings, a sink, toilet and shower.

One had a mirror, cement walls and a small window he could only see out of when he stood on his desk. The other cell had no mirror - only a small window covered in mesh and painted over, he said.

Silverstein, 60, had been convicted of murdering another prisoner at a maximum security prison in Illinois. He said he exercised an hour a day by himself in 10ft by 10ft "dog-kennel-like" cement yard attached to his cell.

He said he was "entombed" by a sound-proof door that prevented him from talking to other inmates, and in any case he was barred from contacting other inmates.

The conditions, he said, caused depression, hallucination, disorientation and memory loss.

He said the damage went "beyond the boundaries of what most human beings can psychologically tolerate".