Eventually, Kupers said, a general numbness often sets in.
"I find that prisoners who have spent decades in solitary confinement give up trying to communicate, for instance not even bothering to speak to the officer who delivers their food tray, or not saying good morning to the prisoner in the next cell," Kupers said.
"Then, in order to suppress the anger that evolves, they start suppressing all feelings so they begin to feel numb, then lifeless, then dead. A large number of prisoners in many states have described this hyper-isolation, even in the context of solitary confinement, and this numbing into a dead state."
Half of successful suicides in prisons are prisoners in solitary confinement, Kupers pointed out -- even though that's only 4-6 percent of prisoners.
The United Nations lead investigator on torture, Juan Mendez, has urged the United States to ban the use of prolonged solitary confinement, citing its mental and physical toll.
Supporters of Wallace hope that his case will spark changes in the penal system.