President Obama arrives at the White House on Monday, after a trip to Walter Reed National Military Medical Center in Bethesda, Md. Obama wrote an op-ed in Monday's Washington Post announcing new limits on the use of solitary confinement in federal prisons. Carolyn Kaster/AP hide caption

toggle caption Carolyn Kaster/AP

The plaintiffs in the case are inmates at Pelican Bay State Prison near Crescent City, Calif. "In the most severe cases," Juan Mendez says, indefinite solitary confinement — like that practiced at Pelican Bay — "can even be considered torture." Rich Pedroncelli/AP hide caption

toggle caption Rich Pedroncelli/AP

Nicklas Trujeque in his solitary confinement cell in New Mexico State Penitentiary. Inmates spend 23 hours a day in these cells, with a one-hour period in an open cell outside. According to the New Mexico ACLU, until recent state reforms, the average length of stay for an inmate here was around three years. Natasha Haverty/For NPR hide caption

toggle caption Natasha Haverty/For NPR

A part of Eastern State Penitentiary in Philadelphia is shown in 2008. The penitentiary opened in 1829, closed in 1971, and then historic preservationists reopened it to the public for tours in 1994. Matt Rourke/AP hide caption

toggle caption Matt Rourke/AP

New York Mayor Bill de Blasio tours and meets with youth Dec. 17 at Second Chance Housing on Rikers Island in New York City. Second Chance Housing is an alternative for incarcerated adolescents, instead of punitive segregation, also known as solitary confinement. Susan Watts/Getty Images hide caption

toggle caption Susan Watts/Getty Images

The U.S. holds more prisoners in solitary confinement than any other democratic country, according to critics of the treatment. Here, an immigrant detainee makes a call from his "segregation cell" at a detention facility in Adelanto, Calif., last November. John Moore/Getty Images hide caption

toggle caption John Moore/Getty Images