A court in Toronto is hearing for a second day why Canada's system of solitary confinement for inmates is unconstitutional.
Civil liberties lawyers say administrative segregation, among other things, is procedurally unfair and structurally deficient.
They say no meaningful review exists of a warden's decision to place someone in isolation.
Inmates in administrative segregation are locked away by themselves for 23 hours a day.
It is used when prisoners are at risk from other inmates, or pose a threat to the security of the prison.
Research, however, suggests the practice can cause extreme psychological harm, especially when prolonged.