A perceptual filter was a component of a holographic program run on a holodeck. It kept the holodeck characters oblivious to anything outside the parameters of the program. This allowed a participant of the program to give computer commands to the holodeck system without the characters responding to such an action. It also kept the characters from realizing that they were holograms. Running a holographic program for an extended period of time uninterrupted can cause the perceptual filters to malfunction.