The Time Lords used perceptual barriers to hide memories. When the person tried to think about the topic, the memories became vaguer. These memories could crop up when the person thought about something else. The Time Lords put a perceptual barrier inside Lucie Miller's mind, stopping her from remembering why she had been taken to the Eighth Doctor for safekeeping. (AUDIO: Blood of the Daleks) The Viyrans used a perceptual barrier to hide Lower Downham, making it seem to disappear from existence and from people's memories. (PROSE: No One Died)