Perception filter A perception filter , also known as a perceptual filter , was a telepathic effect which misdirected the senses around itself or the person using it . Functions and features . Perception filters had the effect of directing attention away from the object or its bearer , rendering them unnoticeable . Martha Jones described the effect on the viewer : " You know it ' s there , but you don ' t want to know it ' s there " . ( TV : " The Sound of Drums " ) It could also " trick " the memory . ( TV : " The Lodger " ) Any inconsistencies would be forgotten . ( TV : " Night Terrors " ) Particularly intelligent people were immune to the effects of perception filters . ( TV : " Human Nature " ) Some were also immune because they truly wanted to see what the filter was diverting . ( TV : " Last of the Time Lords " ) Once a person became aware of a perception filter and the object it was disguising , they could no longer be affected . ( TV : " Utopia " , " Everything Changes " ) Perception filters supposedly did not work on people who were not native to that time . ( PROSE : " The Lions of Trafalgar " ) Usage . While several examples originated from Gallifreyan technology , notably TARDIS es , other species did occasionally use them , frequently to escape notice by humans . ( TV : " The Sound of Drums " ) The TARDIS and associated objects . Several perception filters were created using items associated with the TARDIS , or even created by the TARDIS itself . The Tenth Doctor was able to construct a perception filter around three keys to the TARDIS , activated when they were worn around the neck on a chain . These keys had previously belonged to Martha Jones , Jack Harkness and the Doctor himself , and enabled the three of them to travel around London safely while they were on Britain ' s Most Wanted list . While they were eventually detected by , when Martha escaped she was able to continue to use the TARDIS key ' s perception filter to travel the world unhindered during the Year That Never Was , spreading a message about the Doctor . ( TV : " Last of the Time Lords " ) The Chameleon Arch ' s biodata modules also had their own perception filters , although in this case the filter acted only on the watch itself , making even its wearer unaware of its importance . Both the Tenth Doctor and the Master perceived their watches as " broken " while in human form . ( TV : " Human Nature " , " Utopia " ) At least one perception filter seemed to have accidentally been created by the TARDIS itself , possibly in conjunction with the Cardiff Space - Time Rift : a slab of pavement on Roald Dahl Plass near the Rift . The Torchwood Institute made use of this by having Torchwood Three convert that tile into an entry to the Hub , referring to it as the " invisible lift " . ( TV : " Everything Changes " ) The Eleventh Doctor activated a variation of the TARDIS ' perception filter that not only rendered the ship " invisible " but also caused the Doctor to forget its very existence . ( PROSE : " Dark Horizons " ) Other perception filters . Beginning in the 14th century , the entire country of Goritania was hidden from the world with a perception filter . ( AUDIO : " Death and the Queen " ) The Saturnyn s , led by Rosanna Calvierri , possessed a different type of perception filter when in Venice in the 16th century . These perception filters were in the form of an electronic device worn at the waist . Rather than preventing the Saturnyns from being observed , this filter caused anyone looking at them to perceive them as human , even providing clothing . However , it did not disguise mirror images , leading the human brain ( and the Doctor ' s ) to initially see no reflection when they were unable to process what they saw . Amy Pond broke Rosanna ' s perception filter by kicking her in the side where the device was and Rosanna was stuck looking human even after removing the device . ( TV : " The Vampires of Venice " ) In 1914 , the Testimony stated through a glass avatar that a perception filter would render itself , the First Doctor and the Twelfth Doctor invisible to Captain Archibald Hamish Lethbridge - Stewart when time resumed . The terminology left Archibald confused but he accepted it . The perception filter kept the Doctors hidden from Archibald and his German opponent , but only seemed to last a limited time as the Twelfth Doctor was able to give a confused Archibald a salute that he returned . ( TV : " Twice Upon a Time " ) In 1943 , several Endovorm positioned in Trafalgar Square used perception filters to disguise themselves as stone lions . Not being from that time , the Fifth Doctor , Nyssa and Tegan were able to see them . After a few decades , the filters wore off and the lions were visible . ( PROSE : " The Lions of Trafalgar " ) In 1996 Prisoner Zero used a perception filter to hide a room in Amy Pond ' s house , in an effort to hide from the Atraxi . The room went unnoticed for 12 years , until the Eleventh Doctor arrived and showed Amy how to see past the filter by looking out the corner of her eye . ( TV : " The Eleventh Hour " ) Bilis Manger had the ability to create perception filters . He used them to make his shop look like whatever was expected . ( PROSE : " The Twilight Streets " ) In 2008 , Majenta Pryce and her assistant Fanson used perception filters to send their invitations for Hotel Historia . ( COMIC : " Hotel Historia " ) In 2010 , 79B Aickman Road used a perception filter , not only to disguise the ship as the top floor of a house , but to alter people ' s memories into believing it had always been there , and , when the ship was destroyed , it would seem as though it was never there . ( TV : " The Lodger " ) In 2010 , an alien used a perception filter to hide a warehouse . The perception filter made it look like it burnt down . ( PROSE : " The Night After Hallowe ' en " ) The Tenza were capable of producing powerful perception filters shortly after birth . One , who took the name George , used a perception filter to alter his adoptive parents ' memories , causing them to forget they could not conceive and believe he was their biological son . ( TV : " Night Terrors " ) When the Kchrusivour invaded Earth in the 23rd century , they disguised themselves as Zonian s using perception filters . The Eleventh Doctor was able to break the devices and trap them in the Zonian forms . ( COMIC : " The Kchrusivour Gambit " ) Lum - Tee used a perception filter to disguise himself as a human on Earth . ( COMIC : " Down to Earth " ) A Cerebravore ' s perceptual filter prevented the Tenth Doctor and Gabby Gonzalez from seeing the emergency exit while being chased by a Cerebravore host . ( COMIC : " Revolutions of Terror " ) When converted into a Mondasien Cyberman , Bill Potts ' mind acted like a perception filter , convincing her she was still in human form . It was gradually broken when the Twelfth Doctor revealed what she really was . ( TV : " The Doctor Falls " ) Other references . The Eleventh Doctor suggested that the Weeping Angel s were using a perception filter when pretending to be statues in the Aplan Mortarium of Alfava Metraxis . However , he then dropped the idea , believing that he , his companion s and the soldiers of the Church may simply have been a bit thick . ( TV : " The Time of Angels " )