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, (AUDIO: The Lions of Trafalgar) although they were proven to still work on the Doctor and their companions on numerous occasions. (TV: The Vampires of Venice, Rosa)

Overview
While several examples originated from Gallifreyan technology, notably TARDISes, 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 perceived their watches as "broken" while their human forms, John Smith and Professor Yana respectively. (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 Saturnyns, 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) The Doctor suspected that the power of his regeneration was interfering with the perception filter. (PROSE: 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) He also used one so that Rowena Protheroe, Piers Meadows and Zoe Blake would perceive him as a stranger. (AUDIO: Dead Man's Switch)

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 was given the name George by his foster parents, unconsciously used a perception filter to alter his 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 Zonians 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 Mondasian 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)

Krasko's equipment was protected by a perception filter to disguise it from others noticing it. (TV: Rosa)

During the Second World War, used a "tiny, Teutonic perception filter" to convince Nazi soldiers that he was their commanding officer. However, the Thirteenth Doctor later disabled the filter with her sonic screwdriver. (TV: Spyfall)

During a temporal crisis where incarnations of the Doctor and their comapnions were flung from Waypoint to Waypoint, one of these Waypoints happened to be the Panopticon on Gallifrey. They used perception filters to move through it unnoticed. (GAME: Lost in Time)

Other references
The Eighth Doctor explained eye filters to Liv Chenka by saying that they were similar to a TARDIS's perception field. (AUDIO: The Eleven)

The Eleventh Doctor suggested that the Weeping Angels 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 companions and the soldiers of the Church may simply have been a bit thick. (TV: The Time of Angels)

Origins
The perception filter is very similar to the "Someone Else's Problem field", as mentioned in The Hitchhiker's Guide to the Galaxy books by former Doctor Who writer and story editor Douglas Adams, widely understood to take place in the Doctor Who universe. In fact, the novel in which it is first mentioned, Life, the Universe and Everything, was, originally pitched to be a Doctor Who serial, The Krikkitmen.

Later, the Doctor Who novel A Sourcebook for Field Agents mentioned the concept of perceptual induction, aa condition where "the perceptions of an individual are fooled into believing that reality is not what it seems… The aberration usually takes the form of seeing something that the individual wishes to see instead of the actual physical reality".

Ultimately, the first mention of a perception filter in a Doctor Who-related story, under that name, was in the first episode of Torchwood, Everything Changes. The concept was subsequently used in Doctor Who proper.

Other matters

 * The Doctor Who Confidential accompaniment for Human Nature suggests that Timothy Latimer, the boy who was immune to the perception filter, was a genetic mutant born with an extra piece in his brain. The final script makes no mention of his being a "mutant", although the Doctor does note that he has an extra telepathic synapse in his brain, accounting for his strange abilities. (CON: Alter Ego)