Perception filter

A perception filter was a telepathic effect which misdirected the senses around itself or the person using it. While several examples originated from Gallifreyan technology (especially the TARDIS), other species did occasionally use them, frequently to escape notice by Humans.

The TARDIS and associated objects
Several perception filters were created using items associated with the TARDIS, or even created by the TARDIS itself. These had the effect of directing attention away from the object or its bearer, rendering them unnoticeable. Martha Jones described the effect on the viewer as "you know it's there, but you don't want to know it's there". (DW: The Sound of Drums) It is even possible that the TARDIS had its own associated perception filter, explaining why it was seldom noticed despite landing in many locations through time and space.

The 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 the Master, 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. (DW: Last of the Time Lords)

The fob watches associated with the Chameleon Arch 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. (DW: Human Nature, Utopia) Both the Doctor and the Master perceived their watches as 'broken' while in human form.

At least one perception filter seems 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 3 convert that tile into an entry to the Hub, referring to it as the "invisible lift". (TW: Everything Changes)

Particularly intelligent people were immune to the effects of a perception filter of this type (DW: Human Nature) much like the psychic paper (DW: The Shakespeare Code). Once a person became aware of a perception filter and the object it was disguising, they could no longer be affected. (DW: Utopia, TW: Everything Changes)

Perception filters not associated with the TARDIS
Not all perception filters were of Gallifreyan origin. Prisoner Zero used a similar 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 Doctor arrived and showed Amy how to see past the filter. (DW: The Eleventh Hour)

The Saturnynians, led by Rosanna Calvierri, possessed a different type of perception filter, in the form of an electronic device worn at the waist. Rather than preventing the Saturnynians 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, leaving 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. (DW: The Vampires of Venice)

The Auto Pilot of a damaged Time ship used a highly advanced 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 here. (DW: The Lodger)

Behind the scenes

 * 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 this, although the Doctor does note that he has an extra telepathic synapse in his brain, accounting for his strange abilities.
 * The series has yet to establish when the perception filter was installed or activated in the TARDIS for the first time. The earliest known reference to the TARDIS posessing it occurs in BFA: The Architects of History, meaning it must have been installed some time prior.
 * The perception filter is very similar to the "Someone Else's Problem field", as mentioned in the Hitch-Hiker's Guide to the Galaxy books by former Doctor Who writer and story editor Douglas Adams. In fact, the novel in which it is first mentioned, "Life, the Universe and Everything", was, originally, supposed to be a serial of Doctor Who, "Doctor Who and the Krikkitmen", but was turned down.