Perception filter

A perception filter is a telepathic effect often caused by Gallifreyan technology often associated with a TARDIS and various related objects such as TARDIS keys, which shifts perception away from the object the filter is cast on. As Martha Jones put it, "you know it's there, but you don't want to know it's there". (DW: The Sound of Drums)

Particularly intelligent people are immune to the effects of a perception filter (DW: Human Nature) much like psychic paper (DW: The Shakespeare Code). Once one becomes aware of a perception filter or has their attention spiked, it ceases to work. (DW: Utopia, TW: Everything Changes)

Known Uses
Perception filters are a possible explanation of why the blue box that is the Doctor's TARDIS fails to be noticed when parked in unfamiliar territories across the time and space.

In Cardiff, the Doctor's TARDIS, in conjunction with the Cardiff rift, accidentally applied a perception filter to a slab of pavement on Roald Dahl Plass by transferring its dimensionally transcendental properties (DW: Boom Town). The Torchwood Institute made use of this by having Torchwood 3 convert that tile into an entry their base, referred to as the "invisible lift". (TW: Everything Changes)

In The Year That Never Was, Martha managed to shield herself from detection using her TARDIS key which the Doctor had equipped with one of these filters. It prevented her from being detected and allowed her to set up a plan to defeat the Master. (DW: Last of the Time Lords)

Not all perception filters were of Gallifreyan origin, as Prisoner Zero utilised a perception filter to hide a room in Amy Pond's house, in an effort to hide from the Atraxi, which went unnoticed for 12 years, until the Doctor arrived and showed Amy how to see past the filter. (DW: The Eleventh Hour)

Rosanna Calvierri possessed a perception filter in the form of an electronic device worn on her waist. Rather than preventing her from being observed, it merely caused anyone looking at her to perceive her as human, including providing clothing. (DW: The Vampires of Venice)

The Auto Pilot of a damaged Time ship used a highly advanced Perception filter to not only 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. It would appear to be a relatively recent innovation (possibly Time War-related?), as there are many occasions in the original series and 1996 telefilm where no perception filter effect is in evidence.
 * 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.