Perception filter

A perception filter is a telepathic effect associated with the TARDIS which causes items to appear insignificant, perhaps even to the extent of near-invisibility. It is a possible explanation of why the blue box fails to be noticed when parked in unfamiliar territories across the time and space. Time Lords can use their TARDISes to place perception filters upon small objects such as Chameleon Arch fob watches (DW: Human Nature) in order to keep them safe.

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).

The "Doctor Who Confidential" accompaniment for "Human Nature" suggests that the boy (Tim Latimer) 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.

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).

Using his TARDIS keys, the Doctor is able to create portable perception filters for himself, Martha and Jack while they are on the run from Mr Saxon (DW: The Sound of Drums).