The Cairn were a humanoid species native to the planet Cairn, also known as Vair, and were members of the United Federation of Planets by at least the year 2378. The Cairn were almost indistinguishable from humans except for fleshy patches on the sides of their heads and a high degree of telepathy.
Prior to contact with the Federation, the Cairn had no known knowledge of spoken language, or the concept of privacy.
The Cairn communicate telepathically by mentally sending images allowing for a much more efficient form of communication but could learn to communicate verbally with time, effort and the use of vocal enhancers.
In 2370, two Cairn, Hedril and Maques, joined Lwaxana Troi on a diplomatic mission to the USS Enterprise-D during which the strain of serving as intermediary between the Cairn and the crew of the Enterprise in addition to the physical resemblance of Hedril to her deceased daughter, Kestra, caused Troi to lapse into a coma. (TNG episode: "Dark Page")