The Cairn are a telepathic species who are so far advanced telepathically that they have lost the ability to communicate verbally. Unlike most other telepathic species, Cairn telepathy sends images rather than words, allowing for a much higher rate of information exchange. Due to this fact, other telepathic species have difficulty communicating with the Cairn, though it is possible with training. Privacy was a concept unknown to the Cairn before they started to interact with the Federation.
Ambassador Lwaxana Troi was assigned to help the Cairn integrate with the Federation by both communicating with them telepathically and teaching them to communicate verbally with the help of artificial voice boxes. This caused strain on her and that, coupled with working with Hedril, a Cairn youth, brought back painful memories involving the death of her daughter Kestra. (TNG: "Dark Page")