The telescan cephalic implant was technology developed by the Starfleet Cultural Survey Bureau in 2269 to aid in studying the civilizations of other planets. It was a small device surgically placed behind the user's right ear which established a telepathic link with the mind of one of the planet's natives, allowing instant knowledge of their language and behavioral patterns.
The telescan cephalic implants were tested by members of the USS Enterprise crew when they studied the Kyrosians of Kyros. They discovered that a side-effect of the implants was that some of them began imitating the behaviors of their "dops." This caused problems when Spock's link with Chag Gara, a mentally disturbed religious leader, resulted in him believing that he was the Kyrosian Messiah. (TOS novel: Spock, Messiah!)