The diencephalon was a region of the humanoid brain, posterior to the forebrain. Within this region, the hypothalamus, thalamus, and the epithalamus were located.
After Deanna Troi fell into a telepathically-induced coma in 2368, Doctor Martin reported strange neurotransmitter readings in her diencephalon. (TNG: "Violations")
Soon after, in an attempt to reverse Kieran MacDuff's memory loss, Doctor Beverly Crusher was puzzled by the lack of increase in diencephalic activity in his brain. This was because "MacDuff" had no such memory loss. (TNG: "Conundrum")