The paracortex was a part of the Betazoid brain which was the center of their telepathic abilities. Psilosynine was the key neurotransmitter present in the paracortex.
In 2370, following extended telepathic contact with the Cairn, Lwaxana Troi fell into a coma-like state when all brain activities fell off, except in her paracortex. (TNG episode: "Dark Page")
Other telepathic species also possess a paracortex. (SCE eBook: Lost Time)