The midbrain was a portion of the central nervous system associated with vision, hearing, motor control, sleep/wake, alertness, and temperature regulation.
In the 22nd century, Vulcan mind melds were frowned upon partly because they caused a disruption of neuroelectric impulses in the Vulcan midbrain, which could lead to the early stages of Pa'nar Syndrome. (ENT: "Stigma")
In 2373, in an imaginary scenario in which the Sacajawea had crashed causing the death of Kathryn Janeway, Kes informed The Doctor there was minimal electrical activity in Janeway's midbrain. (VOY: "Coda")