The prefrontal cortex was the anterior part of the frontal lobe of the brain, located at the very front of the brain. The prefrontal cortex controlled higher reasoning.
In 2151, Klaang began to speak gibberish in Klingonese after his prefrontal cortex was hyperstimulated. (ENT: "Broken Bow")
Radiation from a trinary system encountered by Enterprise NX-01 in 2152 had an effect on the crew's prefrontal cortices, causing them to obsess over trivialities. (ENT: "Singularity")
In 2153, vasodilation in Hoshi Sato's prefrontal cortex caused her to experience headaches. (ENT: "Exile")
The Ktarian game affected the prefrontal cortex of a victim, partly explaining their addiction to it. (TNG: "The Game")