Human speech is thought to be centered in and near Broca's area, a clump of brain cells in the front of the brain. Injury to Broca's area produces a loss of both speech production and comprehension of grammar. Deacon found the equivalent of Broca's area in the brains of monkeys. Surprisingly, however, damage to that area does not interfere with the ability of monkeys to communicate by means of some two dozen calls and sounds. In the former technique, selected brain circuits can be temporarily shut down, electrically speaking, and then researchers determine how this affects a person's speech.
PET scans reveal metabolic activity in the brain.
They show that brain cells in front of Broca's area become particularly active when a person makes semantic associations, such as naming all the objects in a room. Don't Cry Therein lies a paradox. Monkeys don't cry, they don't laugh the way that we do, and they still communicate by sounds when their Broca's area is destroyed. Deacon believes that the answer lies in the development of the prefrontal cortex. As proportions of connections in this area increased relative to other pans of the expanding brain, the gray matter of primates became reorganized.