Speech and other language abilities are lateralized brain functions, meaning they are all located on one side of the brain. For most people, the left hemisphere controls language. Shippensburg University states that the lateralized hemisphere varies on the person's dominant hand: 97 percent of right-handed persons have left hemisphere language areas, while 19 percent of left-handed persons have right hemisphere language areas; an additional 68 percent of left-handed persons have language areas in both hemispheres. If injury occurs to one of the language areas in the brain, the patient can have difficulties producing speech.
Paul Broca first discovered this language area in 1861, when he had a patient who could only say one word: 'tan.' After the patient's death, an examination revealed lesions on an area in the frontal lobe. The University of Washington states that Broca's area's function to produce speech. When the area is damaged, a condition called Broca's aphasia, the patient cannot form words properly and has slurred, slow speech. The patient, however, can understand speech.
The other main area responsible for speech is Wernicke's area, which was first discovered by Karl Wernicke in 1876. Located in the temporal lobe, Wernicke's area is responsible for understanding speech. The University of Washington notes that if the area is injured, termed Wernicke's aphasia, the patient will not say words that make sense.
Shippensburg University states that the arcuate fascilicus is a tract of nerves that connects Broca's area and Wernicke's area, which allows a person to create coherent, understandable speech. If the arcuate fascilicus is damage, it does not result in problems with speech production or comprehension, like damage to the two language areas does; instead, the person cannot repeat language that she has heard, a condition called conduction aphasia.