As early as 1836, Marc Dax found that patients who could not speak properly had damage to the left side of the brain, and in 1861 Paul Broca described a patient who could say only one word - Tan (The Brain, 2005). When the patient died, he carried out an autopsy and discovered damage to the patient's left frontal cortex, which became known as Broca's area. In 1876, Karl Wernicke found damage in the posterior temporal lobe (Wernicke's area), connected to Broca's area by a bundle of nerve fibers, the arcuate fasciculus, also caused speech problems. Damage to the arcuate fasciculus causes conduction aphasia, in which people can understand language but their speech does not make sense and they cannot repeat words.
To speak a word that is read, information must travel from the primary visual cortex to the posterior speech area, including Wernicke's area, and from here to Broca's area, and then to the primary motor cortex (The Brain, 2005). To speak a word that is heard, the word must first get to the primary auditory cortex, then to the posterior speech area, including Wernicke's area, and from there to the primary motor cortex. Damage to Broca's area results in the inability to speak, the inability to form words properly, and slow, slurred speech. Damage to Wernicke's area results in a loss of the ability to understand language, and the person can speak clearly but cannot put words together in a meaningful way. In most people, both Broca's and Wernicke's areas are found only in the left hemisphere.
Electrical stimulation experiments have shown there can be large differences in the brain area that is important for language from one person to another (The Brain, 2005). Using amobarbital to anesthetize either the right or left hemisphere has also proved that language ability resides in the left hemisphere: when the right hemisphere is anesthetized, the person can still speak and understand language, but when the left side i...