In most people, the speech area is in the left cerebral cortex. Two major cortical areas are involved in speech: The sensory speecharea (Wernicke area), located in the parietal lobe, functions inunderstanding and formulating coherent speech. The motor speecharea (Broca area), located in the frontal lobe, controls the move-ment necessary for speech (see figure 8.27). Damage to these parts of the brain or to associated brain regions may result in aphasia (ă-fā′ zē-ă; a-, without + phasis, speech), absent or defective speech or language comprehension. The most common cause is a stroke. It is estimated that 25–40% of stroke survivors exhibit aphasia.
Speech-related functions involve both sensory and motor pathways. For example, to repeat a word that you hear involves the following pathway: Action potentials from the ear reach the primary auditory cortex, where the word is perceived; the word is recognized in the auditory association area and comprehended in portions of the sensory speech area. Action potentials representing the word are then conducted through nerve tracts that connect the sensory and motor speech areas. In the motor speech area, the muscle activity needed to repeat the word is determined. Action potentials then go to the premotor area, where the movements are programmed, and finally to the primary motor cortex, where specific movements are triggered.
Speaking a written word involves a slightly different path-way: The information enters the visual cortex, then passes to the visual association area, where it is recognized. The information continues to the sensory speech area, where it is understood and formulated as it is to be spoken. From the sensory speech area, it follows the same route for repeating words that you hear: through nerve tracts to the motor speech area, to the premotor area, and then to the primary motor cortex.