Understanding and speaking a language involve several region of the cortex. Much of what we know about these areas, which are most highly developed in human beings, comes from studies of patients with language or speech disturbances that have resulted from brain damage.
The motor speech (Broca's) area, auditory association (Wernicke's) area, and other language areas are located in the left cerebral hemisphere of most people, regardless of whether they are left-handed or right-handed. Injury to the sensory or motor speech areas results in aphasia ( a = without; phasis = speech), an inability to speak.
Damage to the motor speech area results in nonfluent aphasia, an inability to properly articulate or form words. The person knows what she wishes to say but cannot speak. Damage to the gnostic area or auditory association area (area 39 and 22) results in fluent aphasia, faulty understanding of spoken or written words.
Such a patient may fluently produce strings of words that have no meaning. The deficiency may be word deafness, an inability to understand spoken word, or word blindness, an in ability to understand written words, or both.
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