In lectures 7 and 8 we looked at the way we can infer things about the way the brain processes language by looking at naturally occurring errors in speech and comprehension. Cases where the brain has suffered damage as a result of disease or trauma can also give us clues about the way that things work.
Broca was one of the first to investigate "aphasia" (lit. "without speech") - Trask says "dysphasia" is preferred, since in many cases the ability to speak is not lost, but impaired in some way. Broca performed an autopsy of a patient who had suffered a severe form of aphasia and found damage evident in a particular area of the brain, subsequently identified as "Broca's area". Patients suffering from "Broca's aphasia" appear to have great difficulty in speaking - it seems that they are often unable to find the right word, and so their speech is typically very laboured and slow. Comprehension is not so dramatically affected; Broca's aphasics can understand what is said to them, although more complex sentences can cause difficulties.
Wernicke's aphasia is named after another clinician who identified damage to Wernicke's area which causes a different set of symptoms: patients speak very rapidly and apparently fluently, but their speech often does not make sense. Wernicke's aphasics also have significant difficulty with comprehension. To quote Trask, "damage to Wernicke's area largely destroys comprehension and severely impairs access to vocabulary. Damage to Broca's area, in contrast, destroys grammatical structure and impairs the production of speech" (p148).
Trask also points out that a user of sign language who suffers from Broca's or Wernicke's aphasia shows similar problems when attempting to sign - i.e. the damage appears to affect linguistic ability, rather than the mechanical system responsible for speech production.
Initially, it was thought that brain function was highly localized: recent studies using brain scanning techniques appear to show that language ability does appear to be lateralized in the left hand side of the brain, and that it is a modular faculty: "it is independent of other cognitive systems with which it interacts" (Fromkin p.62)