Aphasia is caused by damage to the language centers of the left hemisphere in the region of the sylvian fissure. Nearly 98% of aphasia cases can be traced to damage in the perisylvian area of the left hemisphere of the cerebral cortex. Remember, however, that in the occasional individual language is localized elsewhere; and in children language is not yet fully localized.
Strokes cause 85% of all aphasia cases; other causes include cerebral tumors and lesions. One in 200 people experiences aphasia, with males more at risk. Gradual recovery is possible in 40% of adult cases; pre-pubescent children are much more likely to recover from aphasia, with the language faculty localizing in another, unaffected area of the brain, usually the perisylvian cortex of the right hemisphere. Generally, the more extensive the injury, the greater the likelihood of permanent damage.
But we have seen that language is a complex of interacting components--consonants and vowels, nouns and verbs, content words and function words, syntax and semantics. Could it be that these components are housed in particular sub-areas of the left hemisperic perisylvian cortex? We haven't pinpointed whether nouns are stored separately from verbs, or where the fricative sounds are stored. There is no conclusive proof for that type of specialization of brain tissue. But there is compelling evidence to believe that two special aspects of language structure are processed by different sub-areas of the language center. We know this because damage to specific areas of the peresylvian area produces two basic types of aphasia.
Each of these two types of language loss is associated with damage to a particular sub-region of the perisylvian area of the left hemispheric cortex.
(1861) Paul Broca discovered Broca's area (located in the frontal portion of the left perisylvian area) which seems to be involved in grammatical processing. (While parsing sentences such as fat people eat accumulates, there is a measurable burst of neural activity in Broca's area when the last word is spoken.) Broca's area seems to process the grammatical structure rather than select the specific units of meaning. It seems to be involved in the function aspect rather than the content areas of language)
Broca's aphasia involves difficulty in speaking. For this reason it is also known as emissive aphasia. Broca's aphasics can comprehend but have great difficulty replying in any grammatically coherent way. They tend to utter only isolated content words on their own. Grammatical and syntactic connectedness is lost. Speech is a labored, irregular series of content words with no grammatical morphemes or sentence structure. (Read example) Grammar rules as well as function morphemes are lost. Broca's aphasia is also known as agrammatic aphasia. Grammar is destroyed; the lexicon more or less preserved intact.
(1875) Karl Wernicke: Wernicke's area (in the lower posterior part of the perisylvian region) controls comprehension, as well as the selection of content words. When this area is specifically damaged, a very different type of aphasia usually results, one in which the grammar and function words are preserved, but the content is mostly destroyed.
Since Wernicke's aphasia involves difficulty in comprehension, in extracting meaning from a context, it is also known as receptive aphasia. Wernicke's aphasics easily initiate long-winded, fluent nonsense, but don't seem able to respond specifically to their interlocutor (unlike Broca's aphasics, who can understand but the have difficulty replying). Wernicke's aphasics often talk incessantly and tend to utter whole volumes of grammatically correct nonsense with relatively few content words or with jibberish words like "thingamajig" or "whatchamacallit" instead of true content words. (Read example.) Because Wernicke's aphasia patients can utter whole monologs of such contentless grammatical babble, hardly letting their interlocutor get a word in edgewise, their affliction is also known as jargon aphasia.
The normal human mind uses both areas in unison when speaking. Apparently, normal adults use the neurons of Wernicke's area to select sounds or listemes. We use the neurons of Broca's area to combine these units according to the abstract rules of phonology and syntax--the elements in language which have function but no specific meaning-- to produce utterances.