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Aphasia, specifically the type that affects Broca’s or Wernicke’s area, is one of the prime examples used by linguists as evidence that linguistic ability is lateralized (located almost entirely in) in the left hemisphere of the brain. Damage to either region will result in speech disorder, losing syntax with lesions or injuries to the Broca region (frontal region) and lexicon when in the Wernicke area (posterior temporal and lower parietal regions). Damage to equivalent areas in the right hemisphere has no affect on lingual ability. (It must be mentioned, though, that the ability to comprehend semantics may be impaired with damage to the right hemisphere.) Studies of children with unilateral damage to the left hemisphere exhibited deficiency in acquiring language, while those with right hemisphere damage acquired language like normal children.

Perhaps the greatest piece of evidence, more final than studying brain damage, is the lack of advanced linguistic ability in left hemidecorticates. Hemidecorticates have only one hemisphere of the brain, either by surgical removal or lack since birth. Right hemidecorticates may still acquire language normally, those without a left hemisphere are severely impaired. Furthermore, children born as left hemidecorticates do not develop language normally, proving that language is not only left hemisphere lateralized but innate in human beings.

Most but not all humans have left hemisphere specialization for language abilities. A left handed individual is somewhat more likely to have right hemisphere speech lateralization than a right handed person is, but still has a better than fifty percent chance of having speech abilities located in the left hemisphere.

The question of lateralization is not always as simple as picking a side. A few people show little lateralized specialization. In addition, the right hemisphere seems to often contribute more to the emotional intonations of speech than the left in people whose speech facilities are otherwise located in the left hemisphere.

Aphasia is a disorder where a person has trouble speaking and comprehending speech. It may be caused by damage to either Broca's or Wernicke's area, but is considered to affect an individual as a whole and not an area of the brain. One of the ways neurologists learned what they know is by studying people with brain damage, and checking what effect if any it had on their speech ability. The areas that were damaged in people with Aphasia were prime candidates for being involved in speech, although neurologists may occasionally remind people this is not as simple as it seems by telling a sick joke about a frog and scientist.

Source: http://www.indiana.edu/~primate/brain.html

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