Language and the brain Many people assume the physical basis of language lies in the lips, the tongue, or the ear. But deaf and mute people can also possess language fully. People who have no capacity to use their vocal cords may still be able to comprehend language and use its written forms. And human sign language, which is based on visible gesture rather than the creation of sound waves, is an infinitely creative system just like spoken forms of language. But the basis of sign language is not in the hand, just as spoken language is not based in the lips or tongue. There are many examples of aphasics who lose both the ability to write as well as to express themselves using sign-language, yet they never lose manual dexterity in …show more content…

These boy eat two cookie. John is work in the factory. These errors are random, not the set patterns of an alternate dialect: the next conversation the same SLI-afflicted individual might say This boys eats two cookies. These sentences, in fact, were uttered by a British teenager who is at the top of his class in mathematics; he is highly intelligent, just grammar blind. SLI sufferers are incapable of perfecting their skills through being taught, just as some people are incapable of being taught how to draw well or how to see certain colors. This is the best proof we have that the language instinct most children are born with is a skill quite distinct from general intelligence. Because SLI occurs in families and seems to have no environmental cause whatsoever, it is assumed to be caused by some hereditary factor--probably a mutant, recessive gene that interferes with or impairs the LAD. The precise gene which causes SLI has yet to be located.
Aphasia
We know which specific areas of the left hemisphere are involved in the production and processing of particular aspects of language. And we know this primarily from the study of patients who have had damage to certain parts of the left hemispheric cortex. Damage to this area produces a condition called aphasia, or speech impairment (also called dysphasia in Britain). The study of language loss in a once normal brain is