Most language processing occurs in one hemisphere called the "dominant" hemisphere. Which hemisphere is dominant shows considerable individual differences, which bear a systematic relationship to handedness. In approximately 98% of right-handed individuals, the left hemisphere is dominant, with the extent to which left hemisphere lesions cause language disorders influenced by the degree to which an individual is right-handed and the number of non-right-handeders in his or her family. Approximately 60-65% of the non-right-handed individuals are left hemisphere dominant, approximately 15-20% are right hemisphere dominant, and the remainder appear to use both hemispheres for language processing. The relationship of dominance for language to handedness suggests a common determination of both, probably in large part genetic (although the nature of the genetic effect remains unclear).

Although language was the first function known to be lateralized and is still the best example of a lateralized function, it is not completely lateralized. Although not as important in language functioning as the dominant hemisphere, the nondominant hemisphere is involved in many language operations. Evidence from the effects of lesions and split-brain studies, experiments using presentation of stimuli to one or the other hemisphere in normal subjects, and activation studies indicates that the nondominant hemisphere understands many words, especially concrete nouns, and suggests that it is involved in other aspects of language processing as well, such as syntactic processing. Some language operations may be carried out primarily in the right hemisphere. The best candidates for these operations are ones that pertain to processing the discourse level of language, interpreting nonliteral language such as metaphors, and appreciating the tone of a discourse as is manifest in, for instance, its being humorous. Some scientists have developed models of the sorts of processing that the right hemisphere carries out. For instance, it has been suggested that the right hemisphere codes information in a course way compared to the left hemisphere. This and other suggestions provide the bases for ongoing research programs on the nature of language processing in the right hemisphere.

Lateralization of language functions can be seen as a broad form of localization—the localization of a function in one of the two cerebral hemispheres. As noted previously, lateralization varies as a function of handedness and even within populations with similar handedness profiles. These facts suggest intriguing similarities between the phenomena of localization and lateralization of language. In both localization and lateralization, the location of a particular language processing component varies across the adult population as a whole. In both cases, however, there are central tendencies with respect to the location of particular language processing components: There appear to be preferred sites for particular language processing functions within the perislyvian region, and there is a strong preference for language processing components to be left hemisphere based. These patterns would result from any area of either perisyl-vian association cortex being capable of supporting any subcomponent of the language processing system at the initial stage of language development, and from different areas of cortex assuming particular language processing roles as a function of intrinsic, genetically determined, developmental patterns, modified by other factors such as the internal organic milieu and the nature of exposure to language.

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