Sign language and the brain.

Analysis of the patterns of breakdown of a visuospatial language in deaf signers thus allows new perspectives on the nature and determinants of cerebral specialization for language. First, these data show that hearing and speech are not necessary for the development of hemispheric specialization--sound is not crucial. Second, the data show that in these deaf signers, it is the left hemisphere that is dominant for sign language. The patients with damage to the left hemisphere showed marked sign language deficits but relatively intact capacity for processing nonlanguage visuospatial relations. The patients with damage to the right hemisphere showed much the reverse pattern. Thus, not only is there left hemisphere specialization for language functioning, there is a complementary right hemisphere specialization for visuospatial functioning. The fact that much of the grammatical information is conveyed via spatial manipulation appears not to alter this complementary specialization. Furthermore, the finding that components of sign language (e.g., lexicon and grammar) can be selectively impaired suggests that the functional organization of the brain for sign language may turn out to be modular. Finally, patients with left and right hemisphere damage showed dissociations between two uses of space in the language--one to represent spatial relations and the other to represent syntactic relations. Right hemisphere damage disrupts the former but spares the latter; left hemisphere damage disrupts the use of space for syntactic relations but spares its use for spatial relations. Taken together with studies of the processing of sign language "on line" by neurologically intact deaf signers, these data suggest that the left cerebral hemisphere in humans may have an innate predisposition for language, independent of language modality. Studies of the effects of brain damage on signing make it clear that accounts of hemispheric specialization are oversimplified if stated only in terms of a dichotomy between language and visuospatial functioning. Such studies may also permit us to come closer to the real principles underlying the specializations of the two cerebral hemispheres, since in sign language there is interplay between visuospatial and linguistic relations within the same system.