were not identical to either those of the monolingual hearing subjects reading English or to the congenitally deaf subjects viewing ASL. For example, when reading English, both hearing groups displayed strong left hemisphere asymmetries over inferior frontal regions. However, the hearing native signers did not display as robust activation over temporal brain regions. It may be that anterior regions perform similar processing on language input independently of the form and structure of the language whereas posterior regions may be organized to process language primarily of one form (e.g., manual/spatial or oral/aural). Further research characterizing cerebral organization during language acquisition will contribute to an understanding of this effect. When processing ASL, both deaf and hearing native signers displayed significant activation of both left and right frontal and temporal regions. However, whereas the activations were uniformly bilateral or larger in the right hemisphere for the deaf subjects, over anterior areas they tended to be larger from the left hemisphere in the hearing native signers. This pattern suggests that the early acquisition of oral/aural language influences the organization of anterior areas for ASL.
The hearing native signers (bilinguals) displayed considerable individual differences during sentence processing of both English and ASL. These results are reminiscent of recent reports of a high degree of variability from individual to individual and area to area of language activation in hearing, speaking bilinguals who learned their second language after the age of 7 years (20, 21). Thus, these data are consistent with the proposal that in addition to individual differences in the age of acquisition, proficiency, and learning and/or biological substrates, the structure and modality of the first and second languages also determine cerebral organization in the bilingual.
In summary, classical language areas within the left hemisphere were recruited in all groups (hearing or deaf) when processing their native language (ASL or English). In contrast, deaf subjects reading English did not display activation in these areas. These results suggest that the early acquisition of a fully grammatical, natural language is important in the specialization of these systems and support the hypothesis that the delayed and/or imperfect acquisition of a language leads to an anomalous pattern of brain organization for that language.‡‡ Furthermore, the activation of right hemisphere areas when hearing and deaf native signers process sentences in ASL, but not when native speakers process English, implies that the specific nature and structure of ASL results in the recruitment of the right hemisphere into the language system. This study highlights the presence of strong biases that render regions of the left hemisphere well suited to process a natural language independently of the form of the language, and reveals that the specific structural processing requirements of the language also in part determine the final form of the language systems of the brain.
The deaf subjects in this study scored moderately on tests of English grammar (Table 1). However, deaf subjects who fully acquire the grammar of English (i.e., score ≥95% on tests) display evidence of left hemisphere specialization for English (48).
We thank Dr. Robert Balaban of the National Heart, Lung, and Blood Institute at the National Institutes of Health for providing access to the MRI system, students and staff at Gallaudet University for participation in these studies, Drs. S.Padmanhaban and A.Prinster and C.Hutton, T.Mitchell, A.Newman, and A.Tomann for help with these studies, Dr. J.Haxby for lending us equipment, and M.Baker, C.Vande Voorde, and L.Heidenreich for help with the manuscript. This research was supported by grants from the National Institute on Deafness and Communication Disorders, the Human Frontiers Grant, and the J.S.McDonnell Foundation.
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