Studies of Neural Processing in Deaf Signers: Toward

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hemispheric specialization for language and nonlan- guage visuospatial ... Many of the early case studies were hampered by a lack ... critical left hemisphere areas are damaged. Of 16 left ... In summary, evidence from lesion studies of deaf.

Empirical Articles

Studies of Neural Processing in Deaf Signers: Toward a Neurocognitive Model of Language Processing in the Deaf David P. Corina University of Washington

The ability to comprehend and produce language stands as a defining characteristic of human cognition and enables the transfer of knowledge and culture within human society. A proper characterization of the human capacity for language is required for the development of interventions that may be used to assist those individuals who have failed to achieve, or who Thii work w»s supported in part by « grant from the University of Washington, RRF-13401 md a grant from NIDCD R29 DC03099-01A1 awarded to the author. I thank my collaborator! on the fMRI studies, Drs. Helen Neville and Daphne Bavelier. I thank Connie Schachtel for editorial assistance. Figure 2: copyright Dr. Ursula Bellugi, the Salk Institute, La Jolla, California 92037. Figures 3 and 4: copyright DT. David Corina, all righo reserved, University of Washington, 98195. Correspondence should be sent to David P. Corina, Department of Psychology, Univenity of Washington, Seattle WA 98195-351525. Copyright O 1998 Oxford University Press. CCC 1081-4159

have lost competence in, a full range of language behaviors (e.g., effective interpersonal communication, reading, writing, etc.). Cognitive psychologists have made great strides in understanding the functional and neural mechanisms underlying the use of spoken language. These studies have led to a wide range of effective educational and clinical programs for enhancing language behaviors (Tallal, Miller, Bedi, Byma, Wang, Nagarian, Schreiner, Jenkins, & Merzenich, 1996; Merzenich, Jenkins, Johnston, Schreiner, Miller, & Tallal, 1996). For signed languages however, models of competent language use are lacking. This lack of knowledge hampers the development of effective assessment measures for deaf children who may be experiencing learning problems beyond those confronting the normal deaf child. Moreover, our lack of knowledge stifles the development of effective intervention strategies for these children. The development of a comprehensive neurocognitive model of sign language processing is crucial if we are to properly serve the needs of the deaf signing community. Development of such a model would also benefit basic science, providing insight into how altered sensory experience affects the development of neural systems underlying cognitive functions. Finally, this model will benefit cognitive scientists interested in functional models of human language. Within the last decade there has been a monumental increase in our knowledge of cognitive processing in deaf signing individuals (see, for example, Hanson, 1990; Hanson, Lichtenstein, 1990; Neville, Mills, & Lawson, 1992; Parasnis & Samar, 1985, Marschark,

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The ability to comprehend and produce language stands as a defining characteristic of human cognition and enables the transfer of knowledge and culture within human society. A proper characterization of the human capacity for language is required for the development of interventions that may be used to assist those individuals who have failed to achieve, or who have lost competence in, language behaviors. For signed languages, models of competent language use are lacking. This lack of knowledge hampers the development of effective assessment measures for deaf children who may be experiencing learning problems beyond those confronting the normal deaf child. I discuss two research avenues that have begun to provide a window into the neural systems involved in sign language processing: studies of language disruptions in adult deaf signers who have suffered brain injury, and studies of functional brain imaging in normal deaf signers. This research provides a basis for the development of a comprehensive neurocognitive model of sign language processing.

36 Journal of Deaf Studies and Deaf Education 3:1 Winter 1998

Sign Language Aphasia In the 1800s noted neurologist Hughlings Jackson broached the issue of neural control of signed language. In a much quoted musing, Jackson said: "No doubt by disease of some part of his brain the deafmute might lose his natural system of signs" (Jackson, 1878). Since this time researchers have looked to case studies of deaf signing individuals to answer two broad questions: first, whether left hemisphere structures mediate the signed languages of deaf individuals, and second, whether deaf individuals show complementary hemispheric specialization for language and nonlanguage visuospatial skills. More recent investigations have focused on the question of intrahemispheric specialization for sign language systems. Important generalizations regarding these two questions are beginning to emerge from this complicated literature. Of the case studies of brain-damaged deaf and/or signing individuals reported to date, roughly 20 involve left-hemisphere damage, while 8 involve right hemisphere damage (see Corina, in press, for a recent review). These case studies vary greatly in their ability to

explain underlying brain processes involved in signing. Many of the early case studies were hampered by a lack of understanding of the relationships among systems of communication used by deaf individuals. For example, several of the early studies discussed disruptions of fingerspelling and only briefly mentioned or assessed sign language use. Anatomical localization of lesions was often lacking or confounded by the existence of multiple infarcts. Rarely were etiologies of deafness or audiological reports presented. Despite these limitations, with careful reading general patterns do emerge. More recently, well-documented case studies have started to provide a clearer picture of the neural systems involved in language processing in users of sign languages. One conclusion that can be drawn from the sign aphasia literature is that right-handed deaf signers, like hearing persons, exhibit language disturbances when critical left hemisphere areas are damaged. Of 16 left hemisphere cases reviewed by Corina (in press), 12 provide sufficient detail to implicate left hemisphere structures in sign language disturbances. Five of these cases provide neuro-radiological or autopsy reports to confirm left hemisphere involvement and provide compelling language assessment- to implicate aphasic language disturbance. In hearing individuals, severe language comprehension deficits are associated with left hemisphere posterior lesions, especially posterior temporal lesions. Similar patterns have been observed in users of signed languages. For example, in the case of WL, reported by Corina, Kritchevsky, and Bellugi (1992), the subject had damage to posterior temporal structures and evidenced marked comprehension deficits. WL showed a gradation of impairment across tasks, with some difficulty in single sign recognition, moderate impairment in following commands, and severe problems with complex ideational material. In contrast, a right hemisphere-damaged signer, SM, who also suffered a large right hemisphere lesion with parietal extension, showed only mild impairment on only the most difficult of comprehension tests (see Figure 1). In users of spoken languages, impairment in language production with preserved comprehension is associated with left hemisphere anterior lesions. The execution of speech movements, for example, involves the

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1993a, 1993b; Emmorey, Corina, & Bellugi 1995; Emmorey, Kosslyn & Bellugi, 1993). In addition, the body of literature exploring online processing of sign language is growing (Emmorey, 1993; Emmorey & Corina 1990; Mayberry & Eichen, 1991; Mayberry & Fischer, 1989). Advances in sign language linguistics have provided a basis of comparison for cross-language and cross-modality studies (Perlmutter, 1993; Corina, 1990; Corina & Sandier, 1993). Taken together these behavioral studies provide a foundation for the development of functional models of cognitive and language processing in deaf signers. An equally important goal is to understand the neural systems that underlie these functional characterizations. Research in this direction is required in order to develop a comprehensive neurocognitive model of sign language processing. In this article I discuss two research avenues that have begun to provide a window into the neural systems involved in sign language processing: studies of language disruptions in adult deaf signers who have suffered brain injury, and studies of functional brain imaging in normal deaf signers.

Studies of Neural Processing in Deaf Signers

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Sign Comprehension Tests 100H

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