Modelling Atypical Syntax Processing

We evaluate the inferences that can be drawn from dissociations in syntax processing identified in developmental disorders and acquired language deficits. We use an SRN to simulate empirical data from Dick et al. (2001) on the relative difficulty of comprehending different syntactic constructions under normal conditions and conditions of damage. We conclude that task constraints and internal computational constraints interact to predict patterns of difficulty. Difficulty is predicted by frequency of constructions, by the requirement of the task to focus on local vs. global sequence information, and by the ability of the system to maintain sequence information. We generate a testable prediction on the empirical pattern that should be observed under conditions of developmental damage. 1 Dissociations in language function Behavioural dissociations in language, identified both in cases of acquired brain damage in adults and in developmental disorders, have often been used to infer the functional components of the underlying language system. Generally these attempted fractionations appeal to broad distinctions within language. However, fine-scaled dissociations have also been proposed, such as the loss of individual semantic categories or of particular linguistic features in inflecting verbs. Here, we consider the implications of developmental and acquired deficits for the nature of syntax processing. 1.1 Developmental deficits A comparison of developmental disorders such as autism, Downs syndrome, Williams syndrome, Fragile-X syndrome, and Specific Language Impairment reveals that dissociations can occur between phonology, lexical semantics, morphosyntax, and pragmatics. The implications of such fractionations remain controversial but will be contingent on understanding the developmental origins of language structures (Karmiloff-Smith, 1998). These processes remain to be clarified even for the normal course of development. In the area of syntax, Fowler (1998) concluded that a consistent picture emerges. Individuals with learning disabilities are systematic in their grammatical knowledge, follow the normal course of development, and show similar orders of difficulty in acquiring constructions. However, such individuals can often handle only limited levels of syntactic complexity and therefore development seems to terminate at a lower level. While there is great variability in linguistic function both across different disorders and within single disorders, this cannot be attributed solely to differences in ‘general cognitive functioning’ (e.g., as assessed by problem solving ability). Syntax acquisition is therefore to some extent independent of IQ. However, adults with developmental disorders who have successfully acquired syntax typically have mental ages of at least 6 or 7, an age at which typically developing children also have well-structured language. The variability in outcome has been attributed to various factors specific to language, including verbal working memory and the quality of phonological representations (Fowler, 1998; McDonald, 1997). Most notably, disorders with different cognitive abilities show similarity in syntactic acquisition. The apparent lack of deviance across heterogeneous disorders has been used to argue for a model of language acquisition that is heavily constrained by the brain that is acquiring the language (Newport, 1990). 1.2 Acquired deficits in adulthood One of the broadest distinctions in acquired language deficits is between Broca’s and Wernicke’s aphasia. Broca’s aphasics are sometimes described as having greater deficits in grammar processing, and Wernicke’s aphasics as having greater deficits in lexical processing. The dissociation is taken to support the idea that the division between grammar and the lexicon is one of the constraints that the brain brings to language acquisition. Dick et al. (2001) recently argued that four types of evidence undermine this claim: (1) all aphasics