It has been suggested that 7% of children in the UK have specific language impairments (SLI); these individuals experience significant difficulties with communication despite having non-verbal skills (and learning potential) in the average or above average range. If the cause of the impairment were simply a case of developmental delay, then it would be reasonable to expect that all development would be similarly delayed. However, although language is impaired (often severely), non-verbal reasoning skills appear to remain intact. As such this condition can reveal much about language and problem solving. There are various theories of the causation of SLI, below are three major accounts:
1. Input deficits - Tallal and colleagues (e.g. Tallal, Merzenich, Miller & Jenkins1998) propose that deficits in processing auditory input, i.e. rapidly presented acoustic signals, underlie language-learning deficits. Chiat (e.g. 2001) proposes an account of SLI based on phonological processing. Although they disagree on the source of disruption, both theories propose that there is defective speech processing.
2. Grammatical deficits - Van der Lely (e.g. 1998) proposes a grammar-specific deficit hypothesis. This suggests that SLI results from impaired representation of syntactic dependencies, affecting, for example, the generation of rule-based regular past tense morphology.
3. General capacity limitations - e.g. Johnston (1991). This account suggests that reduced processing capacity limits the amount of information which can be manipulated, thus, for example, connections between propositions are likely to be lost, resulting in impaired language understanding.
There are many research projects currently examining the underlying causes of SLI, the effect of SLI on general education, and the effects of SLI on children's literacy. Relatively little attention has so far been devoted towards mathematical skills in the context of SLI.
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