Perceptual and cognitive processing limitations in specific language impairment

The experiments presented in this thesis explored the possibility that an underlying cause of specific language impairment (SLI) may be a deficit in perceptual or cognitive information processing. The first three studies tested the hypothesis that children with SLI have impaired perception of the dynamic elements of visual and auditory stimuli, as proposed in the magnocellular hypothesis for developmental dyslexia. The experimental predictions were that a) children with SLI would have poor sensitivity to coherent motion (but not coherent form) stimuli relative to chronological-age matched controls; b) children with SLI would have poorer sensitivity than controls to slow (but not fast) rates of frequency modulation in a tone; c) sensitivity to slow rates of frequency modulation (FM) would correlate with children's performance on a set of tests of phonological skill. Overall, these predictions were not corroborated by the results, and the conclusion drawn from this set of studies is that a magnocellular impairment of the type reported in dyslexia is unlikely to be a causal factor in SLI. The second three studies used a grammaticality judgement task to focus on inflectional morphology, an area of language which poses particularly marked difficulty for many children with SLI. The findings from Study 4 suggested that children's performance on the grammaticality judgement task overall was strongly related to phonological discrimination ability, but was unaffected by the specific inflectional allomorph tested. The final two studies manipulated the information processing load of the grammaticality judgement task, in Study 5 to simulate (successfully) SLI-like performance in a group of typically developing children, and in Study 6 to attempt (unsuccessfully) to improve performance in a group of children with SLI. These results are compatible with the idea that the profile of language difficulties experienced by many children with SLI is due to a processing deficit in the early stages of language acquisition which interrupts the establishment of robust linguistic representations. The nature of this processing deficit is as yet unclear, though the current findings do not support the suggestion of a central auditory impairment. It is possible that a number of distinct deficits, such as poor phonological memory or reduced speed of processing, may produce a broadly similar linguistic profile in different individuals.