Examining the Extent to Which General Cognitive Deficits Can Explain the Grammatical Profile in Specific Language Impairment: A Simulation Paradigm

This thesis set out to examine whether deficits in general cognitive processes could explain the hierarchy of inflectional difficulty seen in Specific Language Impairment. A simulation approach was largely adopted, in which the online inflectional processing skills of typically-developing children were assessed when sentences were processed under conditions of cognitive stress.

Experiment 1 investigated the speed of processing account, which argues that children with SLI experience ‘generalised slowing’. Typically-developing children demonstrated an SLI-like pattern of inflectional difficulty when sentences were compressed by 30%, and this was replicated in Experiment 2. Experiment 2 also increased cognitive load by introducing noise masks and by lengthening sentences, to test the auditory perception and phonological working memory deficit accounts of SLI, respectively. No other stressors resulted in an SLI-like pattern of inflectional difficulty in the typically-developing participants. Experiment 3 re-examined the effect of noise masks as a cognitive stressor and manipulated the signal-to-noise ratio, but still no SLI-like inflectional impairment was simulated. Collectively, the findings from Experiments One, Two and Three suggest that a speed of processing deficit may be central to the inflectional difficulties seen in SLI.

This idea was further examined in Experiment 4, where the simulation paradigm was ‘flipped’: Children with SLI completed an online measure of inflectional awareness when sentences were slowed down, effectively lightening the cognitive load. The children’s morphological performance improved as a result of slowing the sentences down, however deficits in the regular past tense remained.

The results of the experiments contained within this thesis strongly support the notion that children with SLI experience ‘generalised slowing’, and that this plays a central role in the morphology deficits that are so prevalent in the disorder. The results also support the Surface Hypothesis of SLI, which argues that the inflectional deficits are the consequence of an interplay between speed of processing and the phonological properties of inflections. This thesis did not provide support for the auditory processing or phonological working memory deficit accounts of SLI, although it is possible that these impairments are present in children with SLI, but that they do not play a causal role in the difficulties with inflectional morphology.