Bello Lepe, Sebastián; (2024) Grammatical Processing in Aphasia: a usage-based and ERP investigation. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Extensive research has shown that individuals with post-stroke aphasia have different degrees of difficulty in grammatical processing. Nevertheless, the linguistic hypotheses underlying these impairments have failed to provide a comprehensive explanation of the nature of grammatical processing difficulties in aphasia. On the one hand, Universal/Generative grammar claims that grammatical impairments arise primarily because of damage to the innate mental mechanisms of syntax processing. However, Usage-Based approaches consider elements such as the influence of the context and use on processing. This thesis reports behavioural experiments to test Usage-Based hypotheses, and an electrophysiological experiment to test hypotheses based on a generative approach. Experiment One was conducted with neurotypical older adults and individuals with aphasia to explore how statistical properties of language influence cognitive effort in language production, reflected in pausing phenomena in speech. The results showed that the statistical properties of language, especially the probability with which a two-word combination appears together, play a pivotal role in reducing the processing demands on language production, especially in individuals with post-stroke aphasia and reflected in shorter pauses. A second report applies the same methodology to the study of pauses in speakers diagnosed with Alzheimer’s dementia. Experiments Three and Four explored electrophysiological brain responses to noun-verb phrase agreement, comparing neurotypical younger and older adults, and performance of the neurotypical older adults to that of individuals with aphasia. Experiment Two revealed that neurotypical older adults did not show electrophysiological and behavioural differences compared to the younger controls. Experiment Three showed that, although there were behavioural differences between neurotypical older adults and individuals with aphasia, the electrophysiological responses between both groups were similar. The results of Experiment Three raised further questions regarding post-processing costs of sentences in individuals with post-stroke aphasia.

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