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Investigating the Comprehension Impairment in Wernicke’sAphasia
[Thesis]. Manchester, UK: The University of Manchester; 2011.
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Abstract
Wernicke’s aphasia (WA), an acquired impairment of language comprehension andword repetition, results from a cerebrovascular accident to the left temporoparietaljunction. The disorder has been important to the development of neurobiologicalmodels of language however neuropsychological investigations into the nature of thecomprehension impairment have been limited. This thesis presents a series of fourexperiments, investigating the comprehension impairment in WA. Chapter 3, abehavioural neuropsychological study, investigates existing hypotheses of thecomprehension impairment in WA: a phonological breakdown, a semanticbreakdown, a dual phonological-semantic breakdown. A case series comparisonmethodology is utilised. Participants with WA are compared to participants fromtwo other clinical, comprehension impaired groups: semantic dementia and semanticaphasia. Semantic dementia and semantic aphasia provide neuropsychologicalmodels of semantic breakdown, affecting semantic representations and semanticcontrol respectively. Individuals with WA showed disrupted non-verbal semanticanalysis of a similar magnitude to that in semantic dementia and semantic aphasiaand of a qualitatively similar nature to that in semantic aphasia. A significantlygreater impairment on assessments which required acoustic-phonological analysiswas found for individuals with WA compared to semantic aphasia. Overall a dualbreakdown in acoustic-phonological and semantic control best accounted for thecomprehension impairment in WA. In Chapter 4, direct evidence was sought for alink between acoustic-phonological non-word analysis and auditory comprehensionin WA. A novel test of non-word discrimination was created which was perceptuallygraded so as to provide a sensitive measure in severely impaired participants.Individuals with WA were significantly impaired at non-word discriminationcompared to age and hearing matched control participants who performed at ceiling.The degree of non-word discrimination/acoustic-phonological analysis impairmentcorrelated with auditory comprehension in WA. Chapter 5 investigated the extent towhich the established acoustic-phonological impairment in WA was grounded in amore fundamental deficit in non-verbal auditory analysis. The capacity to detectstructural changes in non-verbal auditory stimuli was measured. Participants with12WA had an impaired capacity to detect differences in all but the most structurallysimple auditory stimuli, compared to control participants. The degree of thisimpairment correlated with the degree of auditory comprehension impairment in theWA group. Chapter 6 revisits the semantic impairment observed in WA. Functionalmagnetic resonance imaging was used to investigate the residual neural networksrecruited by individuals with WA, when performing a semantic animate-inanimatejudgment task. Large portions of the inferior and anterior temporal lobes bilaterallywere activated, regions remote from the lesion in WA. Age matched controlparticipants recruited similar regions; however the activation in WA participants wassignificantly stronger. This indicated greater reliance on the residual semanticnetwork in WA in response to damage to posterior temporoparietal semantic regions.The results from this series of studies indicated that the primary deficit in WA is oneof impaired acoustic analysis and co-morbid damage to a phonological system.Additional disruption occurs to the semantic control network, which regulates thetask directed use of semantic representations. A combination of all three factorsaccounts for the comprehension impairment in WA and it is the relative contributionsof each factor that accounts for behavioural variation between individuals.
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10 supplementary audio files - examples of experimental stimuli
Keyword(s)
Wernicke's aphasia; auditory processing; language comprehension; phonology; semantic memory; stroke