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Behavioural Neurology
Volume 25, Issue 1, Pages 23-34
http://dx.doi.org/10.3233/BEN-2012-0346

Unpicking the Semantic Impairment in Alzheimer’s Disease: Qualitative Changes with Disease Severity

Faye Corbett,1 Elizabeth Jefferies,2 Alistair Burns,3 and Matthew A. Lambon Ralph3

1University of Manchester, Manchester, UK
2University of York, York, UK
3Neuroscience and Aphasia Research Unit (NARU), School of Psychological Sciences, Zochonis Building, University of Manchester, Manchester, UK

Received 26 December 2011; Accepted 26 December 2011

Copyright © 2012 Hindawi Publishing Corporation and the authors. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Despite a vast literature examining semantic impairment in Alzheimer's disease (AD), consensus regarding the nature of the deficit remains elusive. We re-considered this issue in the context of a framework that assumes semantic cognition can break down in two ways: (1) core semantic representations can degrade or (2) cognitive control mechanisms can become impaired [1]. We hypothesised and confirmed that the nature of semantic impairment in AD changes with disease severity. Patients at mild or severe stages of the disorder exhibited impairment across various semantic tasks but the nature of those deficits differed qualitatively for the two groups. Commensurate with early dysfunction of the cognitive control, temporoparietal-frontal-cingulate network, characteristics of deregulated semantic cognition were exhibited by the mild AD cases. In contrast, the severe AD group reproduced features of additional degradation of core semantic representations. These results suggest that spread of pathology into lateral anterior temporal lobes in later stage AD produces degradation of semantic representations, exacerbating the already deregulated system. Moreover, the dual nature of severe patients’ impairment was highlighted by disproportionately poor performance on tasks placing high demand on both conceptual knowledge and control processes–e.g., category fluency.