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

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.

Apraxia profile differentiates behavioural variant frontotemporal from Alzheimer's dementia in mild disease stages

“…Nevertheless, accuracy in tasks on pantomimes of object use is positively influenced by semantic knowledge about objects and tool use 31. Impairments of semantic memory and conceptual knowledge become more evident in later stages of AD, possibly due to spreading of the pathology into areas of the anterior temporal lobe 32. This may explain our finding of relatively better performance of pantomime compared with imitation tasks in our sample of patients with mild AD.…”

Deregulated semantic cognition contributes to object‐use deficits in Alzheimer's disease: A comparison with semantic aphasia and semantic dementia

“…Motivated by this goal, Corbett et al . () provided evidence showing that performance on a multimodal semantic battery (including word‐ and picture‐based comprehension tasks) reflected deficits of semantic control from the very earliest stages of AD. In the present study, we extended these investigations by examining 10 AD patients on a battery of object‐use tasks, which probed both comprehension of the actions and functions of objects, and overt demonstrations of object use.…”

“…Our previous investigation of word and picture semantic tasks in AD (Corbett et al ., ) found that semantic impairment in the mildest cases was qualitatively similar to that in SA, indicating that executive‐semantic deficits are apparent even from an early stage of the disease. However, in a more severely impaired sample of AD patients, there was evidence of additional degradation of conceptual knowledge, consistent with the view that damage is likely to encroach on the inferolateral ATL as the disease progresses.…”

“… Executive control is impaired from the early stages of A lzheimer's Disease ( AD ) and this produces deregulated semantic cognition (Corbett, Jefferies, Burns, & Lambon Ralph, ; Perry, Watson, & Hodges, ). While control deficits should affect semantic retrieval across all modalities, previous studies have typically focused on verbal semantic tasks.…”

Análise de aspectos micro e macrolinguísticos da narrativa de indivíduos com doença de Alzheimer, comprometimento cognitivo leve e sem comprometimentos cognitivos

“…Os distúrbios de linguagem são considerados importantes na DA (St-Pierre et al, 2005), podem ocorrer desde as fases iniciais (Taler, Phillips, 2008, Pistono et al, 2016 e há evolução do quadro com a progressão da doença (Groves-Wright et al, 2004, Albert, 2008Corbett et al, 2012).…”