Lexical and articulatory aspects of cognitive decline

Abstract

Background: Dementia may be defined as the attenuation of cognitive powers despite preserved basal consciousness. Its most common cause is Alzheimer's disease (AD). Pathoanatomical staging has made it clear that the disease passes through a protracted preclinical phase with slowly emerging symptoms. Such symptoms may include various forms of forgetfulness, such as forgetting the names of people, things, and places. Because word finding also wanes somewhat with normal aging, objective tests of word finding might help in separating benign word lapses from those reductions in lexical ability that may herald dementia.

Objectives: The main aim of the thesis was to identify speech production tests of relevance to the diagnosis of AD and mild cognitive impairment (MCI).

Material and Methods: The thesis is based on retrospective clinical data obtained from patients admitted to the memory clinic of the Department of Geriatrics, Karolinska University Hospital. All subjects had undergone an examination for possible dementia. Study I explored the interrelationships of three word fluency tests and performance differences between the diagnostic categories. Study II investigated whether reductions in regional brain blood flow affected performance in a verb fluency task and a noun fluency task. Study III examined the production of an automatic word sequence in forward and reverse order as a diagnostic target. Study IV analyzed the capacity for rapid articulatory action in AD and related disorders.

Results: Verb, noun, and letter-based fluency were found to be factorially distinct tests although with covariance between factors. Noun fluency was disproportionately impaired in AD. In MCI, however, verb fluency was more decreased than noun or letter-based fluency. Verb fluency performance was predicted by educational level and temporal lobe blood flow, while noun fluency was predicted by age and parietotemporal-occipital blood flow. Forward word sequence production was preserved across diagnostic categories. In contrast, backward word sequence production was impaired in MCI and devastated in AD. The diagnostic predictive power of backward word sequence production was approximately equivalent to that of the widely-used Mini-Mental State Examination. Sequential speech motion rate was modestly reduced in MCI and AD, but more markedly so in frontotemporal dementia and progressive nonfluent aphasia. In semantic dementia, articulatory capacity was at the level of participants without objective cognitive impairment and superior to the other categories.

Conclusion: Lexical production tasks have application to the diagnosis of MCI and mild AD. The capacity for rapid articulatory action varies both between and within AD and related disorders. The potential neural correlates of this variation remain to be explored by future studies.