Language and cognition in healthy aging and dementia

Abstract

This dissertation investigates how changes in language performance in Alzheimer’s disease (AD) and primary progressive aphasia (PPA) can help shed light on theories of language processing, the mental lexicon, and how language impairments in dementia can help give a more comprehensive understanding of the complex difficulties associated with the different diseases. The dissertation is the first of its kind in Norway, focusing on language impairment in AD and PPA from a linguistic perspective. It contributes with new knowledge on deficits in lexical production and sentence comprehension in AD, PPA and healthy aging, shedding light on the structure of the mental lexicon in these populations. The results are in line with usage-based theories and interactive models of language processing, where the mental lexicon is seen a structured network of smaller and larger units at different levels of abstraction, sensitive to effects of frequency, age of acquisition and cognate status. Furthermore, they support a notion that theories of language should consider the multilingual mental lexicon as default. The results also indicate that there may be two different variants of the logopenic subtype of PPA. The dissertation illustrates the advantages of using a range of different methods for assessing language, to get a detailed picture of possible impairments. By means of eye-tracking, subtle differences in processing speed between healthy adults and persons with dementia could be detected that were not seen in a parallel offline task. A free word association task detected differences that were not captured with traditional naming tasks. Currently, language assessment plays a minor role in diagnosing dementia. However, this dissertation indicates that language data can add to diagnostic criteria for AD and PPA. While language difficulties in healthy aging and dementia can be seen as a continuum, the use of new methods and better assessment tools may contribute to both diagnosis and suggestions for possible treatment.