Aging brain, educated mind : dynamic interconnectedness of semantic cognition

Abstract

Semantic cognition is a core element of the human experience, guiding and supporting innumerable verbal and non-verbal behaviours. Recent studies have demonstrated that semantic cognition is a heterogeneous system, interacting with various subcomponents, mainly semantic representation and semantic control. With shrinkage of brain and prevalent cognitive declines in the late lifespan, aging effects on such basic abilities has drawn researchers’ attention. But whether the aging effects are ubiquitous across all subcomponents or focus on specific ones are still not clear. The underlying neural mechanisms of the effects also need to be clarified. Besides, most of the previous studies are based on WEIRD (White, Educated, Industrialized, Rich, and Democratic) populations, which may not generalize to the whole population, as educational attainment has proved to influence verbal and many other cognitive abilities. There is still a research gap regarding the role of educational attainment and how aging influences semantic cognition in lower-educated older adults. The goal of this dissertation is to shed light on the questions and bridge the gap. To explore the effects of aging on semantic cognition, and whether and how educational attainment would improve cognitive reserve and modulate these effects, three groups of participants – young adults, high-educated older adults, and low-educated older adults – were recruited to complete experiments. A battery of neuropsychological tests and questionnaires was administered to investigate aging and educational effects on cognitive functions and to illustrate individual variation in cognition. Different subcomponents of semantic cognition were examined in terms of semantic representation and semantic control, in order to have a comprehensive understanding of the diverse aging and educational effects. For semantic representation, quantity and structure were measured by vocabulary size and semantic priming effect in a Lexical Decision Task with a short SOA (150 ms). As for semantic control, semantic controlled retrieval and semantic selection were measured by Global Association and Feature (size) Selection Tasks, respectively. Electroencephalogram (EEG) technique was used to explore how aging and education modulate the underlying neural mechanisms of semantic priming, controlled retrieval, and feature selection processes, as well as to dissociate the biomarkers of the latter two processes. Divergent aging and educational effects were found among different subcomponents of semantic cognition. Semantic representations increased during the lifespan, and higher educated older adults accumulated more knowledge than their lower educated counterparts. Semantic retrieval ability declined with advancing age, evidenced through an absence of semantic retrieval N400 and LPC effects in older adults. Semantic selection ability was preserved in higher educated older adults, which might be a result of compensation. It was impaired in the lower educated older adults. Besides, educational attainment had a positive impact on semantic selection by increasing LPC amplitude in the fronto-central region. In terms of the relationships between the subcomponents of semantic cognition, and the relationships between semantic cognition and other cognitive abilities, older adults’ performance was significantly influenced by their size of semantic representations, which indicated that older adults relied more on semantic representations to compensate their declined semantic control and cognitive abilities. The lower-educated older adults failed to attain a comparable performance to younger adults due to a relatively limited store of semantic representations. Based on the changes in relations among the different subcomponents, and the dynamic relations between semantic cognition and other general cognitive abilities, a framework (Aging Brain, Educated Mind) was proposed to illustrate the mechanisms of aging and educational effects on semantic cognition, from the view of semanticization of cognition.