Abstract
Semantic deficits are common in individuals with Alzheimer’s disease (AD). These deficits notably impact the ability to understand words. In healthy aging, semantic knowledge increases but semantic processing (i.e., the ability to use this knowledge) may be impaired. This systematic review aimed to investigate semantic processing in healthy aging and AD through behavioral responses and the N400 brain event-related potential. The results of the quantitative and qualitative analyses suggested an overall decrease in accuracy and increase in response times in healthy elderly as compared to young adults, as well as in individuals with AD as compared to age-matched controls. The influence of semantic association, as measured by N400 effect amplitudes, appears smaller in healthy aging and even more so in AD patients. Thus, semantic processing differences may occur in both healthy and pathological aging. The establishment of norms of healthy aging for these outcomes that vary between normal and pathological aging could eventually help early detection of AD.
Highlights
Semantic knowledge refers to our general knowledge about the world
Regarding Alzheimer’s disease (AD) patients, we found in the literature search that 13 studies investigated the effect of group on N400 amplitude
This systematic review reinforces the idea that differences in semantic processing are observed in healthy elderly adults relative to both young and AD patients
Summary
Semantic knowledge refers to our general knowledge about the world. Semantic processing or semantic cognition enables us to use this knowledge in order to understand the meaning of words, recognize objects in the environment and perform cognitive tasks, such as identifying the presence or absence of links between concepts [1–3]. This was found in AD patients [22,23] and elderly adults [10,24], larger N400 amplitudes were found for semantically congruent or predictable stimuli in these populations [25,26] It remains unclear whether the N400 effect differences observed in healthy and pathological aging are due to changes in the processing of stimuli that are highly associated (e.g., rabbit—carrot), weakly associated (e.g., bird—carrot) or both. It appears important, when studying semantic processing, to consider several semantic related measures (e.g., behavioral accuracy and response time; N400 amplitude, latency, and effect), as well as the studied semantic conditions (e.g., high vs weak semantic association). The identification of the semantic conditions (e.g., highly or weakly associated stimuli) that may differ between normal and impaired aging could allow us to detect early and better monitor the evolution of pathological aging
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