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Purpose: This study aimed to investigate the changes and deterioration in lexical processing caused by Alzheimer's disease (AD). It analyzed the differences in lexical processing between individuals with healthy controls, mild AD, and moderate AD as well as how these groups processed varying lexical aspects. Method: A total of 180 older adults participated in the experiment, including 60 healthy controls, 60 with mild AD, and 60 with moderate AD. Each group was further divided into two subgroups, with each subgroup assigned to one of two different experiments. The experiments assessed the speed and accuracy of lexical processing in both orthography and meaning using compound words. Results: Individuals with mild and moderate AD showed significant differences in the speed and accuracy of lexical processing, both in orthography and meaning, compared to healthy controls. When the prime character shared character-level units with the target word, it enhanced the accuracy of lexical processing in AD patients. Mild AD patients demonstrated a significant advantage in both the speed and accuracy of processing high-frequency words in terms of orthography and meaning, while moderate AD patients only showed a significant advantage in orthographic processing accuracy. The AD group showed no significant differences in the speed and accuracy of processing high- and low-transparency words in terms of orthographic and meaning processing. Conclusions: Lexical processing significantly deteriorated in individuals with AD, with a greater decline observed in those with moderate AD. Differences in lexical processing between mild and moderate AD patients highlighted the varying impact of the disease's severity.
