Impaired Semantic Guidance in Visual Search Differentiates Prodromal Dementia with Lewy Bodies from Alzheimer's Disease

Dementia with Lewy bodies (DLB) and Alzheimer's disease (AD) show substantial symptom overlap at the prodromal stage, complicating early differential diagnosis. To identify markers of cognitive dysfunction, we investigated visual search performance in prodromal DLB (LB-MCI), prodromal AD (AD-MCI), and healthy controls across four levels of target visual richness: Gabor patches, abstract shapes, midlevel forms, and real-world objects. While increased stimulus richness improved search performance across all groups, LB-MCI participants exhibited a selective impairment in target present searches involving real-world objects, despite perceptual sensitivity comparable to that of AD-MCI participants. Moreover, unlike healthy older adults, both clinical groups failed to adaptively shift decision thresholds as stimulus complexity increased. These findings suggest that prodromal DLB is associated with difficulty leveraging semantic information to guide visual search, reflecting impaired deployment of semantic target templates rather than early perceptual discrimination deficits.