Object identification deficits in dementia of the Alzheimer type: Combinedeffects of semantic and visual proximity

Identification deficits in dementia of the Alzheimer Type (DAT) often targe t specific classes of objects; sparing others. Using Line drawings to uncov er the etiology of such category-specific deficits may be untenable because the underlying shape primitives used to differentiate one Line drawing fro m another are unspecified, and object form is yoked to object meaning. We u sed computer generated stimuli with empirically specifiable properties in a paradigm that decoupled form and meaning. In Experiment 1 visually similar or distinct blobs were paired with semantically close or disparate labels, and participants attempted to learn these pairings. By having the same blo bs stand for semantically close and disparate objects and looking at shape- label confusion rates for each type of set, form and meaning were independe ntly assessed. Overall, visual similarity of shapes and semantic similarity of labels each exacerbated object confusions: For controls, the effects we re small but significant. For DAT patients more substantial visual and sema ntic proximity effects were obtained. Experiment 2 demonstrated that even s mall changes in semantic proximity could effect significant changes in DAT task performance. Labeling 3 blobs with "lion," "tiger," and "leopard" sign ificantly elevated DAT confusion rates compared to exactly the same blobs l abeled with "lion;" "tiger," and "zebra." In conclusion both visual similar ity and semantic proximity contributed to the identification errors of DAT patients.