Object identification deficits in dementia of the Alzheimer type: combined effects of semantic and visual proximity.

Identification deficits in dementia of the Alzheimer Type (DAT) often target specific classes of objects, sparing others. Using line drawings to uncover the etiology of such category-specific deficits may be untenable because the underlying shape primitives used to differentiate one line drawing from another are unspecified, and object form is yoked to object meaning. We used 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 blobs stand for semantically close and disparate objects and looking at shape-label confusion rates for each type of set, form and meaning were independently assessed. Overall, visual similarity of shapes and semantic similarity of labels each exacerbated object confusions. For controls, the effects were small but significant. For DAT patients more substantial visual and semantic proximity effects were obtained. Experiment 2 demonstrated that even small changes in semantic proximity could effect significant changes in DAT task performance. Labeling 3 blobs with "lion," "tiger," and "leopard" significantly elevated DAT confusion rates compared to exactly the same blobs labeled with "lion," "tiger," and "zebra." In conclusion both visual similarity and semantic proximity contributed to the identification errors of DAT patients.