No category specificity in Alzheimer's disease: a normal aging effect

The authors examined category effects on tasks of picture naming, naming to definition, and word-picture matching in 38 patients with Alzheimer's disease (AD) and 30 elderly controls. Each task was matched across category on all "nuisance" variables known to differ across domains. Standard analyses revealed significant category disadvantages for classifying living things in AD patients but also for elderly controls on each task. To overcome the ceiling effect in controls, the authors conducted 1,000 bootstrap analyses of covariance, with control performance as a difficulty index covariate. These covariate analyses eliminated the category effect in AD patients on all 3 tasks. Indeed, the authors report that control performance accounted for 64% (picture naming), 49% (naming to description), and 42% (word-picture matching) of variance in AD performance. This suggests that, although category effects in AD patients do not reflect intrinsic variables, the size and direction of the category effect are not different from those in elderly controls.