Selective attention skills in Alzheimer's disease: performance on graded cancellation tests varying in density and complexity.

Nine selective cancellation tasks were administered to nine subjects with Alzheimer's disease (AD), nine elderly control, and eight depressed subjects. Each task consisted of an 8 1/2 x 11 sheet of paper containing line drawings of geometric shapes that were identified as either targets or distractors. The nine forms of the task varied along two independent graded dimensions: (1) the total number of items in the field (Density: 3 levels) and (2) the number of different distractors in the surround (Complexity: 3 levels). Performance of the AD group was worse than that of the other groups, and performance for all groups declined with density. Error analysis of the impaired performance of the AD subjects showed that they made errors of omission that differed significantly as a function of density. There was no evidence that they could adopt a chunking strategy to accommodate the increased load.