Aging, divided attention, and processing capacity.

The performance of young and old adults was compared in two different situations to investigate the reasons why the latter are especially poor at divided attention tasks. Although the two experimental tasks were quite different, one a single task and the other time-sharing, task difficulty was manipulated in a similar fashion in both situations. Specifically, in both tasks difficulty was varied by manipulation of number of items to be rehearsed and number of mental operations necessary for successful performance. The most salient difference in task demands, that is, divided versus nondivided attention, had little effect on results. Performance of both age groups declined as task difficulty increased, with the decline being greater and more rapid for the older subjects. Results are discussed in terms of competition for limited processing capacity in working memory.