Distraction as a determinant of processing speed.

Processing speed is often described as a fundamental resource determining individual (e.g., I.Q.) and group (e.g., developmental) differences in cognition. However, most tests that measure speed present many items on a single page. Because many groups with slowed responding are also distractible, we compared younger and older adults on high-distraction (i.e., standard) versus low-distraction versions of two classic speed tasks. Reducing distraction improved the performance of older adults but had little or no effect on younger adults, suggesting that the ability to limit attentional access to task-relevant information can affect performance on tests designed to measure processing speed.