Figure 1.

The experimental design and paradigm. (A) In a 2 × 2 factorial block design, the factors alertness demand and selective attention demand were each presented at 2 levels. (B) Example time courses for stimulus blocks corresponding to the 2 × 2 design in A (real experimental blocks contained 4 times as many stimuli). Each vertical line represents one sound and its height represents the pitch. The target sound (highlighted with arrows ↓) was identical throughout the experiment and had the highest pitch. In a sustained task, participants pressed a response button whenever they heard the target. Stimuli occurred at regular intervals in the low alertness conditions (left) and at jittered irregular intervals in the high alertness condition (right). The pitch of the nontarget sounds was closer to—and thus harder to discriminate from—the target sound in the high attention condition (bottom) when compared with the low attention condition (top).