The role of preparation in overlapping-task performance.

We examined how performance of two overlapping discrete tasks is organized and controlled. Experiment 1 showed that when stimuli were presented in an unexpected order, expectations rather than actual presentation order determined the order in which the two stimuli were processed. In Experiment 2, when task order was unpredictable, performance was facilitated when task order was repeated on consecutive trials. In Experiment 3, performance of the second but not the first task benefited greatly when task order was held constant as compared to regularly alternated between trials; such effects were especially pronounced at short intertrial intervals. These results suggest that sequential performance of overlapping tasks is scheduled in advance and is regulated by initially allocating the central processing channel to one task and subsequently switching this channel to the other task.