"Just do it when you get a chance": the effects of a background task on primary task performance.

Two experiments investigated multitasking performance with a new "prioritized-processing paradigm" in which participants responded only to a high-priority primary task when this task required some action, responding to a low-priority background task only when no action was required for the primary task. In both experiments, performance was worse on the primary task than on the same task performed in isolation, indicating that this attempt to give absolute priority to the primary task is not sufficient to protect it from multitasking interference. Multitasking interference was present for task-repetition trials as well as task-alternation trials, so the interference could not be completely explained as a task-switching cost. In addition, responses to the primary task were influenced by their compatibility with the responses associated with the stimulus for the background task, indicating that there was some activation of S-R associations within the background task even when this task did not require any response. The findings generalize a number of effects from the psychological refractory period and task-switching paradigms to the prioritized-processing paradigm, thereby providing hints as to the underlying mechanisms responsible for those effects. The "prioritized-processing paradigm" appears to have several desirable features for the study of multitasking interference.