What it costs to implement a plan: Plan-level and task-level contributions to switch costs

Four experiments were conducted to identify the costs of implementing a plan in the task span procedure, which requires subjects to retrieve the task to perform on the current target from a list of planned tasks in memory. Experiment 1 compared switch costs in the task span procedure with switch costs in the explicit taskcuing procedure, which presented cues indicating the task to perform on each target. Switch costs were greater in the task span procedure. Experiments 2–4 were designed to identify the sources of this difference. Experiment 2 showed that the requirement of establishing a correspondence between the list of task names and the list of targets contributed to switch costs. Experiment 3 showed that retaining lists of similar task names produced greater switch costs than did retaining lists of dissimilar task names. Experiment 4 showed that memory load had no effect on switch costs. The results are discussed in terms of the interaction between plan-level and task-level processing in the implementation of plans.