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The measurement of "switch costs" is held to be of interest because, as is widely believed, they may reflect the control processes that are engaged when subjects switch between two (or more) competing tasks. [In task-switching experiments, the reaction time (RT) switch cost is typically measured as the difference in RT between switch and non-switch (repeat) trials.] In this report we focus on the RT switch costs that remain even after the subject has had some time to prepare for the shift of task, when the switch cost may be approximately asymptotic (so-called residual switch costs). Three experiments are presented. All three experiments used Stroop colour/word, and neutral stimuli. Participants performed the two tasks of word-reading and colour-naming in a regular, double alternation, using the "alternating runs" paradigm (R. D. Rogers & S. Monsell, 1995). The experiments were designed to test the hypothesis that RT switch costs depend on a form of proactive interference (PI) arising from the performance of a prior, competing task. A. Allport, E. A. Styles and S. Hsieh (1994) suggested that these PI effects resulted from "task-set inertia", that is, the persisting activation-suppression of competing task-sets, or competing task-processing pathways. The results confirmed the existence of long-lasting PI from the competing task as a major contributor to switch costs. Non-switch trials, used as the baseline in the measurement of switch costs, were also shown to be strongly affected by similar PI effects. However, task-set inertia was not sufficient to account for these results. The results appeared inconsistent also with all other previous models of task switching. A new hypothesis to explain these between-task interference effects was developed, based on the stimulus-triggered retrieval of competing stimulus-response (S-R) associations, acquired (or strengthened) in earlier trials. Consistent with this retrieval hypothesis, switch costs were shown to depend primarily on the S-R characteristics of the preceding task (the task that was switched from) rather than the upcoming task. Further, the effects of the other, competing task were found to persist over many successive switching trials, affecting switch costs long after the stimulus overlap (and hence the principal S-R competition) between the current tasks had been removed. Switch costs were also found to be affected by recent, item-specific experience with a given stimulus, in either the same or the competing task. Finally, the results showed that switch costs were massively affected by the ratio of the number of prior trials, in response to the same stimuli, that had implemented either the currently intended or the competing S-R mappings. None of these effects are predicted by current models of residual switch costs, which appeal to the differences in control processes assumed to be engaged in switch versus non-switch trials.

Action control is shaped by numerous interacting factors. These are sometimes partitioned into internal factors (e.g., current intentions, goals) and external factors, such as the tendency of situations (or stimuli) to prime actions associated with them. Recently, the interaction of these factors has been studied using the task-switching paradigm. In task switching, performance in repeated tasks is compared with performance when there is a requirement to switch between different tasks. Typically, reaction time (RT) and error rate are greater on task switch than on repetition trials. These shift costs appear in many studies, in which a variety of tasks and paradigms have been used (see, e.g., Monsell, 2003, for a review). Interactive Processes in Task SwitchingIt has been suggested that shift costs in task switching are related to a process of reconfiguring the task set (see, e.g., Goschke, 2000;Meiran, 1996;Rogers & Monsell, 1995). The main empirical support for this suggestion has come from studies exploring the effects of the time available for task preparation. For instance, Rogers and Monsell varied the temporal interval from the preceding response to the onset of the next task stimulus (response-stimulus interval, RSI) in predictable, instructed task sequences (e.g., AABBAABB etc.) and found that longer RSIs were associated with reduced shift costs. They took this reduction of shift costs with long RSIs as evidence for advance preparation for the upcoming task.However, lengthening the RSI provides time not only for active preparation of the upcoming task, but also for changes relating to the preceding task, such as the decay of activation relevant to that task (Meiran, 1996). Both of these processes could, in principle, affect the size of shift costs. To dissociate the influence of preparation and decay, Meiran (1996) used explicit instructional cues in a random task sequence.Decay of task activation. The cuing paradigm allows the potential effects of decay time on shift costs to be examined, independently of active preparation. To do so, the cue-to-stimulus interval (CSI), which represents the time available for cue encoding and task preparation, is held constant while the prior response-to-cue interval (RCI) is varied. In fact, it has been found that shift costs decrease with increasing RCI (see, e.g., Koch, 2001;Meiran, Chorev, & Sapir, 2000), in accord with the idea that task activation decays over time following execution of a response. The assumption is that persisting task activation In this study, we investigated the interaction of three different sources of task activation in precued task switching. We distinguished (1) intentional, cue-based task activation from two other, involuntary sources of activation: (2) persisting activation from the preceding task and (3) stimulus-based task activation elicited by the task stimulus itself. We assumed that cue-based task activation increases as a function of cue-stimulus interval (CSI) and that task activation from the preceding trial decays as a function ...

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People find it difficult to switch between two tasks, even if they have time to prepare-the so-called residual task shift cost. We studied a switch of tasks from picture naming to word reading, using picture-word Stroop stimuli. Consistent with previous findings, we demonstrate that a large part of the observed task shift cost was due to priming from prior stimulus-response episodes, in which the current task stimulus was encountered in a competing task. We further show that this task-priming effect generalizes to semantically related stimuli, which opens the possibility that most or all of these residual shift costs reflect some sort of generalized proactive interference from previous stimulus-task episodes.BRIEF REPORTS

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When subjects switch between tasks, performance is slower after a task switch than after a task repetition, even when preparation time is long. We report two experiments that support the idea that a large part of these residual task shift costs can be due to stimulus-cued retrieval of previous task episodes. We demonstrate that there are two different factors at work: (1) facilitation of response to the current distractor stimulus, appropriate to the previously relevant, competing task (competitor priming), and (2) impaired processing of previously suppressed responses (negative priming). Negative priming was contingent on the size of the stimulus set, suggesting that distractor suppression comes into effect only if the distractors are highly activated. Importantly, both types of interference interacted with task readiness: Whereas in the nondominant task (picture naming), switch and nonswitch trials were equally affected, the dominant task (word reading) showed priming effects on switch trials only. Thus, the retrieval of previous processing episodes has a selective impact on situations in which task competition is high.