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In task switching, a common result supporting the notion of inhibitory processes as a determinant of switch costs is the occurrence of n - 2 repetition costs. Evidence suggests that this effect is not affected by preparation. However, the role of preparation on preceding trials has been neglected so far. In this study, evidence for an influence of preparatory processes on n - 2 repetition costs is provided by focusing on the cue-target interval on trials n - 1 and n - 2. N - 2 repetition costs were affected by an interaction of both n - 1 and n - 2 preparation time, being more pronounced when both preparatory intervals were long. The results provide support for the notion of active preparation processes involved in backward inhibition.

The task-switching paradigm is being increasingly used as a tool for studying cognitive control and task coordination. Different procedural variations have been developed. They have in common that a comparison is made between transitions in which the previous task is repeated and transitions that involve a change toward another task. In general, a performance switch cost is observed such that switching to a new task results in a slower and more error-prone execution of the task. The present article reviews the theoretical explanations of the switch cost and the findings collected in support of those explanations. Resolution and protection from interference by previous events explain part of the switching cost, but processes related to task setting and task preparation also play a prominent role, as testified by faster execution and lower switch costs when the preparation time is longer. The authors discuss the evidence in favor of each of these sets of accounts and raise a number of questions that situate task switching in a broader context of cognitive control processes. The role of several aspects of the task set, including task variations, task-set overlap, and task-set structure, is addressed, as is the role of knowledge about probability of task changes and about the structure of task sequences.

The present study used a go/no-go signal delay (GSD) to explore the role of response-related processes in task switching. A go/no-go signal was presented at either 100 ms or 1,500 ms after the stimulus. Participants were encouraged to use the GSD for response selection and preparation. The data indicate that the opportunity to select and prepare a response (i.e., long GSD) resulted in a substantial reduction of task-shift costs (Experiment 1) and n-2 task-repetition costs (i.e., backward inhibition; Experiment 2) in the current trial. These results suggest that interference from the preceding trial can be resolved during response selection and preparation. Furthermore, the shift costs and the n-2 repetition costs after no-go trials with long GSD (i.e., response selection but no execution) were markedly smaller than after go trials. These findings suggest that the interference that gives rise to shift costs and n-2 repetition costs is related not solely to response selection but also to response execution. Thus, the present study demonstrates dissociable contributions of response selection and response execution to interference effects in task switching.

Endogenously initiated transitions between tasks are associated with inhibition of the attentional set for the task preceding the transition, as demonstrated by slowed reactions to a task most recently switched away from (U. Mayr & S. W. Keele, 2000). Using an altered methodological approach, the authors found that this backward inhibition counteracts perseverative tendencies when switching to a new task in that it selectively reduces interference exerted by the preceding task set. The reduction of interference was dependent on endogenous preparation for the new task and did not occur for unpredictable task switches or for task switches that were precued without information about the identity of the new task.

Does the explicit task-cuing procedure require an endogenous a act of control? In 5 experiments, cues indicating which task to perform preceded targets by several stimulus onset asynchronies (SOAs). Two models were developed to account for changes in reaction time (RT) with SOA. Model 1 assumed an endogenous act of task switching for cue alterations but not for cue repetitions. Model 2 assumed no such act. In Experiments 1 and 2, the cue was masked or not masked. Masking interacted underadditively with repetition and alternation, consistent with Model 2 but not Model 1. In Experiments 3 and 4, 2 cues were used for each task. RT was slower for task repetition than for cue repetition and about the same as RT for task alternation, consistent with Model 2 but not Model 1. The results suggest that the explicit task-cuing procedure does not require an endogenous act of control.

Executive processes necessary for flexibly switching between different tasks were studied using a set switching paradigm that requires participants to rapidly switch between different tasks across consecutive trials. Switch cost reflects poorer performance for task- switch trials than for consecutive same-task trials. Significant switch cost was observed even with considerable preparation time before a task- switch, an effect known as residual switch cost. This study tested the hypothesis that one process underlying residual switch cost is inhibition of the previous task-set. We used semantic categorization tasks to compare switch cost between alternating task series (ABA) and nonalternating series (ABC) in order to test the generality of a task- set inhibition effect previously observed with perceptual judgment tasks (Mayr & Keele, in press). The results yielded significant switch cost only for alternating tasks, in both response times and errors resulting from performance of the wrong task. Thus, resolving inhibition associated with previously abandoned task-sets may be the main process underlying residual switch costs, suggesting that task-set inhibition is an important executive control process. Executive control processes that underlie voluntary direction of cognitive resources to some specific goal are not well understood. Much is known about how our minds work to do specific tasks such as process language (e.g., Pinker, 1994), arithmetic (e.g., Ashcraft, 1992), music (e.g., Schmuckler, 1997; Steinke, Cuddy, & Holden, 1997), or to control motor movements such as throwing to a target (e.g., Castiello, 1996; MacKenzie & Marteniuk, 1985). But how we effect decisions to use our minds to process language instead of music at a particular moment is still largely a mystery. Executive processes theoretically function to select and coordinate the operation of multiple processes for any given task, including relevant perception, memory, and motor control processes. Until recently, most of what we knew about executive control was learned by studying those who can no longer use these processes efficiently, especially those with brain damage to frontal cortices (Damasio, 1994; Pollux, Wester, & De Haan, 1995; Robertson, Manly, Andrade, Baddeley, & Yiend, 1997; Shallice, 1988; Spatt & Goldenberg, 1993). One reason for the apparent neglect of executive control in the cognitive literature is the difficulty of designing methods to isolate executive control processes. In any task, the behaviour that is emitted is always a combination of the task-specific processes and executive control processes (Allport, Styles, & Hsieh, 1994; Rogers & Monsell, 1995), making behavioural measurement of executive control difficult. Recently, however, two methods have been developed to study executive processing. Baddeley and his colleagues use dual task paradigms to determine which cognitive skills involve executive processes (Baddeley, 1996; Baddeley, Emslie, Kolodny, & Duncan, 1998; Baddeley & Wilson, 1988), and hypothesize that executive control is needed for tasks that require frequent switches between retrieval plans (e.g., category fluency and random number generation). A second method, set switching, directly examines switching between tasks (Allport et al., 1994; Meiran, 1996; Rogers & Monsell, 1995). In a set-switching study, participants perform two or more different tasks within a series of trials, and performance on trials that involve switching between tasks is compared to that for trials without a switch. For example, Allport et al. (1994) had participants switch between naming the ink colour and reading the word for Stroop colour-word stimuli. Set- switching studies indicate a robust cost for switching tasks, presumably reflecting the time necessary for executive control processes to operate. However, switch cost is not observed if the to-be-performed tasks are uniquely cued by stimulus type within the experimental context. …

Flexible control of action requires the ability to disengage from previous goals or task sets. The authors tested the hypothesis that disengagement during intentional shifts between task sets is accompanied by inhibition of the previous task set ("backward inhibition"). As an expression of backward inhibition the authors predicted increased response times when shifting to a task set that had to be abandoned recently and, thus, suffers residual inhibition. The critical backward inhibition effect on the level of abstractly defined perceptual task sets was obtained across 6 different experiments. In addition, it was shown that backward inhibition can be differentiated from negative priming (Experiment 2), that it is tied to top-down sequential control (Experiment 3), that it can account at least partially for "residual shift costs" in set-shifting experiments (Experiment 4), and that it occurs even in the context of preplanned sequences of task sets (Experiment 5).

Participants performed choice reaction time (RT) tasks on 2-dimensional stimuli such that each task was based on 1 stimulus dimension. A cue preceded the target stimulus and instructed the participant about which (randomly selected) task to perform. Shifting between tasks was associated with an RT cost, which was larger when the (randomly varying) cue-target interval was short as opposed to when it was long. Cue-target interval was not confounded with the remoteness from the previous trial. Hence, it affected the task-shift cost through preparation rather than by allowing carryover effects to dissipate. Similar results were obtained for 2 location tasks and for the object-based tasks (color and shape discrimination). They indicate a time-effort consuming process that operates after a task shift, precedes task execution, and presumably reflects the advance reconfiguration of processing mode.