The surface structure and the deep structure of sequential control: What can we learn from task span switch costs?

A large component of response time switch costs in the cued task-switching paradigm is linked to cue changes without task changes, suggesting costs might reflect passive priming rather than endogenous control. In contrast, the task span procedure requires subjects to guide task selection via sequences of memorized task cues and therefore may be better suited to reflect endogenous switch processes (Logan, 2004). The present experiments combined the task span procedure with a 2:1 mapping between cues and tasks, allowing separation of cue-switch costs from true task-switch costs. Replicating findings with the cued task-switching paradigm, results showed both substantial cue-switch costs and actual task-switch costs (Experiments 1 and 2) as well as sensitivity of cue-switch costs, but not of task-switch costs, to opportunity for preparation (Experiment 2). Apparently, simple action plans use “surface level” phonological or articulatory codes that contain no task information. These results suggest that the distinction between cue-related and task-related processes is critical no matter whether tasks are cued exogenously or endogenously.     

Cue-switch effects do not rely on the same neural systems as task-switch effects

The cued task-switching paradigm is often used to study cognitive control. In this paradigm, people are generally slower and make more errors when switching tasks as compared with repeating the same task. When two cues are mapped to each task, these switch costs could result from a mixture of cue-switch effects (which are thought to reflect cue encoding) and task-switch effects (which are thought to reflect task set preparation). In the behavioral literature, there has been a lively debate on the degree to which cue-switch effects and task-switch effects indeed reflect different phenomena. In the present study, we used fMRI to examine whether and to what extent the neural network underlying task-switch effects is also involved in cue-switch effects. We found task-switch but no cue-switch effects in the frequently observed preparation-related activation in fronto-parietal areas. These results suggest that the fronto-parietal areas displaying preparatory activity in task-switching paradigms are engaged in task preparation but not in cue encoding and that task preparation and cue encoding rely on completely different processes.     

Cue-switch costs in task-switching: cue priming or control processes?

In the explicitly cued task-switching paradigm, two cues per task allow separation of costs associated with switching cues from costs of switching tasks. Whilst task-switch costs have become controversial, cue-switch costs are robust. The processes that contribute to cue-switch costs are under-specified in the literature: they could reflect perceptual priming of cue properties, or priming of control processes that form relevant working memory (WM) representations of task demands. Across two experiments we manipulated cue-transparency in an attention-switching design to test the contrasting hypotheses of cue-switch costs, and show that such costs emerge from control processes of establishing relevant WM representations, rather than perceptual priming of the cue itself. When the cues were maximally transparent, cue-switch costs were eradicated. We discuss the results in terms of recent theories of cue encoding, and provide a formal definition of cue-transparency in switching designs and its relation to WM representations that guide task performance.     

Differential effects of articulatory suppression on cue-switch and task-switch trials in random task cueing with 2:1 mapping

Recent studies have revealed that verbal representations play an important role in various task-switching situations. This study examined whether verbal representations contribute to the actual switching process using random task cueing with two cues per task. This procedure allowed us to produce a trial in which the cue switched, but the task repeated, thereby separating the cue-switching process from the actual task-switching process. Participants performed colour or shape judgements that were initiated by an arbitrary symbol cue (Experiments 1 and 2) or a kanji cue (Experiment 3) under control, articulatory-suppression, and foot-tapping conditions. In Experiments 1 and 2 with the arbitrary cues, articulatory suppression impaired performance in only the cue-switch condition. In Experiment 3, in which a kanji cue indicated the upcoming task name, articulatory suppression did not have any effects. These results suggest that the involvement of verbal representations in random task cueing is based on the cue-switching process rather than on the task-switching process.     

What matters in the cued task-switching paradigm: Tasks or cues?

Schneider and Logan (2006) recently showed that cue-switch and task-switch costs are sensitive to the relative probability of cue switches versus task switches. From this they concluded that taskswitch costs reflect priming of cue-cue transitions rather than actual task-switching operations. However, because this design confounded probability of specific cue transitions with probability of task switches, the results could also reflect task-switch-level adjustments. The present experiment (N = 80) pits the critical prediction of the cue-priming account, namely that costs for high-probability cue-cue transitions are smaller than for low-probability cue-cue transitions, against the main prediction of the switch-probability account, namely that switch probability, irrespective of specific cue-cue transitions, determines switch costs. Whereas the cue-priming prediction was rejected, a specific version of the probability account—that subjects are sensitive to the probability of a task switch, given a cue switch—was fully confirmed. Thus, tasks are in fact the critical representational units that determine task-switch cost.     

Differential effects of articulatory suppression on cue-switch and task-switch trials in random task cueing with 2:1 mapping

Recent studies have revealed that verbal representations play an important role in various task-switching situations. This study examined whether verbal representations contribute to the actual switching process using random task cueing with two cues per task. This procedure allowed us to produce a trial in which the cue switched, but the task repeated, thereby separating the cue-switching process from the actual task-switching process. Participants performed colour or shape judgements that were initiated by an arbitrary symbol cue (Experiments 1 and 2) or a kanji cue (Experiment 3) under control, articulatory-suppression, and foot-tapping conditions. In Experiments 1 and 2 with the arbitrary cues, articulatory suppression impaired performance in only the cue-switch condition. In Experiment 3, in which a kanji cue indicated the upcoming task name, articulatory suppression did not have any effects. These results suggest that the involvement of verbal representations in random task cueing is based on the cue-switching process rather than on the task-switching process.     

Methodological and empirical issues when dissociating cue-related from task-related processes in the explicit task-cuing procedure

In the explicit cuing version of the task-switching paradigm, each individual task is indicated by a unique task cue. Consequently, a task switch is accompanied by a cue switch. Recently, it has been proposed that priming of cue encoding contributes to the empirically observed switch costs. This proposal was experimentally supported by using a 2:1 mapping of cues to tasks, so that a cue switch does not necessarily imply a task switch. The results indeed suggested a substantial contribution of “cue-switch costs” to task-switch costs. Here we argue that the 2:1 mapping potentially leads to an underestimation of “pure” task-switch costs. To support this argument, we report the results of a new study in which we used “transition cues” that indicate the identity of the current task based on the identity of the preceding task. This new type of cue allows a full factorial manipulation of cue switches and task switches because it includes the condition in which a cue repetition can also indicate a task switch (i.e., when the “switch” cue is repeated). We discuss the methodological implications and argue that the present approach has merits relative to the previously used 2:1 mapping of cues to tasks.     

Can the task-cuing paradigm measure an endogenous task-set reconfiguration process?

In 6 task-cuing experiments, with 2 cues per task, the authors varied cue-stimulus interval to investigate G. D. Logan and C. Bundesen's (2003) claim that when cue repetition is controlled for, task-switch cost and its reduction with preparation are largely eliminated and hence cannot index an endogenous control process. Experiment 1 replicates their result, but Experiments 2 and 3, with similar designs, demonstrate a substantial task-switch cost, reducing with increasing cue-stimulus interval. Experiments 4 to 6 show that the critical difference is the probability of a task change: If it is kept low enough to discourage reconfiguration of task set unless and until the cue signals a task change, robust evidence for anticipatory task-set reconfiguration is obtained, even in Experiment 6, modeled closely on Logan and Bundesen's.     

Is task switching nothing but cue priming? Evidence from ERPs

Recent findings suggesting that switch costs in the task-cuing paradigm are largely attributable to a change in the task-indicating cue have been interpreted in terms of a priming model of task-switch costs (Logan & Bundesen, 2003). According to this explanation, participants do not actually switch task sets, but merely use a cue-stimulus compound to disambiguate competing response tendencies associated with bivalent stimuli. Here, we report an event-related potential (ERP) experiment that provides evidence against this notion. In a paradigm with a 2:1 mapping between cues and tasks, we show that cue-switch and task-switch effects are dissociable on a neurophysiological level, indicating that task switching is more than a switch in the task-indicating cue. Moreover, a systematic analysis of the ERPs during the cue-stimulus interval suggests that updating processes can run in advance, before the stimulus is presented.     

Outsourcing control to the environment: effects of stimulus/response locations on task selection

Response-time and accuracy costs as assessed in the context of the task-switching paradigm are usually thought to represent processes involved in the selection of abstract task sets. However, task sets are also applied to specific stimulus and response constellations, which in turn may become associated with task-set representations. To explore the consequence of such associations, we used a task-switching paradigm in which subjects had to select between two tasks (color or orientation discrimination) that were either associated with shared or unique stimulus/response locations on a touchscreen. When each task was associated with unique locations, error switch costs, stimulus-response congruency effects, as well as the characteristic task-switch x repetition-priming interaction were eliminated, and global selection costs were substantially reduced. These results demonstrate that to understand standard task-switching phenomena it is critical to consider links between lower level stimulus/response parameters and task sets.     

Control processes in voluntary and explicitly cued task switching

Explicitly cued task switching slows performance relative to performing the same task on consecutive trials. This effect appears to be due partly to more efficient encoding of the task cue when the same cue is used on consecutive trials and partly to an additional task-switching process. These components were examined by comparing explicitly cued and voluntary task switching groups, with external cues presented to both groups. Cue-switch effects varied in predictable ways to dissociate explicitly cued and voluntary task switching, whereas task-switch effects had similar characteristics for both instructional groups. The data were well fitted by a mathematical model of task switching that included a cue-encoding mechanism (whereby cue repetition improves performance) and an additional process that was invoked on task-switch trials. Analyses of response-time distributions suggest that this additional process involves task-set reconfiguration that may or may not be engaged before the target stimulus is presented.     

Cognitive control in cued task switching with transition cues: Cue processing,task processing,and cue–task transition congruency

We investigated the processes underlying performance during cued task switching with transition cues. To this end, transition cueing and explicit cueing were compared in a design controlling for sequential effects in the two preceding trials in order to further examine the contribution of cue processes, task processes, and cue–task transition congruency during transition cueing. The study confirmed that the task-switch cost in transition cueing is larger than the task-switch cost in explicit cueing and showed that this larger switch cost is mainly due to cue processing. We also successfully decomposed performance in transition cueing into cue processing, task processing, and cue–task transition congruency on both a theoretical (Experiment 1) and an empirical basis (Experiments 2–3). Our empirical dissociation also demonstrates that cue–task transition congruency affects performance during both cue processing and task processing. We discuss the importance of our findings in relation to the different theories on task switching.     

Components of task switching: A closer look at task switching and cue switching

Research using the diffusion model to decompose task-switching effects has contributed to a better understanding of the processes underlying the observed effect in the explicit task cueing paradigm: Previous findings could be reconciled with multiple component models of task switching or with an account on compound-cue retrieval/repetition priming. In the present study, we used two cues for each task in order to decompose task-switch and cue-switch effects. Response time data support previous findings that comparable parts of the switching effect can be attributed to cue-switching and task-switching. A diffusion model analysis of the data confirmed that non-decision time is increased and drift rates are decreased in unpredicted task-switches. Importantly, it was shown that non-decision time was selectively increased in task-switching trials but not in cue-switching trials. Results of the present study specifically support the notion of additional processes in task-switches and can be reconciled with broader multiple component accounts.     

Response selection difficulty and asymmetrical costs of switching between tasks and stimuli: no evidence for an exogenous component of task-set reconfiguration

Four task-switching experiments examined the notion of an exogenous component of task-set reconfiguration (i.e., a process needed to shift task set that is not initiated in the absence of a task-associated figuration stimulus). The authors varied the complexity and familiarity of stimulus-response (SR) mapping rules to produce differentially time-consuming reconfiguration demands. Tasks with more complex or less familiar rules did not display increased switch costs, given that stimulus repetitions were excluded from the analysis. These results do not support the idea of exogenous reconfiguration. Moreover, stimulus repetitions inflated task-switch costs and did so disproportionately for tasks with increased response selection difficulty, thereby demonstrating that insufficient control of the sequence of stimuli may yield results that mimic those predicted by exogenous reconfiguration accounts.     

The effects of recent practice on task switching

Four experiments investigated the effect of recent selective practice on the cost of switching between 2 tasks afforded by letter-digit pairs: alphabet arithmetic and shape comparison. Experiments 1 and 2 found a greater cost associated with switching to the more recently practiced task: evidence that task-set inertia contributes to switching costs. Experiment 3 found this effect to be limited to trials on which a recently trained stimulus followed another such stimulus: a result problematic for all current theories of task-set priming. Experiment 4 showed that the effect of recent practice was eliminated by active preparation for a task switch: It appears that endogenous task-set preparation reduces the effects of task-set inertia. ((c) 2003 APA, all rights reserved)     

Task-set reconfiguration with predictable and unpredictable task switches

Participants switched frequently between high/low and odd/even classification of a digit. The interval between a task cue and the next digit varied between blocks. In Experiment 1, the task switched predictably every two, four, or eight trials. In Experiment 2, switching predictably every four trials was compared with random switching. With predictable switching, the cost was limited to the first trial of a run. Random switching produced a more gradual approach to asymptotic performance. After one performance, control processes attenuate the resulting change in task-set bias if a further switch is likely, but this strategic modulation is soon overwhelmed by task-set priming through further performances. Preparation reduced switch costs but not interference from the irrelevant attribute: Control of interference appears to be reactive, not proactive. Switch costs did not increase with run length, suggesting that retrieval of the task set last associated with the stimulus did not contribute to switch costs.     

Inhibition, interference, and conflict in task switching

The role of inhibition in the task-switching process has received increased empirical and theoretical attention in the literature on cognitive control. Many accounts have suggested that inhibition occurs when a conflict must be resolved—for example, when a target stimulus contains features of more than one task. In the two experiments reported here, we used variants of backward inhibition, or N – 2 repetition, designs to examine (1) whether inhibition occurs in the absence of conflict at the stimulus or response level, (2) when in the task-switching process such inhibition may occur, and (3) the potential consequences of inhibition. In Experiment 1, we demonstrate that neither stimulus- nor response-level conflict is necessary for inhibition to occur, while the results of Experiment 2 suggest that inhibition may be associated with a reduction of proactive interference (PI) from a previously performed task. Evidence of inhibition and the reduction of PI both occurred at the task-set level. However, inhibition of specific stimulus values can also occur, but this is clearly separable from task-set inhibition. Both experiments also provided evidence that task-set inhibition can be applied at the time of the new task cue, as opposed to at the onset of the target or at the response stage of the trial. Taken together, the results from these experiments provide insight into when and where in the task-switching process inhibition may occur, as well as into the potential functional benefits that inhibition of task sets may provide.     

Influence of display type and cue format on task-cuing effects: Dissociating switch cost and right-left prevalence effects

In previous studies of task switching and of the right-left prevalence effect, researchers have used a procedure in which the stimulus on each trial occurs in one of four quadrants, and responses are made by pressing one of two diagonally arranged response keys. Across these studies, discrepant effects of cuing interval have been reported. These discrepancies need clarification because cue-based preparation effects are frequently interpreted as reflecting cognitive control processes. In Experiment 1, we compared performance with display formats used by Meiran (1996; Meiran, Chorev, & Sapir, 2000; small display, cues located at sides of quadrants and displayed until response) to study task switching and by Proctor and colleagues (Proctor, Koch, & Vu, 2006; large display, cues located at center of display and shown until target onset) to study right-left prevalence. We found a decrease in task-switch cost with increasing cuing interval with the Meiran display, but not with the Proctor display, but the right-left prevalence effect was of similar size for the two display formats and was relatively unaffected by cuing interval. To determine the basis of the discrepant task-switch results, we used small and large displays in Experiments 2 and 3, respectively, with cue type and cue offset varied. With the side cues, the task-switch cost decreased in all cases at the longer cuing interval, but with the centered cues, it decreased only when the display size was small. Thus, the effects of cuing interval on switch costs are sensitive to variations of display characteristics, whereas cuing interval and display characteristics have little influence on the right-left prevalence effect, suggesting that prevalence effect is due to processes that are independent from those producing the switch cost.     

Phasic alertness and the residual task-switching cost

Participants switched between two randomly ordered discrimination tasks and each trial began with the presentation of a task cue instructing which task to execute. The authors induced phasic alertness by presenting a salient uninformative stimulus after the task cue was provided, and at variable intervals before the target stimulus was presented (Experiments 1-3) or before the task cue (Experiment 4). When the alerting stimulus preceded the target stimulus or the task cue by an optimal interval, RT was faster, indicating an alert state and the task-switching cost was reduced. These results support the suggestion of De Jong (Acta Psychologica, 1999) that alertness improves the overcoming of retrieval competition through improved goal representation, but also show that the effect is specific to the residual task-switching cost.     

On the role of verbalization during task set selection: Switching or serial order control?

Recent task-switching work in which paper-and-pencil administered single-task lists were compared with task-alternation lists has demonstrated large increases in task-switch costs with concurrent articulatory suppression (AS), implicating a crucial role for verbalization during switching (Baddeley, Chincotta, & Adlam, 2001; Emerson & Miyake, 2003). Experiment 1 replicated this result, using computerized assessment, albeit with much smaller effect sizes than in the original reports. In Experiment 2, AS interference was reduced when a sequential cue (spatial location) that indicated the current position in the sequence of task alternations was given. Finally, in Experiment 3, switch trials and no-switch trials were compared within a block of alternating runs of two tasks. Again, AS interference was obtained mainly when the endogenous sequencing demand was high, and it was comparable for no-switch and switch trials. These results suggest that verbalization may be critical for endogenous maintenance and updating of a sequential plan, rather than exclusively for the actual switching process.