Mike Wendt

Control and interference in task switching--a review.

The task-switching paradigm offers enormous possibilities to study cognitive control as well as task interference. The current review provides an overview of recent research on both topics. First, we review different experimental approaches to task switching, such as comparing mixed-task blocks with single-task blocks, predictable task-switching and task-cuing paradigms, intermittent instructions, and voluntary task selection. In the 2nd part, we discuss findings on preparatory control mechanisms in task switching and theoretical accounts of task preparation. We consider preparation processes in two-stage models, consider preparation as an all-or-none process, address the question of whether preparation is switch-specific, reflect on preparation as interaction of cue encoding and memory retrieval, and discuss the impact of verbal mediation on preparation. In the 3rd part, we turn to interference phenomena in task switching. We consider proactive interference of tasks and inhibition of recently performed tasks indicated by asymmetrical switch costs and n-2 task-repetition costs. We discuss stimulus-based interference as a result of stimulus-based response activation and stimulus-based task activation, and response-based interference because of applying bivalent rather than univalent responses, response repetition effects, and carryover of response selection and execution. In the 4th and final part, we mention possible future research fields.

Sequential modulations of interference evoked by processing task-irrelevant stimulus features

Compatibility level repetition benefits in interference paradigms have been taken to reflect enhanced processing selectivity in response to cognitive conflict elicited by a task-irrelevant stimulus feature. The authors demonstrate such sequential effects in the Simon task which (a) occur independent of previous behavioral conflict effects and (b) cannot be accounted for by selectivity enhancement. Furthermore, when presenting more than one type of irrelevant stimulus features, compatibility level repetition effects occurred in a type-specific manner. The results do not support the notion that cognitive conflict results in enhanced processing selectivity and favor a feature integration account.

Disentangling Sequential Effects of Stimulus- and Response-related Conflict and Stimulus-Response Repetition using Brain Potentials

Conflict monitoring theory holds that detection of conflicts in information processing by the anterior cingulate cortex (ACC) results in processing adaptation that minimizes subsequent conflict. Applying an Eriksen f lanker task with four stimuli mapped onto two responses, we investigated whether such modulation occurs only after response-related or also after stimulus-related conflict, focusing on the N2 component of the event-related potential. Contrasting with previous findings, both stimulus- and response-related conflict elicited enhancement of the N2, suggesting that the ACC is sensitive to conflict at both the stimulus and the response level. However, neither type of conflict resulted in reduced conflict effects on the following trial when stimulus-response (S-R) sequence effects were controlled by excluding identical S-R repetition trials. Identical S-R repetitions were associated with facilitated processing, thus demonstrating that inclusion of these trials in the analysis may mimic results predicted by the conflict adaptation hypothesis.

Location-specific versus hemisphere-specific adaptation of processing selectivity

Current theories of cognitive control assume that processing selectivity is adjusted according to the utility of processing task-irrelevant stimulus features. Consistently, interference evoked by flanker stimuli is reduced when the proportion of incompatible trials—in which flankers are associated with an incorrect response—is increased. Consistent with the idea that the cerebral hemispheres select processing strategies independently of each other, Corballis and Gratton (2003) demonstrated that flanker interference for stimuli presented in either the left or right visual hemifield is affected by the ratio of compatible and incompatible target-flanker pairings presented in the same—but not in the other—hemifield. Presenting stimuli at four different locations, we demonstrated independent effects of the ratio of compatible and incompatible trials for stimulus locations in different hemifields as well as for stimulus locations within the same hemifield. Independent selectivity adjustment regarding the left and right visual hemifields thus appears to be a special case of a larger class of location-specific adaptation effects and might not be informative regarding hemisphere-specific processing.

Conflict-Frequency Affects Flanker Interference

Performance in choice reaction time tasks deteriorates when an irrelevant stimulus feature is associated with an incorrect response (conflict condition). Such interference effects are reduced under conditions of increased conflict-frequency. Although models of cognitive control account for this modulation in terms of conflict-related attentional focusing on the target stimulus dimension, it is possible that the effect reflects practice with specific stimulus ensembles or stimulus feature-response contingencies. Using an Eriksen flanker task, we deconfounded the frequency of conflict trials and the frequency of specific stimulus ensembles (i.e., target-flanker conjunctions). In Experiments 1 and 2, flanker interference varied inversely with the frequency of conflict trials, irrespective of practice with specific target-flanker conjunctions, thereby disputing a stimulus ensemble learning account. In Experiment 3, however, flanker interference was reduced for specific flanker stimuli which occurred predominantly in conflict trials. Taken together, the findings are consistent with flanker-specific attentional adjustment or associative flanker-response priming.

Conflict adaptation in time: Foreperiods as contextual cues for attentional adjustment

Interference evoked by distractor stimulus information, such as flankers in the Eriksen task, is reduced when the proportion of conflicting stimuli is increased. This modulation is sensitive to contextual cues such as stimulus location or color, suggesting attentional adjustment to conflict contingencies on the basis of context information. In the present study, we explored whether conflict adjustment is modulated by temporal variation of conflict likelihood. To this end, we associated low and high proportions of conflict stimuli with foreperiods of different lengths. Flanker interference was higher with foreperiods associated with low conflict proportions, suggesting that participants use the foreperiod as a contextual cue for attentional adjustment. We conjecture that participants initially adopt the strategy useful for conflict contingencies associated with short foreperiods, and then readjust during the trial, in the absence of any additional exogenous cue, when the imperative stimulus has not occurred during a certain time interval.

Context specificity of conflict frequency-dependent control

Interference in the Eriksen flanker task has been shown to be reduced when the (relative) frequency of conflicting stimuli is increased, a modulation thought to reflect a higher degree of processing selectivity under conditions of frequent conflict (Botvinick, Braver, Barch, Carter, & Cohen, 2001). Previous studies suggest that stimulus location acts as a contextual cue, resulting in location-specific adjustment of processing selectivity when different locations are associated with differential conflict frequencies (Corballis & Gratton, 2003; Wendt, Kluwe, & Vietze, 2008). In the current study we extend these findings by showing that not only stimulus location but also stimulus colour can be used for context-specific adjustments. These findings suggest that processing selectivity is adjusted in parallel with current stimulus processing, potentially serving to resolve a current conflict rather than to prepare for an upcoming new conflict.

Conflict-Induced Perceptual Filtering.

In a variety of conflict paradigms, target and distractor stimuli are defined in terms of perceptual features. Interference evoked by distractor stimuli tends to be reduced when the ratio of congruent to incongruent trials is decreased, suggesting conflict-induced perceptual filtering (i.e., adjusting the processing weights assigned to stimuli associated with the target and with the distractor features). In search of evidence for such a mechanism, we administered a flanker task, in which targets and distractors were defined in terms of stimulus location (Experiment 1) or color (Experiment 2). The efficiency of processing stimuli associated with target and distractor features was assessed in intermixed trials of a visual search task, in which a target had to be detected irrespective of these features. In both experiments search times were shorter for stimuli associated with the target feature than with the distractor feature of the flanker task. This effect was increased under conditions of a reduced congruent/incongruent ratio, thereby providing evidence for conflict-dependent perceptual filtering.

Adjustments to recent and frequent conflict reflect two distinguishable mechanisms.

In conflict paradigms such as the Eriksen flanker task, interference has been found to be reduced under conditions of recent and/or frequent cognitive conflict. Using a modified flanker task, we investigated the interplay of conflict recency and conflict frequency by comparing the interference reductions following conflict trials under conditions in which conflict was either frequent or infrequent overall, while controlling for stimulus and response feature repetitions to rule out nonattentional accounts. The reduction of flanker interference after a conflict trial was attenuated when overall conflict was frequent. This result is consistent with models assuming that processing adjustments occur gradually in response to conflict strength, such as the connectionist model of Botvinick, Braver, Barch, Carter, and Cohen (2001), in which both recency and frequency modulations are brought about by the same mechanism. By decomposing response times into initiation times and movement times, we revealed that frequent conflict delayed response initiation but sped up movement. Moreover, whereas frequent conflict reduced interference in both components, interference reduction after individual conflict trials was confined to movement times. Taken together, these results suggest that different mechanisms underlie the two kinds of modulation.

Attentional Adjustment to Conflict Strength Evidence From the Effects of Manipulating Flanker- Target SOA on Response Times and Prestimulus Pupil Size

Current models of cognitive control assume gradual adjustment of processing selectivity to the strength of conflict evoked by distractor stimuli. Using a flanker task, we varied conflict strength by manipulating target and distractor onset. Replicating previous findings, flanker interference effects were larger on trials associated with advance presentation of the flankers compared to simultaneous presentation. Controlling for stimulus and response sequence effects by excluding trials with feature repetitions from stimulus administration (Experiment 1) or from the statistical analyses (Experiment 2), we found a reduction of the flanker interference effect after high-conflict predecessor trials (i.e., trials associated with advance presentation of the flankers) but not after low-conflict predecessor trials (i.e., trials associated with simultaneous presentation of target and flankers). This result supports the assumption of conflict-strength-dependent adjustment of visual attention. The selective adaptation effect after high-conflict trials was associated with an increase in prestimulus pupil diameter, possibly reflecting increased cognitive effort of focusing attention.