Inhibition during task switching is affected by the number of competing tasks.

Inhibition during task switching is assumed to be indexed by n - 2 repetition costs-that is, performance costs when the task in the current trial equals the task in trial n - 2 (sequences of type ABA) compared with two consecutive switches to another task each (sequences CBA). The present study examined effects of a short-term reduction of the number of candidate tasks on these costs. For this purpose, a variant of the task switching paradigm was used in which in half of the trials, a precue that preceded the task cue allowed for a short-term reduction of the number of candidate tasks. In Experiment 1, one out of three tasks could be excluded. In Experiment 2, one or two out of four tasks could be excluded. Experiment 3 served as control condition using the standard cueing paradigm. Significant n - 2 repetition costs were present with three candidate tasks. In contrast, no costs were visible when the number of candidate tasks was reduced to two. This result is interpreted in terms of a task selection mechanism based on antagonistic constraints among task representations, which operates on a rather superficial level when switching among only two tasks, thereby reducing the need for inhibition.

Effects of a dynamically changing response set overlap on n - 2 repetition costs.

N - 2 repetition costs are a marker for inhibition processes during task switching that are supposed to reduce interference from currently irrelevant information. The present study aimed at elucidating effects of response set overlap on n - 2 repetition costs while keeping stimulus set overlap constant. For this purpose, each task was associated with two different response sets. The relevant response set was visually cued in every trial. N - 2 repetition costs were present when the response set overlapped from trial n - 2 to trial n - 1. In contrast, they were abolished when the response set switched. This result is interpreted in terms of stronger interference for overlapping response sets that need to be inhibited to a high degree, resulting in large n - 2 repetition costs. Furthermore, the present results support the notion that two means for interference reduction, task inhibition and task shielding, are deployed in a flexible way depending on environmental demands.

No-go trials in task switching: effects on the task-set and task-space level.

A common marker for inhibition processes in task switching are n - 2 repetition costs. The present study aimed at elucidating effects of no-go trials on n - 2 repetition costs. In contrast to the previous studies, no-go trials were associated with only one of the three tasks in the present two experiments. High n - 2 repetition costs occurred if the no-go task had to be executed in trial n - 2, irrespective of whether a response had to be withheld or not. In contrast, no n - 2 repetition costs were visible if the other two tasks were relevant in n - 2. Whereas this n - 2 effect was unaffected by whether participants could reliably exclude a no-go trial or not, effects of no-gos in trial n were determined by this knowledge. The results differ from effects of no-go trials that are not bound to a specific task. It is assumed that the present no-go variation exerted its effect not on the response level, but on the level of task sets, resulting in enhanced salience of the no-go task that leads to higher activation and, as a consequence, to stronger inhibition. The dissociation of the effects on no-gos in trials n - 2 and n as a function of foreknowledge suggests that the balance between activation and inhibition is shifted not only for single trials and tasks, but for the whole task space.

Motor imagery entails task-set inhibition.

Motor imagery requires the covert execution of a movement without any overt motor output. Previous studies indicated that motor imagery results in the prolonged inhibition of motor commands. In the present study, we investigated whether motor imagery also leads to the inhibition of more abstract task representations. To do so, we investigated the effect of motor imagery on n - 2 repetition costs, which offer an index of the extent to which task representations are inhibited. Participants switched among three tasks and among two response modes: overt and covert responding (i.e., motor imagery). N - 2 repetition costs were present when the current trial required an overt response but absent when the current trial required a covert response. Furthermore, n - 2 repetition costs were more pronounced when trial n - 1 required a covert response rather than an overt response. This pattern of results suggests that motor imagery also leads to the inhibition of abstract task representations. We discuss our findings in view of current conceptualizations of motor imagery and argue that the inhibitory mechanism entailed by motor imagery targets more than motor commands alone. Finally, we also relate our findings to the mechanisms underlying the inhibition of task representations.

Task switching among two or four tasks: effects of a short-term variation of the number of candidate tasks.

In two experiments, using a modified task-cueing paradigm, participants switched among a set of four tasks that was on some trials reduced to two by a pre-cue preceding the task cue. A pre-cue temporarily restricting the options to two candidate tasks facilitated performance on switch trials to a much larger extent than on repeat trials. This observation is interpreted as an indication of a short-term restructuring of a global task representation by implementing antagonistic constraints among the representations of the two remaining candidate tasks.

Tracing the time course of n - 2 repetition costs in task switching.

In order to flexibly adapt to a permanently changing environment, it is necessary to inhibit previously activated but now irrelevant processing pathways. Empirically, this inhibition manifests itself only indirectly in terms of a cost of reengaging a previously inhibited pathway (n - 2 repetition costs). While imaging studies suggest an involvement of the prefrontal cortex in this type of inhibition, it has recently been argued that the underlying processes are implicated not in triggering inhibition, but in overcoming it. To disentangle these processes on a behavioral level, we investigated the time course of inhibition using a cued task switching paradigm. The response-cue interval (between the response of trial n - 1 and the cue of trial n) was varied in five steps to capture its influence on inhibition in a fine-grained manner. The results suggest that the impact of inhibitory processes increases during the first 200-300 ms after the response of the previous trial, reaches its full extent with about 300 ms, and starts to diminish after that. Therefore, future research on the neural correlates of n - 2 repetition costs should employ techniques with a high temporal resolution that are able to capture this presumed time course of inhibitory processes.

Further investigating effects of task repetition proportion on n - 2 repetition costs: Task shielding as a potential modulating factor?

Previous research has shown that n - 2 repetition costs are reduced when the experimental procedure allows for task repetitions. The present study aimed to further elucidate possible mechanisms underlying this effect. To this end, three experiments were conducted in which the possibility of task repetitions was tied to individual tasks. In Experiment I, the specific repetition proportions varied between participants. The reduction of n - 2 repetition costs was stronger when the relative amount of task repetitions associated with a particular task was high, suggesting that inhibition is reduced when the presence of repetitions tied to a specific task can easily be detected. In Experiment II, task preparation time varied between blocks to investigate whether cue-based preparation underlies this reduction of n - 2 repetition costs. While the duration of the preparation interval had no effect in this respect, additional combined analyses of both experiments revealed an influence of stimulus congruency: n - 2 repetition costs were highest when repetitions were precluded and the task stimulus was incongruent, whereas no effect of stimulus congruence on n - 2 repetition costs was observed with tasks that possibly repeated. This data pattern was replicated in Experiment III. This result is interpreted in terms of task shielding which is reduced for tasks without repetitions, making these tasks more vulnerable to crosstalk from competing tasks when they are still in an inhibited state.

Effects of global and local task repetition proportion on n - 2 repetition costs.

In task switching research, one of the most straightforward indications for the involvement of inhibitory processes are n - 2 repetition costs. The present study aimed at investigating effects of different types of repetition proportion on n - 2 repetition costs. In Experiments 1 and 2, repetition proportion was varied globally (i.e., equally for all tasks). The occurrence of 33% task repetitions reduced n - 2 repetition costs when varied within as well as between subjects, but no further decline was visible from 33% to 50% task repetitions. This result is interpreted in terms of a shift of balance between task inhibition and task activation due to the occurrence of task repetitions that is independent of the specific repetition proportion. In contrast, when repetition proportion was varied locally (i.e., by differentially manipulating the occurrence of task repetitions for the three tasks involved), n - 2 repetition costs were reduced monotonically from 0% to 50% task repetitions. This result indicates that when the utility inhibition is tied to individual tasks, the cognitive system is able to adjust the deployment of inhibition accordingly, possibly by modulating processes of overcoming inhibition, not releasing it.

Further evidence for functional differences between guessing versus choosing an upcoming task.

We replicated and extended previous evidence regarding functional differences between guessing versus choosing an upcoming task. Participants switched among four tasks and were asked to predict the upcoming task on each trial. These predictions were instructed to participants as either 'guessing' or 'choosing'. Furthermore, we varied the proportion of trials in which the presented task conformed to participants' predictions on three levels. Whereas with choosing instructions unexpectedness affected task switches and repetitions similarly, leaving switch costs unchanged, with guessing instructions switch costs were reduced, that is, task switches were affected less than repetitions. This interaction was unaffected by the proportion of expected tasks. We propose that with choosing, the impact of a mismatch between chosen and presented tasks is reduced by explicit knowledge regarding the proportion of denied choices. With guessing, task unexpectedness mainly increases task difficulty, which is compensated by an increase of cognitive control that reduces switch costs.

Motor command inhibition and the representation of response mode during motor imagery.

Research on motor imagery proposes that overt actions during motor imagery can be avoided by proactively signaling subthreshold motor commands to the effectors and by invoking motor-command inhibition. A recent study by Rieger, Dahm, and Koch (2017) found evidence in support of motor command inhibition, which indicates that MI cannot be completed on the sole basis of subthreshold motor commands. However, during motor imagery, participants know in advance when a covert response is to be made and it is thus surprising such additional motor-command inhibition is needed. Accordingly, the present study tested whether the demand to perform an action covertly can be proactively integrated by investigating the formation of task-specific action rules during motor imagery. These task-specific action rules relate the decision rules of a task to the mode in which these rules need to be applied (e.g., if smaller than 5, press the left key covertly). To this end, an experiment was designed in which participants had to switch between two numerical judgement tasks and two response modes: covert responding and overt responding. First, we observed markers of motor command inhibition and replicated the findings of Rieger and colleagues. Second, we observed evidence suggesting that task-specific action rules are created for the overt response mode (e.g., if smaller than 5, press the left key). In contrast, for the covert response mode, no task-specific action rules are formed and decision rules do not include mode-specific information (e.g., if smaller than 5, left).

Integration of Advance Information about a Forthcoming Task Switch - Evidence from Eye Blink Rates.

We investigated task switching among four tasks by means of a modified cuing procedure with two types of cues. One type of cue consisted of a standard task cue indicating the next task. In half of the trials, this task cue was preceded by another type of cue that reduced the set of candidate tasks from four to two tasks. In addition, we measured participants' spontaneous eye blink rates (EBRs) at the beginning, in the middle, and at the end of the experiment. Whereas interindividual differences in mean EBR had no pronounced effect on task switching performance, changes in EBRs during the first half of the experiment significantly modulated the interaction of the effects of the two types of cues. We suggest that changes in EBRs in the early phase of the experiment reflect adaptations of dopaminergic projections serving to integrate advance information about a forthcoming task switch.

Effects of absolute and relative practice on n-2 repetition costs.

Recently, Grange and Juvina (2015) found decreasing n-2 repetition costs with increasing practice. However, in their experiment, no differentiation between absolute and relative strength of the three tasks was possible because all tasks were practiced to the same degree. To further elucidate this issue, two experiments were designed in which for one of the three tasks, aspects of the task set changed during the course of the experiment (Exp. I: Stimulus-response mapping, Exp. II: Cue-task mapping). Replicating Grange and Juvina (2015), decreasing n-2 repetition costs with increasing practice were observed, but the change of stimulus-response mappings in Exp. I did not affect n-2 repetition costs. In Exp. II, n-2 repetition costs were affected by the change of the cue-task-mapping, but no effect of absolute practice was visible. These results suggest that absolute practice influences n-2 repetition costs as long as no change in relative strength is introduced on the level of mapping cues to tasks. If, however, relative task strength is varied, its impact overrides the influence of absolute practice. In addition, the data pattern points towards cue-related instead of response-related inhibitory processes causing n-2 repetition costs.

Guessing versus Choosing an Upcoming Task.

We compared the effects of guessing vs. choosing an upcoming task. In a task-switching paradigm with four tasks, two groups of participants were asked to either guess or choose which task will be presented next under otherwise identical conditions. The upcoming task corresponded to participants' guesses or choices in 75 % of the trials. However, only participants in the Choosing condition were correctly informed about this, whereas participants in the Guessing condition were told that tasks were determined at random. In the Guessing condition, we replicated previous findings of a pronounced reduction of switch costs in case of incorrect guesses. This switch cost reduction was considerably less pronounced with denied choices in the Choosing condition. We suggest that in the Choosing condition, the signaling of prediction errors associated with denied choices is attenuated because a certain proportion of denied choices is consistent with the overall representation of the situation as conveyed by task instructions. In the Guessing condition, in contrast, the mismatch of guessed and actual task is resolved solely on the level of individual trials by strengthening the representation of the actual task.

Incorrect predictions reduce switch costs.

In three experiments, we combined two sources of conflict within a modified task-switching procedure. The first source of conflict was the one inherent in any task switching situation, namely the conflict between a task set activated by the recent performance of another task and the task set needed to perform the actually relevant task. The second source of conflict was induced by requiring participants to guess aspects of the upcoming task (Exps. 1 & 2: task identity; Exp. 3: position of task precue). In case of an incorrect guess, a conflict accrues between the representation of the guessed task and the actually relevant task. In Experiments 1 and 2, incorrect guesses led to an overall increase of reaction times and error rates, but they reduced task switch costs compared to conditions in which participants predicted the correct task. In Experiment 3, incorrect guesses resulted in faster performance overall and to a selective decrease of reaction times in task switch trials when the cue-target interval was long. We interpret these findings in terms of an enhanced level of controlled processing induced by a combination of two sources of conflict converging upon the same target of cognitive control.

Effects of reducing the number of candidate tasks in voluntary task switching.

Recently, Demanet and Liefooghe (2014; Experiment 3) reported an experiment on voluntary task switching (VTS) in which the number of candidate tasks to choose from was reduced from 4 to 2 before participants indicated their task choice. This procedure was intended to prevent participants from choosing a task in advance of the presentation of a prompt to do so. This procedure is highly similar to a procedure recently employed by Kleinsorge and Scheil (2013) in a study of cued task switching which yielded evidence for a selective facilitation of task switches by a reduction of the number of tasks to two. In order to examine whether a similar effect would also be observed with VTS, we conceptually replicated the experiment of Demanet and Liefooghe (2014) with an additional control condition in which the number of tasks was not reduced. In this experiment, no evidence for a facilitation of task switching could be observed, pointing to a functional divergence between explicit task cues and the internally generated cues involved in VTS. In addition, we observed evidence for a selective advantage of forced switch trials over repetition-possible trials that was largely independent of the duration of the preparation interval. This effect was accompanied by a massive increase of task indication times in conditions with a reduced number of tasks, suggesting that this manipulation resulted in a pronounced change in the way participants performed voluntary task switches.

N - 2 repetition costs depend on preparation in trials n - 1 and n - 2.

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.