Preparing or executing the wrong task: The influence on switch effects

In a previous study, it was proposed that executing a task leads to task strengthening. In other words, task activation at the moment of response execution determines subsequent switch effects (Steinhauser & Hübner, 2006). The authors investigated this issue by comparing switch effects after task and response errors. However, the use of bivalent stimulus–response mappings might have obscured some of the effects. Therefore, we replicated the experiment using univalent stimulus–response mappings. With this adjusted design, which overcomes some shortcomings of the original study, we were able to replicate the finding of switch benefits after task errors. Closer inspection of the data showed the importance of preexecution processes on subsequent switch effects. In a second experiment, we further elaborated on these preexecution processes. More precisely, we investigated the effect of task preparation on subsequent switch effects. Taken together, our data extend current accounts of task switching by showing that the preparatory processes occurring before the response on trial n influence the switch cost on trial n?+?1.     

Age equivalence in switch costs for prosaccade and antisaccade tasks

This study examined age differences in task switching using prosaccade and antisaccade tasks. Significant specific and general switch costs were found for both young and old adults, suggesting the existence of 2 types of processes: those responsible for activation of the currently relevant task set and deactivation of the previously relevant task set and those responsible for maintaining more than 1 task active in working memory. Contrary to the findings of previous research, which used manual response tasks with arbitrary stimulus-response mappings to study task-switching performance, no age-related deficits in either type of switch costs were found. These data suggest age-related sparing of task-switching processes in situations in which memory load is low and stimulus-response mappings are well learned.     

Implicit task sets in task switching?

In 2 experiments, the authors compare stimulus-based versus task-rule-based task performance. Participants practiced 8 stimulus-response mappings either with or without knowledge about 2 underlying task sets. After practice, 2 transfer blocks with 8 new stimuli were presented. Results show that rule knowledge leads to significant switch and transfer costs, whereas without rule knowledge neither switch nor transfer costs occur. However, significant Task Type x Response Type interactions occurred in both conditions. In a second experiment including only the no rule condition, half of the stimulus-response mappings in the transfer blocks were incongruent to the underlying task rule. Slower response times for these incongruent stimuli as compared with congruent stimuli and the absence of switch costs suggest that participants acquired (presumably implicit) knowledge about 4 different stimulus-response categories.     

The role of task rules and stimulus–response mappings in the task switching paradigm

Switch costs occur whenever participants are asked to switch between two or more task sets. In a typical task switching experiment, participants have to switch between two task sets composed of up to four different stimuli per task set. These 2 (task sets) x 4 (stimuli) contain only 8 different stimulus-response (S-R) mappings, and the question is why participants base their task performance on task sets instead of S-R mappings. The current experiments compared task performance based on task rules with performance based on single stimulus-response mappings. Participants were led to learn eight different S-R mappings with or without fore-knowledge about two underlying task sets. Without task set information no difference between shifts and repetitions occurred, whereas introducing task sets at the beginning led to significant switch costs. Most importantly, introducing task sets in the middle of the experiment also resulted in significant switch costs. Furthermore, introducing task rules at the beginning of the experiment lead to slower RTs when simple stimuli (Experiment 1) had to be processed. This detrimental effect disappeared with more complex stimuli (Experiment 2). Results will be discussed with respect to cognitive control.     

A role for set in the control of automatic spatial response activation

Spatial stimulus-response (S-R) compatibility effects are widely assumed to reflect the automatic activation of a spatial response by the spatial attributes of a stimulus. The experiments reported here investigate the role of the participant's set in enabling or interacting with this putatively automatic spatial response activation. Participants performed a color discrimination task (Experiment 1) or a localization task (Experiment 2). In each experiment, two different S-R mappings were used and a task-cue indicated the appropriate mapping on each trial. S-R compatibility and the time between the task-cue and target were manipulated, and compatibility effects were assessed as a function of (a) the time between the task-cue and the stimulus, and (b) whether the S-R mapping repeated or switched on consecutive trials. Critically, whether response mappings repeated or switched on consecutive trials determined the relation between compatibility effects and the time between task-cue and stimulus. These results are discussed in terms of an interaction between automatic spatial response activation and the participant's set.     

Instruction-induced feature binding

In order to test whether or not instructions specifying the stimulus-response (S-R) mappings for a new task suffice to create bindings between specified stimulus and response features, we developed a dual task paradigm of the ABBA type in which participants saw new S-R instructions for the A-task in the beginning of each trial. Immediately after the A-task instructions, participants had to perform a logically independent B-task. The imperative stimulus for the A-task was presented after the B-task had been executed. The present data show that the instructed S-R mappings influence performance on the embedded B-task, even when they (1) have never been practiced, and (2) are irrelevant with respect to the B-task. These results imply that instructions can induce bindings between S- and R-features without prior execution of the task at hand.     

Effects of response selection on the task repetition benefit in task switching

A task switch typically leads to worse performance than a repetition does. This shift cost can be reduced with sufficient task preparation time, but a residual cost usually remains. We propose that a large part of this residual cost is caused by an activation bias produced by response selection processes in the preceding trial. In our experiments, we manipulated response selection requirements using a go/no-go methodology. The residual shift cost disappeared after no-go trials, suggesting that response selection is crucial to establish an activation bias for the current category-response rules and that this bias persists into the next trial. A comparison with a go-only group confirmed this analysis by revealing no differences in preparatory strategy due to the inclusion of no-go trials. In addition, no-go trials had no significant effects on subsequent trials in a single-task experiment, suggesting that no-go trials are not coded as a task different from go trials and that there is no inhibition of the prepared task in a no-go trial. We thus conclude that a persisting activation bias of response rules plays a major role in task switching.     

Effects of associative learning on age differences in task-set switching

Costs of switching between tasks may disappear when subjects are able to learn associations between tasks, stimuli, and responses (cf. Rogers, R. D., & Monsell, S. (1995). Costs of a predictable switch between simple cognitive tasks. Journal of Experimental Psychology: General, 124, 207-231). The first aim of this study was to examine this possibility by manipulating stimulus-set size. We expected that costs of switching between tasks would be strongly reduced under conditions of small stimulus-set sizes (n=4) as compared to large stimulus-set sizes (n=96) with increasing time on task. The second aim was to determine whether younger as well as older adults were able to create associations between task components. As age differences in task switching are often found to be larger when response mappings are incompatible we also investigated interactions with response compatibility. Results of our study indicated that practice effects on switch costs were much more pronounced for small than large stimulus-set sizes, consistent with the view that the strength of associations between task components facilitates task switching. Furthermore, we found that practice benefits on task switching for small stimulus-set sizes were sensitive to age and response compatibility. In contrast to younger adults, who showed a reduction of switch costs for both response mapping conditions, older adults showed a reduction of switch costs only when response mappings were compatible. That is, older adults showed less associative learning when the currently irrelevant task feature had to be suppressed, supporting the view that older adults have primarily problems in separating overlapping task-set representations.     

The role of response selection in sequence learning

We investigated the role of response selection in sequence learning in the serial reaction time (SRT) task, by manipulating stimulus-response compatibility. Under conditions in which other types of learning, like perceptual, response-based, and response-effect learning, were unaffected, sequence learning was better with an incompatible than with a compatible stimulus-response mapping. Stimulus discriminability, on the other hand, had no influence on the amount of sequence learning. This indicates that the compatibility effects cannot be accounted for by a different level of task difficulty. Relating our results to the dimensional overlap model (Kornblum, Hasbroucq, & Osman, 1990), which assumes that incompatible stimulus-response mappings require more controlled response selection than do compatible stimulus-response mapping, we suggest that sequence learning in the SRT task is particularly effective when response selection occurs in a controlled way.     

Is automatic imitation a specialized form of stimulus–response compatibility? Dissociating imitative and spatial compatibilities

In recent years research on automatic imitation has received considerable attention because it represents an experimental platform for investigating a number of interrelated theories suggesting that the perception of action automatically activates corresponding motor programs. A key debate within this research centers on whether automatic imitation is any different than other long-term S-R associations, such as spatial stimulus-response compatibility. One approach to resolving this issue is to examine whether automatic imitation shows similar response characteristics as other classes of stimulus-response compatibility. This hypothesis was tested by comparing imitative and spatial compatibility effects with a two alternative forced-choice stimulus-response compatibility paradigm. The stimulus on each trial was a left or right hand with either the index or middle finger tapping down. Speeded responses were performed with the index or middle finger of the right hand in response to the identity or the left-right spatial position of the stimulus finger. Two different tasks were administered: one that involved responding to the stimulus (S-R) and one that involved responding to the opposite stimulus (OS-R; i.e., the one not presented on that trial). Based on previous research and a connectionist model, we predicted standard compatibility effects for both spatial and imitative compatibility in the S-R task, and a reverse compatibility effect for spatial compatibility, but not for imitative compatibility, in the OS-R task. The results from the mean response times, mean percentage of errors, and response time distributions all converged to support these predictions. A second noteworthy result was that the recoding of the finger identity in the OS-R task required significantly more time than the recoding of the left-right spatial position, but the encoding time for the two stimuli in the S-R task was equivalent. In sum, this evidence suggests that the processing of spatial and imitative compatibility is dissociable with regard to two different processes in dual processing models of stimulus-response compatibility.     

Unmixing the mixing cost: contributions from dimensional relevance and stimulus-response suppression

When participants repeat the same task in a context in which the task may also switch (a mixed block), performance deteriorates compared to when there is only one task repeating (a pure block). Three experiments were designed to assess how perceptual and motor transitions influenced this mixing cost. Experiment 1 provided three pure block baselines for perceptual and motor transitions. Experiments 2 and 3 examined these transitions in a mixed block. Results show that most of the mixing cost comes from two factors: (a) episodic interference in the mixed block when the stimulus changes and the response repeats, and (b) increased suppression in mixed blocks affecting trials where stimulus-response mappings repeat. We propose that these mechanisms are strategically applied when adopting a sustained "switching set" in mixed blocks. The purpose of this set would be to avoid perseveration errors in the most demanding trials (the task-switching trials), but remaining active during task-repetitions. Results regarding the mixing cost are thus relevant to the assessment of models of task-switching, which at present mainly rely on data from task switch trials.     

Response inhibition under task switching: its strength depends on the amount of task-irrelevant response activation

Under task switch conditions, response repetitions usually produce benefits if the task also repeats, but costs if the task switches. So far, it is largely undecided how to account for these effects. In the present study, we provide additional evidence in favor of the account that each response is inhibited in order to prevent its accidental re-execution. To test this hypothesis, the risk of an accidental re-execution of a given response was manipulated by modulating the activation of the response in the previous task. In Experiment 1, this was done by means of congruent and incongruent stimuli. As expected, on task switch trials, the repetition costs were larger if a congruent rather than an incongruent stimulus occurred in the previous task. In Experiment 2, the same effect occurred for stimulus-response compatible versus incompatible stimuli in the previous task. In Experiment 3, both manipulations were applied together, which produced almost additive effects. Altogether, the results support the inhibition account for the response repetition effects under task switch conditions.     

Task switching and the measurement of “switch costs”

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. Received: 31 March 1999 / Accepted: 23 July 1999     

Determinants of the benefit for consistent stimulus-response mappings in dual-task performance of four-choice tasks

Reaction times are shorter when the stimulus-response mappings for pairs of three-choice tasks are consistent (both corresponding or both mirrored) than when they are inconsistent. The benefit for consistent mirrored mappings is evident at the side positions for each task, for which the responses are crossed, but not at the middle position, for which the response is corresponding. In the present study, we report experiments in which we tested implications of an emergent mapping-choice account of the consistency benefit using pairs of four-choice tasks. This procedure allows crossed responses for all positions when the mapping is mirrored and use of mixed mappings for which one pair of stimuli and responses within a task has a corresponding assignment and the other a crossed assignment. Results showed that when a pure corresponding or pure mirrored mapping was used, there was a consistency benefit for both the middle and side positions. However, when the mixed mapping was introduced, the consistency benefit for that mapping depended on the overall complexity of the set of individual stimulus-response pairings for the combined tasks. A mapping choice between Tasks 1 and 2 is only one of several emergent processes that contribute to response-selection efficiency in dual-task contexts.     

Age differences in response selection for pure and mixed stimulus-response mappings and tasks

Two experiments examined effects of mixed stimulus-response mappings and tasks for older and younger adults. In Experiment 1, participants performed two-choice spatial reaction tasks with blocks of pure and mixed compatible and incompatible mappings. In Experiment 2, a compatible or incompatible mapping was mixed with a Simon task for which the mapping of stimulus color to location was relevant and stimulus location was irrelevant. In both experiments, older adults showed larger mixing costs than younger adults and larger compatibility effects, with the differences particularly pronounced in Experiment 1 when location mappings were mixed. In mixed conditions, when stimulus location was relevant, older adults benefited more than younger adults from complete repetition of the task and stimulus from the preceding trial. When stimulus location was irrelevant, the benefit of complete repetition did not differ reliably between age groups. The results suggest that the age-related deficit associated with mixing mappings and tasks is primarily due to older adults having more difficulty separating task sets that activate conflicting response codes.     

Task switching and response correspondence in the psychological refractory period paradigm

Three experiments examined the effects of task switching and response correspondence in a psychological refractory period paradigm. A letter task (vowel-consonant) and a digit task (odd-even) were combined to form 4 possible dual-task pairs in each trial: letter-letter, letter-digit, digit-digit, and digit-letter. Foreknowledge of task transition (repeat or switch) and task identity (letter or digit) was varied across experiments: no foreknowledge in Experiment 1, partial foreknowledge (task transition only) in Experiment 2, and full foreknowledge in Experiment 3. For all experiments, the switch cost for Task 2 was additive with stimulus onset asynchrony, and the response-correspondence effect for Task 2 was numerically smaller in the switch condition than in the repeat condition. These outcomes suggest that reconfiguration for Task 2 takes place after the central processing of Task 1 and that the crosstalk correspondence effect is due to response activation by way of stimulus-response associations.     

Influence of mapping complexity on negative priming for incompatible spatial mappings

For tasks with an incompatible stimulus-response mapping, whether the compatible response must be inhibited paradigm for four-choice tasks with three different incompatible spatial mappings. For a mapping that did not follow a simple rule, reaction time was lengthened when the corresponding response on the preceding trial became the required response on the current trial, as compared with when it did not, showing a negative priming effect. However, for mappings that followed a simple rule, negative priming was not evident. The present study extends this research to a more complex mapping. On the basis of a two-process model adopted from the negative priming literature, we hypothesized that high mapping complexity should also diminish the negative priming effect for incompatible mappings, because the balance of cognitive resources is allocated to identification of the correct response. Two experiments are reported in which mappings of different complexity were used in six-choice spatial tasks. Analyses of reaction times showed that negative priming diminished with increased mapping complexity, apparently due to increased dominance of response identification processes, rather than inhibition of the corresponding response.     

Exploring task-set reconfiguration with random task sequences

Switching between two different tasks normally results in an impairment in people's performance known as a switch cost, typically measured as an increase in reaction time (RT) and errors compared to a situation in which no task switch is required. Researchers in task switching have suggested that this switch cost is the behavioural manifestation of the task set reconfiguration processes that are necessary to perform the upcoming task. However, an examination of the literature in task switching reveals apparently contradictory results about the nature of task set reconfiguration processes. In Experiment 1, we addressed this issue by comparing participants' performance in two different experimental conditions: predictable task switching and random task switching. In the predictable switch condition the switch cost completely vanished after the first repetition of the new task. However, in the random switch condition, while the difference between switch and repetition trials was not significant, we observed a significant reduction in RT between the first and second repetition of the new task. In Experiment 2, we further investigated the pattern of task set reconfiguration in the random switch situation. The results showed a progressive reduction of participants' response latencies across repetitions of the same task. The present study demonstrates that, whereas the results in predictable switching conditions are compatible with an exogenous-reconfiguration hypothesis, random task switching produces a more gradual, decay-like switch cost reduction with task repetition.     

A switch in task affects priming of pop-out: evidence for the role of episodes

Maljkovic and Nakayama (Memory & Cognition, 22(6), 655-678, 1994) demonstrated that response times decrease in a pop-out search task when target-defining features repeat from one trial to the next. This priming of pop-out (PoP) effect has been explained by some researchers as reflecting low-level modulations in attentional control settings Lee, Mozer, and Vecera (Attention, Perception, & Psychophysics, 71(5), 1059-1071, 2009). The present experiments tested whether a shift in higher order task requirements from trial n - 1 to trial n alters PoP effects. The results of Experiments 1 and 2 demonstrated that a switch in task significantly modulated PoP effects when shape was the relevant pop-out dimension. Experiment 3 failed to show significant modulation of PoP as a function of task switch when the pop-out dimension was color, but the findings of Experiment 4 did show modulation of PoP for color when the relative salience of target and distractors was high. Together, the results strongly support the view that PoP effects can be sensitive to a switch in task, a result consistent with the view that PoP effects are modulated by trial-to-trial episodic integration processes.     

That’s what task sets are for: shielding against irrelevant information

Goal-directed behavior requires the cognitive system to distinguish between relevant and irrelevant information. The authors show that task sets help to shield the system from irrelevant information. Participants had to respond to eight different colored word stimuli under different instruction conditions. They either had to learn the stimulus-response mappings (SR condition), to use one task set (1 TS condition) or to use two different task sets (2 TS condition). In the 2 TS and the SR conditions, participants showed response repetition effects (interaction of color repetition x response repetition), indicating that participants processed the color of the words. Importantly, the 1 TS condition did not show such an interaction. Overall, the results provide evidence for the shielding function of task sets. This benefit turns into costs in classical task switching paradigms. From this perspective, switch costs can be interpreted as the consequence of successful shielding on the previous task.