The role of response selection for inhibition of task sets in task shifting

Response selection in task shifting was explored using a go/no-go methodology. The no-go signal occurred unpredictably with stimulus onset so that all trials required task preparation but only go trials required response selection. Experiment 1 showed that shift costs were absent after no-go trials, indicating that response processes are crucial for shift costs. In Experiment 2, backward inhibition was absent after no-go trials. Experiments 3 and 4 demonstrated that response selection, rather than execution, causes backward inhibition. All 4 experiments showed effects of preparation time in go trials, suggesting that advance preparation must have also occurred in no-go trials. The authors concluded that inhibition of irrelevant task sets arises only at response selection and that residual shift costs reflect such persisting inhibition.     

Response selection and response execution in task switching: evidence from a go-signal paradigm

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.     

Eye gaze cannot be ignored (but neither can arrows)

Recent studies have shown that switch costs (i.e., slower responding on task-alternation trials than on task-repetition trials) are not observed when on the preceding trial a no-go signal instructed the participant to withhold the response to the target stimulus. This finding suggests that neither task set is inhibited on no-go trials, and that the origin of switch costs is located in the application of the task set to the target stimulus. However, these studies also showed that responding after a no-go trial is substantially slower than after a go trial. This suggests that both task sets are inhibited on no-go trials and that switch costs originate from the preparation of a task set. In two experiments we created conditions that revealed an absence of switch costs in conjunction with relatively fast responding after no-go trials. Together these findings confirm that switch costs originate from the application of the task set.     

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.     

Task-switch costs subsequent to cue-only trials

There is abundant evidence that there is a performance cost associated with switching between tasks. This “switch cost” has been postulated to be driven by task performance on the preceding trial, but recent research challenges any necessary role of previous task performance in driving the cost. Across three experiments, we investigated whether it is difficult to switch from a task that was prepared but never performed. We replicated the finding of a switch cost following cue-only trials (involving no task performance) whilst controlling for a potential cue-switching confound. This cost was larger than that following completed trials when preparation interval was short (300?ms), and it reduced significantly with a longer preparation interval (1000?ms) on the current trial. We also found that preparing only to attend to a particular visual dimension (colour or shape) was sufficient to drive a significant subsequent switch cost, which appeared to be residual in nature; we speculate that this cost may reflect the persistence of unfulfilled task intentions and/or a strategic slowing when consecutive intentions conflict.     

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     

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.     

Impact of contingency manipulations on accessory stimulus effects

Accessory tone stimuli facilitate response performance despite being irrelevant for the current task. In order to investigate which processes are affected by accessory stimuli, we presented accessory tones in a simple response time (RT) task while varying the contingencies between accessory stimulation and either responses(Experiment 1) or stimulus conditions (Experiment 2). Accessory tones speeded up responding to a larger degree when they were conjointly presented within go compared with no-go trials. In contrast, contingency variation with stimulus conditions did not alter the impact of accessory stimuli. Additionally, accessory tones increased response force. Thus, we conclude that in simple RT tasks accessory tones influence response-related stages such as response selection and response execution rather than perceptual processes.     

Response retrieval in a go/no-go priming-of-popout task

Priming of popout is the finding that singleton search is faster when features of a target and of nontargets are repeated across trials than when the features switch. Theoretical accounts suggest that intertrial repetition influences perceptual and attentional selection processes, episodic retrieval processes, or both. The present study combined a popout search task with a go/no-go task. In Experiment 1, the nontarget distractors in each display carried the go/no-go feature, and in Experiment 2, the texture of all items carried the go/no-go feature. Results showed that the go/no-go task moderated the intertrial repetition effects. In Experiment 1, the target color elicited retrieval of the preceding distractor color and associated no-go response, resulting in larger interference effects. In Experiment 2, the target color elicited retrieval of the preceding target color and no-go response, resulting in reduced facilitation effects. Additional results from both experiments showed that the colors in a search display also influenced target selection on the following trial. Taken together, the results of both experiments suggest that intertrial repetition influences both early selection and postselection retrieval processes.     

Contextual dependencies in a stimulus equivalence paradigm

Two experiments with human subjects assessed contextual dependencies in a stimulus equivalence paradigm. Subjects learned to form two sets of stimuli in a matching-to-sample training procedure. Each set was presented against one of two different background colours, the contextual cues. At test, the influence of a context change--that is, presenting each set against the other context--was measured on baseline, symmetry, and equivalence trials. These three trial types reflect a difference in task complexity. It was predicted that the magnitude of context-dependent effects would be a positive function of task complexity. In Experiment 1, the context change was realized by switching the stimulus set at test while keeping the background colour constant. In Experiment 2, the stimulus set remained constant, and the background colour was switched. In both experiments, a change in context only resulted in an increase in response latency on equivalence trials; no effect was seen on symmetry and baseline trials. Results were discussed in the framework of switch costs, habituation to contextual stimuli, and a model based on Shea and Wright (1995) that explains the differential influence of a context switch on easy versus difficult tasks.     

An electrophysiological study of the insertion of overt response choice

Using electrophysiological measures, the authors studied changes in prestimulus state, stimulus identification, and response-related processing when, in a go/no-go task, forced choice between 2 overt go responses was inserted. The authors observed decreased prestimulus motor preparation (electromyogram), no change in stimulus identification time (selection negativity), a minor increase in response selection time (lateralized readiness potential), a large increase in response preparation time (lateralized readiness potential), a minor effect on response execution time (electromyogram), and a decrease in the activation of a response-inhibition process on no-go trials (frontal event-related potential). The existence of the response-inhibition process was verified by the presence of inverted lateralized readiness potentials on no-go trials. Pure insertion of response choice in a task seems impossible because the choice between activation and inhibition (go/no-go) always seems already present.     

Conditions under which children experience inhibitory difficulty with a "button-press" go/no-go task

Go/no-go tasks seem to provide a simple marker of inhibitory development in young children. Children are told to respond to one stimulus on go trials but to make no response to another stimulus on no-go trials; responding on no-go trials is assumed to reflect a failure to inhibit the go response. However, there is evidence to suggest that a type of go/no-go task, which we call the "button-press" task, does not require inhibition. We investigated the conditions under which young children (M=3 years 6 months, N=120) experience inhibitory difficulty with this type of task. The data suggest that the speed of stimulus presentation is crucial and that other studies using this type of task have presented the stimuli too briefly. The importance of establishing the inhibitory credentials of a task before it is used as a marker of inhibitory control is emphasized.     

Electrophysiological evidence for temporal overlap among contingent mental processes.

A series of studies assessed perceptual-motor transmission of stimulus information by measuring lateralization of movement-related brain potentials in a choice reaction task with no-go trials. When stimuli varied in shape and size, lateralized potentials on no-go trials suggested that easily recognized shape information was used to initiate motor preparation and that this preparation was aborted when size analysis signified that the response should be withheld. This indicates that movement preparation can begin once partial perceptual information about a stimulus becomes available, contrary to an assumption of fully discrete models of information processing. By contrast, when stimuli varied only in size, no evidence for preliminary response preparation was obtained, contrary to an assumption of fully continuous models but consistent with asynchronous discrete coding models (Miller, 1982, 1988).     

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)     

Eliminating the cost of task set reconfiguration

With insufficient time to fully prepare for a switch in task, a deterioration in performance on the first trial of a new task would be expected. The interest of researchers has been captured by the residual switch costs that, surprisingly, remain despite sufficient time to prepare. We used avery simple task to investigate the costs to reaction time and accuracy associated with changing between two different instructional sets every eight trials. Subjects responded to left and right visual targets by making either spatially compatible or incompatible eye movements (Experiment 1) or buttonpress responses (Experiment 2). The subjects were cued as to whether to make the compatible or the incompatible response by the color of a border appearing on the perimeter of the display. In cases in which the subject alternated between making pro- and antisaccades, the large costs to reaction time and accuracy at the short cue-target stimulus onset asynchrony were completely eliminated when sufficient time was provided to prepare for the switch. This complete elimination of residual switch costs was not obtained when the same alternation was applied to manual responses. This pattern of results links residual costs to response selection processes and suggests that they are not a necessary component of the switch process. We propose that the elimination of "stubborn" residual switch costs is rooted in our use of a hypercompatible task (making saccades toward targets) that places minimal demands on response selection.     

Task switches under Go/NoGo conditions and the decomposition of switch costs

Alternating switches between two simple S-R tasks are combined with Go/NoGo tasks. Non-switches after Go trials are assumed to selectively profit from stimulus driven repetition benefits, whereas switches after NoGo trials are assumed to be selectively delayed by stimulus driven negative priming. Intentionally driven reconfiguration costs are assessed by RT differences between switches after Go trials (no negative priming) and non-switches after NoGo trials (no repetition benefits). Experiment 1 indicates that with short preparation time repetition benefits, negative priming costs, and intentional components contribute approximately additively to switch costs. Experiment 2 confirms that the delay of switches after NoGo trials is indeed due to negative priming. Experiments 3 and 4 show that repetition benefits and intentional components of switch costs are properly assessed only if the settings assure that participants reconfigure the required task set in NoGo as well as Go trials.     

A response-discrimination account of the Simon effect

Simon effects might partly reflect stimulus-triggered response activation. According to the response-discrimination hypothesis, however, stimulus-triggered response activation shows up in Simon effects only when stimulus locations match the top-down selected spatial codes used to discriminate between alternative responses. Five experiments support this hypothesis. In Experiment 1, spatial codes of each response differed by horizontal and vertical axis position, yet one axis discriminated between alternative responses, whereas the other did not. Simon effects resulted for targets on discriminating axes only. In Experiment 2, both spatial axes discriminated between responses, and targets on both axes produced Simon effects. In Experiment 3, Simon effects resulted for a spatial choice-reaction task but not for a go/no-go task. Even in the go/no-go task, a Simon effect was restored when a two-choice reaction task preceded the go/no-go task (Experiment 4) or when participants initiated trials with responses spatially discriminated from the go response (Experiment 5).     

Switching in the cocktail party: exploring intentional control of auditory selective attention

Using a novel variant of dichotic selective listening, we examined the control of auditory selective attention. In our task, subjects had to respond selectively to one of two simultaneously presented auditory stimuli (number words), always spoken by a female and a male speaker, by performing a numerical size categorization. The gender of the task-relevant speaker could change, as indicated by a visual cue prior to auditory stimulus onset. Three experiments show clear performance costs with instructed attention switches. Experiment 2 varied the cuing interval to examine advance preparation for an attention switch. Experiment 3 additionally isolated auditory switch costs from visual cue priming by using two cues for each gender, so that gender repetition could be indicated by a changed cue. Experiment 2 showed that switch costs decreased with prolonged cuing intervals, but Experiment 3 revealed that preparation did not affect auditory switch costs but only visual cue priming. Moreover, incongruent numerical categories in competing auditory stimuli produced interference and substantially increased error rates, suggesting continued processing of task-relevant information that often leads to responding to the incorrect auditory source. Together, the data show clear limitations in advance preparation of auditory attention switches and suggest a considerable degree of inertia in intentional control of auditory selection criteria.     

Reconfiguration of task-set: Is it easier to switch to the weaker task?

Switching between two tasks afforded by the same stimuli results in slower reactions and more errors on the first stimulus after the task changes. This “switch cost” is reduced, but not usually eliminated, by the opportunity to prepare for a task switch. While there is agreement that this preparation effect indexes a control process performed before the stimulus, the “residual” cost has been attributed to several sources: to a control process essential for task-set reconfiguration that can be carried out only after the stimulus onset, to probabilistic failure to engage in preparation prior to the stimulus, and to two kinds of priming from previous trials: positive priming of the now-irrelevant task set and inhibition of the now-relevant task-set. The main evidence for the carry-over of inhibition is the observation that it is easier to switch from the stronger to the weaker of a pair of tasks afforded by the stimulus than vice versa. We survey available data on interactions between task switching and three manipulations of relative task strength: pre-experimental experience, stimulus-response compatibility, and intra-experimental practice. We conclude that it is far from universally true that it is easier to switch to the weaker task. Either inhibition of the stronger task-set is a strategy used only in the special case of extreme inequality in strength, or its consequences for later performance may be masked by slower post-stimulus control operations for more complex tasks. Inhibitory priming may also be stimulus specific. Received: 31 March 1999 / Accepted: 23 July 1999     

Age-related differences in switching between cognitive tasks: does internal control ability decline with age?

The present study tested the hypothesis that older adults establish a weaker task set than younger adults and therefore rely more on stimulus-triggered activation of task sets. This hypothesis predicts that older adults should have difficulty with task switches, especially when the stimuli-responses are associated with multiple, competing tasks. Weak task preparation, however, could actually benefit older adults when performing an unexpected task. The authors tested this prediction in Experiment 1 using a repeating AABB task sequence, with univalent and bivalent stimuli intermixed. On some univalent trials, participants received an unexpected task. Contrary to the authors' predictions, expectancy costs were not smaller for older adults. Similar findings were obtained in Experiments 2 and 3, in which the authors used a task-cueing paradigm to more strongly promote deliberate task preparation. The authors found no disproportionate age effects on switch costs but did find age effects on bivalence costs and mixing costs. The authors conclude that older adults do experience extra difficulty dealing with stimuli associated with 2 active tasks but found no evidence that the problem specifically stems from an increased reliance on bottom-up task activation rather than top-down task preparation.