Dual-Action Costs and Benefits in a Uni-Modal Single-Onset Paradigm.

While performing two actions at the same time has mostly been associated with reduced performance, several recent studies have observed the opposite effect, that is, dual-action benefits. Previous evidence suggests that dual-action benefits result from single-action inhibitory costs - more specifically, it appears that under certain circumstances, single-action representations are derived from dual-action representations by removing (i.e., inhibiting) one of the component actions. In the present paper, we investigated if this is tied to the presence of multi-modal response demands (i.e., responses making use of two different effector systems). We implemented a very simple experimental paradigm where participants responded to a single stimulus with zero, one, or two uni-modal responses. As predicted, we did not observe dual-action benefits, but rather significant dual-action costs. Furthermore, a trial-by-trial sequence analysis revealed that alternations between both single-action responses were associated with significantly better performance than all other types of action switches. This can be accounted for by assuming that actions are represented as "feature bundles" and that switching a single, binary distinctive feature of an action to its opposite is relatively easy.

Are some effector systems harder to switch to? In search of cost asymmetries when switching between manual, vocal, and oculomotor tasks.

In task-switching studies, performance is typically worse in task-switch trials than in task-repetition trials. These switch costs are often asymmetrical, a phenomenon that has been explained by referring to a dominance of one task over the other. Previous studies also indicated that response modalities associated with two tasks may be considered as integral components for defining a task set. However, a systematic assessment of the role of response modalities in task switching is still lacking: Are some response modalities harder to switch to than others? The present study systematically examined switch costs when combining tasks that differ only with respect to their associated effector systems. In Experiment 1, 16 participants switched (in unpredictable sequence) between oculomotor and vocal tasks. In Experiment 2, 72 participants switched (in pairwise combinations) between oculomotor, vocal, and manual tasks. We observed systematic performance costs when switching between response modalities under otherwise constant task features and could thereby replicate previous observations of response modality switch costs. However, we did not observe any substantial switch-cost asymmetries. As previous studies using temporally overlapping dual-task paradigms found substantial prioritization effects (in terms of asymmetric costs) especially for oculomotor tasks, the present results suggest different underlying processes in sequential task switching than in simultaneous multitasking. While more research is needed to further substantiate a lack of response modality switch-cost asymmetries in a broader range of task switching situations, we suggest that task-set representations related to specific response modalities may exhibit rapid decay.

The hard work of doing nothing: Accounting for inhibitory costs during multiple action control.

Performing many actions at the same time is usually associated with performance costs. However, recent eye-tracking evidence indicates that under specific conditions, inhibiting a secondary response can be more costly than executing it, resulting in dual-action benefits. Here, we show that performance gains due to the absence of inhibitory control demands in dual-action trials are not limited to saccades as a response modality. In our study, participants had to react to a visually presented directional word by either reading the stimulus aloud (vocal modality), pressing the corresponding arrow key on a keyboard (manual modality), or both. Crucially, manual error rates were significantly lower when participants had to respond with both a button press and naming than when they had to respond with naming only. More specifically, in vocal-only conditions we observed a significant percentage of false-positive manual responses, suggesting difficulties with inhibiting an unwarranted manual action. Thus, our results indicate that difficulties associated with single- (vs. dual-) action control are a stable, domain-general phenomenon that likely arises whenever execution-related demands are accompanied by substantial additional inhibitory control demands.

Sources of interference in cross-modal action: response selection, crosstalk, and general dual-execution costs.

Performing several actions simultaneously usually yields interference, which is commonly explained by referring to theoretical concepts such as crosstalk and structural limitations associated with response selection. While most research focuses on dual-task scenarios (involving two independent tasks), we here study the role of response selection and crosstalk for the control of cross-modal response compounds (saccades and manual responses) triggered by a single stimulus. In two experiments, participants performed single responses and spatially compatible versus incompatible dual-response compounds (crosstalk manipulation) in conditions with or without response selection requirements (i.e., responses either changed randomly between trials or were constantly repeated within a block). The results showed that substantial crosstalk effects were only present when response (compound) selection was required, not when a pre-selected response compound was merely repeated throughout a block of trials. We suggest that cross-response crosstalk operates on the level of response selection (during the activation of response codes), not on the level of response execution (when participants can rely on pre-activated response codes). Furthermore, we observed substantial residual dual-response costs even when neither response incompatibility nor response selection requirements were present. This suggests additional general dual-execution interference that occurs on a late, execution-related processing stage and even for two responses in rather distinct (manual and oculomotor) output modules. Generally, the results emphasize the importance of considering oculomotor interference in theorizing on multiple-action control.

Effects of a no-go Task 2 on Task 1 performance in dual - tasking: From benefits to costs.

When two tasks are combined in a dual-task experiment, characteristics of Task 2 can influence Task 1 performance, a phenomenon termed the backward crosstalk effect (BCE). Besides instances depending on the (spatial) compatibility of both responses, a particularly interesting example was introduced by Miller (2006): If Task 2 was a no-go task (i.e., one not requiring any action at all), responses were slowed in Task 1. Subsequent work, however, also reported the opposite result-that is, faster Task 1 responses in cases of no-go Task 2 trials. We report three experiments aiming to more precisely identify the conditions under which a no-go Task 2 facilitates or impedes Task 1 performance. The results suggest that an adverse no-go BCE is only observed when the Task 2 response(s) are sufficiently prepared in advance, yielding strong inhibitory control demands for Task 2 that eventually hamper Task 1 processing as well (i.e., inhibitory costs). If this is not the case, encountering a no-go Task 2 trial facilitates Task 1 performance, suggesting that the underlying task representation is reduced to a single - task. These results are discussed in the context of other recent work on BCEs and of recently suggested accounts of the no-go BCE.