Dual tasking is known to yield performance costs. Corresponding research has often focused on early or central task processing stages, that is, on features related to stimulus processing or response selection. Here, we demonstrate the important role of the final (late) stage of task processing by studying effects of effector system combinations. We used pairwise combinations of tasks requiring oculomotor, manual, vocal, and pedal responses, triggered by visual/auditory stimuli. Across task combinations, we compared dual-task costs among effector systems (e.g., oculomotor, vocal, and pedal) under controlled conditions, that is, when combined with the same “context effector” (e.g., manual) in the other task. The dual-task cost pattern was strongly determined by the particular combination of effector systems in line with the assumption of an ordinal effector-based prioritization pattern (oculomotor > pedal > vocal > manual), and could not be explained by classic “first-come, first-served” accounts of central processing. Stimulus modality and its mapping to effector systems affected reaction times (RTs), but the impact on the general prioritization scheme was negligible, suggesting a more substantial influence of output (compared with input) system characteristics on dual-task capacity scheduling. The results call for a distinct effector system weighting mechanism in models of dual-task control.

In situations requiring the execution of two tasks at around the same time, we need to decide which of the tasks should be executed first. Previous research has revealed several factors that affect the outcome of such response order control processes, including bottom-up factors (e.g., the temporal order of the stimuli associated with the two tasks) and top-down factors (e.g., instructions). In addition, it has been shown that tasks associated with certain response modalities are preferably executed first (e.g., temporal prioritisation of tasks involving oculomotor responses). In this study, we focused on a situation in which task order has to be unpredictably switched from trial to trial and asked whether task-order representations are coded separately or integrated with the component task sets (i.e., in a task-specific manner). Across three experiments, we combined two tasks known to differ in prioritisation, namely an oculomotor and a manual (or pedal) task. The results indicated robust task-order switch costs (i.e., longer RTs when task order was switched vs. repeated). Importantly, the data demonstrate that it is possible to show an asymmetry of task-order switch costs: While these costs were of similar size for both task orders in one particular experimental setting with specific spatial task characteristics, two experiments consistently indicated that it was easier for participants to switch to their prioritised task order (i.e., to execute the dominant oculomotor task first). This suggests that in a situation requiring frequent task-order switches (indicated by unpredictable changes in stimulus order), task order is represented in an integrated, task-specific manner, bound to characteristics (here, associated effector systems) of the component tasks.

When processing of two tasks overlaps, performance is known to suffer. In the well-established psychological refractory period (PRP) paradigm, tasks are triggered by two stimuli with a short temporal delay (stimulus onset asynchrony; SOA), thereby allowing control of the degree of task overlap. A decrease of the SOA reliably yields longer RTs of the task associated with the second stimulus (Task 2) while performance in the other task (Task 1) remains largely unaffected. This Task 2-specific SOA effect is usually interpreted in terms of central capacity limitations. Particularly, it has been assumed that response selection in Task 2 is delayed due to the allocation of less capacity until this process has been completed in Task 1. Recently, another important factor determining task prioritization has been proposed—namely, the particular effector systems associated with tasks. Here, we study both sources of task prioritization simultaneously by systematically combining three different effector systems (pairwise combinations of oculomotor, vocal, and manual responses) in the PRP paradigm. Specifically, we asked whether task order-based task prioritization (SOA effect) is modulated as a function of Task 2 effector system. The results indicate a modulation of SOA effects when the same (oculomotor) Task 1 is combined with a vocal versus a manual Task 2. This is incompatible with the assumption that SOA effects are solely determined by Task 1 response selection duration. Instead, they support the view that dual-task processing bottlenecks are resolved by establishing a capacity allocation scheme fed by multiple input factors, including attentional weights associated with particular effector systems.

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.

Abstract. Doing two things at once (vs. one in isolation) usually yields performance costs. Such decrements are often distributed asymmetrically between the two actions involved, reflecting different processing priorities. A previous study (Huestegge & Koch, 2013) demonstrated that the particular effector systems associated with the two actions can determine the pattern of processing priorities: Vocal responses were prioritized over manual responses, as indicated by smaller performance costs (associated with dual-action demands) for the former. However, this previous study only involved auditory stimulation (for both actions). Given that previous research on input–output modality compatibility in dual tasks suggested that pairing auditory input with vocal output represents a particularly advantageous mapping, the question arises whether the observed vocal-over-manual prioritization was merely a consequence of auditory stimulation. To resolve this issue, we conducted a manual–vocal dual task study using either only auditory or only visual stimuli for both responses. We observed vocal-over-manual prioritization in both stimulus modality conditions. This suggests that input–output modality mappings can (to some extent) attenuate, but not abolish/reverse effector-based prioritization. Taken together, effector system pairings appear to have a more substantial impact on capacity allocation policies in dual-task control than input–output modality combinations.