In daily life, eye movement control usually occurs in the context of concurrent action demands in other effector domains. However, little research has focused on understanding how such cross-modal action demands are coordinated, especially when conflicting information needs to be processed conjunctly in different action modalities. In two experiments, we address this issue by studying vocal responses in the context of spatially conflicting eye movements (Experiment 1) and in the context of spatially conflicting manual actions (Experiment 2, under controlled eye fixation conditions). Crucially, a comparison across experiments allows us to assess resource scheduling priorities among the three effector systems by comparing the same (vocal) response demands in the context of eye movements in contrast to manual responses. The results indicate that in situations involving response conflict, eye movements are prioritized over concurrent action demands in another effector system. This oculomotor dominance effect corroborates previous observations in the context of multiple action demands without spatial response conflict. Furthermore, and in line with recent theoretical accounts of parallel multiple action control, resource scheduling patterns appear to be flexibly adjustable based on the temporal proximity of the two actions that need to be performed.

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.

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