Shared or Distinct Attentional Resources? Confounds in Dual Task Designs, Countermeasures, and Guidelines

Human information processing is limited by attentional resources. That is, via attentional mechanisms humans select information that is relevant for their goals, and discard other information. While limitations of attentional processing have been investigated extensively in each sensory modality, there is debate as to whether sensory modalities access shared resources, or if instead distinct resources are dedicated to individual sensory modalities. Research addressing this question has used dual task designs, with two tasks performed either in a single sensory modality or in two separate modalities. The rationale is that, if two tasks performed in separate sensory modalities interfere less or not at all compared to two tasks performed in the same sensory modality, then attentional resources are distinct across the sensory modalities. If task interference is equal regardless of whether tasks are performed in separate sensory modalities or the same sensory modality, then attentional resources are shared across the sensory modalities. Due to their complexity, dual task designs face many methodological difficulties. In the present review, we discuss potential confounds and countermeasures. In particular, we discuss 1) compound interference measures to circumvent problems with participants dividing attention unequally across tasks, 2) staircase procedures to match difficulty levels of tasks and counteracting problems with interpreting results, 3) choosing tasks that continuously engage participants to minimize issues arising from task switching, and 4) reducing motor demands to avoid sources of task interference, which are independent of the involved sensory modalities.