Dual tasks (DTs) are characterized by the requirement for additional mechanisms that coordinate the processing order of two temporally overlapping tasks. These mechanisms are indicated by two types of costs that occur when comparing DT blocks with fixed and random orders of the component tasks. On a block level, task-order control costs are reflected in increased reaction times (RTs) in random-order compared to fixed-order blocks, indicating global, monitoring-based, coordination mechanisms. On a trial level, within random-order blocks, order-switch costs are indicated by increased RTs on order switch compared to order repetition trials, reflecting memory-based mechanisms that guide task-order in DTs. To test the nature of these mechanisms in two experiments, participants performed DTs in fixed- and random-order blocks. In random-order blocks, participants were either instructed to respond to both tasks according to the order of task presentation (sequential-order instruction) or instructed to freely decide in which order to perform both tasks (free-order instruction). As a result of both experiments, we demonstrated that task-order control costs were reduced under the free-order compared to the sequential-order instruction, whereas order-switch costs were not affected by our instruction manipulation. This pattern of results suggests that the task-order control costs reflect global processes of task-order regulation such as engaging monitoring processes that are sensitive to changes in order instructions, while order-switch costs reflect rather local memory-based mechanisms that occur irrespective of any effort to coordinate task-order.

Dual tasks are characterized by the requirement for additional task-order coordination processes that schedule the processing order of two temporally overlapping tasks. Preliminary evidence from functional imaging studies suggests that lateral pFC (lPFC) activation correlates with implementing these task-order coordination processes. However, so far, it is unclear whether the lPFC is also causally involved in coordinating task order during dual-task performance and which exact mechanisms are implemented by this brain region. In this study, we addressed these open issues by applying online TMS during a dual-task situation. For this purpose, participants performed a dual task in fixed-order blocks with a constant order of tasks and in random-order block, in which the order of tasks varied randomly and thus demands on task-order coordination were increased. In Experiment 1, TMS of the lPFC compared with control TMS conditions impaired dual-task performance in random-order blocks, whereas performance in fixed-order blocks was unaffected by TMS. In Experiment 2, we tested for the specificity of the lPFC TMS effect on task-order coordination by applying TMS over the preSMA. We showed that preSMA TMS did not affect dual-task performance, neither in fixed-order nor in random-order blocks. Results of this study indicate that the lPFC, but not the preSMA, is causally involved in implementing task-order coordination processes in dual-task situations.

When two overlapping tasks are processed, they hit a bottleneck at a central processing stage that prevents simultaneous processing of the two tasks. Thus far, however, the factors determining the processing order of the tasks at the bottleneck are unknown. The present study was designed to (re)investigate whether the arrival times of the two tasks at the central bottleneck are a key determinant of the processing order (cf. Sigman & Dehaene, 2006). To this end, we implemented a visual-auditory dual task with a random stimulus order, in which we manipulated arrival time by prolonging the initial, perceptual processing stage (stimulus analysis) of the visual task and compared the effects of this manipulation with those of one impacting the central bottleneck stage of the visual task. Additionally, we implemented two instruction conditions: Participants were told to respond either in the order of stimulus presentation or in the order they preferred. The manipulation of the visual perception stage led to an increase in task response reversals (i.e., the response order was different from the order of stimulus presentation), whereas there was no such increase when the bottleneck stage was manipulated. This pattern provides conclusive evidence that the processing order at the bottleneck is (at least in part) determined by the arrival times of the tasks at that point. Reaction time differences between the two instruction conditions indicated that additional control processes are engaged in determining task processing order when the participants are expressly told to respond in the order of stimulus presentation.